Brass shrinkage factor

[Dan Fruth]

I'm planning my next build and need to make patterns for a trigger guard, butt plate, and patch box lid for an Oerter rifle. I don't know the % of shrink to allow for the brass. I have heard 5% in length and 2% in width, but I don't trust those numbers. Any help in this would be appreciated...……..Dan

[Mike Brooks]

I've been doing  a lot of pattern making for sand casting lately. The shrinkage is so little it isn't worth worrying about.

[ron w]

to put it in perspective,, a 5/8" wide trigger guard will shrink .012" (TWELVE THOUSANDTHS) and if the guard is 8 inches long, it will shrink less than 1/16 inch in length,.....not much to be concerned about.

[44-henry]

For sand castings I seal my wood patterns with an enamel paint to protect them, for the little bit of shrinkage you will get the enamel thickness will help to slightly enlarge the parts and offset the problem.

[Dan Fruth]

An 8 inch guard @5% shrink is .4, or just over 3/8".  I know it doesn't sound like much, but pattern makers used different measuring rulers, each calibrated for the cast materials rate of shrinkage.....Maybe I'm too picky!!

[Randall Steffy]

5% shrinkage for cast brass is simply incorrect. 1.5% is closer to correct if you want to use a percentage. The rule for brass shrinkage is 3/16" per foot and that applies to length, width or thickness. For a multiplier use 0.985 to see what a given dimension will shrink to or 1.0156 to see how much larger your pattern must be to achieve a given dimension. This shrink factor does not take into account excessive shrinkage which may occur if unduly heavy sections in the casting are not protected by a sacrificial riser feeding that heavy section.

[ron w]

I don't know what math you learned , but 8x.05 (5 hundredths) equals .040, or 40 thousandths of an inch......1/16th of an inch is .0625"..0625 minus.040 equals .0225,.... not even 1/32 of an inch.

[Ky-Flinter]

I don't know what math you learned , but 8x.05 (5 hundredths) equals .040, or 40 thousandths of an inch......1/16th of an inch is .0625"..0625 minus.040 equals .0225,.... not even 1/32 of an inch.

Ron W,

Not that it matters since Mr. Steffy has given a very succinct explanation, but you need to check your calculator.  8 x .05 (5%) does indeed equal .40     -     8 x .005 = .040

-Ron

[jerrywh]

  From my experience you can figure 3% over all but remember this. In a sand casting you will file off a significant amount in cleanup also.  Investment castings are not so much of a problem in that respect.

[Dan Fruth]

Thanks for the help guys....Ron W had me goin there for a minute!