Four Carriages on Two 15mm Rails or a Single Carriage on a Single 45mm Rail ???

I'm designing a CNC Mill at the moment. It is a 12" x 8" x 12" (XYZ) for Milling Aluminum and Mild steel. I did some reading over here and have come across 2 Linear guide based designs. (Please check the attached pictures)
Design 1: Each Axis has 2 Rails (15mm) and 4 Carriages (2 Per Rail)
Design 2: Each Axis has Just One Bigger Sized rail (45mm) and One Carriage
Now I compared the load ratings or 15mm and 45mm Linear Guides and found them to be WIDE APART. The 45mm guideways seem to be WAY higher in load bearing capacity than the 15mm ones.
However I was wondering if using just One 45mm Rail is a Good Idea?
Will it give better Axis rigidity?
I checked the prices with an AMT Linear Guide supplier and found Two 15mm Rails with 4 Carriages is pretty much priced the same as a Single 45mm Rail with a Single Carriage. So its even economic to use a Single Rail concept. Plus since there is just one rail, we don have to go through the time consuming effort to make sure the rails are mounted Perfectly Parallel too. So Prima Facie the Single Rail concept seems better. But if that is so, Then Why is it soo unpopular? in all the posts I have read here, I have come across just One Design where the single rail concept is used.
Here are the Specs for the AMT Guideways

Carriage Name : MSA 15 S
Length (mm) : 56.3
Width (mm) : 34
Height (mm) : 28

Dynamic C : 11.8
Static C0 : 18.9

Mp :0.12
My :0.12
Mr :0.14

Carriage Name : MSA 45 LS
Length (mm) : 169.5
Width (mm) : 86
Height (mm) : 70

Dynamic C : 102.4
Static C0 : 157.3

Mp : 3.13
My : 3.13
Mr : 3.43
If you have used Larger sized rails in your projects, Please post your experiences as it would be very helpful in deciding which of the two methods is better. I hope someone can help me with choosing the better idea of the two.
Thank You!
PS: The Attached Pictures do not belong to me Nor are they my Designs. I have attached them ONLY for reference. All Credits goes to the Original Designers.




I think both designs are great. Either choice would be fine for a small mill too I think. The 15 mm are a little small for my taste, but the 45's are almost twice the size of what I typically have bought.
With the 15's the wear would be evenly distributed over a lot more balls, albeit much smaller.
The 45's would be easier to mount. The screw would need offsetting with these, but that is doable as the second design shows. If you went with the single truck on a single rail, it would need to be the wide rail type shown. Otherwise I would opt for the 4 truck design.
Maybe someone else has some specific experience with these sizes. Regardless of which you choose, I doubt you will ever wear either out on a benchtop machine.

I would go with option nr 1. The double 15's. Here's why:
- The load ratings are wide apart yes, but both overkill for your application. You can park your car on those 15's and they still wont break.
- Using 2 rails make for a wider support of the cutting bed. This prevents your X-axis acting like a teeter-totter.
- With the double rail you can mount the leadscrew in between. It's always best to drive an axis from it's center.
That's what I would do. However, the 45 will also work fine, and may be easier to align. Bottom line, your machine is so small, it doesnt make much difference.

Joe, Just finished a design for a larger machine. Two 25mm rails would have been more than adequate, in fact overkill. Customer buys by the pound , so I put two 45mm rails on. Now he's happy, I'm happy and the rail sales guy is ecstatic!! LOL
Dick Z

Thanks Lee, Sieg, Richard for your inputs.
I contacted the Original Designer of the Single Rail Design in the pictures I posted earlier. He goes by the nick Jason3. He has just completed another Mill with a similar design and the machine looks great. But somehow I think he is using rail sizes wider than 45mm. There is an HRW series from THK which offers rails almost Twice as wide. But out here the HRW series is not very popular and that makes it a lil more expensive (The dealers dont keep HRW series in stock) and most sellers in E-Bay don ship to India not to mention the customs formalities out here... So I'm still wondering if the 45mm single rail single carriage per axis will work...??? if not I might have to resort to the tried n tested 2 rail 4 block Design!
Richard... You just mentioned about using 45mm rails in your project...Can you share your experience using them? My Axis are 12" x 8" x 12" Do you think a Single Carriage on a Single 45mm rail per axis would be sufficient? (the specs for the 45mm rail and carriage is mentioned in my last post)
Please Advice!
Thank You.

If I had them in my hand already, I might use the 45 single rail, but if you can't get the HRW type, then the nod goes heavily toward the smaller 4 rails. You just won't have enough area even on a 45 rail if it's not HRW.

The options are not even close. Either will handle the load; but only the two 15's will handle the moments you are likely to have.
Use the two 15's. Space the rails as far apart as you can. Space the carriages as far apart as you can on the rails.

There are three basic reasons you use 2 rails and 4 bearings.
They are pitch, yaw, and roll.
Every linear bearing has some amount of clearance or compliance.
Picture a 6 foot long table with one rail bearing in the middle.
Now if you grab one end on the table an try to twist it side to side it rotates around the bearing in the middle.
If you put a bearing in each corner of the table this twist is drastically reduced. Also the 4 bearings average out any localized errors reducing the error of the overall assembly.

Joe, It appears you want to use those 45's. lol I agree with the other guys, I'd recommend the 15's with (4) blocks.
I've used the single wide HRW's in longest, widest flange type form. This was a drilling/boring application where the forces were mainly in line with the axis of travel. Very little moment load.
If you want rigid look at a set of IKO "Super MX" crossed roller system in the longest style blocks. (I have nothing to do with IKO other than being a customer)
Dick Z

You may not notice much of a difference, but the 4x15mm blocks are preferable.
Most hobbyists install their linear motion components incorrectly due to lack of resources. The proper way to install a rail is to mill a very accurate slot with a 90 degree shoulder on one side and a 60 degree shoulder on the other... not easy if you don't already have a decent sized VMC to play with. Tapered wedges are installed every few inches to push the rails against the 90 degree shoulder before they're bolted down from the top. The blocks are installed in a similar fashion. That is why the sides of the rails and blocks are ground.
With a single HRW-45 block, the area of the mounting pad is something like 3.5" x 3.5", whereas four HSR-15 blocks can be spaced out to whatever you want. Without a mounting shoulders, a larger mounting area is preferable.
But again, for these very small mills, you probably won't notice any difference.

Hi Joe,
For the size of your machine Both the options are equally good. But as you say you will have to look at the prices & availability. But 2 Rails & 4 Carriages is the most widely used design in commercial machine building. But i consider the issue of the carriage clearance with the rail when you move the Y axis on the edge with a heavy Job loaded which will tend to lightly shake up only on heavy machining.But for your machine size i would think you can have Both the options open.Please let me know the prices.

Hi Joe,
You're right, my machine used THK's HRW series guides. There's a single HRW50 block and rail for the Y, 2 HRW35 blocks on a single rail for the X and a single HRW35 block and rail for the Z. For this design they work beautifully, and will never even come close to their rated capacities on this mill, being way oversized for the application. It runs really smooth and quiet.
The machine is designed for complex 3D parts requiring a good accuracy and good surface finish, and where minimizing the cycle time per part is less critical. I use a high speed spindle, taking light cuts at higher feedrates, not hogging off serious metal in a single pass... It's capable of a nice high resolution (0.00025 mm / 0.00001") and limiting the ballscrews to a fairly safe 2500 rpm gives feedrates up to 5,000 mm/min or 200 ipm. I run brushless servos and a smoothstepper board (Mach3 won't come close to driving it at that speed directly, even with the kernel at 100 Khz).
If you can't get the HRW blocks or similar (NSK do a wide, low profile series too, others may as well), I'd suggest you go with the 4 blocks and 2 rails - the 45 mm rails are just too high. The height alone will contribute to a greater moment load due to inertia and cutting forces, and that series isn't designed to be used in that configuration as the HRW blocks are.
If you want to go nuts with big caged roller linear bearings, check out THK's SRW series
Best regards,

Thanks Lee, Ken, Bob, Richard, Zumba, Vishnu and Jason. I have finally decided on using the 4x15mm design.
I will be starting a Build log thread once I start my build. Thank you once again for your inputs.

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