Bob La Londe wrote:
I've got a little project I am working on. Steel shaft in steel hole. At
first I was just going to bore close, and skim cut the bore until the shaft
just barely fit then secure it with a set screw. Some strength against
twisting is important. Then I was wondering if I might get a better
concentricity by heating the part, and letting it cool around the shaft.
Still with a flat on the shaft and a set screw just to be safe.

The part with the hole has an outside diameter where the hole is of about
1.00". The hole is 0.500" apx. Final cut dimension of the hole will be
determined by trying to figure out how much to heat the part and how much
expansion I can get. The shaft is hardened tool steel with a diameter of
0.4995" apx. The part is 1018. My thought was to bore to .49 then ream to
.499 with an under reamer. Heat the part, insert the shaft, snug up set
screw momentarily to align the flat, and allow part to cool. Then tighten
set screw. This is to be a modestly accurate permanent installation.

How much heat do I need on the part? I think I'll need a little more growth
than necessary to fit due to rapid cooling while assembling, and the slow
clumsiness of working with heat heat resistant gloves on and/or using tongs.
Ambient in my shop is around 70F in the afternoon lately.

Please lets not go off on tangents. Thanks for any help or suggestions.


550-600 degrees will get you around .0025" of expansion. Then if
possible toss the shaft onto dry ice for 20 minutes. That will give you
..0005" more.

You are working with almost the same dimensions and materials as a
connecting rod with a pressed pin.

Watch your heating method, you want the heat even and slow to prevent
thermal distortion. Make sure the hole is perfectly clean and the shaft
is perfectly clean. Anything that causes a snag will screw you up. As a
twist inhibitor you could put striations on the area of the shaft where
the part will stop. Just make sure they are below the surface so they
don't inhibit the assembly. This is done on some pins that have oiling
provisions to prevent rotation.

In the case of the pins (which are hollow) you have around 5-6 seconds
before the parts are locked. With the solid shaft you might get a bit
more but I wouldn't count on it.

Do NOT use any forced cooling, let them air cool slowly. Any type of
quench will cause thermal distortion and probably thermal shock enough
to damage the part.


--
Steve W.