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Printrbot Talk Forum • View topic - LM12UU Bearing Block

LM12UU Bearing Block

Discussions about CAD use and designing

LM12UU Bearing Block

Postby benwis » 2014-May-Sun-03-May

Hello!
I've built my Printrbot almost entirely from scratch, and I recently started work on the XR Upgrade. I was wondering if anyone had the cad file for the LM12UU Aluminum bearing blocks. I was wondering what diameter hole they used for the correct press fit of the two individual bearings. I was also wondering what would be correct for the LM8UU, because I have some of those lying around as well, and the 8mm shaft that goes with it. I'm afraid I don't know much about press fit tolerances and what not.

Can anyone shed any light on press fit tolerances? I'd rather waste as little time on the CNC mill as possible.

Thank you,
Ben
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LM12UU Bearing Block

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Re: LM12UU Bearing Block

Postby REPRAP SQUAD » 2014-May-Sun-10-May

I am not the master of press fit parts into aluminum but, I have a friend who is. I can ask about the lm12uu bearings. Just an FYI, there is a difference between the lm12uu that printrbot used in the earlier days and the lme12uu that they use now. A lot of the files that printrbot released are for the lm12uu that they only used for a little bit.

I'll ask about the lm12uu for you and get back to you asap.
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Re: LM12UU Bearing Block

Postby REPRAP SQUAD » 2014-May-Sun-11-May

My buddy is out and about right now so he will give me an exact number when hes home. He said if he remembers correctly its a couple of a thousands smaller but, I can get a better number when he has time to look at them.
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Re: LM12UU Bearing Block

Postby benwis » 2014-May-Sun-12-May

Thank You. If I could get numbers for the LM12UU, LME12UU(they're different?), and the LM8UU that would be awesome!

I'm also curious how you arrive at those numbers. I think there are different classes of fit as described here:
search URL engineersedge class fit

Odds are I'm going to have to shove bearings in some other place soon, so I'd be awesome to know how to do it correctly for different diameters of bearing. Thanks again!
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Re: LM12UU Bearing Block

Postby REPRAP SQUAD » 2014-May-Sun-16-May

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Re: LM12UU Bearing Block

Postby benwis » 2014-May-Sun-17-May

That's very nice. I'm building my own new machine as well, though I don't have any photos. I hope it looks at least as good as yours.

I've got access to a positively gigantic allen press, so I'm not worried about getting the thing in there if the holes are right.

No terrible hurry :) I'm probably going to do the milling work next weekend, I can always change the CAD model. I just wish I could find some kind of tutorial or something on press fits. I'm thinking I'm going to need to find a good machining book. Maybe Machinery's Handbook or fitting and machining by ron culley
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Re: LM12UU Bearing Block

Postby Jon Lawrence » 2014-May-Sun-18-May

Here is one source. Scroll down to page 9 for housing tolerances
http://www.lunabearings.com/new/Linear_Ball_Bushing.pdf
Press fit is a J7. On a 30mm translates to +14 / -11 micrometers.
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Re: LM12UU Bearing Block

Postby REPRAP SQUAD » 2014-May-Sun-18-May

Thanks. We've poured a ton of time designing, printing and building this plus. I have a plus v2/2.2 hybrid that was built from scratch as we'll. the only parts we didn't make was the wood pieces and a couple of months ago we added pbs dual extruder.

It's a great platform and I've done a ton of mods to it so if you ever have any questions pls let me know.
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Re: LM12UU Bearing Block

Postby benwis » 2014-May-Sun-19-May

Why are the fits both over and under the desired hole size? At any rate if I read that datasheet and this one correctly:
mdmetric CH6.8wGO.pdf (Still won't let me add links)

Then the hole size for press fit is as follows:
Lm12UU Outer Diameter: 21mm
Press Fit Hole Size for J7 fit: 20.991 to 21.012mm

LME12UU Outer DIameter 22mm:
Pressit Fit Hole Size for J6: 21.995-22.008mm

LM8UU Outer Diameter: 15mm
14.99mm to 15.010mm

LM8EUU Outer Diameter 16mm:
15.995mm-16.006mm

They give a moderate range, so I'm not sure where to aim in that area. I'm kinda thinking slightly smaller than the desired hole, but I've got no experience with these things. I'd be curious to see what the other guy has to say :)
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Re: LM12UU Bearing Block

Postby Jon Lawrence » 2014-May-Sun-20-May

The reason for the + AND - range is reality hits somewhere in the middle. Those tolerances are based on a statistical deviation that you can reasonably assume to be somewhere in the middle. In reality, all machinists worth their salt will make holes on the small size of the tolerance and shafts on the large size of the tolerance. The reason, there is still room for correction if you leave a little meat on the bone.

In addition, just because the theoretical number may be over the ideal dimension, in reality the hole will not be perfectly round or perfectly smooth. Combine those and you still get a light press on a long bearing surface like the LM12UU.

It's late, and I am tired, so hope that made some sort of sense.
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Re: LM12UU Bearing Block

Postby REPRAP SQUAD » 2014-May-Mon-20-May

If you want to pm me your email info I can send you his contact info as he is quite the machinist. I'm sure you can pick his brain.
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Re: LM12UU Bearing Block

Postby benwis » 2014-May-Mon-20-May

That would be helpful. It looks like I'm going to need to hire a machinist to get that fit cut, and it's pretty hard to find any. Would he be interested in running a couple prototypes for a fee?
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Re: LM12UU Bearing Block

Postby REPRAP SQUAD » 2014-May-Mon-20-May

Im sure he probably could. Ill ask.
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Re: LM12UU Bearing Block

Postby REPRAP SQUAD » 2014-May-Mon-20-May

My email is reprapsquad@techie.com. email me and ill get you guys connected.
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Re: LM12UU Bearing Block

Postby FACTEngineering » 2014-Jul-Tue-18-Jul

Greetings,

I'm Cary and I own and run the machine shop that makes the LM12UU bearing blocks for Printrbot. As far as the hole sizing for the press fit goes we simply tune the size for a light feel on the press. We also have to chase the hole size throughout the day as the temperature of the machine changes. It seems that the best fit for the 12uu bearings is about .0005" undersize, but again, we adjust if by feel at the press.

One thing to be aware of is that if the hole is too tight you can actually compress the bearing down to a smaller size and then when you force the 12mm rod into it it will not roll nicely or it will destroy the bearings or it will quickly cause the rod to pit and flake off the hardened surface.

-Cary
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