GlossaryKind of term: Manufacturing termInternal corner radius
What is an internal corner radius?
An internal corner radius is the rounding in an inside corner, such as that of a pocket or slot; when milled, it is at least as large as the cutter radius.
It is the corner between two walls. The lower limit of at least the cutter radius applies when milling with the periphery of the cutter; wire EDM, for example, can produce a much smaller radius. On the drawing it appears as an R at the corner or as a general note for all internal corners. If nothing is given, a general edge requirement or the agreement with the customer decides what is allowed. The radius between wall and floor is a different feature: it comes from the corner radius or ball nose of the cutter.
What does an internal corner radius depend on?
The internal radius limits the largest cutter that can make the corner, and the depth sets how far that cutter has to stick out. A slender cutter deflects: by the beam formula for a clamped cylinder, deflection grows with the cube of the overhang and falls with the fourth power of the diameter. That is an approximation, but it explains why a small radius in a deep pocket calls for light cuts, a low feed and more passes. A cutter radius slightly smaller than the corner radius keeps the engagement in the corner limited; a cutter with exactly the corner radius engages over a large arc in the corner and tends to chatter.
How does an internal corner radius work in practice?
Often you rough with a large cutter and finish the corners with a smaller one: one more tool and one more pass. An internal corner tighter than the smallest usable cutter radius needs another process, such as wire EDM for a through contour, sinker EDM or slotting in a pocket with a floor, or a design change. Even then a small radius remains, set by the wire, electrode or tool. If a rectangular mating part has to fit right into the corner, a relief beyond the corner is often an alternative. What it means in time and cost depends on depth, material, number of corners and the processes you have in house.
What is an internal corner radius often confused with?
An internal corner radius is not a rounded outside edge: that is a design choice, made with a corner-rounding cutter or as a contour, while in a milled internal corner the tool sets how small the radius can be. Nor is it the floor radius between wall and floor, which comes from the cutter's corner radius.
Frequently asked questions
Which internal radius should I propose when the drawing gives none?
Check whether a general note or edge requirement covers internal corners. If not, propose a radius that suits a cutter able to reach the depth and record it in the quote. Otherwise the corner can need an EDM step that is not in your price.
Why is a radius just above the cutter radius better?
Because the cutter then takes the corner in an arc instead of engaging a large part of its circumference at once. That keeps the load more even, limits chatter and can allow a higher feed.
What is a dog-bone corner relief?
A small round cut-out beyond the corner, so a rectangular mating part still fits right into the corner. It takes a few extra moves with the same cutter and is often cheaper than eroding a sharp corner, if the function allows it.