Feed and speed equation

After burning out a router and fitting a spindle alone with a VFD and having to do a complete rewire with cy cable I am now happy with the result.
Now I am trying to get my head round speed and feed.
But I must be going wrong some where. I'm Cutting 6mm birch ply with a 4mm single flute end mill at a depth of 4mm per pass, but the bit is still very hot to the touch when done, The chip load for ply cut with a 4mm end mill. I have found is around 0.20 to 0.24 the equation that I use is ( Number of flutes x chip load x spindle speed ) = feed rate.
So this comes out as 1 x 0.24 x 15000 = 3600 mm/min I have been adjusting this and I am now at a feed rate of 6500mm/min and the bit is still hot. It looks like I will have to drop the spindle speed down but at what speed will the spindle not have enough power to keep cutting ( my spindle is only a 0.8kw water cooled ) or will I keep going with increasing the feed rate?
Also where am I going wrong with the equation or is it the chip load I got of the Internet wrong?

For slotting with a 4mm HSS 1 flute cutter at 4mm DOC I am seeing 18881rpm and 2333mm/min on my calculator. You are right 0.238mm chip loading.

Carbide is 24000rpm and 2471mm/min with the other parameters the same

That's with a 15mm stick out.

That's for a finishing pass - roughing would be way higher - carbide 24,000rpm and 4745mm/min

Thanks for the quick reply Washout. I'm using carbide bits. tomorrow I will run the spindle at a higher speed 24000 rpm with the feed rate at 4745mm/min just to try it. Could be I'm asking to much from a 4mm end mill, but all that I have been reading about how when the correct speed and feed that the end mill should just be warm after cutting. May I ask what calculator are you using.

Is the bit getting discolored? If not, then your feedrate is probably fine. A general rule with wood, is to cut as fast as you can, while still getting an acceptable quality cut. That will achieve that maximum tool life.

Interesting question and I know I have lots to learn about CNC machining. Based on years of experience using a router you would be better using a down cut spiral twin flute cutter on birch ply as this will give less tear-out, used with a manual feed rate of around 2Mtr per min taking around 1-1.5mm per pass so a feed of around 2.5Mtr sounds about right on a CNC machine to me. BTW what make of router / wattage did you burn out?
Mike :)

Hi Longy,
The router that burnt out was a Black and Decker KW780 600watt the bottom bearing seized and over heated the router it was just working to hard trying to cope with the bearing seizing. It was my fault it had been cutting continuously for just over one hour, what I should have done is to let it cool down after 20 minutes. So my fault.

Hi Ger21,
I started with a new carbide 4mm bit this morning and started cutting two 600mm lines with a 4mm depth cut the bit did not discolour but was still hot, so after playing with the feed rate and spindle speed, going by noise the best rate has turned out to be 5500mm/min with a spindle speed of 12000, it cuts with very little noise this is a lot faster that I have worked out with the equations but when I cut at 4800feed and 20000 spindle speed the wood starts to blacken after about 900mm cut.
I will speed it up again later but for now I will cut at a feed of 5500mm spindle speed 12000 RPM but I will have to learn a lot more about chip load.

Quote:

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Originally Posted by Bush Flyer

Black and Decker KW780 600watt the bottom bearing seized and over heated the router it was just working to hard trying to cope with the bearing seizing.

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Don't blame yourself mate, over the years B&D have cut quality to cut the selling price, in other words made down to a price not up to a quality. Having said that I have a B&D router still going strong after 40 years and it's only 500W they don't make em like they use too LOL