On 2014-01-15, PCS wrote:
On Wednesday, January 15, 2014 12:21:42 PM UTC-5, 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.

[ ... ]

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.

[ ... ]

IIRC, shrink fits are generally .003" for the first inch of diameter,
.001" per inch larger.

So, you might bore your 1-inch part's hole .4965" for your .4995"
shafting.

Sounds reasonable.

Slowly and evenly heat the part with the hole to a dullish red. Don't
let it develop any scale.

Easiest to avoid if you can heat it in an inert atmosphere --
Nitrogen or Argon for example.

Insert the cool shaft in one swift motion, you won't have time to mess
with your set screw alignment. The shaft instantly starts to expand
with the heat, further motion will not likely be an option.

If you want a bit more clearance to work with, get some liquid
nitrogen, and *cool* the shaft while you heat the collar (or whatever
kind of part -- I'll use collar for simplicity). You can get the Liquid
Nitrogen from a welding gas supply place usually. Take a big thermos if
you don't have a Dewar made for the task. Ideally, one of the fragile
glass-lined thermos jars. If not that, several large styrofoam cups
nested, and handle with good gloves. Drive with the windows open so you
don't pass out from the nitrogen which boils off displacing the oxygen
in the car.

The Liquid Nitrogen is at 77 K (-231.07 F), so that will add a
bit more difference in size.

Can you place the collar on a support with a hole sized to pass
the shaft? If so, mount another ring above it a loose fit on the shaft
at room temperature, and a few inches above the collar. This will let
you drop the shaft in with a pretty good alignment, and the support will
be at the same temperature as the collar, so it will keep that warm a
few seconds longer. Set a stop below the support so the shaft goes the
right distance into the collar. Then heat the one, cool the other, drop
in and stand back.

Then cool it all off with water.

Your set screw will probably be redundant.

Agreed. That will be a serious interference fit. If it isn't,
the assembly can always be drilled for a taper pin to lock it.

Enjoy,
DoN.

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