I handle fastener and aluminium extrusion orders for a systems integrator. Eight years of it. In that time I have made 11 significant ordering mistakes that I actually wrote down, and they cost roughly $6,400 in scrap, rework, and expedited replacements. This is the checklist I now hand to every new buyer on our team, because I got tired of watching people repeat my errors.
Eight steps. They run in order. Step 8 is the one almost everyone skips.
Who this checklist is for
If you are speccing stainless screws, T-nuts, slot nuts, truss plates, connector bolts, or leveling hardware for a frame, a bench, a machine guard, or a window profile assembly, this applies. It is written for the person who actually places the order, not the person who signs off on the drawing. Those are usually not the same person, which is basically the root of half my mistakes.
It assumes a mixed bill of materials: some catalogue parts (m8 slot nut, m6 t nut, 16mm end cap, hex head screws), some custom items (custom machine screw, custom aluminium extrusion). If your order is 500 pieces of a single item, you need less of this.
Step 1 — Lock the profile series before you look at nuts
In 2020 I ordered 400 tee nuts for a 30x30 frame with a 6mm slot. What arrived were nuts for an 8mm slot. 400 pieces, about $310, and they sat in a bin for two years. We did eventually use them on another job, so not a total loss, but the schedule damage was real.
The rule: the nut follows the slot, never the other way around. Rough mapping — 20 series is around a 6mm slot, 30 series around 8mm, 40 series around 10mm. Approximate on purpose. A '4040' from one extruder is not always the same slot geometry as a '4040' from another, and an aluminium profile 40x80 sometimes has two different slot faces on the same extrusion.
Step 2 — Write the nut spec in full, in writing
This is the step where I stopped trusting my own shorthand. 'M8 nut' means nothing. Write it as: M8 T-nut, 40 series, 10mm slot, slide-in, A2 stainless. Same discipline for the small parts — m3 t slot nuts and 3mm t nuts get confused constantly on our floor because both get called 'the tiny ones'.
Same for the m6 slot nut, the 6mm t slot nuts, and the 12mm t slot nuts. If the drawing shows a hammer-head and you order a slide-in, the head will not seat, and you find out at assembly instead of at receiving. That one cost me a Saturday.
Step 3 — Full thread callouts, including the standard
M6 and 1/4-20 are close enough to thread into each other badly. We caught that one early. A supplier of ours did not: 600 custom machine screws arrived with the wrong pitch, and we were three days behind on a panel build.
Format I use now: M6 x 20, full thread, A2, pan head, ISO 7045 equivalent. If it is a socket head cap screw, state the class — 12.9 and A2-70 behave very differently under torque. If it is a flat head screw, state the angle, which is the next step.
Step 4 — Decide the stainless grade before you compare price
Honestly, this is the step that separates buyers who have been burned from buyers who have not. A2 (304 family) covers most indoor and general outdoor work. A4 (316 family) is what you want anywhere chlorides show up — coastal air, road salt, food equipment washdown.
Per ISO 3506-1, property classes run A2-70, A4-70, A4-80 and so on (as of 2025, still the reference most suppliers quote against — verify against your own spec). Three practical notes:
- Zinc-plated steel labelled as stainless does happen. A magnet is a crude test — A2 and A4 are essentially non-magnetic when annealed, but cold-worked screws can read slightly magnetic, so do not over-trust it.
- No stainless fastener is 100% rust-proof under every condition. If a supplier says that, they are telling you something about themselves, not the material.
- Galvanic pairing sometimes matters more than grade. A stainless bolt into an aluminium profile in a wet environment will eat the aluminium, not the bolt.
Step 5 — Head style and countersink angle, together
In September 2022 I approved 600 countersunk screws for a panel job. The panels were machined at 82 degrees. The screws were 90 degrees. Every single one sat proud of the surface. $450 in replacements plus a week of rework, and the customer noticed before we did.
Pan head is the default for general clamping. Flat head only when the hole is actually countersunk, and then the angle has to match. Socket head cap screws are for when you need torque in a small footprint — and those need a counterbore, not a countersink.
Step 6 — Order the connection as a kit, not as parts
Truss plates without the matching connector bolts and cap nuts are a half-order. Tooth plate connectors, steel truss connector plates, right angle connectors, angle brackets — these are designed as systems. Mixing a plate from one range with a bolt from another gets you a joint that passes inspection on paper and fails in service.
The part people forget is the cap nut. Cheapest line on the order, and it is the one that stops the shipment. Same with end plugs and caps: an aluminium t slot frame with open extrusion ends looks unfinished, and a 16mm end cap that does not match the wall thickness of the profile is worse than leaving it open.
If I remember correctly, our worst single delay in 2021 was a missing bag of T nuts and screws on a t slot extrusion order — 200 frames, three days idle. Not a big-money mistake. A very visible one.
Step 7 — Leveling feet and casters: load per foot, not total
I went back and forth between m10 adjustable feet and a caster with leveling foot for about two weeks on a 300kg bench. The feet were cheaper and simpler. The casters gave us mobility we would need maybe twice a year. I chose feet, and I was wrong — we moved that bench four times in six months.
Worse was the sizing. Four stainless steel leveling feet rated 75kg each sounds like 300kg capacity. It is not, once the load is offset. Two feet carried most of it, the M10 thread deformed, and the bench settled 4mm out of level. Heavy duty furniture feet ratings assume even distribution. Real benches do not have even distribution.
Step 8 — Incoming inspection (the step everyone skips)
This one. Fifteen minutes at receiving has saved us more money than any negotiation I have ever run.
- Thread gauge on three random pieces per box. Go/no-go, not 'it feels fine'.
- Test fit on a scrap piece of the actual profile. Not a good profile — a scrap one.
- Count one box, weigh the rest. Small count discrepancies are normal in fasteners; a 10% variance is not.
- Photograph the labels with batch numbers visible. When something goes wrong six months later, that photo is the difference between a claim and an argument.
What still trips people up
The lead time number is not your lead time. Here is something vendors will not usually spell out: a quoted 'standard lead time' often includes buffer the supplier uses to manage their own production queue. It is a queue-management figure, not a promise about your order. Ask when the material gets cut, not when it ships.
End caps and hinges are their own checklist. If the build includes doors or a window aluminum profile frame, the hinge spec is a separate exercise — hinge price tiers vary enormously for what looks like the same part, and a good hinge on a sample board is not necessarily the same hinge in a 200-unit order. Aluminum end caps and end plugs all have tolerance bands; check them against the profile wall, not against the catalogue drawing.
Wholesale pricing has a floor that moves. Wholesale self tapping screws and standard hex head screw supplier pricing shifted a lot between 2021 and 2024 (freight, alloy surcharges, energy). I still see buyers using a two-year-old price sheet as a benchmark and concluding that the new quote is a rip-off. It probably is not.
The fundamentals have not changed. The execution has. What was best practice in 2020 does not fully apply in 2025 — spec sheets are better, tolerances are tighter, and most reputable suppliers will send a real 3D model before you commit to a custom aluminium extrusion order. But the rule from my first year still holds: if it is not written on the purchase order, it is not in the box.
We now source most of our stainless screws, T-nuts and connector hardware through JINGFA, mainly because the spec sheets match what arrives, and the checklist above is quicker to run when the documentation is already there. That is the whole test, honestly. Everything else is price negotiation.