Press fit question for the machinists/engineers
 

We are having a friendly dispute here and I am looking for some input.

We have a pin (.234" dia 12L14 stock) that is getting pressed into a knurled knob (.375" thick also 12L14). What diameter should the hole in the knob be for a solid press fit? What would be "too tight"/"too loose"?

Thanks.

Are you planing to heat one (the knob) and freeze the other (the shaft)prior to assembly?

Makes a bit of difference in the tolerances.

I'd add .0005" interferance and you will be fine;)

I would suggest an FN2 medium drive fit of 0.0008 to .0027" interference. Of course I'd need material and application for a more accurate number. Really, 0.001 is probably plenty.

Get out your Machinest's Handbook and look it up. I have the 1917 edition that was my Grandfather's.

How big is the hammer?

Thanks guys,

My machinists handbook is in my desk and I'm at home. I told them I thought a half a thou to a thou and a half would be fine, but they had it set for MUCH more than that. Can you believe .008"?

Thanks again.

OF course the easy way might be to taper the end of the shaft and the hole in the knob. Very slightly of course than assemble in a hydraulic press.

Ya, we can still get it in there(we use an air over oil press) but you can bulge the pin with the required force.

Is it being held with just the fit i.e. no key, no set screw, etc?

If so I'd go with a minimum .001" interference and no more than .002"
So, make the hole .232" to .233", but mic the pin up to make sure it is in tolerance prior to reaming the hole.

Just interference, no locking fluid or set screw.

Not sure what you just said, but it sounds like a recipe for a night of serious fun!!

;)

Don't forget some lube, otherwise getting it in there might gall a bit.

:D

A good press fit is about 0.005" to 0.008" interference in most materials. It does depend on the diameter. For a small pin, like yours, an interference of about 0.003" should be sufficient. NO less. Anything less will not stay put.

Don't forget to chamfer the end of the shaft!!

Did you mean 0.0005? Because 0.005" will be little difficult to assemble.

Why not go with a slip fit and red loctite?

If the pin is .234" , make the hole the following dimension (diameter) for a press fit.

.2340/.2335

this is the case if both are metal

Isn't it easier to size the pin to the hole, than size the hole to the pin?
That way you can ream the hole to a standard size then turn the pin to whatever size you want to fit the hole. It's usually more difficult to get a hole to the right size when you already have the pin, so it would probably be a good idea to make the hole first in future.

However, if you already have the pin then here's my suggestion. For an interference fit I would think you'd be looking at 1 to 2 thou and a transition fit would be roughly half a thou either side of nominal. But if you want a press fit I would think that anything between 0.0005" to 0.0015" would be fine. You'd probably want to aim for the middle of that range and make it 0.001", but 12L14 is a lead bearing, free cutting, and relatively ductile steel so you shouldn't have any problem if you make it a little on the tight side.

8 thou is WAY too much!

This is a production part and the components are already made incorrectly. It is much easier to ream the hole than to turn down the pin at this point.

I meant 0.003", not 0.0005". You probably need to heat the hole, freeze the pin and use a 5 ton press to fit the pin, and yes it needs tapering on the leading edge. The materials are important. If the coefficient of expansion of one material is 3 times that of the other, its very important.

One example is a valve seat in a aluminum head. The valve seat may have an expansion rate of 6ppm/deg C and Al 22 ppm/deg C. You better have at least a 0.003" interference fit or the seat will fall out. For larger holes, like a cylinder sleve in a 4" bore, you may need 0.005" to 0.008" interference and thats for a steel sleeve in a cast iron bore.

Materials, operating temp range, and stress conditions are all important so there is not a single good answer to your question, but only 0.0005" for a 0.3" dia is marginal.