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

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
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If it is that RC28 pre hardened from J&L, and old tap should cut it.
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Hi Clark,
Please let me clarify my question, when I said "tap fit" I meant that the pin will be tapped with a hammer through the hole in the housing. There are no threads involved.
My concern is with the min. and max. hole size that will allow the pin to be lightly hammered through the housing hole fairly easily, but I don't want the housing rotating on the pin. The housing and pin must rotate together, and the ends of the pin rotate in needle bearings.
Sorry for any confusion.
Thanks John
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John2005 wrote:

Sorry, I screwed up. It seems to me that for a long pin, the calculation on the pin would be bulk modulus, and the calculation on the hole would be Lame's formula.
The equilibrium between those two forces times the co efficient of friction would give the force needed to drive in the pin.
The heat treat on the hole would make the math the same up to the point that the hole permanently stretched. The hole when heat treated could stretch larger and still spring back. The heat treat would then be a good thing, if the forces are not so high that you can't get the pin in.
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Whack the hole with a ball pein hammer on the exit side and let the pin stretch it back.
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Thanks for the feedback guys,
"Mechanical Magic" from the Sci-eng group gave me the following from Machinery's handbook...
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
I can rig up a small press for this thing but it's still best to have the easiest press possible.
As far as the bearings at each end, they are just drawn cup needle rollers with no inner ring. The rollers roll right on the shaft. I will slide the dowel through the bearing ID and then press or lightly hammer the pin into the 4140 housing and on through to the other bearing. I will probably start a pin in at the other end just a little to align, and then the pin I'm driving in will push the other pin out when it gets through.
Thanks again, John
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Your fit will depend also on how you machine the bores - i.e. the finish. Bored, reamed, honed, burnished? A roughly bored finish with little interference might give you a light press fit, whereas a bore burnished to the same size might be a heavy press or shrink fit.
There's always Loctite.....
John Martin
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Loctite is almost certain to migrate into the needle bearings. When I wrote 'Whack it with a hammer' I meant swage the exit hole down a little and let the pin expand it back to a very snug press fit but that might score the needle bearing journal and it wouldn't stay tight for long under a load.
I've had some success adjusting a press fit without precise diameter control by not reaming full depth, so the interference fit section is short. If it's too tight I just ream a little deeper and try again. This is the prior-planning version.
Move the needle bearings into the housing, if there's room.
Change to a drillable pin, install a roll pin in the shaft and matching slot on one side of the housing and a snap ring on the other, assuming you have enough room between the assembled 4140 housing and the bearing cup to push in the roll pin and install the snap ring. Lots of low-end equipment uses mild steel for roller and needle bearings.
Press the needle bearing cups in last. Then you could clean Loctite off the dowel pin. If you can tolerate heavy grease it will keep the needles in place pretty well, otherwise put in a temporary plug.
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Thanks for the additional info Jim
I had considered moving the needle bearings into the housing, but It seemed that when pressing the pin through the upright support and then into the bearing bore, I would not be able to feel if it were going into the bearing or perhaps hurting the bearing. If the bearings are pressed into the supports first, then I can slide the pin through the bearing ID, feel it start in the housing hole, then push it through to the other side. I was going to start a pin just a little on one side for alignment, then press another pin in from the other side, the second pin will push out the first pin as it goes through.
If I don't put the bearings in the housing, I like your suggestion to press the bearings in last. This way I could press the pin through and not worry about hurting the bearings, and then press the bearings into the upright supports last. The full complement needles are retained by the ends of the cup, so theres no problem with them coming out. I have to use a hard shaft for this application so that's why I'm sticking with a dowel pin, it comes in the exact length and OD I need, plus their fairly cheap.
Thanks again, John
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You are talking about a press fit. I would shoot for a hole that is .0005" smaller than the pin.
The bearing is the next consideration. If you will consult the bearing specs, you should see that the bearing is designed to have either the shaft or the housing press fit to the bearing to take up the slop. This depends on the permissible slop and the amount of change that will occur when the bearing heats up and swells at operating temp.
On a surface grinder where you want to keep wobble as near to zero as possible, all of the cumulative tolerances need to me considered to select the proper fit. If you are talking about a less critical application, then the running clearance can be looser.
If I understand your application correctly. you want the pin to be fixed in the hole a little snugger than the pin to the inside of the bearing.
--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
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John2005 wrote:

To begin with I wouldn't "pound" it in. It should be PRESSED. ...lew...
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