Machining Guides

Practical guides for buyers and design engineers: tolerances, finishes, DFM rules, documentation and sourcing.

How to use these guides

These are working guides, not tutorials. Each one is written for a specific decision point in an order — how to prepare a file, how to choose a tolerance, how to read an inspection report, how to handle a rejected batch — and each ends with the rule that matters most rather than a summary of everything covered.

Read them in the order you meet them. Most first orders need only four: the RFQ checklist, CAD file preparation, the DFM checklist and tolerance grade selection. Everything else becomes relevant once you are placing repeat orders or auditing a supplier.

Start here: the four guides that cover most first orders

If you have never placed a machining order before, these four remove the causes of most delays and most price surprises. The RFQ checklist tells you what to send. CAD file preparation covers the format and modelling decisions that decide whether a shop can quote at all. The DFM checklist catches the features that add cost without adding function. Tolerance grade selection stops you paying for precision the part does not need.

Together they take about twenty minutes to read and typically remove one full round of questions from a first quote.

Guides by stage of the order

Stage	The question you are answering	Guides
Before you send anything	What does the shop need, and in what format?	RFQ checklist, CAD file preparation, STEP vs IGES, NDA and IP protection
Choosing a supplier	Can this shop actually make my part?	How to choose a CNC supplier, supplier audit checklist, second-source strategy
Designing the part	What will this feature cost me?	DFM checklist, cost reduction design, corner radius rules, minimum wall thickness, workholding for thin parts
Specifying it	How precise, how smooth, how hard?	Tolerance grade selection, surface finish specification, GD&T basics, thread specification, material hardness guide
Placing the order	What am I agreeing to?	MOQ explained, payment terms, tooling cost amortisation, Incoterms, HS codes
Receiving the parts	Is this batch acceptable?	First article inspection, reading a CMM report, AQL sampling, rework and warranty
Scaling up	When does the process have to change?	Prototype to production, small batch vs mass production, PPAP and ISO requirements
Special situations	The part that does not fit the pattern	Stress relief, thermal expansion and tolerance, importing from China, RoHS and REACH

What these guides will not do

They will not replace a conversation about your specific part. Tolerance, material and geometry interact in ways a general rule cannot capture — a rule that holds for a 20 mm aluminium bracket can fail on a 200 mm stainless one because of thermal expansion alone. Use the guides to know which questions to ask, then ask them.

Where a guide states a limit, treat it as the point at which a conversation starts rather than the point at which a feature becomes impossible. Most of the hard numbers in machining can be beaten with enough time, cost and the right tooling; the guides tell you when that trade is worth making.

Frequently asked questions

Do I need to read all of these?

No. Most buyers use four or five regularly. The rest are reference material for situations that come up once a year — importing, auditing a supplier, handling a rejected batch.

Are these written for engineers or for buyers?

Both, but the language is chosen so a non-engineer can act on it. Where a technical term has to be used, it is defined on first use rather than assumed.

Do you apply these rules to your own work?

They are written from how we quote and inspect, so yes — with the caveat that we will break a general rule when the drawing justifies it, and we will say so in the quote rather than silently pricing around it.

Can I use these guides inside my own company?

Yes. Use them for onboarding, in design reviews, or as a checklist for your own supply chain. Attribution is appreciated but not required.

A guide contradicts my supplier’s advice. Who is right?

Possibly both. Rules of thumb shift with equipment, volume and material. Ask your supplier for the reasoning behind their recommendation — if they can explain the mechanism, their answer may be the right one for their process.

Something not covered?

Tell us the decision you are stuck on. If it turns out to be a common one it belongs in this section, and either way you will get an answer you can use.

Tooling references

CNC tooling reference — the full chain from cutter body to insert to screw to wrench.
Indexable insert screw and Torx wrench cross-reference — APKT, APMT and SEHT inserts: mounting hole, thread, length and wrench size.
Indexable milling cutter body reference — decode a body code such as BAP400R-50-22-4T or KM12 and find the insert class it takes.