Gibbs and Ways OR Linear Rails

[sbirdranch]

There are so many topics on this forum, I have picked this one to start....
I am working on selecting a CNC mill to add to my shop. CNC is not new to me. I have/am building two of my own. Now it is time for a mill.

I have seen various machines and the one option I have no insight into is how the axis are built. Some have gibbs and ways, and some have linear rails.

I understand the difference in design, but not performance, longevity, accuracy, etc. Do any other you have insight into how they perform compared to each other?

I plan to buy a new machine.

Preston.

 

[T Bredehoft]

No experience with rails, but cast iron ways would impart (IMHO) rigidity that rails would not. Perhaps for a CNC router they would work, but I can't imagine cutting 1018 steel with a cutter mounted on rails. ( or with the work mounted on rails)

 

[cjtoombs]

This is my take on the pros/cons of each:

Linear rails pros:

You don't have to build them, they come pre made, which speeds up the build

Linear Rail cons:

They are probably more costly than plain slides.

Because of thier ball bearing nature, they are not as well coupled as plain slides, so they do not transfer vibration from one stage of the machine to the other. So that means having a very massive base to the machine may not do you as much good, as not as much vibration is transmitted to it. This means the machine is likely to be less stiff and more prone to chatter.

Linear Rails it probably doesnt matter for a mill:

Linear rails support much higher movement speeds than plain slides, which probably doesn't make much difference for a hobby mill, since you won't be able to or need to move it at very high speeds. This would be more relevant to a 3D printer, router or a plasma cutter, where very high movement speeds and the need to change direction quickly are more important than they are for a milling machine.

Plain slide pros:

Can be cheaper, depending on how you make them.

Transmit vibration from stage to stage better than linear bearings and rails.

Plain slides cons:

Unless you convert an existing mill or use a comercial xy table, you have to build them. This will likely require grinding or hand scraping the surfaces, which is either time consuming or expensive.

For my money, for a mill that is primarily for metal milling, especial steel, I would choose the plain slides. I have plans to someday build a multipurpose shop bot that would do plasma, oxyfuel and light milling on which I planned to use linear rails, but the milling would not be it's primary function and I would be willing to live with low metal removal rates from that aspect of it. Hope this helps, and good luck.

 

[sbirdranch]

This is my take on the pros/cons of each:....

I really appreciate the time it took to put that out there. The issue of vibration isolation is enough to make me lean in the plain slide direction.
Thanks for your input. ~P

 

[jbolt]

Tormach has a nice write up on their design analysis for a personal CNC.

https://www.tormach.com/engineering_pcnc1100.html

 

[TomS]

Looking through the Haas Automation website it looks like several, if not all, of their machines have linear guides. I didn't dig any deeper so can't say whether other manufacturers use linear guides or dovetails. Obviously Haas are industrial size/type of machines and there a lot of them in service so the linear guide approach works. As it applies to hobby type machines there are a couple of threads on this site where a mill was converted using linear guides. Try this link - https://www.hobby-machinist.com/threads/x2-mill-table-and-saddle-with-linear-rails.59991/. Here's another link where linear guides were used on a lathe build - https://www.hobby-machinist.com/threads/slant-bed-cnc-lathe-build.58711/#post-484372.

 

[sbirdranch]

Looking through the Haas Automation ....

Well that is interesting. Haas are well respected machines. I would buy one out of hand if I could justify it. Thank you. ~P

 

[cjtoombs]

Looking through the Haas Automation website it looks like several, if not all, of their machines have linear guides. I didn't dig any deeper so can't say whether other manufacturers use linear guides or dovetails. Obviously Haas are industrial size/type of machines and there a lot of them in service so the linear guide approach works. As it applies to hobby type machines there are a couple of threads on this site where a mill was converted using linear guides. Try this link - https://www.hobby-machinist.com/threads/x2-mill-table-and-saddle-with-linear-rails.59991/. Here's another link where linear guides were used on a lathe build - https://www.hobby-machinist.com/threads/slant-bed-cnc-lathe-build.58711/#post-484372.

I expect the linear rails they are using are not the same ones that you find on eBay from China. Machine tool manufacturers have gone to linear rails to eliminate the high labor costs of hand scraping plain slides. They also need to move at very high speeds, as these machines have very high rapid speed capapbility, which plain slides do not support. Hobby machines don't have the need or the power to have rapid speeds equivelant to industrial machines, where seconds per cycle can make a large difference in profitability. They still have the same tradeoffs as the hobby machines, they just have diffferent requirements that make linear rails (specificaly designed for those machines) more atractive from a cost and performance standpoint. That doesn't change my assesment of linear rails for home built hobby machines.

 

[Robert LaLonde]

Tormach has a nice write up on their design analysis for a personal CNC.

https://www.tormach.com/engineering_pcnc1100.html

Tormach also has a white paper that basically claims steppers are superior to servos. (plane steppers not closed loop steppers) I do own a Tormach mill, but have not yet finished setting it up.

Linear rails are basically worth what you pay for them. The bigger, heavier, and more expensive they are the better... generally. I have three Chinese machines that are very similar. Two have square profile linear rails and the third has your typical dovetail ways. Other than the fact that the dovetail machine is lighter, and has a punier spindle the performance is similar. Rapid, acceleration, feed, accuracy, etc. In theory the linear rail machines should be faster, but they really aren't. I haven't tweaked them until they stall to see what the failure point is, but nobody works right at the failure point anyway.

 

[cs900]

as someone who just converted a hobby mill from dovetails to linear rails I have (biased) opinions on this, haha.

Linear rails have the advantage of not needing to be adjusted. Dovetails inherently have a small amount of slop to keep them from binding and to provide a place for the lubrication film. And as we all know, where there is clearance there is adjustment that needs to be made to maintain that clearance as things wear. This also means if things are too tight you loose your lubrication film which induces premature wear and can cause binding, and if things are too loose, you will get excessive vibration and poor surface finishes. On the flip side linear rails can come with a preload in the bearing already which keeps things nice and tight all the time. I just got done with my conversion and I'll never go back. In fact, I'm so tickled with them i got a set for my cnc lathe as well.

I suppose i should also note that I did not get cheapo chinesium rails either. I spent the money and got nice (used) NSK rails and bearings.