Do we have what it takes to install some press-fit drill bushings in an aluminum fixture I’m designing? The bushings are 0.25" OD and 5/8" long.
And if so, can anyone help me do it?
Thanks!
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Somewhere we have a hand-operated press that you can use to press the bushings into the holes.
In my limited experience it takes a super-precise hole (i.e. reamed, not just drilled). I can give you some info but I’m squeamish about doing this myself because I just know that I would be the person who breaks the reamer …
In theory you need a size C drill bit to pre-drill for a 1/4" reamer. Somewhere we have reamers (possibly locked up so they don’t get trashed) but I can’t guarantee we have a size C drill bit. Easy enough to check in drill index in the tall grey cabinet across from the Bridgeport.
If they don’t quite fit you can freeze the bushings and then rush out to the Machine shop to press them into your fixture.
EDIT: That’s a pretty deep hole. Is it blind?
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No not blind. I have a complete set of carbide drills in fractional, letter and number sizes so that’s not a problem.
I’m having the fixture machined in China. Maybe I should have them make the holes undersize. I don’t trust their tolerances but they claim 0.05mm.
The bushings themselves are slightly oversized by a fraction of a thousandth I think.
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Maybe you can spec out a hole diameter tolerance of +.000 / -.0005 to force your supplier to ream the holes for you. That’s a pretty deep hole to ream at that diameter and if you can force them to take the risk, that’s best. We have pin gauges of every possible size in Machine Shop so you can inspect the holes when they arrive. I’m assuming you can reject the fixture if the holes don’t meet your spec?
Other option is to have the holes undersized and plan on reaming them. The correct hole size should actually be 6.1mm for aluminum but I suggested size C because I’d bet money that machine shop doesn’t own a 6.1mm drill. Either one would work for you here.
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The recommended hole size for the bushing is .2507 - .2510
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Hmmm. That’s interesting. At that size you might be able to use a simple 1/4" drilled hole because unless it’s a worn bit the hole may be a little oversized. You could chuck up a drill bit, drill the hole, and measure the hole with pin gauges.
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The gauge pins are in one mil increments so being precise to the tenths can’t happen. The drilling will likely be a wee bit over sized; you might, indeed, find that the .251 gauge pin goes through! If the .251 pin goes in and the bushing feels too tight to enter the hole, the press fit might work straight away. If the resulting hole feels too tight to take the press fit, the most practical approach may be to chuck up the bushing, first with half out, then the other half out, sanding a few tenths off each half to get a size that will allow a press fit. The hole diameter can be measured to a precision of tenths if one takes a rod down to 0.252 and lathes on a very slight taper, which can be achieved either by setting the compound angle or by merely extending the rod about four diameters out from the chuck and depending on the usual deflection (that one typically trys to avoid) to give the needed taper. Part off the rod at the chuck. Then the narrow end
of the turned piece is inserted into the bushing hole until it is stopped by the increasing diameter. Mark this point prior to with drawing it. Measure with a micrometer, not calipers, to find the diameter to the nearest ten thousandth.
Regards,
Bob
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So when you get the parts let’s take a look at the actual dimensions. Using a snap gauge and a Micrometers we get the actual dimensions. If it needs to be reamed slightly larger, then it will need to be fixtured and then reamed. We can get away with I feel using a .250 reamer and either anchor lube or a piece of paper towel to get it slightly oversized.
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