Lead Screw Advice

Hello Everyone,
I have a basic machine going, along the lines of a joescnc made mostly of 8020 extrusions and angle iron.
It won't be long before I need to select a lead screw for a CNC router used mostly for wood and plastics of a slightly larger scale, mine is 6X6 foot in size, with a workable size of at least 5x5' in size.
I have read the FAQ and took note of mention of whip in the thread when traveling at higher speeds, so I would like the highest resolution possible but would like to avoid sagging and whip over 6 feet of acme threaded rod. I was shooting for a 10 tpi with a double screw, meaning at 200 steps per rotation .001 of resolution. If I calculated that correctly, that should be more than high enough "resolution" for me for lack of the proper term.
I was thinking that 1/2 acme thread might sag under its own weight so much so that it will cause havoc, and was curious if anyone who has built a similar machine had any advice on how thick a screw would be sufficient to span that distance, I do plan to pen up the one end in two pillow blocks, and the other one, possibly two if I go the belt drive route. I would make the gear captive between two pillow blocks, or mounted bearings.
Thanks, I would certainly provide more information if needed. All I am really shooting for is a basic machine that get me up and rolling at this scale.
Thanks for you help,

Check out the used market for 1" + ground ball screws. No whip, no slop.
Amplexus

Mine is 60x60 inch and I have 4 steppers and use 3/4" x 6tpi screws from Enco. I did make my own nuts from bronze for a better fit. I use a Xylotex
driver and can get 80 ipm and have no whip. They might whip if turned very fast but I haven't had any problems. See video
"http://www.youtube.com/watch?v=GCLdQtIm32I"]YouTube - CNC doing text
Nick

I got some 5/8" rollerball screws of the rolled variety, from a guy on E-Bay together with two nuts at very very reasonable prices. These would be ideal for what you require. He also sold a preloaded version of the nuts which only meant you require one nut. They are pretty accurate as well (as in 0.001 per ft) with a 5mm pitch. The roller shafts where something like $15 per foot and the nuts $25 and preloaded nuts like $45. If you really want to pursue this I can go lookup the original invoice to get his deatails. He had a web site you could purchase directly as well - something like HobbyCNC.com

Hi,
I built a 4ft x 2ft machine and made the mistake of using 1/2" leadscrew which whips and makes quite a noise too. I'm now planning my next project a 6ft x 3ft, i've made a few phone calls to various suppliers who say i would need to go to either 22mm or 25mm ballscrew which in the UK you seem to need a mortgage for. The other thing is what size motor would i need to drive it? So i'm now considering rack and pinion drive, hope this helps.
Simon.

Hello Everyone,
I have a basic machine going, along the lines of a joescnc made mostly of 8020 extrusions and angle iron.
It won't be long before I need to select a lead screw for a CNC router used mostly for wood and plastics of a slightly larger scale, mine is 6X6 foot in size, with a workable size of at least 5x5' in size.
I have read the FAQ and took note of mention of whip in the thread when traveling at higher speeds, so I would like the highest resolution possible but would like to avoid sagging and whip over 6 feet of acme threaded rod. I was shooting for a 10 tpi with a double screw, meaning at 200 steps per rotation .001 of resolution. If I calculated that correctly, that should be more than high enough "resolution" for me for lack of the proper term.
I was thinking that 1/2 acme thread might sag under its own weight so much so that it will cause havoc, and was curious if anyone who has built a similar machine had any advice on how thick a screw would be sufficient to span that distance, I do plan to pen up the one end in two pillow blocks, and the other one, possibly two if I go the belt drive route. I would make the gear captive between two pillow blocks, or mounted bearings.
Thanks, I would certainly provide more information if needed. All I am really shooting for is a basic machine that get me up and rolling at this scale.
Thanks for you help,

From some quick napkin calculations, it looks like a 1/2" ball screw would sag roughly 1/8 of an inch and have a maximum speed around 150 ipm.
It that what sort of information you're looking for?
Let me know,
Matt Adams
IMAC Motion Control

Matt,
would you agree that 25mm ballscrew is the way to go and what size steppers would you go for?
Simon.

Matt,
would you agree that 25mm ballscrew is the way to go and what size steppers would you go for?
Simon.

Simon,
How fast are you looking to move during rapids and during cutting? A 25mm screw would probably only sag .040" over that span, which I personally like a lot more than the 1/8" I was calculating before.
Matt

Matt,
I'd like to aim for 300 ipm for the rapids, which with a 2 lead ballscrew I hope should be achievable. Most of the time i'll be cutting oak, iroko or softwood so cutting speed will be a case of trial and error to start with plus i havn't decided what size router yet.
Simon.

Matt,
I'd like to aim for 300 ipm for the rapids, which with a 2 lead ballscrew I hope should be achievable. Most of the time i'll be cutting oak, iroko or softwood so cutting speed will be a case of trial and error to start with plus i havn't decided what size router yet.
Simon.

With what you specified, whipping occurs right around 310 ipm at 25mm diameter. Most of the time a 20% safety factor is put in there, so 250 ipm is what suppliers might quote.
I can get great prices Thomson ball and lead screws, which is a large manufacturer here in the states. If you get me a drawing, I can work up a quote for you. I'm not sure how things would work over seas, but I'd try if you're interested. PM me.

I have a 42" 1/2" 10 TPI 2 start that whip is noticable starting around 160 IPM as the gantry gets 10" of so from the end of travel. While my drive/motor/ps combo will go faster I had to tune it considering the whip limit. it's not that the 1/2" sags, it's the rotary torque that is the issue.
Ballscrews are great IF, one you can afford it and two you don't have to machine them. Ballscrews are HARD.
This might help you some to understand how stepper torque vs rpm plays out: http://pminmo.com/PMinMOwiki/index.p...chanical_Power

Matt/pminmo
Thanks for your help, just shows what a great site this is! I'm still working on the drawings but when i get to that stage i might just take you up on that offer matt.
cheers for now,
Simon.

I didn't see anyone mention it, but what about fixing the screw and spinning the nut?
I saw this recently underneath a Techno machine and it was a nice setup. The motor was mounted to the lower arm of the gantry and the nut had a toothed gear on it. A timing belt arrangement was driven by the motor. No whipping at all. Somebody must have a picture of this somewhere...
Steve

I've seen posts on that done successfully. It makes a lot of sense on long installations. Accelerating and decelerating a 1" diameter screw 6' will take more motor power.

If you use a 1/2" acme with 1 or 2 turns per inch, whip really shouldn't be a problem. If you want more resolution or more power, use a belt reduction. 2 pulleys and a belt are about $10-$15.

ROTATING-NUT. The lead-screw don't turn so no whip is possible.
Yannick

ROTATING-NUT. The lead-screw don't turn so no whip is possible.
Yannick

Not true according to Nook. See the Critical Speed section on page 7.
http://www.nookindustries.com/pdf/NookAcmeTechnical.pdf

By using rotating-nut, you can go faster without need of using huge screw or support bearing.

Pretty much, and thanks for the replies everyone, and for the calculation.
Of those that use ball screws how long was your run and what did it cost you ? I see that they pretty much offer the complete solution with bearing blocks, etc, all in one package. I was thinking the acme screw, mounted bearings and nut being that I can mill slots and set screws in my garage with no problem.
Spending a little bit more for less hassle and more precision is not out of the question, while I can find many ball screw companies on the internet, none are quick to quote a price without a call.
I was thinking 3/4 10 double start, in a 1" bearing blocks and just buy a machinable lead nut that I can throw into the block with a grub screw and a small flat milled on the screw. McMaster prices seem to climb a bit high doing it this way, maybe i can turn down the edge of a acme nut to accomplish the same thing.
The one thing I probably can't do is turn down the end of a 6' rod, it would be easier for me to mill the nut to fit into a bearing block that was larger than the diameter of the rod.
I will check out the other links for various off the shelf fixes for this,

In the US, look at the price on Roton screws: http://www.roton.com Specifically the price on their ballscrew nuts seem to considerably less expensive. Online quotes and ordering. I've seen a number of acme mfg'rs and Roton impresses me more for the price. Same for the ballscrew combo when you add up the cost of the bearing plus the screw. Good peple to deal with also.

Of those that use ball screws how long was your run and what did it cost you ? I see that they pretty much offer the complete solution with bearing blocks, etc, all in one package. I was thinking the acme screw, mounted bearings and nut being that I can mill slots and set screws in my garage with no problem.
Spending a little bit more for less hassle and more precision is not out of the question, while I can find many ball screw companies on the internet, none are quick to quote a price without a call.
Go to HOMESHOPCNC.COM they supply ptices on their web site.
Doug

Check ebay or the surplus market for 1" plus ball screws if you are patient bargains are out there.
Amplexus

I don't use screws that long, but I use 5/8 ball screws on my router. I think it is about 48" long. It did have a small amount of whip at 300 IPM. I installed a spring and second nut and that took care of all my whipping issues. Smoothed it right down even at 350 IPM. That was in testing. I just don't need those kinds of rapids. I cut at 150 IPM on straight cuts and 80 IPM on tighter curves. I think I have it set at 200 IPM rapids. Cycle times is like 3 minutes and less for my main products on that machine.

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