This avoids leaving a chunk of material in the middle of the hole when milldrilling holes larger than twice the cutter diameter, which can cause various problems.

It isn't quite finished, but it would be good to get some feedback on the general ideas before polishing it.

Currently when holes are milldrilled, they are always cut in a single pass. For holes larger than about twice the cutter diameter, this can cause problems:

• For holes larger than twice the cutter diameter, a piece of material will be left in the centre of the hole. When this breaks loose, it can get stuck between the cutter and the work.
• For holes around twice the cutter diameter, the cutting geometry is not ideal; the cutter may rub rather than cut the thin unsupported part near the centre.

The idea is to avoid this by cutting larger holes in multiple passes. The existing millhole routine is used to make each pass, so the code change is quite minimal, but the parameters are the tricky bit.

parameters

The new parameters are:

  --min-milldrill-entry-diameter arg (=150 %)                                                                   
                                        minimum initial hole width for                                          
                                        milldrilling when multiple steps are                                    
                                        used, as a fraction of                                                  
                                        milldrill-diameter                                                      
  --max-milldrill-entry-diameter arg (=190 %)                                                                   
                                        maximum initial hole width for                                          
                                        milldrilling in a single step, and the                                  
                                        minimum size of the initial hole when                                   
                                        multiple steps are used, as a fraction                                  
                                        of milldrill-diameter                                                   
  --milldrill-stepover arg (=50 %)      maximum hole radius increase when                                       
                                        milldrilling in multiple steps, as a                                    
                                        fraction of milldrill-diameter                                          

Multiple passes are used for holes larger than max-milldrill-entry-diameter. When multiple passes are used, the first pass is between min-milldrill-entry-diameter and max-milldrill-entry-diameter, with the radius increasing by up to milldrill-stepover each time.

This is in addition to min-milldrill-hole-diameter, which is unchanged. Originally, I used the value, for min-milldrill-hole-diameteras min-milldrill-entry-diameter, but this doesn't quite work right.

For example, it might be acceptable to use a 1mm cutter to make 1.1mm holes. This will go slowly (milldrill slows it down to the vertical feed), and recut a lot of chips, but maybe it works.

If these parameters were the same, making a 2mm hole with a 50% stepover would result in two passes, at 1.1mm, and 2mm. This is not ideal, because there's no reason in this situation for the first pass to be so small -- the first pass is not ideal for chip evacuation, and the second pass removes much more material than the first.

The current parameters would result in passes at 1.5mm and 2mm, which should be better on both counts.

min-milldrill-entry-diameter and max-milldrill-entry-diameter also seem like they could be one parameter, but having a range increases the chance that a single pass can be used, and avoids having passes too close together for holes just larger than the maximum for a single pass.

These are all relative to the cutter size, because that seems like the most natural way and makes the defaults make sense. I was also thinking about adding support for multiple milldrill cutter sizes (which would be handy if you want to make small slots/holes and mounting holes on one board), and percentages make more sense here too.

compatibility

Currently this change does affect the default behaviour; if the old behaviour was considered to be problematic than maybe that's OK? There's currently no clean way to switch to the old behaviour, except for setting max-milldrill-entry-diameter to a large number (the parser doesn't seem to accept inf).

results

Here's what making holes and slots sized 2.17, 2.59, 2.73, 3.57, 3.99 and 4.13mm with a 1.4mm bit (chosen to show the maximum variation in stepover) and default settings looks like from the side/top:

And a zoom on one showing the travel between passes (which is one turn above the work):

default parameters

The defaults are a bit arbitrary, and relatively conservative. I'm most unsure about the default stepover; 50% seems conservative compared to slot milling (100%), but these situations are a quite different, I haven't had a chance to try them out yet; if anyone else wanted to try it and report back that would be very helpful.

todo

• validate the new parameters (0 < stepover <= 100, 100 < min, < max)
• update the man page
• add integration test with the above example