Sizing the blank: allowance, clamping and stock

What goes into a blank?

Stock is the block, bar or plate a part is cut from. The blank is the piece you take from it for one part, sawn or parted off, and it is always bigger than the finished part. The smallest box around the model, the bounding box, is only where the sum starts.

Material is added in three steps. First an allowance on every side, so the first cut gets under the skin of the material. Then a grinding allowance on faces that are ground after milling. Then the material the setup needs to hold the piece. Only after that do you pick what is on the rack. Every millimetre ends up in the kilos.

What decides the allowance per side?

There is no single right allowance. A sawn face is never quite square and has a rough skin, and the first cut has to get fully beneath it, or a patch of sawn face survives into a finished dimension. Rolled plate comes with a thickness tolerance and drawn bar with a tolerance on diameter. That tolerance and the rough skin together set the allowance.

The tolerance a plate or bar arrives with is set by its delivery standard. For hot-rolled aluminium plate from 2.5 up to and including 400 mm thick that is EN 485-3, and for cold-drawn aluminium round bar from 3 up to and including 100 mm it is EN 754-3. If your supplier names the standard, look up the thickness or diameter tolerance there and build it into the allowance.

A casting or a forging needs more than a sawn block, because the shape itself already deviates. Write the values down per stock form and per operation, as part of your shop's rules. Then every estimate uses the same millimetres.

How much extra material does each clamping method need?

There are six common ways to hold a part, and each kind of workholding asks for its own extra material. So choose the method before you fix the cut size, not after.

In a vice, the clamping margin comes off in the last setup, once the piece is resting on machined faces. That follows from planning setups back to front: you fix the last setup first, and then you know which margin the first one needs. How tall that margin has to be depends on the vice jaws, the mass of the piece and the cutting force.

Which size is on the rack?

The blank that comes out of the sum is rarely on the rack as it is. You step up to the next plate thickness or bar diameter your supplier carries. Or you saw a block off a size you already hold, and the rest goes back on the shelf or in the bin.

Every saw cut costs material, and on a batch from one length you count it per piece. Cut to size costs more per kilo, but it leaves no offcut and no sawing time of your own. On a small piece, carriage and sawing can outweigh the material itself, and then a block from your own rack is cheaper, even one size too big.

Weight is the volume of the sawn piece times the density; for 6082 that is 2.7 kg/dm³ according to the AZoM material data. The price per kilo comes from your material catalogue, with your own purchase price and its date. Why it takes precedence over a market quotation is set out in purchase price and market price.

Worked example: a 6082 housing in two setups

Take a 6082 housing of 120 by 80 by 40 mm, milled in two setups: first +Z in the vice, then reclamped for the underside, -Z. The allowance is 2 mm per side, and the vice gets a clamping margin of 8 mm under the part. In this example 52 mm plate is not on the rack, and the next thickness up is 60.

The sawn piece is well over one and a half times the volume of the bounding box, and the bounding box is already larger than the housing with its pockets and bores. In the second setup the piece is held on its machined sides, the margin comes off and the underside is machined. At an example price of €8 a kilo the sawn piece costs €13.76. Costed on the bounding box, the line shows €8.32, which leaves €5.44 a piece uncovered.

How do you size the blank for a shaft from bar?

In turning, the bounding box is the wrong question. A shaft of Ø36 by 150 mm comes out of round bar, so the bar size counts: the first bar diameter that covers the largest diameter plus allowance, in this example Ø40.

The bar length per piece is the length of the finished shaft, plus the allowance for facing the end, plus the width of your parting tool, which you find in the tool data. The weight per piece is then π/4 × d² × bar length per piece × density, where d is the bar diameter, here Ø40, and not the diameter of the finished shaft.

From bar with a sub-spindle, the end of the bar leaves a stub the collet can no longer hold. You spread that remnant over the pieces in the batch. As a sawn billet in soft jaws, you add allowance on length and diameter. More on the process is on the turning page.

When does the material itself ask for a bigger blank?

A thin wall or one-sided stock removal distorts. Machining releases stresses locked into the plate, and how you split the removal between top and bottom decides how far the piece moves. Research on thick aluminium plate (Li and colleagues, 2023) found far less distortion when removal was shared between both faces than when it all came off one side: in their simulation the largest distortion fell from 1.94 mm to 0.065 mm.

For the blank, that means leaving enough on both faces to share the removal, and preferring a thicker plate to a size that already sits close to the part. Hardening and stress relieving between roughing and finishing make the dimensions move. The finishing allowance for that belongs in the blank from the start.

The material specification on the drawing can ask for a bigger block too. An alloy or temper your supplier does not stock in a smaller size forces you onto the size that is available. That difference belongs in the price, with its reason.

Three pitfalls, and what to write down

First, the STEP model that sits askew in space. The bounding box is then measured around a tilted model and the block comes out too big. Align the model to its machining direction before you measure.

Then the semi-finished part the customer sends in, say a milled part that only needs an EDM operation. No material cost for the whole part belongs there. If the customer supplies the material, the quote states that as an assumption.

And the sum that lives in one head, or in a spreadsheet with a flat surcharge on part weight. That surcharge knows nothing about the clamping method: in the worked example the sawn piece is well over one and a half times the bounding box, and on a shaft from bar the remnant counts as well. The same percentage underprices one part and prices the next out of the market. Write down per part which clamping method sized the blank and which stock size you chose, as inputs to the estimate line for material. Then a colleague can read back later why the block is that size.