Why an internal corner can never be square, and how to pick a radius that does not triple the price.
This is written for buyers and design engineers rather than machinists. It covers what to put on the drawing, what it costs, and the mistakes that turn a straightforward part into an expensive one.
The rules that matter
- A flat end mill always leaves a corner radius equal to its radius
- A 6 mm end mill needs at least a 3 mm internal radius
- Smaller radius means smaller tool, slower feed, higher cost
- A 0.5 mm internal radius in a deep pocket needs EDM
Common mistakes
- Dimensioning a sharp internal corner
- Using a corner radius smaller than the pocket depth allows
Both of these are cheap to fix at the drawing stage and expensive to fix after the parts are cut. If you are unsure which side of the line your design is on, send it over before you release it.
How this shows up in a quote
A quote is built from material, machine time, setup, finishing and inspection. Anything that adds a setup, a secondary operation or a tighter inspection method moves the number. When we quote, we show the process route, so you can see which requirement is driving the price and decide whether it is worth keeping.
Frequently asked questions
Does this apply to both milling and turning?
Yes. The principles are the same. Turning is usually cheaper for concentric geometry and milling for prismatic geometry, but the tolerance and finish rules carry across both.
What is the cost of getting this wrong?
Usually one extra setup, one extra operation or a scrapped batch. In practice, fixing it at the drawing stage costs nothing; fixing it after the first article costs a week and a re-quote.
Can you review my drawing before I release it?
Yes, and we would rather do that than quote a design that is going to be expensive. Send the STEP file and drawing and we will come back with DFM comments alongside the price.
Do you work from a PDF drawing or do you need CAD?
We need a STEP file for the geometry and a dimensioned PDF for the tolerances. A PDF alone is not enough for anything with curved surfaces.
Related pages
- Guides — the full index for this section.
- Thread Specification Guide
- Hole and Shaft Fit Selection Guide
- Minimum Wall Thickness for CNC Parts
- Draft Angle Basics for Molded and Cast Parts
- Instant CNC quote
- Tolerance quick reference
- Frequently asked questions
Get a price for your part
Send a STEP file and a drawing with tolerances, material and quantity. We quote within one working day and machine in our own facility in Shenzhen — milling, turning, EDM, grinding and finishing under one roof, no trading middleman.
Start an instant quote or send your drawings directly. English-speaking engineering support, DFM feedback before you commit to a batch.