CNC bearing question

Dave38

Dave38

theemperorhasnoclotheson
Silver Member
This is a question for the experienced CNC people here. I am rebuilding a CNC gantry machine that I bought a few years ago and found that it had way too much play. I have most of it done, but the problem area right now is the X axis still has too much play. The thomson bearing blocks seem to have play. They are SPB-120pn and were bought brandnew with the rails as a set. They are adjustabl;e, and have tightened them as much as possible, but they still have side to side wobble. I am thinking of going to another type, but don't know which ones to use. My rails are Thomson 3/4" dia. I will post some pictures in a bit.
Any help appreciated.
Dave
 
The play is in the red circled areas. If I grab the router mount plate, and try to wiggle side to side, It rocks about .030" of movement in each direction, side to side and is visiably coming from the bearings. According to the spec's these have a self-alignment ability of about 1 degree in any direction, and I think that is the flex that I'm experiencing.
Thanks,
Dave

Picture 002.jpg
 
On the bottom red circle to the right, there is a grub screw (set screw). That is the adjustment for the closing up of the block around the shaft.
Note, that as this closes up,the main bearing block may need to be repositioned slightly to get every thing back into a neutral alignment. As it only closes on one side, it is an asymmetrical adjustment and that will need to be compensated for.
Usually there is enough clearance in the mount block to accommodate for this.
Neil
 
I'm by no means an expert but i'm guessing that with single bearings "in lieu of typical 2 per rail" it will be very hard to beat. I think your only real shot of tightening it up the way it sits now would be to move the rails up/increasing the distance between the 2, effectively binding them a bit. Of course the binding will likely cause a whole other group of problems.

The bright side to the machine is that it has a ton of room for improvement and will be a great way for you to increase your knowledge when it comes to the mechanical side.

I think that if you come up with a good plan, set aside a few hundred bucks, and do some smart ebay shopping you could easily turn it into something solid.
 
Two bearing blocks per side would be best. I am not a big fan of round in supported rails either. If you want the easy route add she extra bearings.

Jim
 
Thanks for all the responses guys. I guess a rebuild of the gantry assy. is next on my list to accomodate the wider footprint of 2 blocks. Would two blocks per side be better than one 4" long block per side? I am going to try to find non-self aligning blocks, as I really think that is the main source of the sway. I have tightened the adjusters on the blocks as tight as I can, but it didn't help much.
Dave
 
Dave38 said:
Thanks for all the responses guys. I guess a rebuild of the gantry assy. is next on my list to accomodate the wider footprint of 2 blocks. Would two blocks per side be better than one 4" long block per side? I am going to try to find non-self aligning blocks, as I really think that is the main source of the sway. I have tightened the adjusters on the blocks as tight as I can, but it didn't help much.
Dave
Click to expand...

A quick answer "Yes" but not much. Those bearings actually give in both horizontal and vertical directions. The bearing on the bottom helps prevent vertical movement. You will want two on each rail as far as you can get them apart. This will help on horizontal movement. If you add the bearings and are still not happy, consider linear bearings. They will take up less space and are more rigid. Also I notice your Z axis does not even have any type of ball bearings. If I am not mistaken, it looks like rods sliding on bronze bushings. You will have slop and binding with those. One last note, and again I am sorry as I missed it the other day, get rid of those Luv Joy couplers. They are junk. Stick with a good anti-back lash coupler.

Jim.
 
Mc2 said:
A quick answer "Yes" but not much. Those bearings actually give in both horizontal and vertical directions. The bearing on the bottom helps prevent vertical movement. You will want two on each rail as far as you can get them apart. This will help on horizontal movement. If you add the bearings and are still not happy, consider linear bearings. They will take up less space and are more rigid. Also I notice your Z axis does not even have any type of ball bearings. If I am not mistaken, it looks like rods sliding on bronze bushings. You will have slop and binding with those. One last note, and again I am sorry as I missed it the other day, get rid of those Luv Joy couplers. They are junk. Stick with a good anti-back lash coupler.

Jim.
Click to expand...

Thanks Jim for your help and expertise. You are correct about the lack of bearings on the Z axis. I bought 8mm linear bearings and rod to change that, just haven't done it yet. As far as couplers, I am changing those out also, I was going to make my own steel sleeves with setscrews, is that a good idea or do I have to use the anti-back lash coupler you mentioned?
I am a real nubie to this stuff, I have mechanical ability, and I'm just trying to salvage the $1400 investment that I made a few years ago, that looking back I realize was a foolish purchase.:o
Dave
 
Dave,

As Jim mentioned, I'd go with square linear rails on all axis. If you're going to have to change one, then that's really the best time. No need to have to change it twice.

As for the couplers, you don't want to make anything for this. Couplers are a very important part of your machine. They allow for slight misalignment of the stepper motor and the screw, and still don't add any play or backlash. If you use a solid coupler, like you would most likely make yourself, you would end up with binding and trashed bearings in your steppers. These are cheap and work great

http://www.ebay.com/itm/320851035033?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Good luck with your machine! Sometimes I think building the machines is the most fun.
 
Dave38 said:
Thanks Jim for your help and expertise. You are correct about the lack of bearings on the Z axis. I bought 8mm linear bearings and rod to change that, just haven't done it yet. As far as couplers, I am changing those out also, I was going to make my own steel sleeves with setscrews, is that a good idea or do I have to use the anti-back lash coupler you mentioned?
I am a real nubie to this stuff, I have mechanical ability, and I'm just trying to salvage the $1400 investment that I made a few years ago, that looking back I realize was a foolish purchase.:o
Dave
Click to expand...

Dave, the coupler makes up for any miss alignment of your motor shaft to the leadscrew or ballscrew. If you just use a piece of steel that will not flex, where the setscrew is the leadscrew will rock back and forth and eventually come loose. This is a good area to spend a little money. I have purchased some good ones on E-bay from china and received them fairly quickly. You will want that flex at that coupler.

Jim.
 
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