How Much Stock Is Required?

If you think that just multiplying the part length by the number of parts needed is the answer you will be quite disappointed…

The number of these needed is more than just the number of parts times the part length in inches...

The number of these needed is more than just the number of parts times the part length in inches…

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 set-up 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 under 2 inches in length, allow for 5% extra material needed to make the desired quantity;
  • For parts between 2 to 3 inches inclusive allow for 6.5% extra material needed to make the desired quantity;
  • For parts between 3 to 4 inches inclusive allow 8.5% extra material needed to make the desired quantity;
  • For parts 4 inches and over allow 10.0% 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.

Getting the setup correct the first time is key to effective use of material too.

Getting the setup correct the first time is key to effective use of material too.

Note, do not confuse this for for scrap loss by weight. Heavy stock removal parts may have up to 90% 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 assure you can deliver the required number of parts ordered.

Curly adjusts the machine.

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4 Responses to How Much Stock Is Required?

  1. Thomas Pauley says:

    Cutoff saw to save on mat’l cost: excellent idea, but i have only used std. swiss cutoffs, made from a micro 100 blank. Wouldn’t the extra cycle time required to saw the part off negate the savings in material? There i go, trying to think!

  2. John Detterbeck says:

    Just love “Rules of Thumb”.

  3. How Much stock is required is

    decided by the material/Tolerance and machine accuracy etc.Have seen Very Good C.N C Turning centres used for roughing and finishing allowance is 25 to 30 microns total.This is done with a single point Diamond tool and repeatability is + _ 2 Micron at 22 deg centrigate.Th ease produced for specific
    industry.

  4. Dave Koepper says:

    Coming from the land of Multiple Spindle Bar machines in the Aerospace sector, we also had to allow material for face off, in order to hold tight OAL dimensions.

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