Knowledge Base · Part 2 of 2
DFM Cost-Saving Guide (Part 2)
Part 1 covered the shape of the part — walls, holes, corners, tolerances. Part 2 covers what a drawing tends to treat as an afterthought: threads, counterbores, edge breaks, burrs — and then the four things a drawing never states at all, which decide why two shops quote the same file at prices that look unrelated: quantity, fixturing, inspection level and material condition.
1. Threads — "M6 × 15 deep" is not a specification
A tapped hole only carries load where the thread is full. At the bottom, the tap's lead-in cuts a tapering, incomplete thread profile. So "15 deep" can mean 15 mm of drilled hole and only 11 or 12 mm of thread that actually holds. On a part that vibrates, that gap is the difference between a joint that stays tight and one that strips.
| What the drawing says | What the shop does | What the bolt gets |
|---|---|---|
| M6 × 15 | drills 15, taps to depth | about 11–12 mm of usable thread — the rest is taper |
| M6, full thread 12 min | drills deeper, taps to leave 12 full | exactly what the joint needs |
| M6 through | drills through, taps through | no depth question at all |
For reference, the tap drill for a metric coarse thread is close to nominal minus pitch. Basic dimensions and pitch series are given in GB/T 196-2025 (current; it replaced GB/T 196-2003) and tolerances in GB/T 197-2018.
| Metric coarse | Pitch | Tap drill ≈ |
|---|---|---|
| M3 | 0.5 | 2.5 mm |
| M4 | 0.7 | 3.3 mm |
| M5 | 0.8 | 4.2 mm |
| M6 | 1.0 | 5.0 mm |
| M8 | 1.25 | 6.8 mm |
| M10 | 1.5 | 8.5 mm |
Traps:
- Fine pitch has a bigger tap drill and a shallower thread — use M8 × 1 only when you actually need adjustment or thin walls.
- A threaded hole that bottoms out needs a bottoming tap. That is a second operation, not a deeper drill.
- If the part is anodized after tapping, the coating grows the flanks of the thread and shifts the pitch diameter. Say so on the drawing — M4 and M5 in 6H go out of spec quickly.
2. Counterbores — you probably do not need ±0.05 on the depth
A counterbore exists so a screw head sits at or below the surface. What matters is that the head does not stand proud, and that enough material is left under it. Neither of those needs a tight depth tolerance — but a tight depth tolerance is what gets drawn.
| What you write | What the shop does | Effect |
|---|---|---|
| depth ±0.05 | stop, measure, adjust — on every part | measuring cost added to a feature that does not need it |
| depth 6.0 min, 6.5 max | one pass, spot-check | the head is still clear, and nobody stops mid-cut |
| depth 6.0 min only | one pass, no measuring | cheapest, and usually correct |
Counterbore sizes are already tabulated against screw heads, so they do not need to be invented. GB/T 152.2-2014 covers countersinks for countersunk head screws; GB/T 152.4 covers counterbores for hex head bolts and nuts.
Traps:
- A counterbore on a curved, cast or forged face needs a spotface first — an extra tool and an extra operation.
- Measuring counterbore depth from a rough surface gives a number that means nothing. Measure from the machined face.
- A small-diameter counterbore that is deep needs a long, thin tool that deflects. Keep depth under about 2 × diameter.
3. Edges — "sharp" is the most expensive word on the drawing
Almost no drawing says what to do with the edges. In the shop, every edge gets broken anyway: a razor edge cuts whoever packs the part, and it is a stress riser in service. But if the drawing stays silent, the work is done by hand — and hand work is the one cost that does not fall when quantity rises.
| What the drawing says | How the edge gets done | How the cost behaves |
|---|---|---|
| nothing | hand file or scraper, part by part | fixed cost per part — does not scale down |
| "break sharp edges 0.3 max" | chamfer tool, inside the program | almost nothing per part |
| chamfer or fillet dimensioned | same tool, already in the toolpath | almost nothing, and it is inspectable |
| sharp corner required by a seal or a datum | hand work, then inspection | most expensive of the four |
Traps:
- Edges on a curved or free-form surface cannot be followed by a chamfer tool — the hand work comes back.
- A 0.1 mm chamfer on a large part is harder to hold than a 0.3 mm one. If the reason is appearance, 0.3 is enough.
- Breaking the edge of a thread can damage the first turn. Chamfer the hole before tapping, not after.
4. Burrs — they live where one hole crosses another
A drill entering or leaving a surface raises a burr. On an outside face you can see it, and someone takes it off. The expensive ones are where a drill breaks into another hole: that burr ends up inside the part, where no file reaches. This is not a small point — flatness and freedom from burrs are written into standards for current-carrying parts, precisely because a loose burr inside an assembly is a failure waiting to happen.
| Situation | Where the burr lands | What it takes to remove |
|---|---|---|
| drill exits a flat outer face | on a face you can reach | hand or tool deburr — cheap |
| drill breaks into a cross-hole | inside the cross-hole | reachable only if the cross-hole is wide enough for a tool; otherwise rework or redesign |
| drill exits a curved or angled surface | a ragged edge on the curve | reduce the exit angle, or add a flat pad for it to break through |
| slot milled in from the side | along the whole slot edge | tool deburr if the tool can reach the full length |
Traps:
- Two holes crossing near a wall leaves a burr in a cavity with one narrow opening. Specify the larger hole as the crossing one, so a tool can get in.
- Blind holes collect burrs at the bottom, where nothing reaches. Where it matters, make it a through-hole.
- Deburring adds an operation, and a second clamping. Budget for it on the drawing rather than discovering it in the quote.
5. Quantity — somebody has to pay for the setup
Everything up to here described the part. From here on it is about the paperwork around the part — and this is where two shops quote the same file at prices that do not look like they belong to the same job.
A quote is always per this batch. Setup — programming, fixturing, first-article, dialling in — does not shrink when the batch shrinks. It has to be paid by whoever is in the batch. Ten pieces carry the whole setup; five hundred pieces share it. That is not a discount policy. It is arithmetic.
Which means ten pieces and five hundred pieces are not the same part made the same way. At ten we run slow, measure more, use general-purpose workholding. At five hundred we build a fixture, run at cycle, and sample. Both are honest. They are simply two different trades that happen to share a drawing.
| What the enquiry says | What we quote | What you get later |
|---|---|---|
| "10 pieces" | prototype process, all setup absorbed by those 10 | a high unit price, and a large drop when volume arrives |
| "10 pieces, roughly N per year after that" | two prices — prototype and volume — plus the conditions behind the volume price | the volume price is real from the start instead of renegotiated later |
| "500 pieces, one-off" | production process, fixture included | the lowest unit price — but the fixture is wasted if the design changes |
Traps:
- Partial delivery is not partial order. "Ship 100 now, 400 later" is still a 500-piece order — we buy material and plan for 500, and you get the volume price. Say it up front and it is simply true; say it after the invoice and it is a dispute.
- A number that is too small can only be quoted small. Inflating the quantity to push the price down does not work either — it comes back at delivery, as a delay or a surcharge.
- Annual usage matters more than batch size. Batch size decides the cycle time; annual usage decides whether a fixture is worth building at all.
6. Fixturing — the cost that never appears on a quote
No quote line says "fixture". The cost is folded into labour hours, which is why two quotes for the same part can differ by a number that appears to come from nowhere.
The choice of workholding is usually decided by two things you know and we can only guess: how complex the part is, and whether it will be revised. Both belong in the enquiry.
| Workholding | Good for | Bad for |
|---|---|---|
| General-purpose vice / soft jaws | block stock, one-offs, prototypes | thin walls, irregular shapes, five-sided work |
| Bored soft jaws (turning) | second op on an already-machined diameter | thin rings and parts that crush easily |
| Dedicated fixture | complex shape, stable quantity, design that will not change | parts that get revised — the fixture is then scrap |
| Vacuum or magnetic table | thin plate, large flat area | small parts, or parts with through-holes that leak the vacuum away |
Traps:
- On a thin-wall part, the first operation is not cutting — it is holding the part without deforming it. That decision sets every dimension that follows.
- The trigger for a dedicated fixture is not quantity alone. It is annual usage × shape complexity ÷ how often the design changes. Quantity alone gives the wrong answer on both ends.
- A part that needs more than three setups is usually telling you something about the design, not about the fixture.
7. Inspection level and material condition — two lines that move the price
A drawing states tolerances. It almost never states how they will be verified. That gap is worth money, and it is one of the few places where the buyer can lower the price without changing a single dimension.
| What you ask for | What we do | When it is the right answer |
|---|---|---|
| "It has to fit" | functional dimensions held, in-process sampling, certificate of conformance with the shipment | structural parts, cosmetic parts, non-mating faces |
| Critical dimensions controlled | first article + last article + 100% on the critical dimensions | anything with a fit or an assembly relationship |
| Full dimensional report | every feature measured, report issued | first-article approval, or preventing an incoming-inspection argument |
| 100% CMM with raw data | every piece measured, data file supplied | medical, aerospace, safety-critical parts |
The trap here runs the other way from the rest of this guide. "Provide an inspection report" sounds careful and gets you a full dimensional report at a full report's price. "Only dimensions A, B and C need a report" is cheaper, and it is also the clearer instruction.
Material condition works the same way. The same grade can be supplied as generic stock or as certified, heat-traceable bar — the part behaves identically on the machine, but the documentation burden does not.
| Material condition | What it means | When you need it |
|---|---|---|
| Generic stock, grade guaranteed | the alloy is what the drawing says | general structural work |
| With mill certificate (3.1) | a document travels with the material | export shipments, customers who audit traceability |
| Heat-number traceable, certified bar | the exact heat is known and recorded | medical, aerospace, pressure equipment, safety parts |
Traps:
- "Good material" is not a specification. Write the grade, the temper and the documentation requirement — 6061-T6 and 6061-T651 machine very differently, and the certificate you get depends on which one you named.
- A material certificate is a document that ships with the goods. If it is not on the purchase order, it is not on the pallet.
- Asking for 100% CMM on a part with two functional dimensions is paying for certainty you will never look at.
8. When the drawing says nothing, we have to guess
This is the quiet version of every point above. Missing information does not disappear — it becomes an assumption, the assumption becomes a price, and the price becomes an argument at delivery. These seven are the ones we meet most often.
| # | Usually missing from the drawing | So we assume | And the bill arrives as |
|---|---|---|---|
| 1 | finishing happens after machining | quote as machined | threads and tolerances eaten by coating; rework |
| 2 | which general tolerance standard applies | our in-house default | your inspection standard and ours disagree |
| 3 | stock form — bar, plate, forging, casting | whichever wastes least material | material utilisation lands in the price |
| 4 | which dimensions are actually critical | treat them all as critical | you pay inspection on dimensions nobody checks |
| 5 | what the part assembles into | guarantee the part on its own | it does not fit |
| 6 | quantity, and what comes after it | quote this batch only | the volume price never materialises |
| 7 | delivery date and acceptance method | our standard lead time | a lead-time argument in week three |
None of these is a trick. They are all things a designer knows and simply did not expect to have to write down. The cheapest DFM review is the one where you go through this list before sending the file.
9. Eight things to check before you send the drawing
Part 1 gave five geometric rules. This is the companion list — all small, all cheap to fix on paper.
- Threads. Does the callout say how much full thread is needed, or only a depth? Could any of them be through-holes?
- Counterbores. Is the depth tolerance actually needed, or would a minimum do? Is any counterbore sitting on a rough or curved face?
- Edges. Is there one line telling the shop how to treat edges? If not, the work is being done by hand.
- Burrs. Does any hole break into another hole, or exit a curve? Can a tool reach the result?
- Quantity and annual usage. Does the enquiry say what this part is — a prototype, or the first batch of something bigger? A single number can only be quoted as a single number.
- Setup count. How many faces have to be machined, and is it obvious which ones must stay in the same setup? If it is not obvious, we will assume.
- Inspection. Which dimensions actually need a report? "Provide a report" and "A, B and C need a report" are two different prices.
- Material and finishing. Grade, temper, certificate required — and does anything get anodized, plated or painted after machining? The shop needs to know before it cuts threads or holds a tolerance.
None of these changes what the part does. All of them change what it costs. That is the whole point of a DFM review: the drawing should decide the price, not the shop's guess about what the drawing meant.
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