Guides / Materials
How Much Stock to Leave on a Heavy Weldment or Casting
September 12, 2026
Machining allowance is the stock left on a blank for the cutter to remove. On a small part it is an afterthought. On a heavy part it is one of the few numbers that visibly changes both the price and the risk, in opposite directions.
Leave too little and a face fails to clean up, which on a three tonne weldment is an expensive phone call. Leave too much and you are paying for shifts of roughing that produce nothing but chips.
The two allowances
Most heavy parts carry stock twice, not once.
Roughing allowance is what is on the blank when it arrives. It has to cover the blank’s own tolerance, its distortion, and the fact that nobody knows exactly where the finished part sits inside it yet.
Finishing allowance is what is left after roughing, before stress relief and the final cuts. It exists to absorb the movement the thermal cycle produces.
Quoting one number for a part that needs both is the most common mistake in a heavy RFQ.
Practical starting points
These are typical figures, not rules. The shop that machines it confirms what it needs on your geometry.
| Blank | Typical roughing allowance per face | Finishing allowance after stress relief |
|---|---|---|
| Flame cut plate, small fabrication | 5 to 10 mm | 1 to 2 mm |
| Large welded fabrication | 10 to 20 mm | 2 to 4 mm |
| Sand casting, medium | 5 to 10 mm | 1 to 2 mm |
| Large sand casting | 8 to 15 mm | 2 to 3 mm |
| Forging | 5 to 12 mm | 1 to 3 mm |
| Rolled plate, aluminium | 6 to 12 mm | 2 to 4 mm |
| Sawn solid stock | 3 to 6 mm | 1 mm |
Bores are usually left much smaller than faces, often 3 to 6 mm on diameter for roughing, because a bore that has to be opened up a long way is slow work and the position is set at finishing anyway.
Why bigger is not automatically safer
Every millimetre of extra stock on a large part is real money, for three reasons.
Roughing hours. A 3 000 x 1 200 mm face with 10 mm extra stock is 36 litres of steel to remove. That is a shift, sometimes more, on top of the plan.
Tool cost. Roughing through a flame cut skin destroys inserts faster than anyone budgets for.
More movement, not less. Removing more material releases more residual stress. Very generous stock on a welded structure can make the part move more, which is the opposite of what the extra stock was for.
There is a real optimum, and it is usually tighter than people’s instinct.
Where to put it, not just how much
The distribution matters as much as the quantity.
Symmetric is better than generous. A part with 10 mm on one face and 2 mm on the opposite face will bow when the 10 mm side is cut. Even stock on opposing faces keeps the stress balance.
Put it where the datums will be. The shop has to find a best fit for the finished part inside the blank. Extra stock on the datum faces gives it somewhere to move the part to.
Leave nothing where nothing is machined. Stock on surfaces that stay as cast or as welded is pure waste on the blank cost.
Watch for wall thickness. On castings and fabrications, generous outside stock combined with a bore that gets opened up can leave a wall thinner than the design intended. Check the worst case before setting the numbers.
How to state it on the drawing
Two lines is usually enough.
Blank to be supplied with 12 mm machining allowance on all machined faces, 4 mm on bore diameters. Unmachined surfaces as welded.
Or, if you want the shop to decide:
Supplier to determine machining allowance. Blank supplied by supplier. All machined surfaces to clean up fully.
The second version moves the risk to the shop, which is usually the right place for it if the shop is also buying the blank. Say which you mean, because the difference is not a detail.
If the blank is already made
Sometimes the fabrication exists and the allowance is what it is. In that case measure it before quoting, and tell the shop the real numbers rather than the drawing numbers. A blank that is 6 mm undersize in one corner is something the shop would very much like to know before it books three days of machine time.
What to do next
Decide who buys the blank, then set the allowance accordingly and put it in the RFQ in millimetres. If you are not sure, ask two shops what they would want. Their answers usually agree within a few millimetres, and that band is your number.
Related: weldment or casting and what a large part quote is made of.
Frequently asked questions
Who decides the machining allowance, me or the shop?
Whoever buys the blank. If you supply it, you decide and the shop machines what arrives. If the shop supplies it, let the shop decide and say so in the RFQ, because it carries the risk of a face that does not clean up.
What happens if a face does not clean up?
The shop stops and calls you. The options are usually to weld and re-machine, to accept a partly machined face, or to scrap the blank. All three cost more than the extra stock would have.
Does the allowance change after stress relief?
Yes. That is exactly what the second allowance is for. Roughing takes the bulk off, the thermal cycle lets the part move, and the finishing allowance absorbs that movement.
Need a quote for a part like this?
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