Metal buying guides / Fit and processing
Ordering Metal Blanks for Machining: Leave the Right Allowance
A drawing shows the size of the finished part. The machine shop may need a larger piece to start with because it will remove metal and needs somewhere to grip the work. Order the starting piece after the shop has confirmed its size.
Updated
Quick answer
Ask the machine shop for the minimum starting thickness, width and length before buying a blank. Then check that the supplier’s allowed size variation cannot put it below those minimums. There is no single amount of extra metal that suits every part.
A blank is the piece of metal you supply for machining. Machining allowance is the extra metal left for the shop to remove. Workholding means how the shop grips or supports the piece while it cuts.
Separate three sets of dimensions
On a small screen, scroll the table sideways to see every column.
| Dimension set | What it describes |
|---|---|
| Finished part | The geometry the completed component must meet |
| Required blank | The starting dimensions accepted by the machine shop |
| Ordered nominal size and tolerance | The range the material supplier will provide |
Nominal size is the size you order. Tolerance is the amount it may vary. Check the smallest size the supplier can deliver within that tolerance, because that is the size the machining plan must still work with.
A tolerance example
Suppose a part must finish at 6.000 inches wide. You order a blank at 6.125 inches wide with a ±1/16-inch cut tolerance. The ± symbol means the supplier can cut it up to 1/16 inch smaller or larger. These are example values, not a recommended allowance or a Rapid Metals tolerance promise.
On a small screen, scroll the table sideways to see every column.
| Step | Inches |
|---|---|
| Ordered width | 6.1250 |
| Allowed reduction: 1/16 inch | 0.0625 |
| Smallest permitted blank: 6.1250 − 0.0625 | 6.0625 |
| Finished width | 6.0000 |
| Extra width in total: 6.0625 − 6.0000 | 0.0625 |
| Extra on each side, if centered: 0.0625 ÷ 2 | 0.03125 |
The order looks like it leaves 1/8 inch of extra width. At the smallest permitted size, only 1/16 inch remains in total. Splitting that equally between the two sides leaves 1/32 inch per side.
Whether that is enough depends on the machining plan. Ask the shop for a minimum acceptable blank width. Also ask for a maximum if a larger piece would not fit its setup.
For more on interpreting the range, see cut tolerances and fit.
Include thickness and workholding
Thickness needs the same check. Facing means machining a broad surface flat. If both faces of a plate need machining, metal will be removed from its thickness. A finished thickness of 1/4 inch does not automatically mean you should buy nominal 1/4-inch plate.
The shop may need extra material for a vise or clamps to grip. It may also need the piece placed in a particular direction. Send the finished drawing so the machinist can plan both the cuts and how to hold the piece.
If the part needs a flat face, square edges or a particular surface finish, mark those requirements on the drawing. A rectangle cut from stock has not necessarily had those surfaces machined.
Protolabs’ CNC machining design guide, page 5 explains that trying to use all the starting material can create workholding problems. Buying the smallest piece that contains the finished shape can leave the shop without enough material to grip.
Copy the material specification from the drawing
Copy the full material description from the drawing, including the grade and any temper designation. For example, keep the T651 in 6061-T651 aluminum. Have any substitute reviewed before buying, and include any required material certificates in the request.
For several parts, ask whether the shop wants separate blanks or one larger piece. It may plan to machine multiple parts in one setup.
Send an order the next shop can use
Material and temper:
Quantity of blanks:
Nominal requested blank dimensions:
Minimum acceptable blank dimensions:
Maximum dimensions, if relevant:
Critical stock condition or documentation:
Further machining performed by:
Finished drawing reference:Order standard cut blanks when the available material and tolerance satisfy the machine shop's requirements. For another size, condition, tolerance, or added processing, send a custom request.
Keep the accepted blank specification with the finished drawing. Use the same agreed dimensions when you reorder, unless the part or machining plan changes.