# Shrink Fitting



## ollie (Oct 13, 2008)

I plan to install a 1.250" o.d. bronze rod (1.5" long) in an aluminum block 1.5" thick. I will then flycut the bronze level with the aluminum, and bore the bronze to 1" i.d. The problem is, that I do not know how big to bore the aluminum block to receive the 1.250" bronze rod to achieve an adequate shrink fit. I plan to put the bronze on ice prior to installing, and I plan to heat the aluminum. I'm flying in the dark at this point. Could anyone recommend how big I should bore the aluminum block as well what temperature to heat the aluminum prior to installation? Any help would be greatly appreciated. Thanks, Ollie.


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## rickharris (Oct 13, 2008)

I don't know what the "official" answer is but last week i turned an aluminium boss to adapt a cycle gear to run on a shaft. The boss fitted on a steel flange that already had the keyway for the shaft but had a raised centre about 4 mm high. 

I turned the recess in the aluminium to the same size as measured by my trusty digi calipers. tried it for a fit and it went on just with a gentle hand push.

I left it in the lathe and when I returned after an hours teaching the aluminium had cooled and I couldn't remove it with considerable hand pressure. I would therefore start with them the same size and see of you can get a fit with one hot to the touch (aluminium) and the other frozen.


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## Maryak (Oct 13, 2008)

Ollie,

It depends to some extent what the final use and environment of the finished assembly is.

For steel on steel the range is 0" to 0.0015" per inch of shaft dia. depending on how tight you want it to be and the wall thickness of the outer shell. i.e. if the outer shell is thin then there is a risk of it bursting when the assembly cools as you approach the tighter fit.

Aluminium has a slightly higher co-efficient of expansion than bronze, (0.0000124" per inch v 0.00001" per inch per Deg F).

What I,m trying to say is if your assembly is in a hot environment you need to be towards the larger difference so it won't come loose.

I had an experience where I shrank in a cast liner in a cast body using liquid nitrogen and a 0.0005"/inch interference. Worked a treat until the steam hit it, at only 400 F and bingo the liner went up and down with the piston till the whole thing came to a grinding halt. (Was later advise by the same expert that sorry it should have been 0.0015"/inch)

To say I was peeved was putting it very mildly. 

If you can sneak past SWMBO stick the Ally in the domestic oven on high. : Also dry ice may help with the bronze.

Hope this helps

Regards
Bob


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## CrewCab (Oct 13, 2008)

Here's a thread from a few months ago which may be useful *Click Here* 

It's not the one I was searching for, there has been something more recently, have a poke around in the archives and you'll probably find it  

CC


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## rake60 (Oct 13, 2008)

Sweating a solid piece of bronze into aluminium may not be the best approach.
When it is drilled and bored it's going to grow with the heat.
Bronze is stronger than aluminium and it may stretch the bore causing the press 
fit to be lost when all cools.

If I were doing that I would bore the aluminium first. Size doesn't matter much at
this point, just make it somewhere under the size of the bronze stock.
Then I would drill and rough bore the bronze .030 to .060" under the finished size
and turn the OD for .004 to .005 press on the bore in the aluminium.

High heat is not good for either bronze or aluminium.
Lay the aluminium in the sun or warm it up to not more than 250 Degrees F 
while the bronze bushing spends a an hour or so in the freezer.
It will go into the bore quite easily.
Actually at size your talking about the rough bushing could be coaxed into the 
bore at .005 press with a block of hard wood and a 4 pound hammer without the
sweating process. You just need to turn a 15 degree start on one end about 1/8"
long to help it enter the bore.

Theoretically you can make a brass or bronze bushing that doesn't require finishing
of the bore. If you make the bore .004" oversize and the OD to have .004" press,
Installing the the bushing brings the ID to the perfect size.

I've had the explained to me several times over the past 20 years.
I've NEVER seen it work! 
Obviously it's because I did something wrong in the process... :

Rick


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## Maryak (Oct 13, 2008)

Rick,

I don't think you did anything wrong ;D

I think :-\ the ally would expand more than the bronze would shrink.

Same on same and maybe yes ???

Regards
Bob


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## rake60 (Oct 13, 2008)

There is no perfect answer here.
Different alloys will act as they will.

If you ask two engineers how much press to put on a sleeve
or bushing you will get different answers.
I got tired of asking years ago and use the middle line of 
.004 to .005.

I've put steel sleeves in a lot of cast iron bearing fit over the
years. Cut the sleeve for .004 press, pound it in the oversized
bore, drill and tap it for 3 set screws to be permentally sealed
into place to dowel in in then finish machine the ID of the sleeve
for the bearing fit.

If you happen to work in an industrial job shop and are given a 
cast iron wheel to replace an already repaired sleeve be careful
machining that old sleeve out!

If it's one that I worked on at least one of those dowels just might
be a broken tap that might not cut too easily! :-[

Rick


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## ollie (Oct 14, 2008)

Gentelmen: Thank you all for your help and suggestions. I had planned to press the bronze into the aluminum prior to boring the bronze, however I had not realized that as the bronze heated during boring that it could stretch the aluminum and cause the shrink fit to fail when cooled. Therefore, I will bore the bronze to near final size before pressing it into the aluminum using your recommended clearances.  Thanks again, Ollie.


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## zeusrekning (Oct 14, 2008)

Ollie, Pre boring would be a good idea. We use to press bushings (ID) and sleeves (OD) at work. I would usually shoot for .002" interferance and would use green loctite. We would also turn a small step on the end of the sleeve. The diameter would be .001-.002" smaller than the bore to be pressed into. Turned .100-.200" back. This alows you to get the bushing started in the bore straight. 
Tim


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