Small design choices decide how quickly – and how cheaply – a part can be milled on a vertical machining centre. These ten design-for-manufacturing (DFM) tips help you get accurate VMC parts without paying for avoidable machine time.
Why design for manufacturing matters
A VMC (vertical machining centre) cuts with rotating tools from above. Features that suit that way of cutting are fast and accurate to make; features that fight it need special tools, extra set-ups or slower cutting. Designing with this in mind – often called DFM – keeps cost and lead time down without changing how the part works.
The figures below are general guidelines. Always discuss critical features with your machining supplier.
Tips 1–5: pockets, corners and walls
1. Add internal corner radii
Round cutters leave a radius in every internal corner. Specify an internal radius a little larger than a standard cutter radius so the tool can move smoothly around the corner instead of stopping.
2. Keep pockets reasonably shallow
Pockets up to about three to four times the cutter diameter are straightforward. Deeper, narrower pockets need long tools that must cut slowly to avoid deflection.
3. Avoid very thin walls
Thin walls vibrate and can bend during cutting. Thicker walls machine faster, hold tolerance better and give a better finish.
4. Use a consistent pocket floor radius
If you need a radius between the floor and wall of a pocket, use a standard value that matches common ball-nose or bull-nose cutters – or allow a sharp floor corner where function permits.
5. Put external chamfers instead of fillets where possible
On outside edges, a 45° chamfer is usually quicker to machine than a rounded fillet and does the same job of removing a sharp edge.
Tips 6–8: holes and threads
6. Use standard drill sizes
Standard hole sizes can be drilled with stock tools. Non-standard sizes need interpolation milling or special tools. For precise fits, use standard reamed sizes such as H7.
7. Limit hole depth
Holes up to a few times their diameter are easy to drill. Very deep, small holes need peck drilling or special drills and take longer.
8. Don’t over-specify thread depth
Thread engagement of about 1.5 times the nominal diameter is enough for most steel applications. Very deep tapped holes add time and increase the risk of tap breakage. See our guide to threads on CNC parts.
Tips 9–10: set-ups and tolerances
9. Reduce the number of set-ups
A 3-axis VMC reaches features from above. Every time the part must be turned over or re-clamped to reach another face, time is added and alignment must be re-established. Keeping features on as few faces as possible reduces cost.
10. Tighten only what matters
Use a general tolerance note and apply tight tolerances only to functional features. Read machining tolerances explained for practical guidance.
Quick DFM checklist
- Internal corners have a radius.
- Pockets are not deeper than needed.
- Walls are thick enough for the material and height.
- Holes and threads use standard sizes and sensible depths.
- Features are reachable in as few set-ups as possible.
- Tight tolerances are limited to functional features.
A well-designed part is cheaper and faster to make – see what decides CNC machining cost. VR Engineering is happy to review your design before quoting: send us your drawing or 3D model.
Frequently Asked Questions
Why can’t a VMC make sharp internal corners?
Milling cutters are round, so they always leave a radius in internal corners equal to at least the cutter radius. Truly sharp internal corners need other processes such as EDM, which add cost.
How deep can a pocket be on a VMC part?
As a general guideline, pockets up to about three to four times the cutter diameter are straightforward. Deeper pockets need longer tools, slower cutting and careful planning, which increases cost.
What is the minimum wall thickness for milled parts?
It depends on material and height, but as a general guide, metal walls below about 1 mm and plastic walls below about 1.5 mm become prone to vibration and distortion. Thicker walls machine faster and more accurately.
CNC and VMC machining specialists in Metoda GIDC, Rajkot. Questions about your part? Send us your drawing.
