Hello, this is Ryuta Hamamoto from TIMEWELL.
When you cost material for a sheet metal quote, do you divide part area by sheet area and stop there? That calculation is usually wrong.
Three reasons. You need a web between parts. You need a clamp deadzone at the edge. And the cut itself has width. Then there is a fourth thing on top: parts per sheet only moves in whole numbers.
This piece builds the calculation up in order. There is a sheet at the end you can put in front of a customer as the basis for a price.
The short version:
- Yield is a ratio of areas, but material cost is set by parts per sheet, and that is an integer
- Work from the effective area after subtracting web, clamp allowance, and kerf
- Quote offcuts as scrap. If one gets used, that is profit later
- Pick from distributed standard sizes. A few millimetres off a part can add a whole row
Agree on what yield means first
The same word gets used differently at different stages. Here it means material.
Material yield = area used by parts ÷ area of material consumed
Take a hundred sheets, and if the parts total 75% of the sheet area, yield is 75%. The other 25% is web, clamp allowance, kerf, and offcut.
But the number that drives a quote is not the yield percentage. It is parts per sheet.
If one standard sheet gives eight of a part, material cost per piece is the sheet price divided by eight. Whether yield is 80% or 85%, if you still get eight, the cost has not moved.
Rearrange the nest and get nine, and material cost falls by more than a tenth. Parts per sheet moves only in whole numbers, so the question is always whether the ninth part fits.
Area yield is a fine indicator of how much room for improvement is left. But the quote is built on parts per sheet. Confuse the two and you get "our yield is great and the material cost never comes down."
Four things come off the sheet
Web. The strip left between parts. Take it to zero and parts tack together at the cut, or the heat distorts them. It varies with process and thickness, but something between one material thickness and a few millimetres is typical. On a laser you can sometimes share a cut line between neighbouring parts and tighten the web accordingly.
Clamp deadzone. The machine holds the sheet, so a band along the edge cannot be processed. Position and width are fixed by the machine. Forget it and you get a nest that fits on screen and not on the table.
Kerf. The width the cut itself consumes. Fractions of a millimetre up to a few on laser or plasma. With enough parts it accumulates.
Offcut. Even after subtracting the above, a corner is left over. If its shape and size suit the next job it goes to stock. Otherwise it is scrap.
Subtract all four, and how many parts fit in what remains. That is your parts per sheet.
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We have prepared materials covering ZEROCK case studies and implementation methods.
Work backwards from standard sizes
You buy material in the sizes the market carries. In Japan, steel sheet trades in a handful of standard formats, commonly described in shaku — three by six, four by eight, five by ten. Aluminium and stainless follow similar conventions.
Specify anything outside that and it is either more expensive or slower. And in most cases it gets cut from a standard sheet anyway, so the offcut just moves upstream.
There is a practical consequence. If you work out the nest before fixing the part dimensions, the material cost changes.
Suppose that across the short dimension of the sheet, three millimetres smaller would fit one more column. If those three millimetres are not functionally required, trimming them adds a whole column. Four parts per column turns eight-up into twelve-up. Material cost drops by a third.
This is the sort of improvement that rarely happens when design and production are separated. Do not think about nesting after the drawing is released. Fix the drawing after the nest. Of all value engineering moves, this is one of the easiest to predict the payoff on. There is more on cost definitions in VA, VE and what cost actually means.
Quote offcuts as scrap
Shops differ on this. Here is my view.
For quoting purposes, offcuts with no committed use should be treated as scrap.
The reason is simple: at quoting time you do not know whether they will be used. Price the material cheaply on the assumption that the next job will absorb the remainder, and when it does not, that is a straight loss. Offcuts also take up floor space, cost time to search, and eventually rust.
If you genuinely manage offcut stock and reuse it, by all means reflect your measured reuse rate. But "we use them" needs to be a number, not a feeling. That means recording offcut receipts and issues. Without records, the impression that they get used is not reliable.
One more point. With customer-supplied material, settle who owns the yield risk before you start. Parts come out of the supplied sheet; what happens to the remainder, and who procures more when it runs short? Leave that vague and it will become an argument. It belongs in the RFQ — there is a checklist in writing an RFQ for machined parts.
The order of the calculation
Putting it together:
1. Pick a standard size. From what is actually distributed. Compare more than one candidate.
2. Work out the effective area. Subtract the clamp deadzone from the sheet.
3. Decide the layout. Change part orientation, rotate, interlock. Holding web and kerf, count how many fit. Try both portrait and landscape. Testing only one costs you more than ten percent on parts per sheet often enough.
4. Fix parts per sheet. An integer.
5. Divide. Sheet price ÷ parts per sheet = material cost per piece.
6. Assess the offcut. Usable shape goes to stock. Otherwise it stays in the material cost.
7. Change the standard size and go back to step 2. Compare candidates and the cheaper one sometimes flips.
Running that by hand every time is heavy, which is why nesting software exists. But if you adopt software without understanding the steps, you cannot judge whether its answer is reasonable.
The sheet
There is a nesting and yield calculation sheet that takes this workflow directly. Enter sheet size, part dimensions, web, clamp allowance, and kerf, and it returns parts per sheet, yield, and material cost per piece. Candidate sheet sizes sit side by side for comparison.
It is laid out to be shown to a customer as the basis for a price. The download link is at the end of this article.
Getting material cost out of the drawing
Material is one component of a quote. Machining is covered in estimating machining time. Add those two plus outside processing, and you have the skeleton of a price.
Our enterprise AI, ZEROCK, helps with building that skeleton from the drawing: reading the developed dimensions, pulling up which standard size similar past jobs used and how, and giving you a starting point for parts per sheet. What the person decides is whether to push the layout tighter, and how to treat the offcut.
Same caveat as the other articles: this assumes the manual build-up is already settled in house. If web width and offcut valuation vary by who is doing it, automation just spreads the variation faster.
In summary
- Yield is a ratio of areas, but material cost is set by parts per sheet, and that is an integer
- Subtract web, clamp allowance, kerf, and offcut from the sheet
- Choose from distributed standard sizes. Off-standard is dearer or slower
- A few millimetres off a part dimension can add a whole column
- Quote offcuts as scrap. Reuse only counts if it is recorded
- With supplied material, settle who owns the yield risk first
- Always compare portrait against landscape, and one standard size against another
If you want to straighten out how material cost is derived, or get it started from the drawing, get in touch.
