CNC conversion advice wanted

[Mtnmac]

My major project this winter will be converting a DM-45 mill to CNC. This won’t be the build thread, I will start that later. I’m interested in some general opinions and advice from people who have done similar conversions. Nothing has been purchased yet, so I’m open to any changes. Feel free to make any comments or point out possible problems with my plan. I’m glad I found this forum, there is a wealth of information here.

I’m a retired machinist so don’t need advice on design/cad/cam software.
I have another mill and lathe, so making all the necessary parts is no problem.

The machine is a typical column type 45 series bench mill with geared head.
Head will be counterweighted.
Ballscrews with double nuts, 16mm XY, 20mm Z.
X axis motor will be mounted parallel to ballscrew, on front of table.
Y axis motor will be mounted parallel to ballscrew, on side of base casting.
X, Y driven by timing belts, ratio to be determined, maybe 2:1 for torque?
Z axis will be direct drive.
Drive components:
Closed loop steppers, drives, and power supplies from Steppersonline.
X,Y. 679 in/oz.
Z. 1274 in/oz,
Stepper motors operating at 60V.
Centroid Acorn operating system and hardware.
Intel NUC computer with touch screen, as recommended by Centroid.
Spindle motor will remain stock at this time, will replace with 3ph and VFD later.

So, opinions on using the timing belts vs direct drive? I’m aware this is a more complex setup, but it reduces the space required for the machine. I have a small shop, but If there are any real advantages to going direct drive, that could be done.

Anyone have suggestions on where to buy ballscrews? I’m not finding kits for these size mills. If necessary, I will buy longer screws and machine the ends myself.

Size of the motors? Am I in the ballpark with these motors? I’ve been reading build threads here and other places for months, and motor sizes vary widely. I haven’t done any calculations.
Any specific comments about the Centroid/Acorn set up?
Anything I have missed? General suggestions?

Thanks to all of you for blazing the trail for these conversions.

 

[RJSakowski]

As a benchmark, my Tormach 770 uses 500 oz-in. stepper on the x and y and 770 oz-in on the z axis. Your motor choice sounds OK. The Tormach doesn't use any counterweight for the head. Head wt. is around 150 lbs.

I guess that I would be concerned about lost motion using timing belts rather than direct drive.

 

[Reddinr]

I have an RF-45 CNC conversion with timing belts. It works well but the belts on my machine are pretty stout. Not sure your budget but I have a couple of custom application machines that use Clearpath "stepper killer" servos that have worked very well. Clearpath has been easy to work with and they have good torque data for their servos. A bit on the pricy side but compact.

 

[Karl_T]

i have done CNC builds to both direct and timing belt drives. IMHO, both are fine, very little difference in accuracy etc.

Glad to see you are going closed loop. I had LOTS of trouble with open loop stepper mills, and went away from that 20 years ago.

 

[Bluedog]

According to their website board comparison, the Acorn doesn’t do closed loop control.

 

[matthewsx]

Not sure if I can give any advice but if you need a hand with anything I'm just down the hill. I'll be changing the spindle out on my CNC project soon and eventually getting my new Mesa Electronics controller going.

John

 

[Mtnmac]

According to their website board comparison, the Acorn doesn’t do closed loop control.

Yes, I saw that. There is a fault circuit wired in... when the driver senses the motor is missing steps, an alarm appears on the Acorn screen. So while it won’t correct the problem, it will let you know it’s happening.

 

[Mtnmac]

Not sure if I can give any advice but if you need a hand with anything I'm just down the hill. I'll be changing the spindle out on my CNC project soon and eventually getting my new Mesa Electronics controller going.

John

Thanks, Matthew, It’s possible I may need help in the process, especially moving the large iron parts around while doing the mechanical modifications.