The machine and tool will usually let you know if it’s not happy. My personal rule of thumb is ‘guestimate’ the speed, the smaller the cutter the faster the speed, and maximum depth of cut you think you need to apply. Obviously, finishing cuts will be lighter. You have to take into account the material you are dealing with, what you think your machine can handle and be prepared, initially, to make mistakes. The experience is easily gained with a little practice but cannot be taught, IMO, because a lot of it is down to ‘feel’. As I have stated on a previous occasion, if you just apply common sense and logic to any job you will find that you know more than you ever thought you did.

My rule of thumb goes like this:

1. Select RPM based on 10000/diameter of cutter.  The answer is correct for HSS cutting mild-steel. I’ve got a table showing how to correct RPM for other materials and carbide with simple multipliers, but it’s gone walkabout!
2. DOC no more than 25% of cutter diameter.  This avoids bending the cutter and won’t stress the machine much.  In this, I’m conservative, so others are likely to recommend rather deeper.  They’re not wrong, just prepared to push their tools harder than I like.
3. Having set RPM and decided a DOC, it’s important to get he feed-rate about right.  This I do by ear, feeding the cutter gently at first, then increasing the feed until the mill’s motor and drive train sound loaded, but not over-loaded.  Don’t ignore sounds of distress!  A WM16 isn’t an industrial grade metal hacker.  No need to crawl, but ploughing the cutter into the job regardless of the machine’s limitations is incompetent.  As the old guys recommended, ‘let the tool do the work, don’t force it’.

Important to make sure swarf is removed quickly.  Also, coolant/lube reduces cutting stresses, allowing faster feed-rates.

Apart from the rigidity of the machine, it’s ability to remove metal depends entirely on the power output of the motor.  Roughly, it takes 1HP (750W out), to remove 1 cubic inch of steel per minute.  That’s the maximum a WM16 will do, but only in short bursts, with the motor running at top speed with the right diameter cutter.  Electronic speed control is great, but it reduces the available torque especially at low speed.  Do not expect the motor slowed down to spin a 20mm milling cutter at 500rpm to deliver the power needed for a fast feed-rate.  But the same motor set to run at 1500rpm in low gear to spin an 6mm diameter cutter will sustain a much faster feed-rate without complaining.   Always keep in mind that hobby machines aren’t designed to run continuously – if a lot of metal has to be removed, working hard for tens of minutes, always stop for a few minutes every so often to let the motor and electronics cool down.  Say a 75% duty cycle, no more.

Dave

If you read my articles you will see that I too feel the traditional 1/4 cutter dia does not suit the hobby mills and the reasons why.

Fast and light is the way to go.

I’d also say you would be better sticking to 12mm dia max, if it is just flattening the top of a part then go down the insert face mill route as it is cheaper than wearing the corners off an expensive 16mm dia cutter.

On 14 October 2024 at 15:24 Robert Bowen-Cattry Said:…

I’ve been calculating RPM on a slightly different basis which suggested around 540 for a 16mm cutter, I will try your method (for mild steel) which would be 625 RPM. 25% of cutter diameter is 4mm though, I would never have expected my machine to manage that DOC!

540 and 625 rpm are in the same ball-park, so don’t worry about that.

I should have said that the depth of cut also depends on how much cutting edge is cutting.  In this example a lot less metal is removed to cut the step on the left.   Full DOC might do it one go.

Much more metal has to be removed to make the step on the right, so the DOC and/or feed-rate have to reduced.   Rules of thumb help get the machine set about right, but it’s the actual cutting load that matters.  Don’t flog the mill:  if it sounds heavily loaded, back off.    As Jason puts it ‘fast and light’.

With experience, one gets a feel for what suits a particular machine best: the sweet spot is a balance between motor/drive capability, cutter type & diameter, DOC & Feed, rigidity, how the work is held, and the material.  Take practice, more the better,  Initially, I spent a couple of months experimenting, trying various combinations to see what worked best.  Though rules of thumb are a good start, don’t be afraid to tweak ’em.

Dave

Listening to the speed of the motor should tell you if it starts to labour, and the speed of the cut then slowed, and smaller depths of cut taken in later passes. Cuts in aluminium can be deeper, but remember to add a small ammount of lubricant or the metal will stick to the cutter and thats not recommended. With lower power machinery for hobby use, you can take your time, which is not a vital factor. Sharpness of your tools will make a big difference in the ammount of metal that can be removed in a given time.