Re: Recommendation for tap fit hole tolerance in 4140 pre-hard steel

On Nov 29, 6:32 pm, John2005 <johnjmechani...@xxxxxxxxx> wrote:
Hi everyone,

I would like to ask if anyone can please recommend a tap fit hole Min.
& Max. tolerance for a 3/8" hardened steel dowel (.3751" Min to .3753"
max OD) ? The pin will be tapped into 4140 Pre-hard steel.

The pin is being tapped through a bearing housing, 1" OD X 1.5" long.
The bearing housing fits between two upright supports and there is one
drawn cup needle bearing pressed into each of the upright supports. I
want to insert the dowel pin through a bearing at one end, and then
tap it through a hole in the 4140 housing and on through to the
bearing at the other upright support. The housing and pin will rotate
together, and the pin rotates in the bearings.

I will probably build simple fixture to locate and hold the housing as
I tap the pin through. I want it to be fairly easy to tap the pin
through, so assembly goes smooth, but I don't want the housing to
rotate on the pin, the pin and housing must rotate together in the
bearings.

In order to get a nice easy tap fit, can I have a very small amount of
interference to do I need some clearance ? I was going to shoot for a
hole size of .3748" Min. to .3751" Max.

I'm just curious what's usually recommended for an easy tap fit.

Thanks
John

John,
I think a light press fit would be .0001" interference. Given the
hard materials.
If you get a hole that's .3748" and a pin that's .3753" you end up
with a half thou interference, that will require some pushing.

Machinery's Handbook 19th calls for a shaft of 1/4-13/32 to be:
"Easy Driving Fit" to be .00028-.00047 interference.
"Close Driving Fit" at .00047-.00083
"Forced Fit" .00083-.00146
Dave
.

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