Depth of Cut advice needed please – Warco WM16b

[Robert Bowen-CattryParticipant @robertbowen-cattry70600]

Good afternoon all.

Something I have been trying to learn that has also been recently mentioned in another thread, what depth of cut to apply.

Internet searches seem to conflict, I have read some that advise 10x the cutter diameter, some that advise half the cutter diameter. I currently have endmills that are 6mm, 10mm and 16mm diameter.

The 16mm cutters I tend to take either a 0.5mm or 1mm DOC, the 6mm only 0.25mm. I’m curious if this depth is enough to avoid the cutters rubbing and blunting them?

Is there a rule of thumb or best practice to use? I assume that DOC is also at least partly based on the horsepower of the mill, and as mine is relatively low (1hp I believe) I have been keeping my cuts at what I thought were a modest depth.

Any advice, as always, is gratefully received.

Thanks,

Bob

[Chris CrewParticipant @chriscrew66644]

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.

[Robert Bowen-CattryParticipant @robertbowen-cattry70600]

Thanks Chris,

That’s kind of the approach I have been taking to date.

[SillyOldDufferModerator @sillyoldduffer]

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

[JasonBModerator @jasonb]

Have a look in the “workshop section of the forum, there are some copies of recent articles I did for MEW which suggest the way to get the best out of a hobby mill, the last half of  part 2 deals with solid cutters.

 

You don’t say if that depth of cut is vertical or sideways or what sort of metal you are cutting or type of cutter. But this is a guide

If vertical and at or near the full width of thecutter I would suggest starting at 1mm DOC for the 6mm, 0.75mm for the 10mm and 0.5mm for the 16mm on steel, 50% more in aluminium, brass or bronze.

If it is the side of the cutter then for the 6mm go 6mm vertical 0.6mm sideways, for the 10mm 10mm vertical and 0.5mm sideways and for the 16mm 10mm vertical and 0.5mm sideways. Again this is for steel

Rubbing is not so much to do with depth of cut but the amount each tooth takes off as it comes round to take a cut. My articles give some formulas to work things out. Aim for each tooth to take 0.025mm and work the speed of feed out from that. using teh speed for the given material and utter diameter.

 

These will be a reasonable starter, ask if you need any more help

[Robert Bowen-CattryParticipant @robertbowen-cattry70600]

Thanks for your advice Dave.

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!

[Robert Bowen-CattryParticipant @robertbowen-cattry70600]

Jason, thank you, I will take a look at your article.

[JasonBModerator @jasonb]

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.

[Robert Bowen-CattryParticipant @robertbowen-cattry70600]

Interesting. I bought the 16mm thinking it would be quicker to take less passes (when squaring stock) with a larger cutter than more passes with a small cutter.

For finishing I made a fly cutter with a button type insert holder which seems to work well, but will probably get a face mill somewhere down the line.

[Robert Bowen-Cattry]

On 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

 

[Robert Bowen-Cattry]

On Robert Bowen-Cattry Said:

Interesting. I bought the 16mm thinking it would be quicker to take less passes (when squaring stock) with a larger cutter than more passes with a small cutter.

 

It depends. A smaller cutter can be run at a higher spindle speed and that allows for a faster feed rate so each pass does not take as long. So the area being flattened will decide which is faster.

Based on the same cutting speed for your 3 cutters.

4-flute 6mm cutter @ 2000rpm, 0.025mm chip load can be feed at 200mm/min

4-flute 10mm cutter @1200 rpm, 0.025mm chip load can be feed at 120mm/min

4 -flute 16mm cutter @ 750rpm, 0.025mm chip load can be feed at 75mm/min

Now try some pieces of metal

100mm long x 5mm wide, all three cutters can machine that in one pass so the 6mm will be the quickest option doing it in 30seconds, the 16mm the slowest at 1min 20

100mm long x 15mm wide. 6mm cutter needs 3 passes taking  a total of 1min 30, 10mm cutter needs 2 passes taking 1min 40 and the 16mm needs one pass taking 1min 20 so not a lot in it

100mm x 20mm wide 6mm needs 4 passes takes 2mins, 10mm really needs 3 passes taking 2mins and 16mm needs 2 passes also taking 2mins 40. so in that case the 16mm is the slowest.

That ignores vertical depth of cut which could well see the 6mm coming out even better.

Other advantages of the 6mm particularly on the narrow stuff is that you will likely end up with at least 1 cutting edge engaged all the time so smoother cutting and less knocking of those gears as mentioned in another thread. Also as each corner will be taking off the same amount of metal be it 6m or 16mm the corners will wear at the same rate so both cutters will become blunt after doing the same amount of work, the 6mm will be a lot cheaper to replace.

And if tram is out as per another recent thread multiple passes of a small dia tool will give a flatter surface than one pass of a large diameter. Lots of very shallow scallops will be flatter than one big concave surface.

[old martParticipant @oldmart]

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.

[Robert Bowen-CattryParticipant @robertbowen-cattry70600]

Good advice, thank you everyone.

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