Shrink fit steel pin into an aluminum piece

[WobblyHand]

After reading through a few threads on the subject, and then consulting the Machinery's Handbook 17th Edition, I'm still confused about things. I have a 0.2495" 1144 steel pin, that I want to be permanently held by a piece of 0.300" thick 6061 aluminum. From what I understand, the hole in the aluminum should then be about 0.001" * 0.2495 or about 0.00025" smaller than the steel pin, at room temperature. Is this right? Sure would be more convenient if it was 0.0005" as reamers are available in 0.0005" steps. Can I use a 0.2490" reamer for the hole instead with 0.300" thick material? The aluminum piece is about 5/8" wide.

What I am confused about are the units of thermal expansion. For aluminum is it 13 micro-inches / degree F? And for steel, about 6.5 micro-inches / degree F? If I leave the steel pin at room temperature, let's call it 72F, and toss the aluminum in boiling water, then the hole diameter would increase by 13e-6 * (212-72) inches from the ambient diameter? (0.0018" larger?) If the hole started at 0.2490 at 72F, then it would be 0.2490 + 0.0018 = 0.2508" at 212F? So there would be 0.2508-0.2495 = 0.0013" diameter difference between the pin and the hole? I would probably put the aluminum in an oven and maybe heat it to 300F, for more clearance, but is my thinking correct on this? Seems like one won't have much time to pull this off! I have to set at least 6 pins, so I'd like a way to do this quickly and accurately.

If I am not thinking about this correctly, please let me know. I'm aware of other ways to get this done, but I'd really like to learn about shrink fitting a steel pin into aluminum in this thread.

 

[Ken226]

Your thinking correctly about it.

Look up the coefficient of cubic expansion for the parts, calculate the change per temp you need, then fire up the oven and freezer.

It was always my go-to method for installing V8 wrist pins and 2 stroke crankcase bearings. Anyone whose ever rebuilt a small block Chevy is familiar with the concept. Put the rod in the oven and freeze the wrist pin.

Just get the parts together quickly, because the temps quickly equalize. A pin stuck halfway into a blind hole is a ***** to get further in, and near impossible to pull out.

 

[WobblyHand]

Your thinking correctly about it.

Look up the coefficient of cubic expansion for the parts, calculate the change per temp you need, then fire up the oven and freezer.

It was always my go-to method for installing V8 wrist pins and 2 stroke crankcase bearings.

Just get the parts together quickly, because the temps quickly equalize. A pin stuck halfway into a blind hole is a ***** to get further in, and near impossible to pull out.

Guess the things I am wondering about is 1) if the 0.00025" interference is enough, 2) the thermal mass of the pin and bracket and 3) how to fixture this to avoid fumbling about. Low thermal mass means you don't have much any time before everything locks up. Guess I need to cruise fleabay to see what reamers I could get.

 

[WobblyHand]

Found a 0.2490 reamer, and a 0.2485 reamer, but no 0.24925 reamer on the bay. Think I will get the 0.2490 reamer and hope it is on size. $9.89 + $5 shipping from Zoro. Worth a shot.

 

[JRaut]

I found a simple calculator online:

ANSI Limits and Fits Calculator

Using the following:
- Force/Shrink fit (FN)
- Medium drive fit (FN2)
- Nominal size 0.25"

The calculator says you should be targeting an interference between 0.0004 and 0.0014". So your math certainly seems right.



These calculators are all made for steel/steel. Because you're using aluminum/steel, I'd shoot for the tighter side of things, and if it were me, I wouldn't hesitate to go to 0.002" of interference.

As far as actually performing the operation: I typically just use a propane torch to get the outer part nice and hot, unless there are seals / bearings / etc. involved that don't appreciate getting too hot.

Get that bad boy nice and hot and just drop the pin in. Make sure you do it right next to your arbor press. Press'er home quickly if things hang up on you.

** EDIT ** I mis-read your post. Seems you're shooting for 0.001" of interference per inch hole diameter. I'd definitely go with more than that if you don't want them to separate.

 

[Winegrower]

I frequently and reliably press a steel pin into aluminum with 0.001 interference, all at room temperature. As JRaut says, 0.002" if you are worried about it ever coming out.

 

[jwmelvin]

I agree with @JRaut I’d use 0.0005-0.001” interference.

 

[JRaut]

I frequently and reliably press a steel pin into aluminum with 0.001 interference, all at room temperature.

Yup.

0.001" should be easy, even without heat.

Even pressing 0.002" at room temp for steel into aluminum may be manageable, but heat'll help things go a little easier.

0.00025" doesn't seem near enough.

 

[WobblyHand]

Yup.

0.001" should be easy, even without heat.

Even pressing 0.002" at room temp for steel into aluminum may be manageable, but heat'll help things go a little easier.

0.00025" doesn't seem near enough.

0.00025 is according to machinery's handbook, for a 1/4" pin, but I think is for steel to cast iron, or steel to steel. 0.001"/" interference.
I have ordered a 0.2490 and 0.2485 reamer for the aluminum. I'll try a test piece or two first. Suspect it will take a couple tries before I figure out the technique.

 

[SLK001]

You're not shrink fitting, you're force fitting. It should be no problem pressing your pins into a 0.002" undersized hole - even 0.003" should be doable. Heat shouldn't be required, but it can't hurt to heat up the Al bar before you press. Any press will do this with ease. I've even used a mill vise to do the pressing.