Improved gear cutters.
The gear cutters described here work very well on soft metals such as aluminium and brass using my small micromill (Sieg X1L). On steel they also work but the micromill has to work very hard to make a full depth cut (typically 2.2 mm for a MOD 1 gear) and it really struggles. One way to reduce the cutting force is to put some top rake on the cutting tool.
The diagram above shows that by offsetting the tool from the centre it effectively acquires top rake. With my current cutter holder design the holder is 16 mm diameter and the tool sweeps a circle of 26 mm diameter. To achieve a 5 degree top rake then the tool needs to be positioned off centre by around 1.1 mm.
Unfortunately, it is not possible to use my 3 mm thick cutters made for use mounted "on centre" in the offset arrangement for two reasons:
1. The clearance behind the tool is reduced to only 5 degrees and the tool would rub on the back edge when sweeping a circle of 26 mm diameter.
2. The geometry of the cutter would be modified and the circular cut outs that shape the form tool will actually make an elliptical cut on the gear. This effect will be very small for small rake angles.
Both of these problems can be overcome by modifying the geometry used to create the cutters.
Unfortunately, it is not possible to use my 3 mm thick cutters made for use mounted "on centre" in the offset arrangement for two reasons:
1. The clearance behind the tool is reduced to only 5 degrees and the tool would rub on the back edge when sweeping a circle of 26 mm diameter.
2. The geometry of the cutter would be modified and the circular cut outs that shape the form tool will actually make an elliptical cut on the gear. This effect will be very small for small rake angles.
Both of these problems can be overcome by modifying the geometry used to create the cutters.
The drawing above shows the geometry for making the improved gear cutter.The cutter blank is held sloping downwards at an angle of 5 degrees and the tool is shaped using a 20 degree (10 degree half angle) cone drill. By adopting this geometry the clearance behind the cutting edge is maintained so that the tool does not rub and the elliptical distortion effect is eliminated on the gear teeth.
A new toolholder has been made with an MT2 shank so that it can be directly mounted in the mill quill. I have also made the toolholder so that it has two M4 clamping screws rather than the one screw on the previous version.
This shows one of the new cutters mounted in the toolholder. The slot is about 5 mm wide and the tool can be clamped on the centre line or, as shown here, shims can be inserted to give the tool a positive rake.
Trials with this new cutter have been made and the ease of cutting metal has been greatly enhanced. In steel it is possible to take a full depth cut (2.2 mm) and the micromill does not struggle nearly as much as with the previous arrangement.