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ed_h
04-26-2013, 10:34 AM
I'm considering building a 2-axis CNC sub table for my manual knee mill. I'm looking at using open linear bearings on continuously supported round shafts. One basic choice is ball-type vs plain (ball-less) bearings.

Plain bearings are cheaper, require no lubrication, don't require hardened shafts, and have higher load bearing ratings. They may have more friction and lower speed limits.

I'm leaning toward the plain bearings. Is this a mistake?

Evan
04-26-2013, 02:09 PM
I have been using PTFE plain bearings on my CNC mill for several years now. No wear at all, the shafts look slightly polished. Zero clearance fit with no wipers required. They are self wiping.

http://ixian.ca/pics10/ptfebearing1.jpg

ed_h
04-26-2013, 02:24 PM
Just the kind of input I'm looking for, Evan. Thanks.

Did you make the bearings yourself? What is the span of your shafts? Do you see deflection under load? Anything you'd do differently?

macona
04-26-2013, 02:34 PM
I would use square profile linear guides over round. You can get them used off eBay pretty cheap. They are much more rigid. You really only see round rail stuff in the wood Cnc world commercially.

Evan
04-26-2013, 08:41 PM
I made all of it myself not counting the hardened and ground 1045 round rod ways.

This is the lead screw nut holder but it is the same as the linear bearings. I bored them individually on the lathe, mounted them on the pair of parallel shafts already mounted in the end plates. Then they were drilled and tapped in operating position with pre drilled holes through the bottom plate of the table. Everything was very accurately aligned as it would be in operation since that part of the structure was complete.

The linear bearings holders and the PTFE bearing are slit on the bottom for adjustment. The adjuster uses two screws, one to change the gap and one opposing it to lock it in position. I haven't needed to adjust it for a couple of years. It needed a couple of adjustments at first as it was broken in. The PTFE is good for about 1000 psi per sq inch without creep and each has several sq inches of bearing area under load so they do not change under load.

I tested the complete system by placing about 200 lbs on the end of the table (me). I measured the deflection at the other end at about 0.020". At 25 lbs on the end the deflection was nearly unmeasurable. I have used this mill to machine everything from plastics to hardened steels with no problems.

However, the rails are not the entire support system for either axis. They are the guideways. Additional support is provided by flat ways on both axes. I spent a lot of time designing this in accordance with the design principles in the course notes of Alexander Slocum of MIT. His book is available on line and I highly recommend it.

http://ixian.ca/pics10/lsnut7.jpg

Doozer
04-26-2013, 08:58 PM
http://www.hiwin.com/images/lg/lg_main.jpg

http://www.hiwin.com/html/lg/lg.html

Hiwin are excellent quality rails at a reasonable price.
I have used them and they are super nice.
No affiliation.
--Doozer

Jaakko Fagerlund
04-27-2013, 01:46 AM
I would also build using Hiwin linear ball slides, but the carriages are expensive at around 50 EUR per pop. So a good milling XY table would need 8 of them, which equates to 400 EUR. As a hobby that gets expensive fast, but I would definately go that route if I had to make one. Easy to install, easy to use.

QSIMDO
04-27-2013, 03:49 PM
http://ixian.ca/pics10/lsnut7.jpg

Evan, are the bearings mechanically fixed in the holders?

ed_h
04-28-2013, 02:24 AM
I would also build using Hiwin linear ball slides, but the carriages are expensive...

I've been looking at the Hiwins. The new price is probably a little rich for this project, but there are used ones on ebay at around half the price. They all seem to be the same, though--the LG25, for which the carriages are 4" long, like the larger ones in Doozer's picture. Two of these eat up a lot of track. I'm trying to keep this thing relatively compact.

Evan
04-28-2013, 02:56 AM
Evan, are the bearings mechanically fixed in the holders?

Yes. There are a pair of small flush cap screws at one end on each bearing threaded into the PTFE. It is also glued in place using special glue for PTFE.

Incidentally, there is a lot of nosense passed around about unfilled PTFE and cold flow. It does not cold flow at pressures under around 1000 psi.

outback
04-28-2013, 06:28 AM
I made 2 cnc milling machines with linear bearings and round shafts. They were fast and smooth but not the least bit rigid. For me the
linear bearings and shafting was a waste of my time. But still not bad for my first machine build. I did lead to bigger and better things.


Buy some cast iron and machine dovetails for ways. Won't be that expensive or difficult and you will have a machine capable of almost
anything.
Don't build a machine like this (http://img.photobucket.com/albums/v30/jglass/CNC%20projects/cncmill7.jpg)

My Dovetail project:http://img.photobucket.com/albums/v30/jglass/CNC%20projects/CNC%20%20Mill%203/CNCMill3-2.jpg (http://smg.photobucket.com/user/jglass/media/CNC%20projects/CNC%20%20Mill%203/CNCMill3-2.jpg.html)



Jim

Tony Ennis
04-28-2013, 09:41 AM
nevermind!