CNC for Milling machine.

Making intermittent progress on converting my knee milling machine to CNC. This is random notes on my researches to date

Those that have gone before: Hafco HM-52 CNC Conversion

Torque goals:

1. Maximum handle pressure in normal operation is probably under 5 kilograms.
2. Handle radius is ~ 3 inches.
3. This is ~ 3.3 newton-meters of torque.

Speed goals:

1. Table is ~ 1200mm
2. Would like to be able to go one of the table to the other in substantially less than a minute.
3. Say 12 seconds?
4. Speed circa 6000mm/minute.
5. Leadscrew is ~ 3mm/turn from memory (8 tpi)
6. So need around 2000 rpm at the leadscrew for this speed.

Which gives around 600 rpm/Nm required from motor. So something around 5000 rpm @ 1.5 Nm peak torque would be ideal. (Using timing belt to gear it down to 2k rpm @ 3.3Nm).

Would like to use brushless DC motors for efficiency, heat, ease of sealing et al.

Can use of-the-shelf brushless controllers but they're expensive, and require hall-effect sensors on motors.

Can use Takao Shimizu effort, and modify slightly for sensor'ed operation. This would require doing boards, a fair bit of testing and mucking around, but probably substantially cheaper, particularly if I do a bunch of such boards.

Looking to use EMC2 for motion control, so the Mesa boards seem popular and vestitile. Particularly the 7I43-U which is USB or parallel port @ $USD90. Mesa also sell the 7I39-LV which will drive 2 BLDC motors at up to 250 Watts. ( ~ $USD300 for 4 axis). This is currently ~$AUD600 plus shipping. Hmmm.

On the other hand, running brushed motors enabled using the 7I30 which does 100W/channel over 4 channels for ~$USD90. This would effectively drop the top speed to ~2500mm/minute for the X and Y axis, and something much less for the Z-axis (which will need to be geared way down anyway).

Still need to find a suitable source for servo motors (i.e. cheap!).

GCAM is looking for more useful for simple CAM tasks.

Need to change over to anti-backlash nuts for X and Y axis (can probably rely on weight to handle Z-axis). Simplest thing is probably to use delrin split nut? May need to make/buy an suitable ACME tap for nuts (or find an already made one on ebay?)

APT looks like an interesting thing to play with for CAD/CAM.


Cheap rotary encoders are available for $USD25 ea. Example of one being fitted is here.