PDA

View Full Version : x,y axis linear bearings



level head
04-20-2006, 11:51 PM
i cant quite figure out what kind of sturdy/ridgid linear bearings to use on my x.y axis cnc mill table .I have seen in evan s mill he is using some kind of a brass type linear bearing ....
Link..

http://www.homeshopmachinist.net/bbs/showthread.php?t=12222&highlight=cnc+mill

I am building my table on round ways

Does any one have any ideas that would work out on my project (similar to evan s mill)table
-or any that i could purchase from someone on this board

thanks

darryl
04-21-2006, 12:54 AM
I think Evan is using teflon blocks on his mill. IIRC, these are restrained within metal structures to help keep it rigid. I'm going to suggest using some sheet teflon and sandwich it between the round ways and a machined block which will carry the table. Consider the block as a bushing which is bored oversize to allow the teflon sheet to fill the gap. Because you'd be using a thin sheet, the forces are most directly transferred straight to the bushing or block, so that satisfy's the rigidity requirement, and keeps the vagaries of the teflon to a minimum. . You'll need some kind of way wipers to keep grit out of the bearing- I made mine from uhmw. What I did was turn some rings with enough meat left for mounting holes, then bored to a thou or so larger than the round ways. Then I cut a v groove on the face of each ring very near to the bored diameter- this pushes a ridge of the uhmw material inwards, making a lip which has to expand slightly to fit on the rods. So far these have been working fine for me, but it probably would be better to get some ready made ones- something like what motorcycle forks have.

Evan
04-21-2006, 02:25 AM
I have both bearing bronze bearing shells on the bottom axis where the greatest loads are and teflon on the other two axes. The teflon is calculated to be able to take at least 1000 lbs per bearing without measurable cold flow but that depends on it being well contained in aluminum housings and a very close fit to the ways. The teflon bearings are especially remarkable for the total absence of stick/slip compared to the bronze bearings and it appears they will work out extremely well. I could easily have used only teflon bearings.

I'm not putting up any more pictures of the mill project at this time until it is closer to completion. I will say that it is working out better than I expected for rigidity. If I stand on the end of the table it only deflects about 20 thou. I weigh 185 lbs.

Millman
04-21-2006, 03:33 AM
If you're 185 and have a deflection of .020, that's not bad. I'm sure you would not be loading that kind of weight on such a small table. Now you know by standing on it; you no longer have the 0 tolerance of flatness that you once had. Gotta' give you a hard time, so fire those lasers back up!

level head
04-21-2006, 10:48 PM
hey i was wondering if i bought a piece hollow stock of bronze and instead of boring a hole at the end of pipe and inserting a bronze bering .i would use a long piece of bronze as a whole of a 1 piece silde

Fasttrack
04-22-2006, 11:08 AM
Maybe i'm just an idiot, but i'm not sure i understand what you are asking...

Evan
04-22-2006, 12:22 PM
I think I know what he means. I made my linear bearings by boring blocks of aluminum and carefully fitting bronze shells inside the blocks. Level head wants to know if the entire bearing could be made from just bronze. The answer is yes, of course but it would be very expensive.

If you are thinking of making the bearing as one long piece instead of as two pieces at each end of the carriage then that isn't a good idea. The stick/slip friction will be much too high.

level head
04-22-2006, 12:42 PM
thanks for that information evan

i have 1 more question did you use stainless steel solid tubing on the ways

i want to use stainless steel but it seems to expensive do you a cheaper metal that would work good

Evan
04-22-2006, 01:03 PM
I used precision ground 1040 steel.

level head
04-22-2006, 01:05 PM
one more question Evan
what size did you use on all your ways thanks

Evan
04-22-2006, 01:16 PM
1 1/4" for all ways.

level head
04-22-2006, 01:43 PM
when you put the bronze shells at the end did you use a set screw to hold them in
thanks

Evan
04-22-2006, 01:46 PM
Yes. You have used up all your questions for today. The oracle has chores to do. :D

IOWOLF
04-22-2006, 04:03 PM
Evan, You owe me a keyboard. :)

darryl
04-23-2006, 04:05 AM
Better order a whole case of keyboards. :)