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It's my first time working with bearings, but this is a necessary part of my robotic arm project. I have some $10 bearings that are 24mm OD. I want to fit these tightly in a hole, ideally getting a press fit or interference fit (steel bearing, aluminum block).

I ordered a reamer that's 24mm and plan to ream the hole on a lathe, but I am worried - what if the bearings are themselves OD undersized (I don't have any fancy measuring calipers by the way). I think ideally I want a hole of maybe 23.99mm.

Thinking ahead, if I run into this problem, could I cool the workpiece in ice before placing it on the lathe? This way the workpiece would be reamed in a shrunken state, and then expand as it warms up to room temp? The last thing I want to do is break a tool, but this seems like it could work.

Wikipedia says aluminum has a volumetric thermal expansion coefficient (av) of 69, and for Silicon Carbide (the reamer is made of this) 8.31.

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  • $\begingroup$ Usually all the info is to be found in the catalogue of the bearing, at least that's what we do when we choose a bearing. $\endgroup$ – Sam Farjamirad Jul 12 at 12:21
  • $\begingroup$ @SamFarjamirad does that not depend on the "fit" required: a sliding fit, or a fit that needs gentle persuasion or a fit that is will not move unless heated... The details of "limits and fits" can be challenging... $\endgroup$ – Solar Mike Jul 12 at 12:24
  • $\begingroup$ @SolarMike Yes and no, it so depends on the manufacturer. We choose always FAG or SKF, SKF does offer a comprehensive online solutions about mounting and dismounting of bearings. I guess we have access to the enterprise version of its toolbox, not quite sure. $\endgroup$ – Sam Farjamirad Jul 12 at 12:27
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    $\begingroup$ @SamFarjamirad Watched SKF bearings being fitted that were 12" internal diameter - heated to 250 degrees C so they could be fitted to the shaft... Most amusing when one oven was opened to show a brand new bearing glowing red (the oven 'stat had failed), the only person not laughing was the manager... :) probably due to the cost of the bearing... $\endgroup$ – Solar Mike Jul 12 at 12:32
  • $\begingroup$ @SolarMike seen that video on the very first training day at work:) memorable! for small bearings usually a warm oil bath (the same oil as one recommended by the manufacturer) should be enough. For the gigantic bearings magnetic induction is very popular. In either case the temperature shouldn't rise above the viscoelastic temperature of the polymeric cage (small bearings usually), otherwise the cage melts ... $\endgroup$ – Sam Farjamirad Jul 12 at 13:01
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If you cool the workpiece and the reamer is at "normal" temperature then your final hole will be larger.

So if you heat the workpiece and it cools down after reaming then the final hole will be smaller...

You need to know the size of the bearing before purchasing the correct reamer.

I fitted new valve guides into a cylinder head once - not having a press I heated the cylinder head to about 200 deg C and put the guides in the freezer. They slid in a treat...

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In addition to the other answers about just using the correct sized reamer, If you are using a lathe, you can use a single point boring bar/tool instead of a reamer to bore the hole to the size. If on a milling machine, you can also just use a boring head/tool.

If you have to make multiple holes and time is of the essence, then, go with a reamer.

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You stated that the bearings are 24mm OD. Why would you be worried that they are undersized? I think your best bet, since you're not sure would be to get a scrap piece of stock and ream the 24mm hole and see if the bearing fits. If the bearing is 24mm it won't fit but you can emery the hole, easy to do on a lathe. Once your experimenting is done, start with the real piece. Good Luck.

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  • $\begingroup$ But the bearing may well fit with an interference fit if you heat the piece... A technique often used and heating bearings so they easily fit onto shafts is also done. $\endgroup$ – Solar Mike Jul 12 at 12:22
  • $\begingroup$ Very true, I've done it both ways. I was just thinking that since he's already on the lathe, a little emery paper would do it quicker... $\endgroup$ – JACK Jul 12 at 12:43
  • $\begingroup$ My suggestion is to get a lot of scrap stock and practice until you're consistent! $\endgroup$ – TimWescott Jul 12 at 20:50
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Why do you want to do this the hard way by trying to cool or heat the component while it is on a lathe?

To change the diameter of a 24mm in aluminum by 0.01mm, you need to change the temperature by about 20 degrees C, and maintain the complete piece of metal at that temperature for the whole time it takes to ream the hole - and allowing for the heat generated by the reamer itself, of course.

If you want to make a 23.99mm diameter hole, just use the correct size reamer to do the job. Problem solved!

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