Since I got the 4th axis working, I am going to finish the mill's CNC conversion. I am 100% convinced that the knee needs to be motorized, so that I can change tools and alternate between short tools (small end mill) and long tools (long drill bit in a chuck), while retaining all offsets and relative Z positions.
Otherwise I cannot really do a lot of jobs nicely.
With the knee motorized, I can machine more or less anything that fits into the mill's envelope.
The axis with motorized knee is called W. It is parallel to Z, but has the opposite sign, which actually makes perfect sense if you think about it.
Chris Radek from EMC mailing list gave me a great suggestion, which is to store W offsets in the tool table. This way, when I change a tool, the knee automatically moves to accommodate the tool height.
The best picture of the knee of my mill is from a while ago:
I already have a servo capable precision DC gearmotor ($45 on ebay), with a shaft for encoder sticking out of the back. This gearmotor goes up to 4k RPM and is reduced 15:1. So, at 3k RPM it would give me 200 RPM of the knee crank, which is all I care to have, it would be about
60 IPM speed if I got my math right. I can probably limit it even further and will still be great.I bought an encoder from US Digital, hopefully I will get it this week.
Additionally, I have a dual limit/home switch that I also need to install on the knee. I am pretty sure that it will take me a month at least to get done, maybe more, as I barely have any time.
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