CNC Cutting Bits

I’ve been using milling machines for my hobby of model making for over 40 years, but CNC is new to me and CNC cutting bits are new.

end mill on brass

The primary first task for the CNC engraver is to machine the brass smokebox door on my miniature traction engine.

The CNC engraver in question is a Sain Smart Genmitsu 3018-Pro.

engraver, end mill and slot drill
  • Left = Engraver, 20 degree
  • Middle = 1.2mm End Mill
  • Right = 2mm Slot Drill

Ten of the engraver bits come with the Sain Smart version of the Genmitsu 3018-pro. These are solid carbide bits.

Engraver Bit: found these make a lot of noise and I think this is due to them having a poorly defined cutting edge. I need some more practice to see if I can change the cutting depth and feed rate.

End Mill: I thought a 1.2mm end mill would be small enough to allow me to machine brass using very small 0.05mm depth cuts and a 100mm/minute feed rate. I tried this a couple of times unsuccessfully, it tended to wander and vibrate. This is what I would expect, normally I would cut on the side of an end mill. The end mill tends to wander if you plunge it into a material. The other issue is the Genmitsu 3018 is not very stiff and has play in the Z-axis stage along with a poorly controlled spindle. This means it doesn’t take much to cause vibration and wandering of the cutter.

Slot Drill: this slot drill made by Rennie Tools is very sharp and has just 2 flutes. When using a traditional manual milling machine this is the type of cutter that I would use for this type of cutting operation. As the name suggests this type of cutter is used to machine a slot.

slot drill cutting purple heart

I used the slot drill to machine a piece of purple heart hardwood. At the moment I’m setting the speed to maximum, using a reasonable feed rate of 300mm/minute and taking small 0.1mm cuts.

As yet I’m just a beginner in the world of CNC, however, it is no surprise that most of the lessons from manual milling machines can be transferred: the machine needs to be very stiff, the cutters very sharp and of the right type.

The best CNC cutting bits so far are the Rennie Tools slot drills.

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