Speeds and Feeds

You're at the machine – whether that's a router, a lathe, a milling machine, drill or the CNC. You know what needs cutting, you've selected a cutter and everything is ready to go, apart from one question remaining: what spindle RPM should I use for this operation? The following guide should explain the theory behind that a bit and help you decide.

Originally written by Craig Merritt.

Cutting Speed (RPM!)

Between any pair of cutting material and workpiece material (whether working with metals, plastics or wood) there is an optimum linear cutting velocity – called ‘cutting speed’ (most commonly described in units of SFM: Surface Feet per Minute, but sometimes metric m/s is used too). ‘Linear cutting velocity’ is a scary term, so think of a carpenter’s hand-plane moving in a straight line through a piece of timber, or a metal ‘shaping machine’. Most cutters don’t cut in a straight line like those examples, though: they rotate at a given RPM (drills, lathe, router bits etc). The cutting tips of the spinning cutter still have a linear (tangential) velocity, though! This velocity, which is directly proportional to the RPM, is the ‘cutting speed’ of a rotating cutter (or lathe workpiece) – and the same optimum speed as before applies.

If it helps to visualise, picture a large and a small cutter – both rotating at the same RPM. The large cutter has a larger diameter, and therefore a larger circumference (circumference = pi * diameter). That means its cutting tip has to travel a longer distance per rev than the tip of the smaller cutter, but in the same amount of time (same RPM). This means larger cutters don’t need to spin as fast to generate the same linear cutting speed as a smaller cutter. Or, flipping it on its head: smaller cutters need to spin faster to generate the same cutting speed.

Feed Rate

Of the ‘feed and speed’ duo, we have considered only cutting speed (or just ‘speed’) so far; that still leaves ‘feed’. Cutting tools don’t just spin: they advance into the workpiece (pushing the wood through the router, or moving the milling machine table, or plunging the drill in etc). The speed at which this motion occurs is called ‘feed rate’. Feed units are sometimes given as a linear velocity, in inches per minute or mm/min – but are usually specified as distance per revolution of the cutter (inches per rev or mm/rev). In other words, how far the cutter is advanced into the workpiece per full rotation of the cutter.

Chip Load (Feed AND Speed)

The combined result of feed and speed is that each cutting tip on the cutter describes a sort of spiral pathway through the workpiece (the shape you might draw to sketch a slinky spring on a sheet of paper). This leads to one more parameter (that depends on both speed and feed): ‘chip load per tooth’. Chip load (for short) is the amount of material that each individual cutting tooth needs to remove every time it swings back around. If you had a weird cutter with only one tooth (e.g. fly cutters) the chip load would be exactly the same as the feed per rev. If you have a 4-flute end-mill (or router bit) then the chip load would be 1/4 of that: in a given revolution of the cutter, each tooth chips away 1/4 of the total amount the cutter is pushed into the workpiece). For a 2-toothed router bit, it would be 1/2. Etc.

If you increase feed, you increase chip load, because each tooth has more it needs to cut off per rev. If you increase speed (keeping the same linear mm/min feed velocity through the material), you decrease chip load because the feed distance per rev goes down.

Turning Theory Into Useful Info!

The two variables that we want to keep controlled during cutting are the linear cutting speed (in SFM / mm/min) and the chip load (in mm/tooth).

If cutting speed is too fast (and feed is too slow), you can end up rubbing / generating heat / scorching / work-hardening / stressing the material etc – the cutter is just spinning against the work and not cutting through it. If your cutting speed (RPM) is too slow (and you don’t also feed much slower to compensate), your chip load per tooth goes up – that’s like taking a very deep cut with a hand-plane: really hard work and likely to break something. Too large a chip load and it can end up tearing, rather than cutting.

So the gist is: when using big cutters, spin slower to reduce cutting speed, and feed slower accordingly. Smaller cutters want to spin much faster. Smaller cutters typically have a smaller rated (by the cutter manufacturer) chip load per tooth, so they don’t necessarily want to be fed faster to compensate. Number of teeth don’t affect the optimum RPM (speed), but do affect the optimum feed rate (because of chip load): more teeth = feed faster / less teeth = feed slower (don’t stress the cutting edges with massively deep cuts!)

But gee, that’s a lot of theory! How on Earth do you know what speed and feed to choose when you’re at the machine? Well, you’ve got several options:

1. Look up speed / feed rates using tables online (loads and loads exist)
2. Look up what speeds / feeds other people have successfully used (forums etc)
3. Use the Machinery’s Handbook (there is a copy at the Space) – don’t know if that contains woodworking info, though
4. Use a speeds/feeds app on your phone (many exist, probably including ones for woodworking)
5. Give it your best educated guesstimate and send it, then find out what happens!

Outside of commercial shops, I think it’s safe to say most people just choose that last option! (I certainly do, usually).

With a general understanding of the variables involved and why things need changing, and some experience of just doing it (including getting it wrong!) and learning, you’ll find it is surprisingly easy to just eyeball the right speeds and feeds for things 95% of the time, and get perfectly acceptable results. If you’re working on a CNC machine and need to optimise for part runtime and maximum material removal rate, that’s a whole science. But for you and me? Just guess, and send it.