Speeds and Feeds Calculator
RPM, feed rate and material removal for milling — including the radial chip thinning correction most calculators leave out entirely.
Light cuts need a FASTER feed, not a slower one
This is the correction most calculators leave out. When a cutter is engaged less than half its diameter, each tooth enters and leaves the cut on an arc, so the chip it actually produces is thinner than the feed per tooth you programmed. At a 10% stepover the real chip is only about 60% of what you commanded.
That sounds like a conservative error and it is the opposite. A chip that is too thin means the edge rubs instead of cutting — which puts the heat into the tool rather than the chip, work-hardens the material ahead of the edge, and destroys cutters far faster than pushing them does. Feeding faster as the cut gets lighter is the whole basis of high-speed and trochoidal machining: light radially, deep axially, and a feed rate that looks alarming until you know why.
- The factor depends only on the ratio. A 10% stepover is 1.67× on any diameter; 25% is 1.15×; at half diameter and beyond it is exactly 1, because each tooth already takes a full-thickness chip.
- In stainless this stops being an optimisation. 304 work-hardens the instant the tool rubs, so the next pass has to cut a hardened layer with an already-dull edge. It is the commonest way to ruin an endmill in a single pass.
- Chip load scales with diameter. A ⅛″ endmill at a ½″ endmill's chip load snaps. Small tools are limited by deflection long before they are limited by speed.
- If the spindle cannot reach the speed, scale the feed down too. Running slower without reducing the feed raises the chip load instead — the opposite problem, and just as hard on the tool.
- These are starting points. Machine rigidity, holder runout, workholding and whether coolant reaches the cut change what is achievable more than the material does.
How to use
- Pick the material and cutter, then enter diameter and flute count.
- Set the radial stepover and axial depth for the cut you are taking.
- Read the corrected feed — light radial cuts need a FASTER feed, not slower.
- Override the surface speed or chip load if your tool data differs.
Frequently asked questions
What is chip thinning and why does it matter?
When a cutter is engaged less than half its diameter, each tooth enters and leaves the cut on an arc, so the chip it actually produces is thinner than the feed per tooth you programmed. At a 10 percent stepover the real chip is only about 60 percent of what you commanded. That sounds conservative and is the opposite: a chip that is too thin means the edge rubs instead of cutting, which puts the heat into the tool rather than the chip and destroys cutters far faster than pushing them does.
So I should feed faster on a lighter cut?
Yes, and it is backwards from most people's instinct. The correction is the cutter diameter divided by twice the square root of the engagement times the remaining diameter, which comes to 1.15 times at a 25 percent stepover and 1.67 times at 10 percent. That is the entire basis of high-speed and trochoidal machining: light radially, deep axially, and a feed rate that looks alarming until you understand what it is compensating for.
When does chip thinning stop applying?
At half the cutter diameter and beyond. Past that point each tooth is already taking a full-thickness chip, so there is nothing to correct and the factor is exactly one. A full slot needs no correction at all. The factor depends only on the ratio of engagement to diameter, so a 10 percent stepover gives the same 1.67 on an eighth-inch cutter as on a one-inch one.
Why is stainless steel singled out?
Because 304 work-hardens the instant a tool rubs rather than cuts. Once that happens the next pass has to cut through a hardened layer with an edge that has already been damaged, and the process runs away from you. In most materials an under-fed cut is merely inefficient; in stainless it is the commonest way to ruin an endmill in a single pass, which is why the thinning correction matters more there than anywhere else.
Why does chip load depend on cutter diameter?
Because a small cutter cannot physically take a big bite. The core of an eighth-inch endmill is a fraction of a millimetre across, and the deflection and bending stress from a half-inch cutter's chip load will snap it. Small tools are limited by deflection long before they are limited by surface speed, which is why they want shallow radial cuts, short stickout and patience.
What if my spindle cannot reach the calculated RPM?
Run slower, and scale the feed down by the same ratio. The mistake is dropping the RPM while leaving the feed alone, because that raises the chip load per tooth instead of lowering it — the opposite problem, and just as hard on the tool. Reduced surface speed costs you tool life gradually; an overloaded chip load costs you the tool immediately.
How accurate are these numbers?
They are starting points rather than specifications, and they are deliberately middle-of-the-road. Machine rigidity, tool holder runout, how securely the part is clamped and whether coolant actually reaches the cutting edge all change what is achievable more than the material choice does. A tool manufacturer's published data for the specific cutter you are holding beats any general table, including this one.
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