Finishing without a price: leave the line open

What goes wrong with a zero on that line?

A subcontract machining shop of twelve is pricing a housing in 6082. The order: 25 pieces, Ra 3.2 general, black anodising sent out to an anodiser. Material, milling in two setups, setup time and programming are already in the estimate. Only the anodiser's price is missing. Put a zero on that line and the per-piece total looks finished. It is too low, and nothing in the number tells you so.

A figure from memory does not solve it either. The total looks just as finished, and the number has to be quietly swapped once the supplier's quote arrives, if anyone remembers. Either way the open point has dropped out of sight and the error sits in the quote. A spreadsheet makes it easier still: an empty cell in a sum formula counts as zero, without a warning.

Better to leave the line visibly open than to fill in a figure nobody can check. That is an open value. It is not a figure yet; it is a question someone still has to answer. The worked example below puts both versions side by side.

Ra 3.2 and black anodising are two lines

Ra 3.2 general is a surface roughness requirement. It belongs to the milling: cutting data, tooling and the final cut bring the face to that value, and the time for it sits in the machining line. Black anodising is a treatment carried out by a supplier, with a price and a lead time of its own. So the finishing gets a subcontract line in the cost price, beside material, machining, setup time and programming.

The price on that line depends on how the anodiser charges: per piece, per rack or per batch, often with a minimum charge. Layer thickness, colour, masking and the certificate the customer asks for all count. Fold the finishing into the machining line and you can no longer tell, later, which part of the price was the milling and which part the anodiser.

For inside sales the plain word comes first: finishing, for example anodising. Whoever has the customer on the phone can then tell at a glance which line is still waiting on a supplier.

What can you calculate while the anodiser replies?

A missing anodising price is no reason to set the estimate aside. The blank at your purchase price, cycle time per setup, setup time and programming do not depend on the anodiser. Those lines can be calculated now. At 10, 25 and 50 pieces the quantity break for the machining is already there before the supplier has answered.

What is still missing is one line and the per-piece total. Leave that total open with the missing line beside it, and do not park it on a provisional sum. The total exists once the line does. A material without a purchase price works the same way; how it differs from a market reference is covered in purchase price and market price.

When do you ask for the finishing price?

A quote with subcontracted finishing waits on the supplier's price. Someone has to ring and ring again, and until the enquiry is back the quote stands still. So the enquiry belongs with manufacturing engineering, straight after the drawing has been read, not with sending the quote.

For each finishing step, write the specification, ask for price and lead time, choose the supplier and record the dimensional change and the masking instruction. If the treatment is in your subcontract library with a price, nobody needs to call. If it is not, that is an open line and a question you ask today, not the day before the deadline.

The subcontracted step costs time as well. The Subduxion glossary makes the point that an outsourced operation brings lead time as well as cost into the route, and often a shipment out to the supplier and back. The delivery time on the quote is your own shop's lead time plus the anodiser's, with transport in between, and that sum has to fit the customer's delivery date.

How much size does an anodised layer take?

Anodising converts part of the aluminium into oxide, so the layer grows partly into the metal and partly out of it. The European space standard for black anodising requires the anodiser to tell the customer how this affects final dimensions, and the customer to agree in writing before work starts. According to Vereniging ION, the Dutch surface treatment association, in most processes between a third and a half of the layer thickness ends up outside the original surface.

For indoor use, the same source puts a sulphuric acid layer at usually 5 or 10 µm, for outdoor use 15 or 20 µm. On a diameter the share that grows outwards counts twice, because the layer grows on both walls. A 15 µm layer that grows half outwards makes a bore about 15 µm smaller and a shaft 15 µm larger; with a third outwards it is about 10 µm. On a close fit, that alone can take the size out of tolerance.

Nor is the layer equally thick everywhere. According to Vereniging ION, the ends of a workpiece and the pieces on the outside of the rack draw more current and get a thicker layer. In a cavity the layer reaches only a short way in; how far depends on the diameter. On a close fit, the spread in layer thickness counts alongside the nominal value.

If the 6082 housing has fitted bores and threaded holes, mask them or rework them after anodising. Fasteners go in after the treatment: the space standard requires the part to go into treatment without screws, rivets or any other fastener.

For the other dimensions, carry the layer thickness into the size you mill to. Whether the dimensions apply before or after anodising belongs on the drawing; if it is not there, ask before you price the part. If they apply after anodising, the manufacturing engineer builds a correction into the programme. How an allowance for finishing works through into the stock is covered in sizing the blank.

Finishing sits in the middle of the chain

Finishing is rarely the last step. Hardening comes before grinding, stress relieving sits between roughing and finish milling, and anodising is sometimes followed by an operation or a measurement. So the estimate carries an allowance before the treatment and sometimes a line after it.

With hard anodising this weighs more. It gives thick, wear-resistant layers for engineering parts; for one of the processes Vereniging ION describes, the layer is usually 50 to 100 µm. By the same source, such a layer usually comes out of the bath rougher than the metal underneath and is often finished afterwards; when the part has to hold size closely, that means lapping or honing. That puts a line after the finishing, with its own time, and an Ra requirement on that face depends on it.

The anodiser's quote therefore settles three things at once: the price of the line, the lead time of the order and part of the route. In a spreadsheet the finishing is a closing entry, one amount at the bottom that someone fills in once the rest is done. Work that way and you miss the lead time and the route, and find out at the first piece. An ERP then keeps what the anodiser invoiced, not the layer thickness, size change and assumption behind the line. On the next order the cost engineer starts again from a phone call or a figure from memory, and if that figure is too low, the difference comes off the margin. So when the batch comes back, measure layer thickness and size, and put that measured change in your subcontract library, for the next order with the same anodiser.

What goes in the quote if the price is late?

Sometimes the quote has to go before the anodiser has replied. There are then two honest routes. Quote the machined part and the finishing as separate items, with the finishing as a stated reservation you fill in within an agreed time. Or hold the quote and tell the customer when the price will be in.

An indicative price from the anodiser can work too, as long as it is marked as indicative on the quote, with the supplier and the date. What does not belong there is a figure that looks fixed and is not. When the real price arrives, issue a new quote version, so it stays visible what the customer had first and what changed.