The Home Machinist!

Hi All,

Im getting ready to start making the cylinder pattern for my 5" scale project. If I have a 6" diameter cylinder bobbin dimension to hit on the finished casting, would i jist simply turn the wooden pattern to fit an outside caliper set to 6" off of a shrink rule, or would i need to use a double shrink allowance? Thanks again.

jlakes85

I hope I properly understand your question.
If the part requires machining, you must also include material for that operation, so you'd add an appropriate amount for machining.

If the resulting surface is a bore, you'd think in reverse, and make the cast hole well undersized, so machining would open the hole to the desired size, plus allow for any mislocation of a core.

A shrink rule allows for shrinkage and does not address that issue.
H

jlakes85 wrote: ↑Mon Sep 16, 2019 10:25 am Hi All,

Im getting ready to start making the cylinder pattern for my 5" scale project. If I have a 6" diameter cylinder bobbin dimension to hit on the finished casting, would i jist simply turn the wooden pattern to fit an outside caliper set to 6" off of a shrink rule, or would i need to use a double shrink allowance? Thanks again.

jlakes85

Double shrink is typically applied when the original or "master" pattern will be used to make another pattern.

For instance; your master pattern might be made of wood or plastic and relatively fragile in nature and might not be suitable for use in a production scenario. In this case you might want to cast the original in aluminum first and then this would become the actual pattern to be used in the production run. In this example you would need to include double shrink on the original master pattern (one shrink for the aluminum and the second for whatever final metal would be used.) As Harold mentioned on top of this you need to include your machining tolerance.

My advice would be to discuss what shrink rate and machining tolerance to use with the foundry that will be casting your parts. Last time I spoke with my local iron foundry they told me to plan on 1/8" per foot for a shrink rate and also plan on at least 1/8" to 3/16" of material where ever the part will be machined in order to fully remove the rough outside skin that the cast iron will have.

My "Shrink Rulers" say this...
The brass 12" ruler is actually 12 - 3/16" long
The aluminum & steel 12" ruler is actually 12 - 1/4" long
The cast iron 12" ruler is actually 12 - 1/8" long.
It says on the back " Machining Allowance" should be 1/8" for aluminum & cast iron, 1/4" for steel and 1/16" for brass. Added to the overall ONE TIME.
If making a Master Pattern from which Production Pattern are to be made, double the shrink allowance on the master which will give the proper for the subsequent production patterns.
This is not totally from first hand knowledge, I am parroting off my rulers.
It also has some verbage about larger patterns not requiring as much shrinkage allowance per foot. But I don't think we are in the realm of 36" or greater castings so I ignored that part.
I am reinforcing what Harold and FKreider posted.
So, by my estimation, no need to double that shrink allowance dimension. Altho... why not add that margin for error, just a little more turning to do.

Scotty

ChipsAhoy wrote: ↑Sun Sep 22, 2019 8:35 pm
It says on the back " Machining Allowance" should be 1/8" for aluminum & cast iron, 1/4" for steel and 1/16" for brass. Added to the overall ONE TIME.

It should be noted that the machining allowance should be added to the face intended to be machined. I make mention because that may not always be obvious, so extra material could possibly be added where it would do no good.

Of interest, perhaps. There's an alloy of gray ductile iron that require no shrinkage allowance. If anything, it grows minutely upon solidification.

Armed with the knowledge that all metals do not shrink uniformly, imagine the surprise one receives when making the decision to cast an item in steel when it is usually cast in gray iron, using the same pattern. Undersized parts, anyone?

H