mtngun
posted this
01 September 2013
MM's wheelweights are late model composition, about 3.2% antimony and 0.3% tin. My mix of WW's from the last 20 years cast about 0.001+” larger than what his model states
I have not noticed any change in wheelweight characteristics since I began casting about 30 years back -- providing I'm the one who smelts the weights.
If other people smelt the weights, they may include stick-on weights or even zinc weights, so all bets are off. Unfortunately, wheelweight is getting very hard to find in my part of the country so I am increasingly forced to purchase pre-smelted WW.
As-cast diameters may vary considerably depending on temperatures (both the pot temperature and the mold temperature) and on casting technique, as well as on alloy.
Also, if you put some sort of mold prep on the block halves, or if lead splatter gets on the block halves -- a common occurrence -- that changes the as-cast dimension.
In general, I don't view casting as a precision process. It frustrates me that the general public does not understand that. They think there is a predictable relationship between the cavity diameter and the bullet diameter and that as-cast tolerances are merely a function of machining tolerances. Not so ! :fire CNC machines can usually hold 0.0005” tolerances, but the cavity diameter changes with mold temperature, fill-out may be inconsistent, and if heat shrink kicks in then diameters may vary wildly.
On mold/alloy combos that are prone to heat shrink, it is not unusual to observe 0.002” variation from one pour to the next, especially on the bottom band and on the check shank. It is not unheard of to observe 0.004” or even 0.010” difference between a frosty bullet and a shiny bullet, if heat shrink is happening. Not all mold/alloy combos are prone to heat shrink, but some are, and the mold maker must make allowances for the worst case when he promises a certain as-cast diameter.
I personally prefer bullets to drop about 0.002” larger than the size-to diameter, to ensure that the sized bullet will be “big enough” despite reasonable real-life variations. Sizing is a precision process and sizing dies can easily be fined tuned by honing. Even noses can be sized, as CBA competitors are well aware.
