After mucking around for a few days now with small changes to the blank shape we (RonB and yours truly) had a pretty big breakthrough today.
The one problem that seemed to be chasing us all the time was an 'off center' blade shape which caused the cutter to bite air a lot of the time and a very large blank compared to the resulting blade.
We have made two major changes to solve these problems:
- the blade is now centered in the 'y' direction, which means that there is taper on both the leading and the trailing edge. This is at the expense of a very little bit of efficiency, but makes the blank much more efficient.
- the blade is now centered in the 'z' direction, which means that the whole blade 'side view' is a lot flatter while maintaining the required twist angle.
The blank went from 95 millimeters to 51 ! That saves a lot of wood and cutting time.
The rationale for this is that the blade functions as an infinite number of small sectional blades that each have to have proper twist and taper for that particular station. Changing the blade like this makes it almost impossible to shape by hand, but for the computer operated router that is no problem (it uses 800 stations to calculate the toolpath), every station would have a different front and back 'drop'.
We spent the rest of the day working out a method that would allow us to maintain perfect 'z' registration at the moment of 'flipping' the blade over between passes and further optimized the toolpath by selectively speeding up when we know we're making small cuts (such as the cuts on the final passes).
I think we're getting really close to a final design now, one more blank tomorrow morning to see if we have it all perfect, and then - finally - some real blades. I'd better get on with it too, because snow has been forecast for the region.
pictures follow:
the new blank shape, much simpler than yesterdays:
Last pass on the back, looks pretty smooth
Backside finished: (note the funny little 'hump' caused by the segments rotating about their 'z' center)
Roughing out the front, you can clearly see the 'staircase' steps during the roughout phase here, in this foreshortened (25%) blade that is exaggerated even further.
The finished blade, backside:
The finished blade frontside:
