oversize tapped holes for heat treat

[rgollar]

I have tapped holes in A2 and S7 material that gets heat treated. Do you oversize taps in these two materials before heat treat. And they have dowel holes in the parts, Do you adjust the hole size for heat treat in these materials? The dowel holes are just press fit but my concern is will they become to tight after heat treat? Thanks in advance.

[jeff]

Oversize the tapped holes before heat treat with a + .005 tap (or GH11 if metric)

finish dowel holes after HT especially if location needs to be held.

[Machineguy]

Rgollar,

 

How the parts are heat treated will effect this to a point but here is a common rule;

 

A2 will move .0002/in in heat treat. S7 will be close to that. S7 does not move as much in heat treat.

 

For dowel pin holes, a .0002 U/S is a press fit. A 3/8" dowel pin is .3752". I run a .3752 to a .3755 reamer before heat treat. Dowels are still tight in that range, but not too tight.

For small thin parts stay on the tighter side.

Also it helps if you are doing this day after day to keep some barrel hones and lapping compound in the shop just in case.

 

Tapping, I just use a a loose tolerance tap. Don't use a tight tolerance tap, or a worn out one. You can go to a .001 O/S tap if the holes are deep. That helps a lot but not always needed.

[Guest]

I'm with Jeff,

 

I prefer a much more controllable situation, O/S taps, final size and location after HT

[rgollar]

thank you all for the responses

[JAMMAN]

I did almost 100 die retainers from A2, one inch thick about 4 inches across and a foot to 18" long depending on the die several years ago. I did extensive measuring before and after heat treat and found it to swell about .0009 an inch especially along the grain. If dowel placement was necessary leave it way small and send it to the jig grinder after final grinding. Or if you have a wire handy drill a start hole and wire the dowels after grind.

 

Anyone else notice that much movement or did my heat treat place suck? It was a place called special research in madison heights michigan, bet it isn't there still.

[jeff]

Anyone else notice that much movement or did my heat treat place suck?

 

I've had about the same results in A2.. it doesn't move a whole lot in heat treat.