This is a build thread for the conversion of a Hafco HM52 to CNC control. The HM52 is a low cost entry level vertical knee mill sold in Australia by Hare&Forbes https://www.machineryhouse.com.au/ I believe it to be identical to many other "badge engineerined" low cost mills.

This is a hobby machine, not a production machine, as such I am not expecting the sort of rigidity and accuracy that you might expect from more expensive machines.

After some discussions with DaveJ (on another forum) I understand the base of the current production HM52 (HM52B) machines is strengthened by additional ribs in the casting, this might also apply to grizzly and other brands of the same machine I suspect. Makes me feel a little happier about proceeding with the conversion.

The inspiration for this project comes from chich2, who travelled down this path a few years back, see his thread here... http://www.cnczone.com/forums/showthread.php?t=25895

Some of the design decisions have been made based on problems that chich had with his conversion, so I owe a debt to his pioneering work.

1. ballscrews, I'm going for ballscrews first up. The costs have become much more reasonable in the last few years.

2. Whether to "Z" the knee or the quill. I am reluctant to go down the path of counterbalancing the knee and stiffening the column to base join, I feel (at this point) that it will be easier to deal with the quill run-out and flex than try to solve the base/column flexing. So I am going to drive the quill, and see what new problems might arise. The knee will be locked as normal while machining.

3. Selection of motors.
If we take mariss's example (from an earlier thread somewhere on cnczone)

Just starting somewhere... 120 ipm and 10lbs on a 6" handwheel

At 120 ipm with a 5 tpi ballscrew == 600 rpm on ballscrew
The torque is 10lb x 3 inches == 480 oz-in on ballscrew
The power required == 213 watts ( 600*480/1351)

Looking around ebay I found some yaskawa 400 W AC servos that will run on single phase 240V, there were some 750W Yaskawa's from the same supplier but they required 3 phase 240V, which I sadly don't have. So my "tim the toolman" approach to design has to be moderated a little.

Having decided on the 400W Yaskawa, we can now repeat the calculations above, and get some idea of the drive reduction.

If I go for about 3:1, what does that mean for speeds and torque?
The Yaskawa is rated at 3000 rpm, so that means ballscrew will be 1000 rpm at 5 tpi (actually the ballscrews I'm looking at are metric 5mm) the maximum will be 200 ipm, which is plenty fast enough for what I'm thinking.

Torque at the motor shaft will be 180 oz-in (from motor data) times 3 (reduction drive) == 540 oz-in (at the ballscrew) with peak torque (during accell/decell ) of 1620 oz-in

4. Control Systems,
The servo amplifiers are yaskawa which came with the motors, and can be set up in a number of different ways, The encoder is matched in some way to the drive, I think I read somewhere that the drive actually communicated with the encoder to identify motor type etc.. for what I am intending this doesn't really matter, the drive generates quadrature encoder and index signals (nicely opto-isolated btw) that will be fed to a mesa 5i23 (buffered by a mesa 7i33TA interface board.

The mesa 5I23 is a FPGA pci bus board that handles the encoder interface and servo control.

The mesa 7I33 is an interface board for the mesa 5I23, and provides noise filtering for the encoder pulses, and also generates the +-10V drive signals for the servo amp.

The Yaskawa drive can also run in step/dir mode with a breakout board and Mach3, in fact I'm running that way at present (bench testing), while waiting for the mesa boards to arrive. You just have to set some parameters in the drive.


The CNC software will be EMC2, still haven't decided on a PC platform as yet, but running the live CD, it looks to have everything I will need and more..

That's about it for the time being, I should be picking up the mill in the next few weeks, so I'll post some more then, I hope that others can learn from my screw ups as I go along..

Regards
Ray