bullet shrinkage allowance

[danyboy]

I need some help. I purchased a blank mold from Lee and a neighbour working as a machinist will cut it according to what I need using a CNC machine. How much bigger should I allow for bullet shrinkage when I cut the mold block ?
Danyboy

[Calamity Jake]

It all depends on the gun barrel and or throat dia, your ever changing alloy, what side of the bed you got up on that morning :veryconfu and a few other things.
For example if your making a 38 cal pistol boolit then .358-.359 should be your sized boolit dia. then I would cut the mold to .360-.361 this allows .001-.0015 alloy shrinkage.
You may also want to put some draft angle in the grooves, this allows easy boolit exit.

[danyboy]

Calamity Jake,
I'll give it .0015" extra and see what bullet it drops.
Thanks

[Cap'n Morgan]

To take the guess work out of mold making, I use a simple formula to calculate the cavity diameter: Multiply the bullet diameter with 1.005 to get the size of the cavity This is based on WW alloy with 0.5% shrinkage. For a softer alloy you may increase the factor to 1.007 (0.7%)

The advantage of this method is that it will work for all calibers regardless of size.

[fredj338]

Rather slightly large than slightly small. So I would allow for the lightest/largest alloy + 0.001".

[theperfessor]

Why not just get an existing mold made of the same material and of the same approximate caliber/diameter and measure it? Then cast some bullets with the alloy you plan to use and measure them. You can figure it out after that.

Remember that the mold expands when it heats up, and the bullets shrink as they cool, and they are made of different materials with different expansion rates undergoing different overall temperature changes.

Nothing wrong with using scaling factors such as the one shared by Cap't Morgan, you just need to understand what those factors are based on. And note that the Cap't did specify the alloy but not the mold material. I've seen one of his nice molds and it was aluminum.

Why not be sure the mold makes bullets the size YOU need under the conditions you will be using it? See first paragraph.

[Cap'n Morgan]

Nothing wrong with using scaling factors such as the one shared by Cap't Morgan, you just need to understand what those factors are based on. And note that the Cap't did specify the alloy but not the mold material. I've seen one of his nice molds and it was aluminum.

I don't worry too much 'bout the the thermal expansion of the molds as I only use aluminum or brass, and both materials has approx the same expansion:
(from http://www.matweb.com)

Aluminum:
(metric) 23.0 µm/m-°C @Temperature 20.0 - 100 °C
(english) 12.8 µin/in-°F @Temperature 68.0 - 212 °F

Brass:
20.5 µm/m-°C @Temperature 20.0 - 300 °C
11.4 µin/in-°F @Temperature 68.0 - 572 °F

Steel and cast steel, like Meehanite is a different kettle of fish. It has only about half the expansion of aluminum, and must be cut larger to compensate for the lesser shrinkage.

Meehanite:
11.9 µm/m-°C @Temperature 37.8 - 538 °C
6.60 µin/in-°F @Temperature 100 - 1000 °F

If you would like to calculate how much a mold cavity of a given size expand, there's a neat online calculator here:

http://www.engineeringtoolbox.com/linear-thermal-expansion-d_1379.html

Running the numbers through the calculator it suddenly becomes clear why our boolits varies in weight even if the alloy has a stable temperature, and fill-out is perfect: Going from room temp. to 300F will increase a .450 aluminum mold cavity to .4512, and if the mold is further heated to 500F the cavity grows to .4523... Isn't that great; one more thing to worry about :veryconfu

[303Guy]

This thread is real handy. I'm about to head out to my lathe and make me a mold! It's gonna be brass (or bronze) so the info is exactly what I came here to find! Thanks folks.:drinks:

[danyboy]

please delete