The short answer
Undercuts are sometimes necessary. They are also a common sign that the design is asking the machine to do something awkward when a simpler geometry would work.
Why undercuts cost more
Undercuts usually need special cutters, awkward tool access, extra setups, or slower feeds. Internal undercuts can force long-reach tools or secondary processes. All of that pushes cost and risk up.
That does not mean never use them. It means use them on purpose.
When undercuts are justified
Thread reliefs, seal grooves, bearing clearances, and assembly escape features are legitimate reasons. Decorative or inherited CAD geometry is not. If the undercut does not protect function, it should be the first thing challenged.
The more hidden and inaccessible the undercut, the more aggressive the quote tends to get.
Design alternatives
Increase corner radius, change mating geometry, use a standard relief form, redesign the interface for through-access, or split the component if assembly economics allow. Sometimes a turned groove is trivial while a milled blind undercut is a headache.
A small redesign can save more than any sourcing tactic.
Specification discipline
If the undercut must stay, define its width, depth, radius, and finish only to the extent function demands. Do not create a precision trap on a non-functional relief feature.
And if a standard tool form is acceptable, say so.
What an experienced buyer does next
Related reading: Common DFM mistakes that increase your machining quote by 30%+ and Thread callout guide: UNC, UNF, metric - how to specify them correctly.
The right move is usually to define the real functional requirement, remove the decorative requirements, and let the supplier build a route around what actually matters.
Comparison table where relevant
| Case | Undercut status | Better move |
|---|---|---|
| Thread relief | Often justified | Use standard relief form |
| Blind internal corner | High cost risk | Add radius or redesign access |
| Seal groove | Function-driven | Specify only critical dimensions |
| Decorative recess | Usually waste | Delete it |
How to specify this in your RFQ
Call out whether the undercut is function-critical, which surfaces matter, and whether a standard cutter radius is acceptable. If a relief exists only for assembly clearance, say the intent. That helps the shop propose alternatives instead of just pricing pain.
A clean RFQ does not just list requirements. It separates must-haves from preferences so the supplier can optimize where it is safe.
Have a part that needs quoting? Email your drawings to rfq@precisionmachining.co -
we return a competitive quote within 24 hours. Phone: +1 312-579-0808.