Bearing reamer fit

We’ve been looking into the possibility of buying a custom reamer to finish our bearing holes, but could use some help figuring out exactly what size to order.

Normally we just use an endmill on our CNC router to cut an approximated hole, with roughing and finishing passes for better surface finish. We’ve done a lot of trial and error to find an offset that works most of the time, but every once in a while makes a hole under- or over-sized enough that we have to remake the part. We bought a bearing reamer from Vex a few years ago and have experimented with using it instead of the finishing pass, but it is designed for a loose fit (1.126”) whereas we prefer a slight press fit so the bearing stays in the plate when the mechanism is disassembled.

We’re located near a fairly large industrial center so I’m pretty confident we can find somewhere that can make us a custom sized reamer. But we need to know what size to tell them to make. Technically I covered this material in undergrad, but tolerances was never my strong suit. Is there a good way to calculate exactly what size we should have the reamer made? Or to reliably measure the “good” holes from our CNC to get their true diameter?

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Do you want a tight slip fit on your bearing o.d., or a light press? 1.126 is clearly loose.

We want a light press fit. The Vex reamer is 1.126” which is definitely loose. The question is what size should a light press fit reamer be.

I would use a housing criterion of zero to +10 microns loose (10 microns is ~ 0.0004"). So your reamer should be 1.1252", because the bearing tolerance range is typically -9 to zero microns.

I would do some test fits. That’s what I did.

What you REALLY want is to hit Fleabay for an adjustable reamer in 1 1/8" size and tune that!

Maybe 0.001" per inch of diameter interference at minimum material condition (max hole bore and min bearing od. It’s probably going to be 0.002-0.003" under 1.125" to deal with bearing and hole tolerance.

You really need to test it… you will want less press on 3/8" steel plate than 1/16" Aluminum.
The press will tighten the bearing clearances. With high precision (ABEC 9) bearings you can easily lock them up (done it, re-bored it…).

Referencing the descriptions for Standard Tolerance Limits and Fits, I’d call this an LC1. Looking at this Fit Chart, (also below) the tolerance range for nominal hole size 1.125 would be 0 to +0.0005.

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If the hole is oversized and the shaft (bearing) is undersized, wouldn’t that make it a loose fit? I would think we want a small interference fit if we want the bearing to be lightly pressed into the plate.

We went with a 1.1235 reamer to get a snug/press fit for a bearing. Iirc the 1.123 will work as well.

The issue with hand reaming is misalignment so you’ll take off more than expected.


Our (my) thought was to chuck the reamer in the CNC spindle and run it as a final pass instead of the finishing pass we use now. So hopefully we shouldn’t have too big of a problem with misalignment since the CNC can already hold within a few thou.

What size cnc? Omio status? If so I’d be worried about putting this tool in there to ream some holes.

Have you had poor luck using a 6mm endmill or something equiv to finish bearing holes?

PRO4848 from AvidCNC (formerly CNC Router Parts)

Our current method for bearing holes is a 6mm endmill with a finishing pass to get to the right diameter. This works most of the time, but every once in a while without changing anything it will give us an under- or over-sized hole and we’ll have to remake the part. I assume this is due to tool wear, spoil board warping, coolant issues, etc. My thinking was that it would be nice to have a properly-sized reamer that we could use to get the hole to the right size every time within a few microns without having to rely on the accuracy of the CNC’s positioning.

I see your issue. Yeah I would be worried reaming with that large of a tool on that machine but you can give it a shot.

We’ve been hand reaming for years with good results and if parts come out loose you can peen the inside + loctite.

You make a good point. In my own experience, bearings I’ve measured end up ever so slightly oversized, which better aligns with this chart for Locational Interference Fits.


You may find you have trouble slowing down to an appropriate surface speed for the reamer. Plus the torque may be kinda high for a router spindle.

Also, depending on how you do your work holding you may have issues with clearance below the part for the reamer.

I’d consider using a taper ended expansion reamer in a hand drill for this job. OR a classic hand adjustable reamer (though that requires a lot of clearance on the far side).
The one I bought looked more like this one, and would have been called “adjustable”:
Another good type. The trick with this one is that it MAY not adjust down below 1.125". BUT, you could get it ground undersize maybe 0.005" and be sure you could expand it enough to hit your required fit.
Spiral version, needs lots of room on the far side.

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Don’t. The reamer is too large and the machine not accurate enough for this to be not dangerous. If the machine holds a thou (optimistically), you can still be off to one edge and the reamer will chatter badly.

Hand-ream it by chucking it in a drill press and reaming it by hand (not under power) to assure alignment. Loosen the belts if necessary.


What Don said. The reamer is for taking off ten-thousandths of an inch — think microns. Five microns is about two ten-thousandths.

Basically it’s like sanding. You are making aluminum dust, not chips.

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Alright then, advice taken.

So for manual use in a drill press is the right size 1.1252" like @Richard_Wallace suggested, 1.1235" like @R.C said they use, or something in between? Because that’s a huge difference when we’re talking about removing microns.

Use R.C.’s figure to get a firm press into a thin wall. If you’re making a full depth blind pocket, use my figure.

I think @Richard_Wallace your figure will give you a close slip fit on a normal bearing OD (+0, -0.0005). A dab of bearing retaining compound will stick it in place nicely.
If you want them to stay put dry, you need to have some interference. 1.1235" nominal ream seems reasonable for Aluminum, which stretches a lot easier than steel.

Another way to go at this is to make a set of graduated pockets, measure the ID, and press fit them. See what you like. When I was setting our adjustable reamer I reamed until I got a bore I was happy with. Empirical setting :wink:

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Yes, and when the reamer, spinning at (what’s the slowest speed on an omio?) hits the hole “a couple thou” off center, how do you think it will react?