A buyer sends the same drawing to five machine shops and gets five prices that do not resemble each other. The instinct is to negotiate the hourly rate. That is almost always the wrong lever.
By the time a shop quotes, most of the cost is already fixed — by the number of setups, the tools the geometry demands, the quantity of inspection and the finishing steps written into the drawing. How to reduce CNC machining cost is therefore mostly a design question, not a purchasing one. Below are ten changes we ask buyers to consider before a custom metal parts enquiry goes out. Each one should be priced both ways, because some of them are not worth making.
The ten changes at a glance
| # | Change on the drawing | What it removes from the quote | When to leave it alone |
|---|---|---|---|
| 1 | Put a general tolerance in the title block and call out only mating dimensions | Slower feeds, extra finishing passes and 100% inspection | Sealing faces, bearing fits, safety-critical features |
| 2 | Enlarge internal corner radii | Small cutters, multiple passes and tool changes | Where a mating part has a matching sharp corner |
| 3 | Keep hole depth to a few times the diameter where possible | Long-reach tooling, peck cycles and chip-clearing time | Hydraulic or cooling channels with a fixed path |
| 4 | Use one hole size across the part | Tool changes and separate setups for each diameter | Where function genuinely needs two sizes |
| 5 | Design so the part can be finished in one setup | Re-fixturing, re-datuming and the scrap it can cause | Parts that need features on opposing faces |
| 6 | Avoid undercuts and deep internal pockets that need a fifth axis or EDM | Specialist machine time and secondary operations | When the geometry is the point of the part |
| 7 | Specify a common grade and a stock size the shop already buys | Minimum-order material, waste and waiting time | Corrosion, strength or certification requirements |
| 8 | Apply surface finish only to the faces that need it | A subcontracted process plus a second inspection | Parts exposed to weather or handled by the end customer |
| 9 | Give thin walls enough support to survive cutting | Chatter, rework and rejected parts | Weight-critical designs that have been validated |
| 10 | Ask for prices at three quantity tiers, not one | The wrong process being quoted for your volume | Never — this one is always worth doing |
Tolerances are usually the first place to look
The most common cost problem we see is not a difficult shape. It is a drawing where every dimension carries the same tight value, copied from a template. A shop cannot tell which dimensions matter, so it prices all of them as critical. A general tolerance note such as ISO 2768-m covers everything else, and the few functional dimensions can be marked individually. Our ISO 2768 tolerance guide explains how those drawing notes turn into machining hours.
Ask the supplier to price the tight tolerances as a separate line. If relaxing three non-critical dimensions removes a measurable chunk of the quote, you have learned something useful. If it changes nothing, the cost was elsewhere and you can stop arguing about tolerances.
Geometry decides how many times the part is touched
Every time a part is unclamped and re-positioned, someone is paid to do it and there is a chance the datum shifts. Small internal radii, deep holes and undercuts all add operations that do not show up as “material” on a quote — they show up as time. Standardising hole sizes is unglamorous, but on a bracket with a dozen holes it can remove several tool changes from every single part.
There is also a threshold where a part stops being a three-axis job. Features on opposing faces, or pockets an end mill cannot reach, push the work onto a five-axis machine or into a secondary EDM operation. Sometimes that is unavoidable. Often a small change to how two halves are joined keeps the whole job on a cheaper machine. This is the same question as choosing between CNC, stamping and sheet metal, and it is worth answering before the drawing is frozen.
Material and finishing: two quiet cost drivers
A grade that a Shenzhen shop stocks in several diameters is cheaper and faster than a grade bought in for one order. The same applies to plate thickness. If a dimension is not functionally fixed, letting the shop use its nearest stock size can remove both a minimum-order material charge and a week of waiting. Do not do this where strength, corrosion behaviour or a certificate depends on the grade — those are exactly the cases where verifying the supplier’s capability matters more than the unit price.
Finishing is similar. Anodising, plating or powder coating is frequently done by a partner, which means transport, a queue and another inspection. Mark the faces that need it rather than writing a blanket finish note. And specify the finish in the RFQ so it appears in the quote rather than in a change order later; the CNC quote cost breakdown shows the lines a comparable quote should contain.
Quantity changes the answer completely
At ten pieces you are paying for programming and setup. At a thousand, the conversation is about fixture design, tool life and whether a die or a casting makes more sense. Asking for three tiers in one email gives you a curve rather than a number — and the shape of that curve tells you whether the supplier has understood the part. If the prototype price barely moves at volume, something in the design is forcing manual work that tooling cannot remove.
For parts with a stamping die or custom fixture, settle who owns the tool before any of this is amortised into a piece price. That is covered in tooling cost and ownership. And remember that the cheapest ex-works unit price can still be the most expensive landed one once dense metal parts are packed and shipped — see landed cost in 2026.
When not to make the change
None of the above is worth a part that fails in service. Sealing faces, bearing fits, fatigue-loaded features and anything covered by a certification should stay as specified. A good shop will tell you which of your ten changes it would not make. A shop that agrees to all ten without asking a question is probably planning to recover the difference somewhere else — in a looser interpretation of a drawing note, in a cheaper finish, or in a change order once production starts.
The practical way to test this is to send the same RFQ twice: once with the drawing as-is, once with the proposed changes, and ask for both quotes itemised. Compare the lines, not the totals. Our custom metal parts sourcing guide sets out how we compare quotes on that basis.
Frequently asked questions
Is negotiating the hourly rate the best way to reduce CNC machining cost?
Rarely. Hourly rates between competent shops are usually close; the spread in a quote comes from setups, tooling, inspection and finishing. Push on those instead. If a supplier will not itemise them, you cannot tell what you are actually buying, and a lower number may simply mean a step was left out.
Does a tighter tolerance always cost more?
Only when it changes the process. A tight value on a non-critical dimension adds slower passes and inspection; the same value on one mating bore may be routine. Mark the few features that matter and use a general tolerance for the rest, then ask the shop to price the difference explicitly.
Should I change the material to a cheaper grade?
Only with engineering sign-off. A common stock grade can cut both cost and lead time, but strength, corrosion resistance, heat treatment and any required material certificate all depend on the specification. Confirm the substitution in writing before a sample is approved, not after production.
How many quantity tiers should I ask for?
Three is usually enough: a prototype quantity, a small production run and your expected annual volume. Ask for tooling and setup as separate lines in each, so you can see where the fixed costs sit. That makes the break-even point visible instead of hiding it inside a unit price.
Before you send anything, make sure the request itself is complete. Our CNC machining RFQ checklist lists the twelve files and specifications a shop needs in order to price a part properly.
What to do next
Pick the two changes most likely to apply to your part, send the drawing both ways, and ask for itemised quotes. If you want a second opinion before you send anything, send us your drawing for a free review and we will tell you which changes are worth pricing — before any money changes hands.