How Much Stock is Required?

If you think just multiplying the part length by the number of parts needed is the answer, you will be quite disappointed. The part length usually accounts for the vast majority of stock required.

But the amount of material lost by cutoff tooling (kerf loss), the first piece and remnant in the machine, “steal” parts makeable from the material you purchase.

Parts can also be lost from production by failure to conform to requirements for dimension or during an extended campaign to get the setup dialed in.

Quick rules of thumb to allow for bar end and scrap loss based on the length of the finished part (plus cutoff loss) include:
• For short parts less than 2 inches in length, allow for 5 percent extra material needed to make the desired quantity;
• For parts between 2 to 3 inches inclusive, allow for 6.5 percent extra material needed to make the desired quantity;
• For parts between 3 to 4 inches inclusive, allow 8.5 percent extra material needed to make the desired quantity;
• For parts 4 inches and longer, allow 10.0 percent extra material to make the desired quantity.

Your mileage may vary. If you use narrower than usual cutoff tools, this may be reduced a bit. If you use cutoff saws, you may achieve a significant savings. But if your team can’t get the setup and dimensional control right, these numbers are downright optimistic.

Note: Do not confuse this for scrap loss by weight. Heavy stock removal parts may have up to 90 percent of total material (by weight) removed to create the desired geometry during machining. These guidelines are just an estimating tool to give you a minimum order quantity to ensure that you can deliver the required number of parts ordered.
 

Originally posted on PMPA Speaking of Precision.