Funny Dovetail

Hi guys !
I've been inspired by the design shown here
I'm a beginner non machinist & don't know how to use Solidworks yet (i'm having troubles with my graphics).
You can see my idea in the drawing below it's a very basic/crappy MS Paint drawing... i hope you'll understand !
I want to make the bearings of the right side adjustable by excentric for preload but will have to mod the design a bit...Any help ?
I don't know the effect of hardned steel bearings rolling on an iron dovetail, i'm thinking to use grey iron machined from solid. What do you think ?
How much will it cost in a basic machine shop ? (aprox price or expensive/lowcost or hours of work)
I want to build a cnc knee mill the size of an X2/X3 and use this design for XYZ, the mill will be cast aluminium. What do you think ?
The cool thing about this design is that you can lengthen/shorten it to suite your needs, IMHO.
Any input is much apreciated.
Thanks !
cnc2.

Attached


IMAGE(http://factorydaily.com/fdattachs/fdattachs6/112620533313592.JPG)

I think a lot of hobbyists greatly underestimate the cost of getting parts made at a machine shop. For the cost of your intended design, I would bet you can buy some linear rails and blocks for about the same cost. These will give you a better and more durable machine without so much aggravation.
One example of decently priced rails:
http://www.homeshopcnc.com/LinearGuide2.html
Matt

Thanks for your reply Matt !
I know about ready made linear rails & wanted to buy brand new HiWin linear rails from automation4less.com they have resonable pricing but, when i checked the shipping fees the price was doubled BECAUSE I'm outside of USA & very far from there...No one comming from USA to bring'em to me, not talking about the size & weight of 6 rails & 12 blocks.
I want to build a cnc knee mill the size of an X3 & want to use this funny dovetail design for XYZ one wide rail per axis with a heavy & long block(will cast it myself) with maybe 4 rows of bearings rolling on each side of the iron dovetail.
Will the bearings have a lamination effect on the dovetail & wear it out quickly ?
What do you think ?
Thanks !
cnc2.

I don't think your design is very good for durability or stiffness. I assume you want to be able to machine metal. The design you are "inspired" by does not look to be used for metal cutting, and as such can be much lighter.
There are hundreds of places to buy linear rails, maybe some close to you. If you specified your location as something a bit more specific than the planet you inhabit, maybe I could suggest an alternative source.
Matt

I'm outside of USA & very far from there.

Well, we are you located? "Earth" is clever but really doesn't give us much to go on when offering advice.
I agree with Matt, stock parts are often the best solution. Also, it is much easier to buy another in the event of a failure. i.e. What happens if part of your custom design wears out? You have to get another one made = $$$. If it was a stock part, you will probably spend much less.
There must be a source for linear rails in or close to your country; let us know where you're at and I'm sure someone could point you in the right direction.
Chris Kirchen
EDIT: Sorry for the double scolding about your location; Matt must be a faster typer than me

It's ok you don't have to be sorry Chris !

"Earth" is clever but really doesn't give us much to go on when offering advice

:
Well, i'm in Algiers, Algeria & i defy you to find a linear rails supplier in here. (who knows if you succeed i also need a supplier for servos or i'll be obliged to go with photocopier servos)
I also agree with the idea of ready made linear rail & all the hassle it will save me(not sure it's good english) but getting HiWins for double their price will cost as much as a ready made X3 buying a ready made X3 will also double its price, no supplier in here too.
All the automation shops in here sell only pneumatic equipment, i asked for a ball screw in a shop that had a big Festo sign at its door and the guy an old machinist told me we sell "automation parts" if you want a screw the hardware store is rigth there in the corner : then i said no it's not a simple screw it's especially used in automation... to believe me he took a big catalog...in fact it does exist aahh but it not used in here the market needs pneumatic spare parts so, that's what we offer...even if you want to buy it or you could order it, it would be VERY expensive (more than if i order it & pay shippment from a foreign country) it's industrial supplys it's intended for factories...too expensive for you...blablabla.
I found a "linear stage" but made from extruded aluminium with a built in hydraulic cylinder and the rail on it had plastic where balls should be rolling & was not intended to sustain any lateral loads...it was 80cm long, used and for aprox 350Euros.
Automation in here still means rough pick&place used in food industries & cnc shops can be counted on the fingers of the thumb i heared some Airbus parts were made in here so, those few cnc shops won't talk to the little guy.
Thanks for you help !
cnc2.

I don't think your design is very good for durability or stiffness. I assume you want to be able to machine metal. The design you are "inspired" by does not look to be used for metal cutting, and as such can be much lighter.
There are hundreds of places to buy linear rails, maybe some close to you. If you specified your location as something a bit more specific than the planet you inhabit, maybe I could suggest an alternative source.
Matt

Sorry Matt ididn't see your post !
Yea i want it to machine metal, for stiffness the iron dovetail will be 100mm wide or so & there will be one per axis. This will require to have the lead screws aside the main rail but should be ok.
Close countries, maybe Germany or France, my biggest problem is shipping, it costs a LOT +CUSTOMS, and you can't be sure to recive you package.
Sombody i know was comming from Germany for holidays & he brought me the UHU chips from Uli Huber & the electronics from reichelt.de...that's how i got the electronics, if Uli had sent me the chips by mail i might'd never recived 'em.
Linear rails are heavy weight the size is also a problem, i couldn't ask someone comming for holidays to bring me a ~15 kg package, so i'm stuck !
Thanks !
cnc2.

You might want to contact HiWin and some of the other automation component manufacturers and ask them who you closest distributor is.
When I built my first machine, I sourced the ball screws and ball nuts through a automation company. The parts weren't part of what they regularly sold, but it was in one of their supplier's catalog. They were overpriced and the attitude of the staff was poor; they must have felt that they were doing me a huge favor.
A couple of months later, I found out from Thompson's head office that there was another distributor in my area. When I went to talk to them, it was a completely different experience. The staff was courteous and pleasant, and the parts were about 30% to 40% cheaper. Maybe it was because they sold these parts all the time.
All I'm saying is that you might need to find the right shop to buy from.

You might want to contact HiWin and some of the other automation component manufacturers and ask them who you closest distributor is.
When I built my first machine, I sourced the ball screws and ball nuts through a automation company. The parts weren't part of what they regularly sold, but it was in one of their supplier's catalog. They were overpriced and the attitude of the staff was poor; they must have felt that they were doing me a huge favor.
A couple of months later, I found out from Thompson's head office that there was another distributor in my area. When I went to talk to them, it was a completely different experience. The staff was courteous and pleasant, and the parts were about 30% to 40% cheaper. Maybe it was because they sold these parts all the time.
All I'm saying is that you might need to find the right shop to buy from.

Thanks for the reply Chris !
The HiWin website has changed, on the old one there was no supplier in North Africa nor there is in the new one. In their new website the nearest HiWin plant is in Germany & there's no sellers network on it i think you're right about mailing'em. i also looked on hiwin worldwide and they directed me to the german plant.
My BIGGEST problem is shipping, packages are often lost when you ain't a big importation company with transit agents, not to talk about customs on ~700eur worth package. Even if i find someone comming from germany the package will be big & heavy...who can accept this ?
Even if i find someone that accepts to bring me that package, i'll sitll need a webstore to see the prices & order the parts for the guy to pay'em from there (like i did with reichelt.de for the electronics).
Will mail'em & see.
Thanks !
cnc2.

Hello,
Dear staff,
I saw many american webstores that are selling HiWin products where you can see products prices & order online, but i couldn't find any german webstore selling HiWin products to the public in Germany.
The nearest HiWin plant to me is in Germany, as i don't read/speak german, would you please point me to a german webstore that supports the english language, where i can see the prices & order the parts online ?
I am interrested in linear guideways, so please point me to a webstore that sells to the public & at public intended prices.
Many thanks !

That was my email [email protected] do you think about it ?
I need you advise because i'm used to forum english & my english isn't that good & i have to impove it. So please tell me the naked truth !
Thanks !
cnc2.

That was my email [email protected] ...what do you think about it ?

I would mention to them where you are located. Even though it appears as if the German webstore is the closest to you, there may be a local distributor that you don't know about. The local distributor will have the pull to be able to ensure that shipment does not get lost or loaded up with excessive customs fees. In the eyes of customs and the freight company, you are a one-time importer, and may get treated differently than a seasoned importer, such as a local distributor. Though, you will have to cover the distributor's customs and freight fees, but if your parts are part of a larger shipment, they will be much less than what you would pay by yourself.
You're in Algiers; a distributor in your town would be great, but anywhere in Algeria is still good. Unless the customs laws are radically different there, you don't have to pay customs fees if the package doesn't cross a country's border.
Chris Kirchen

Thanks for the reply Chris !
You're right, i forgot the main point.
I wrote an email to [email protected] asking'em for the closest supplier to me in here...i was waiting for a reply & forgot it was the weekend.
Thanks !
cnc2.

No reply from HiWin

Give them a week or so to fet back to you, sometimes companies take their time answering emails. A phone call may be better if you can arrange it.
Are there other people near you who are interested in building a machine? Maybe it would be possible to arrange a group buy and hire a pro to import all your parts at once. Then you wouldn't get charged so much in taxes and would hopefully reduce some of your headaches trying to get things in. Just make sure you get everything you need at once.
Good Luck
Matt

Give them a week or so to fet back to you, sometimes companies take their time answering emails. A phone call may be better if you can arrange it.
Good Luck
Matt

Thanks for the reply Keebler !
A week Sieg that is a smaller company than HiWin, was a lot faster, i was replied the day after & told there was no supplier in here, i inquired for the X3 & SX3 & they replied the day after with prices & leaflets (shipping from China ?$?$)...Companys, the bigger they get the less they care about the little guy.

Are there other people near you who are interested in building a machine? Maybe it would be possible to arrange a group buy and hire a pro to import all your parts at once. Then you wouldn't get charged so much in taxes and would hopefully reduce some of your headaches trying to get things in. Just make sure you get everything you need at once.

None i know about, there's a guy in here that made a cnc plasma cutter, i knew about him in factorydaily (but i don't know him nor been in contact with him) but take a look at his machine (My CNC Plasma Cutting Table) you'll see how far it is from what i want to build. It seems this country wasn't designed to reduce headaches but i'll have to find my way to succeed in this project.
Thanks !

cnc2.

Matt, Chris,
Here's the reply from HiWin.com(Asia) they redirected me to HiWin Germany.

Dear Sir:
We do not have in Algeria, maybe you can buy from HIWIN-GERMANY.
Pls find http://www.hiwin.de to get touch information.
Best regards,
Jason Liao

I'm still waiting for the reply from Germany (i mailed'em first).
EDIT: it seems that the german won't reply, i've seen many of german wabsites where there was no price list, the kind of companys who you tell: i need a replacement for this part Ref:XXX-XXXX without asking for the price, big guys working for big guys.
Anyone has an idea to improve my concept ? (i'm thinking about chrom plating the dovetail to get a harder surface)
Thanks !

cnc2.

I think that your main problem will be rigidity, not wear. If you want to give it a try, you can of course, but I think you would be sadly disappointed if you tried to machine much metal. Maybe it would work alright with light cuts. It all depends on the quality of the construction I guess. It just seems like that design would be difficult to get everything lined up just right so it works well.
Matt

I see !
I've thought this design will be stronger than the linear stages built from 2x20mm round rails salvaged from copiers with bronze bushings sliding on them, if you say that wear won't be a problem on my design i think i'll have to try it or try a modded version of it.
Today i saw this picture & thought about making a block from cast aluminium with more bearings
IMAGE(http://www.thebackshed.com/cnc/images/Dscf1613.jpg)
and make the bearings roll on the outside of the square bars just like my dovetail, i'm thinking about filling the gap between the bars with aluminium to keep'em from flexing...this method id good to avoid machining but how about accuracy ?
Please, in which direction it won't be rigid ?
Thanks !

cnc2.

Why not just use conventional milling machine slides (no rolling bearings) as in the X2 and X3 mills?

Why not just use conventional milling machine slides (no rolling bearings) as in the X2 and X3 mills?

What about the friction & gib adjustment ?
The more weight is on the table the higher is the friction & it will require a beefy $$$ servo motor to overcome it.
Am i wrong ?
Thanks !
cnc2.

What about the friction & gib adjustment ?
The more weight is on the table the higher is the friction & it will require a beefy $$$ servo motor to overcome it.
Am i wrong ?
Thanks !
cnc2.

If you are going to scratch build a machine you will need to build in the adjustments.
You are correct about more friction but these slides are not a problem in an X2 or X3 machine. Power required will obviously depend on many factors too many to go into here.
Are you sure the servos that are available to you will supply enough power even with rolling bearings?
John

If you are going to scratch build a machine you will need to build in the adjustments.
You are correct about more friction but these slides are not a problem in an X2 or X3 machine. Power required will obviously depend on many factors too many to go into here.
Are you sure the servos that are available to you will supply enough power even with rolling bearings?
John

Thanks for the reply John !
Not sure about the servos ! but if they ain't powerful enough it'll be easier to upgrade the servos than to redesign/rebuild the whole machine.
Excuse me guys, i've been away from the thread for a long time due to a prob with my dsl provider, i'm now over dialup 52 Kbit/s SLOW (i had to check my mail), i hope the problem to be fixed this week.
Thanks !
cnc2.

Hi guys !
I'm back in business with my new dsl provider
I've been contacted by HiWin Taiwan, they wanted to know about the parts i was requesting, i told'em what i was needing & that shipping would be easier & cheaper from Germany, they told me that HiWin.de had a factory in Germany & gave me the mail of a guy to contact they also introduced me to the guy in Germany... i mailed that guy on 3/19/09 & i'm still waiting for a reply.
here's my mail to the German guy:
**************************email start************************
Hello,
Dear Michael:
This is the Algerian customer that was introduced to you by Jason Liao (by email from Taiwan).
I'm in the process of gathering the parts for building a small cnc mill, aprox the size of a Sieg X3.
I'm interested in Linear guideways and blocks HG series between 20mm & 30mm wide, i'm not sure about the best suited guideway size(width) and options to choose for the blocks. I don't want it to be overkill and overpriced for my application, so i need your advice here.
Please send me a price list per block & per 10cm of rail, for 20mm 25mm and 30mm guideways & blocks without forgetting the price of the series/size/options you advice me to choose.
For shipping to Algeria & paying, i know somebody in Germany that will care about it.
Many thanks,
********************end of email**************************
Any comments ?
Thanks !
cnc2.

Hi, I do have a comment, and you might find it useful or completely useless, depending on your viewpoint.
There are many small and large parts needed to build a machining tool, and it is difficult to make a complete list.
If you are already having so much trouble with just obtaining a simple part like a linear bearing, then it is unlikely you will be able to build your own system in any reasonable amount of time.
If you are having this problem, then most likely others face the same challenge and this opens up a real opportunity for you and the local region to improve business conditions there.
I suggest instead of trying to build a simple machine now, attempt to buy a complete, good quality, new or already rebuilt cnc mill with strong capabilities. There certainly are many machine shops in the west with excess equipment right now, and many equipment vendors are stuck wtih demo machines and rebuilt new units.
I don't mean a small machine, I mean a real, working, commercial size mill, like a Haas or something.
Find a University or other training school for mechanical engineering or similar and visit a professor there. Ask them for help to obtain the equipment. There is a good chance that they have connections that can help you, perhaps even with finding partial funding or discounted purchases.
Companies understand better how to deal with a University or other education related entity in the middle of no-where, while they have no idea how to deal with an individual in the same position.

Hi, I do have a comment, and you might find it useful or completely useless, depending on your viewpoint.
There are many small and large parts needed to build a machining tool, and it is difficult to make a complete list.
If you are already having so much trouble with just obtaining a simple part like a linear bearing, then it is unlikely you will be able to build your own system in any reasonable amount of time.
I suggest instead of trying to build a simple machine now, attempt to buy a complete, good quality, new or already rebuilt cnc mill with strong capabilities. There certainly are many machine shops in the west with excess equipment right now, and many equipment vendors are stuck wtih demo machines and rebuilt new units.
I don't mean a small machine, I mean a real, working, commercial size mill, like a Haas or something.
Find a University or other training school for mechanical engineering or similar and visit a professor there. Ask them for help to obtain the equipment. There is a good chance that they have connections that can help you, perhaps even with finding partial funding or discounted purchases.
Companies understand better how to deal with a University or other education related entity in the middle of no-where, while they have no idea how to deal with an individual in the same position.

Thanks for the reply harryn !
I like your viewpoint & you're almost right BUT my 2nd biggest problem is that i'm a student, so forget about the HAAS & the big ladies alike... i sure would go that way if i had the ca$h, as you might guess, as a student i've way more time than cash so i got to begin small (or little, not sure about my english).

If you are having this problem, then most likely others face the same challenge and this opens up a real opportunity for you and the local region to improve business conditions there.

"Great Minds Think Alike" you're right at least for the SMBs, there are FEW cnc shops in here & they're all top end shops having their own suppliers etc... (some AirBus parts are made in here) they'll be no rivalry beteen us.
There are very few small chinease cnc routers & they cost aprox 11k$ for 800*600mm work area(this is called theft)
If i can build a working machine for myself, why not to make small series & sell'em, it's all about the ins & outs finding the suppliers etc... this is a challenge in here, while in the US you can order evrything online & wait for UPS at your door...i've got my encoders from Digikey this way, but the heavier & bigger parts are a problem.
If i succeed i might make money out of it, if i fail i'll still have a crappy geek project to play with
Thanks !
cnc2.

sorry this was a double

The Germans replied !
Here's their reply:
*********************start*******************************
many thanks for your interest in our products.

To quote for you, please let us know the following:
1. complete designation of the requested HIWIN product
2. quantity

Concerning the price list, we do only quote for inquiries case by case (we do not have a gerneral price list for customers).
Please send us your demand.
Thanks for your understanding

Best regards

HIWIN GmbH
******************end**********************************
So, as you might guess i need your help to make the right choice...i don't need it to be neither overkill/overpriced nor too "weak" and wear out too soon.
Anyone who can help me choose the right part numbers is WELCOME !
Thanks !
cnc2.

I think 15 or 20mm rails will be plenty of overkill for your machine. The 25 and 30mm sizes are wayyyy too big. If you want to get a better idea, look up the load capacities of those big boys, probably upwards of 600 kg, way more than you will ever need. People seem to like to go overkill on the linear guides but they are much stronger than people give them credit for. It is good to overbuild a little to get better life, but at some point it becomes ridiculous.
Matt

Thanks for the reply Matt !
I agree with you !
I sure have to agree with you since i don't have the money to go ridiculous
15 or 20 is hard to tell, i heared that it was good to look at the momentum loads etc... but i have no idea about the aprox values.
Do you think it is wise to make my decision on the price ?
(if there's no big difference i'll go for the 20mm)
Any idea about the options for the blocks ?
Thanks !
cnc2.

On the blocks, make sure you get ones with the wiper seals to keep crap out of the bearings. Also get ones with the mounting type you need, tapped holes, through holes, etc. Other than that, I'm not sure you need to specify anything special.
Matt

OK - then the next step is to start reading the build logs for routers and machines like this. There are so many experiences posted in this forum - just start looking at the machines others use and build.
I did not realize that you were a student. As such, take advantage of your school to help you find and buy things. Certainly there will be a teacher there who will help you, possibly with the technical and business aspects.
You might be surprised at the resources that can become available. It might even be possible to obtain school funding and use the "project" as part of your education credits.
I read a little about your country - as a major oil and gas exporter, you can be certain that there is a lot of machining going on there. Processing oil destroys parts, and broken parts need replacement and repairs.

On the blocks, make sure you get ones with the wiper seals to keep crap out of the bearings. Also get ones with the mounting type you need, tapped holes, through holes, etc. Other than that, I'm not sure you need to specify anything special.
Matt

Thanks for the reply Matt !
wiper seals ? is it the tough seal that keeps metal pieces from entering the bearing or the one for dust ?
i need to be sure because i saw an option like this that costs +20$ and making the blocks at 60$ instead of 40$, i think it was at automation4less.com.
Any help is welcome.
Thanks !
cnc2.

Thanks for the reply harryn !


I did not realize that you were a student. As such, take advantage of your school to help you find and buy things. Certainly there will be a teacher there who will help you, possibly with the technical and business aspects.
You might be surprised at the resources that can become available. It might even be possible to obtain school funding and use the "project" as part of your education credits.

Sorry, i didn't mention the filed of my studies. Well, i'm a student in the computing & networking field and can take no advantage of my school & for the teachers maybe there's one of'em who knows what a mill or a lathe is.

I read a little about your country - as a major oil and gas exporter, you can be certain that there is a lot of machining going on there. Processing oil destroys parts, and broken parts need replacement and repairs.

Yes, but it is at the top end...too high for the little guy, unlike the US this industry is in the desert far(+1000km) from major cities, and the price of used machines is ridiculous....to give you an idea, a worn out machine that would quote for 500$ in the US would easily quote for 2000$ in here
I found a company that "repairs", installs and import CNCs but it's way over my budget...will see, they might have refurbished rails but it will have to fit my needs etc... and when it comes to selling refurbished (no, industrial) parts the seller finds himself like selling fresh water in the middle of the desert, buy or die attitude.
I will have to make my way through this sh*t !
Thanks !
cnc2.

I'm not sure which seals are best. You should design your machine so the chips never get to the rails or screws anyway. I guess it would mainly be for incidental contact. It all depends on how well they are protected. If you ran them in a clean room, they wouldn't need any seals at all.
I don't think I would pay $20 extra for it, but I'm not sure what the alternative is.
Matt

Hi guys !
Can anyone help me with the dynamic loads ?
I have the data sheets for the rails & blocks, but i have no idea about the real world dynamic loads a machine can create on the rails & so for me the datasheets are meaningless because i can't tell if the ratings are overkill or under rated for my application.
I've selected the most relevent sizes (IMO):
Code:
HGW 15CA are rated at 11.38 kN for basic dynamic load &amp; at 16.97 kN for basic static load &amp; static rated moments are Mg/Mp/My 0.12/0.10/0.10 kN-m. (CA flange type, mounting from top)<br /> HGW15CC (CC flange type, mounting from top or bottom) same dynamic load &amp; 25.31 kN for basic static load. Mg/Mp/My 0.17/0.15/0.15 kN-m.<br /> HGW20CC are rated at 17.75 kN for the Basic Dynamic Load &amp; at 37.84 kN for the Basic static Load. static rated moments are Mg/Mp/My 0.38/0.27/0.27 kN-m.

I've been told by HiWin Taiwan to choose the just fit for the dynamic loads & this way it won't be overkill/overpriced.
Can anyone help me to do so ? (
Thanks !
cnc2.

I'm not sure which seals are best. You should design your machine so the chips never get to the rails or screws anyway. I guess it would mainly be for incidental contact. It all depends on how well they are protected. If you ran them in a clean room, they wouldn't need any seals at all.
I don't think I would pay $20 extra for it, but I'm not sure what the alternative is.
Matt

Thanks for the reply Matt !
And so do i ! (for extra 20$)
In the datasheets the blocks have standard end & bottom seals, i think it won't be too hard to cover the rails, it's been done before.
Can anyone help me with the machine design software ? i have no idea about the effective rake angle for the mill cutters.
What is the depth of cut (slab milling) achived by most of the mills ? 2"? (i have no idea)
Thanks !
cnc2.

I think any of the rails you list would be fine. Even the smallest has a dynamic load capacity of the weight of a small car. Remember these are ratings for a single rail/block( I think, have to check the datasheet). I assume you will have 2 rails and 4 blocks?
I would say the cheapest of the listed options would be the best.
Are you looking at depth of cut to get forces? That is probably limited by the strength of your motors. There are datasheets for leadscrews which specify how much torque is required for a given force. You can use your motors to calculate the max force and then your depth of cut will be limited by that and the power of your spindle.
Matt

Thanks for the reply Matt !
Yes, i'll have 2rails & 4 blocks.
You're right about the dynamic load, but what about the momentums ?
If say the cutter is applying a horizontal force at 300mm from the table what momentum will it generate ? i can't tell because i have no idea about the force needed to cut at a certain depth.
What cutting depth is common in milling machines ? what force does it generates on the cutter ?
If i go for HiWin rails i think i should size the motors & spindle to get the best performance that i can afford, that's why i need to know about common depths ...etc.
I RFQed for the 15 & 20mm with standard(0.1mm) & high(0.03mm) accuracies to compare prices, 600mm for X&Z & 400mm for Y what effective working area you think can be reached ? did i go too big on the size ?
Any comment ?
Thanks !
cnc2.

you are asking about moments, not momentum. The moment calculation is force times distance. Your max force is determined by your axis motors. The moment is not a big concern because with the 4 blocks the moment is transferred into a force on the opposite block.
Think of a diving board in a swimming pool, it can only support a given weight on the end. If another support is placed at the end, the load can be dramatically increased. This is what the multiple blocks do for you.
You should figure out how much work area you need and get the parts to suite. The rail is relatively cheap once you buy the blocks so get what you want/need. On a smaller machine you might want around 100-150mm of separation between the blocks, so your travel can be calculated like this:
length of rail - dist between blocks - (2*length of block) = travel.
So say you bought a 300mm rail, used a distance of 100mm between blocks and each block is 20mm long:
300 - 100 - 2*20 = 160mm of usable travel.
Normally cutters are run somewhere around 3-5mm deep. It all depends on what you are cutting, how you are cutting etc.
The forces involved depend on your material, size of cutter, feedrate, spindle speed, etc. I can't give you some magic number. You should be able to find resources online to help you calculate that. I would say that it is not a big concern for you. Just get some sturdy components and you can adjust the depth of cut and feedrate to get the most from your machine.
Matt

Thanks for the reply Matt !
Ouch !!! you're right i was meaning moments, my english is showing its limits Thanks for correcting
For the diving board example, i don't think it compares well to the forces on the blocks, i think it compares better to the forces that try to flip a car in a turn(or curve) look at the diagram & tell me what you think.
I hope the moments will be under 0.38 kN, i think the wider the space between the rails the lower will be the moments.
Thanks for the formula on calculating travels, does it remain true for a table like the VM1's in the pic ? (i think i went too short on the rails)
Thanks for the info about Normal cutting, i used the software machine design(works in DosBox under WinXP) to help me figure out some of the machine's numbers, i choosed iron milling at 3000rpm & 90 IPM at 2" deep the result was something like 60 HORSEPOWER !!! for spindle & other crazy numbers... it also asks for cutter angles i'd put 3°(is it common ?).
The effective table size of an X3 is 550x160mm what does it means ? is it the machinable area on the table ? i'm asking these questions because the table doesn't seem to be 1 meter long, am i wrong ?
Thanks !
cnc2.

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The blocks are 77mm long do i need to leave 100mm between'em ?
Thanks !
cnc2.

Maybe it was a bad example, all I could think of. Your picture is better, as you make the stance wider, the moment is reduced. It should not be any concern for your design.
Travels are all the same, obviously the block can't travel past the end of the rail so that is what limits your travel in each direction, see attached.
You got crazy numbers on your calculator because you input crazy numbers. I have never heard of a 2" deep cut. This is a $70,000 machine and you see the depth of cut they are taking. It is more efficient this way and does not heat the tool so much.
"http://www.youtube.com/watch?v=XbvzQDxtPzc"]YouTube - brother CNC Milling Steel, 30 taper w/ Lyndex-Nikken holder
Matt

Thanks for the reply Matt !
That mill is cutting steel like butter & the diameter of the cutter (looks like 1" dia) amazing... What are the feed rates (IPM) for these kinds of cuts ?
What feed rates are common when cutting into materials at 5mm deep ?
Take a look at the pic, i entered more coherent numbers this time, do you think it is reachable & the ways can stand this ?
Can anyone help me choosing the ACME leadscrew diameter ? i want to have a lead of 4 or 5mm/turn.
(I want to implement the DIY roller screw designed by rokag3.)
I want to make the mill's frame from cast aluminium, what do you think ?
with a difference of 40°C the aluminium should change in size by 0.0005mm is it acceptable knowing the rails have 0.03mm Dimensional tolerance of width ?
Thanks !
cnc2.

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Probably 3-400 IPM, not sure really.
When I cut steel on my Bridgeport Series 1 I am normally going around 10-15 IPM.
Your numbers are perfectly doable if the machine is designed correctly. I am not sure that it is realistic, but doable yes.
I don't know if the ACME roller screw design is suitable for milling steel. I do know that virtually no one uses ACME screws on a machine for cutting steel. The forces are higher therefore the wear is higher and then backlash quickly becomes a problem. I have no real experience with this, its just my gut feeling.
Frame from aluminum? Again you won't find much aluminum on machine made to cut steel. Design becomes a lot more important with aluminum as it is 3 times more flexible than steel. That means that your frame will be moving around unless its super beefy. Not much can beat the bulk of cast iron for machine bases.
I am not sure where the 40C temp comes from but you would be dreaming to hit better than .05mm accuracy with ACME screws.
On your block spacing, I would go more than 100 if your length is 600. Look at your VM1 picture: The separation on the X axis is something a little more than 1/4 of the total length. If you keep the proportions from that machine, you should be fine.
I think you might be wanting the "perfect" machine and so you are making it much harder on yourself. Why not make a smaller machine with aluminum components and not so much power to start. You will get a lot of good experience doing that and then you can make another one when you feel you are ready. I would hate for you you to spend a lot of money on a machine only to find out it will not do what you wanted it to.
Matt

Thanks for the reply Matt !
Thanks for the ipm you're running, i got more doable estimates using'em.
For the ACME screws here is the kind i think i should use to implement the DIY roller screw (A different (better) kind of screw), i've no idea on how the hardness of the screw can be scaled to stand a certain load, the rollers are made from bronze a bit softer than the screw(i might have to go for a hardned ACME screw), though.
Can anyone help on how to choose a good affordable leadscrew ? i can't afford ball screws.
For the aluminium frame i think it'll have 30mm wall thickness(look at the diagram) for an example of VM1's like X axis. Do you think it's beefy enough ?
If i can't achive beefyness (lol) i'll have to get my molds to a local foundry to get'em filled with iron.
40°C is a theorical max difference in the temperature of the frame to have an idea on how much it can change in size (expand i think). If i can't expect more accuracy than 0.05mm without using ballscrews, i'll have to live with it. (ball screws are too expensive)

I think you might be wanting the "perfect" machine and so you are making it much harder on yourself. Why not make a smaller machine with aluminum components and not so much power to start. You will get a lot of good experience doing that and then you can make another one when you feel you are ready. I would hate for you you to spend a lot of money on a machine only to find out it will not do what you wanted it to.

Yes, i want the "most perfect" machine i can afford to build & i want it to be a scaled version of the VM1, to build the farme molds i'll need styrofoam paterns, to make these patterns i'll need a smaller machine for wich i have most of the parts, it'll use motors & round railways from photocopiers & it might be cable driven or using simple threaded rods & the cheapest way to avoid backlash, & a dreamel like powertool for cutting.
What money do you think reasonable to spend on a homebuilt machine(a scaled VM1) ?
Thanks !
cnc2.

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Hi guys !
I've recieved the quote from HiWin Germany for the blocks & linear rails.
I'm not sure if it's fair to publish the prices on the forum, so, anyone who wants to know about the prices, let me know & i'll PM you.
They also told me about ball screw they have, i'll try to RFQ maybe it'll be a reasonable price.
Thanks !
cnc2.

Hi - would you mind posting your calculations for the effect of a 40C temperature differential and your assumptions? I would have expected a much more dramatic effect. Perhaps there is a decimal point location that needs adjustment?
I will just repeat what has already been stated
- Al framing is fine for wood cutting
- High strength Al alloy is ok for Al cutting, but for steel is better
- Machining steel - Al is not going to get you there.
I am more of a wood working person, and my brother mostly machines steel (for himself) and Al (for me)
The cutting forces for machining steel are a whole different animal, and just clamping it all down is exciting. Take a piece of scrap Al and scrap steel and try digging them into each other. Which one gets all dinged up, and which one kind of laughs it off ? Seeing is believing.
Can you do a few cuts - sure. Will it hold up for even 1 year of use - probably not.

Hi - would you mind posting your calculations for the effect of a 40C temperature differential and your assumptions? I would have expected a much more dramatic effect. Perhaps there is a decimal point location that needs adjustment?
I will just repeat what has already been stated
- Al framing is fine for wood cutting
- High strength Al alloy is ok for Al cutting, but for steel is better
- Machining steel - Al is not going to get you there.
I am more of a wood working person, and my brother mostly machines steel (for himself) and Al (for me)
The cutting forces for machining steel are a whole different animal, and just clamping it all down is exciting. Take a piece of scrap Al and scrap steel and try digging them into each other. Which one gets all dinged up, and which one kind of laughs it off ? Seeing is believing.
Can you do a few cuts - sure. Will it hold up for even 1 year of use - probably not.

Hi,
Thanks for the reply harryn !
No, i don't mind ! & any help is welcome !!
Look at sigma relief's post #164 (Actual limit on encoder frequency)
For most aluminiums the Coeficient of Thermal Expansion (CTE) is
13 x 10^-6 in/in wich equals(to me) 0.000013x2.54 cm/cm (the error might be in the conversion from in/in to cm/cm).
I calculated this way (from memory): (13x10^-6)x40°C=0.00052 cm/cm instead of mm/mm (i'd mixed the metric with the imperial & forgot to x2.54)
It should be :
(13x10^-6)x104°F= 0.001352 in/in (for 40°C <=> 104°F)

0.001352 x 2.54 = 0.00343408 cm/cm
0.00343408 x 10 = 0.0343408 mm/mm
If my second calculations are good, using an aluminium frame is a nightmare thanks for openning my eyes on this one !
I know that steel & iron are way harder than aluminium but i thought that a beefy Al frame could do it because the load was evenly repartited...
If there's still something wrong, let me know.
So for casting the frame from iron i'll have to look for a local foundry to get the molds filled because i don't think i my furnace can handle this.
Now an other question arises...what thickness of iron would be enough for the frame to get it strong & as light as possible ?
Thanks !

cnc2.

Thanks Mike !!!
It can still be done for cheap if i don't look for high precision & i'm happy for that !
Thanks !
cnc2.

Hi guys !
I finally got a quote for ballscrews from hiwin & it's way over budget, so i'll have to go with ACMEs for a first step then retrofit with RBS when money will permit.
I saw in the news paper that a factory(manufactures trucks) was selling used machines in an auction(not sure it's good english): lathes,milling machines, drilling machines, threading lathe, rectifying machines, automatic cycle revolver lathe(what's that ?)...etc. Most of'em are manual machines.
The universal milling machines Yea they're TOO BIG for me, but i still need your advice if you know the model or feel something about it...if it's really good & i can get it for a killer deal i'll try to throw it in the back yard & make a kind of roof above it.
There's also a lathe that i think is small enough for me, i don't know if i'll really need it but the model seems to be ok, it's an A.M.C P240 horizontal lathe, i didn't see the machines yet, but i need your advice wether it's a good choice or a mistake ?
If you can give me a price figure above which such old machines aren't worth it, it'll help the newbie i am.
Thanks !
cnc2.

A big old universal mill might go for 1-2K USD. It really depends on the condition and what tooling comes with it. It would be a shame to let a nice machine sit outside and rust though. Unless you have at least an enclosed shed, it would probably rust away and not be worth anything to anyone after that.
I have no idea what the size of the lathe is but I have seen some older small lathes (around 8" swing) go for around 500 USD.
Matt

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