[DanielSiew]

New to cnc cutting and i keep on breaking my bits

Need help on cutting pvc board

Bits im using 1/8" single flute Spiral Bit
18mm pvc board

Tired 3000/6000/9000/12000/15000/18000 rpm
And 600mm per sec to 1000mm per sec

Anyone can enlighten me about how much rpm and speed i need to use?

Thanks

[joeavaerage]

Hi,
I think you are trying to cut too fast.

With a small diameter tool you need to spin it fast. I think 18000 rpm is right.

The surface speed:
S (m/min) = PI x Diameter (m) x rpm
=3.14 X 0.003 x 18000
=169.6 m/min which sounds about right for plastic.

The chipload:
T (mm per tooth per rev)= 600 (mm/min) / rpm
=0.033 mm per tooth per rev
For small diameter tools, I use down to 0.4mm endmills I allow 1% of the diameter as chipload. With larger and stronger tools you might allow 2-3%.
Lets do the calculation for a 2% chipload:

T (mm)= 0.02 x Diameter (mm)
= 0.02 x 3
= 0.06 mm
So your chipload is close to correct at a federate of 600mm/min. I think you are going to have to take two or three cuts to get through it,
3mm tools are fairly weak. Either that or slow the feedrate down until you don't overload it, maybe 200-300mm/min.

Craig