Shrinkage

[Ed T]

Does anyone worry about shrinkage factors when casting small parts typically used in small engines. Depending on where you look, the shrinkage for aluminum is around 10% or (in another place) .156"/" . This seems significant even for a small part. Thoughts?

 

[woodnut]

Hi Ed

I usually allow 3/32" for shrinkage and machining for Ali, not sure on Brass or Bronze.

John

 

[JorgensenSteam]

It seems like a multiplier of between 1.02 and 1.04 is about right for aluminum.

This size multiplier seems like a small amount, but it makes a difference when trying to fit parts together, and especially when several parts connect together in series, the error can become very significant.

An example: For a 6" dia. flywheel, the pattern is made between 6.12" and 6.24" dia.

 

[Dan Rowe]

I usually allow 3/32" for shrinkage and machining for Ali, not sure on Brass or Bronze.

3/32" per what distance?

Dan

 

[woodnut]

3/32" per what distance?

Height, width, depth. I add 3/32" to all my measurements. Still experimenting, so far this has worked.
If the piece is fairly large then I add a bit more.

John

 

[ksor]

????

Adding ... shouldn't you be multiplying ?

Lots of diff. measures here ... 10% compared to 1.02-1.04 ?????

Look at what "tubelcain" does in his video here - I think he knows what he's doing:

[ame=http://www.youtube.com/watch?v=h-c4_Ukqgx4]http://www.youtube.com/watch?v=h-c4_Ukqgx4[/ame]

 

[Mainer]

1/8" per foot for cast iron, 3/16" per foot for aluminum and brass.

 

[woodnut]

Now that's a great Video series!
Thanks Ksor, Learned something new tonight.

Now were do I get shrinkage rulers?

John.

 

[Ed T]

Thanks all for the feedback. 3/16"/ foot seems to be the consensus. Now, echoing the previous post, where do you get shrink rulers. A fairly extensive search brought me nothing. Not too hard to make, but easier to buy if reasonably priced.

 

[ksor]

Run an ordinairy one through a roller ;D

 

[mklotz]

Now, echoing the previous post, where do you get shrink rulers. A fairly extensive search brought me nothing.

Gee, that's strange. I plugged "shrink rule" into Google and the second entry returned was...

http://www.freemansupply.com/StarrettSteelShrin.htm

 

[Ed T]

Marv,
Thanks. Google is a fickle mistress. I think I tried every combination of words I could think of. Closest I got to yours was "shrink rules". "s" makes a big difference apparently or maybe today is an odd numbered day. Who knows. Anyhow, it'll be interesting to see the cost of a special order Starrett ruler, but I'll probably just go ahead and make one for now.

 

[MachineTom]

Look at ebay Yup 108 shrink rule's now including a 25" tape measure, nor seen one of those.

 

[Ed T]

Thanks again. I'm starting to wonder if I dreamed that I looked around. Maybe a senior moment? Anyhow, thanks for the lead.

 

[ksor]

In Denmark we have a dianose for that - dementia light

I myself never had that .... as I remember !

 

[mklotz]

Marv,
Thanks. Google is a fickle mistress. I think I tried every combination of words I could think of. Closest I got to yours was "shrink rules". "s" makes a big difference apparently or maybe today is an odd numbered day. Who knows. Anyhow, it'll be interesting to see the cost of a special order Starrett ruler, but I'll probably just go ahead and make one for now.

Maybe you did dream it, Ed. I hate to tell you this but, if you put "shrink rules" (with/without the quotation marks) into Google, you still get the URL I noted as the second return.

 

[Entropy455]

Cast Aluminum has a thermal expansion coefficient of 'about' 13 millionths of an inch, per inch, per degree F.

Assuming a 1200 degree F melting temperature, a cast aluminum object will shrink 0.01456 inches per inch (or 0.17472 inches per foot), as it cools from a solidification temperature of 1200 degrees F, down to an 80 degree F room temperature.

 

[ksor]

->woodnut

I have a whole collection of these videos on my web-site:

(Let Google do the translation for you !)

http://kelds.weebly.com/form-og-stoslashbning.html

and more here:

http://kelds.weebly.com/fra-start-til-slut.html

I plan to do some in danish language showing the same.

 

[JorgensenSteam]

How much aluminum shrinks is not really what you are after, but what you need to know is how much bigger you need to make your pattern.

For the particular alloy of aluminum that I use (356), if you want to make a piece that is 12" long, then the pattern needs to be (12*1.02) = 12.24" long.

You can fine tune your multiplier by cutting a 1/4" square piece of wood to 12.24" length, casting it, and then comparing it to 12".

When casting AL 356, 1.02 is pretty much on the money, but other alloys may vary, but you can easily find out the multiplier for any alloy, as noted above.

Another method that has worked well for me is to plot out a CAD drawing using a 1.02 multiplier (for AL), glue the CAD drawing to the block of wood, and then carve out the pattern. A shrinkage rule is not required.

If you really want a shrinkage rule, just draw a standard 12" ruler in CAD, and then plot it to whatever multiplier is required for the various metals. You can print it on card stock and then laminate it in plastic. Works fine.

 

[woodnut]

Thanks Ksor

I will definitely take a look.
Google translate is pretty good, but it comes up with some interesting translations sometimes :big:

Thanks again.

John