Guides / Design
The Nine Things a Delrin Drawing Needs Before You Send It Out
September 21, 2026
Two shops quoting the same acetal part can come back a long way apart, and the reason is rarely that one is expensive. It is that the drawing left decisions open and each shop made different ones.
Here are the nine items that close those gaps.
1. Grade and family, with substitution rules
“Acetal” or “Delrin” alone tells a shop to use whatever is on the rack. That may be fine. If it is not fine, say so.
Material: acetal copolymer, Tecaform AH or equivalent. Substitution with homopolymer requires approval.
Naming a single brand grade with no alternative allowed is the other failure mode, because it can add a week while stock is ordered in.
2. Which dimensions are critical
On most acetal parts, two or three dimensions do the work and the rest are clearance. If the drawing does not say which, the shop must treat every dimension as critical and inspect accordingly. That is real money.
General tolerance: ISO 2768 medium. Critical dimensions marked with a box.
3. Inspection temperature
Acetal expands near 110 micrometres per metre per kelvin, roughly ten times steel. A 100 mm part measured at 20 degrees C and again at 28 differs by nearly 0.09 mm from temperature alone.
Without an inspection temperature on the drawing, you and the shop can both measure correctly and disagree. Twenty degrees C is the usual choice.
4. Edge condition
Acetal holds a fine flexible burr. Deburring is hand time and it belongs in the quote, but only if the drawing asks for something specific.
Break all sharp edges 0.2 to 0.4 mm unless otherwise noted.
5. Service environment
Temperature, whether the part is wet, and what cleans it. This is what decides homopolymer against copolymer, and a shop that machines acetal regularly will push back if the grade and the environment do not match.
6. Surface finish, on the faces that need it
A maximum Ra on the sealing or sliding faces, and an explicit release for everything else.
Ra 0.8 max on faces A and B. All other surfaces as machined.
A blanket finish specification across a whole part is one of the most common ways a simple acetal drawing gets expensive.
7. Threads and inserts
Thread callout with the standard, full thread depth separate from drilled depth, insert part number if one is used, and an assembly torque. Threads in plastic strip at torques that metal shrugs off, so the torque note is not optional.
8. Quantity and repeat expectation
Not strictly a drawing item but it belongs on the RFQ. Setup and programming are paid once per order, so the same part at 10 pieces and at 200 pieces gets quoted on completely different logic. Saying that the part repeats twice a year changes how a shop prices it.
9. Paperwork
Material certificates, food contact compliance, first article inspection reports, batch traceability. Each one is a real cost and each one is cheap to agree up front and awkward to add afterwards.
A worked example of a note block
MATERIAL: Acetal copolymer, natural. POM-C. Substitution with
homopolymer requires approval.
GENERAL TOL: ISO 2768-m. Boxed dimensions are critical.
INSPECTION: 20 degrees C, part at temperature for 2 hours.
EDGES: Break all sharp edges 0.2 to 0.4 mm.
FINISH: Ra 0.8 max on faces A and B. Others as machined.
SERVICE: 5 to 55 degrees C, daily washdown at 70 degrees C,
mild alkaline detergent.
DOCS: Material certificate with each batch. FAI on first order.That block is nine lines and it removes almost every assumption a shop would otherwise have to make.
What it gets you
Quotes that are comparable. Not necessarily cheaper quotes, though cutting an over specified tolerance usually is cheaper, but quotes where the difference between shops is actually the shop rather than the assumption.
It also gets you a shorter question cycle. A drawing that answers these nine items gets a price back rather than an email asking what grade you meant.
Have a drawing ready? Send your RFQ and we will match it with up to three shops that machine acetal. Reply within 24 business hours.
Frequently asked questions
Why do quotes for the same drawing differ so much?
Usually because the drawing left something open and each shop filled the gap differently. One assumed a [stress relief cycle](/blog/annealing-delrin-and-pom-parts/) and another did not. One read the general tolerance as applying everywhere and another did not. The gap is in the drawing, not in the shops.
Is a STEP file enough without a drawing?
For a rough budget, sometimes. For a real quote, no. A STEP file carries geometry but no tolerances, no material, no finish and no inspection conditions, and in acetal those are most of what decides the price.
Do I need to name a specific brand of acetal?
Name the family and give a grade as an example with substitution rules. That lets a shop offer what it stocks, which often saves a week of lead time, while making clear what the part actually needs.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that machine acetal and reply within 24 business hours.
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