[Bob La Londe]

I'm in the process of converting the mii lathe over to CNC, and I want to figure out the optimum spindle motor. The stock motor is a universal DC brush motor. I want to setup something that is going to have very high torque at low speeds ad still be able to turn higher speeds. Say 300-1500.

I think the stock motor is only rated at 1/3 horsepower, but I am not adverse to putting something else on in its place.

Some type of control with feedback?

VFD / 3ph?

Servo?

[Al_The_Man]

For those specs, you may want to look at a DC brushed with a KB/Baldor 4 quadrant drive, if you pick up a larger DC motor with a tach on it, the KB's 4 quadrant models usually have tach input for tight control.
Unless you get a VFD with an encoder option, you will not get high torque at low rpm, or you could go with a 4 pole motor & VFD, run it at 120 HZ and reduce with a 2:1 reduction, this will double the torque of the motor and give you close to 1800rpm at the chuck.
Very unusual to have a Universal motor on a lathe spindle?
Universal usually implies AC/DC brushed with a wound field?
Al.

[Bob La Londe]

Well, technically its just a DC brush motor. It will not run on AC. The original (now dead) speed control provided DC to the motor. I have a PWM control speed controller on it right now that gets by for manual operation but the noload speed and the load speed have little relationship to each other now. It is setup to take an external pulse source and I originally thought I might use it when I CNCed the lathe, but the lack of consistent speed and torque at low settings makes that a poor choice IMO.

[Al_The_Man]

A DC motor has maximum torque at zero rpm, this curve is usually fairly flat up to its rated speed.
Are you sure it is just not a failing of the drive?
Like I mentioned, the KB's are pretty good at maintaining rpm, especially with the Tach version.
Al.

[DICKEYBIRD]

"...pick up a larger DC motor with a tach on it, the KB's 4 quadrant models usually have tach input for tight control."

You got my attention with that statement Al, I have a KBCC-125R drive (I think it's the 4 quadrant type with the tach input) and a 3/4 HP Baldor motor (no tach) on my CNC lathe. Is it possible to add a tach to the Baldor? If so what would I look for? Hopefully ebay'able & cheap.

I want to do threading and I understand Mach3 likes a steady spindle speed to work well.

[Al_The_Man]

You need a DC tach something around 10v to 20v/1Krev.
One way if nothing can be found cheap, is a miniature quality DC motor such as a ~24vdc Maxon and use it as a tach generator and drive it off the end of the motor shaft.
A couple of years ago I was fortunate to pick up a quantity of new high end tachs off ebay, I see they are fetching a good price now!.
Al.

[DICKEYBIRD]

Thanks Al. I looked around a bit and you're right, what few I found were very pricey.

I just happen to have a new 24v Maxon motor on hand. I read the KBCC manual and they've documented the tach thing very well. They claim 1% speed accuracy which sounds great!

I'm trying to get my head around Mach3 tool tables right now but when that's done I think I'll try some threading with the machine as-is before jumping into the tach thing. I'm pretty tired of wiring & software & spending money...I'm ready to gnash some metal.

Bob you might look around for a used KBIC-125 control on ebay. I've bought several for $30 or less (used) They're rock solid and completely adjustable. You'd need to pick up a KB HP Plug In resistor to match your 1/3 hp motor but they're cheap.

Any of the KB's are controllable from Mach with a CNC4PC or Homann DigiSpeed control board. (Dunno but I'd guess the same about LinuxCNC but I ain't smart enough yet.) I've used the -125's for several years now and they've been bullet-proof. I got the KBCC-125R so I could easily have safe, reliable spindle reversing along with dynamic braking.

[Bob La Londe]

A simple tach can drive the coordination in Mach 3 for single point threading. Index pulse off an encoder. Simple single slot optical sensor, proximity sensor etc. Its my understanding that it does not control RPM, but rather it controls the feed rate based on sensed RPM. It works directly with Mach rather than with the speed controller. Now if you meant rigid tapping... its a little more complicated than that. Most rigid tapping appliations recommend an encoder on the spindle. Usually about 300 lines do to processing speed of the LPT port and the computer. I have not gotten that far as of yet, so I don't know if its just better speed sensing or if its more than that.

One thing Mach 3 can do is treat a stepper or a step direction servo control like a spindle. This is one way where you should be able to get accurate speed control as long as you do not exceed the power of the motor. It will also allow you to do rigid tapping. If using a servo then you can use an encoder with an index pulse as a for basic RPM sensing as well for simple single point threading. The converse is you can then add live tooling and treat your spindle like an axis.

[DICKEYBIRD]

A simple tach can drive the coordination in Mach 3 for single point threading. Index pulse off an encoder.

Yup, got a C3 w/opto sensor and single hole disc on the spindle. It's all working great and threading may be fine with the 3/4 hp spindle motor & Mach; dunno yet. There's a lot of chatter on the web about Mach threading problems even after the driver was re-written a while back. The problems appear to go hand in hand with small spindle motors & subsequent speed fluctuations under load when the cut gets deeper. Al's mention of the 4 quadrant KB's ability to work with a direct tacho input from the motor woke me up. I didn't realize dey could do dat!

I've been amazed at how close a well adjusted KB controller (of any type) maintains the speed setting from Mach, both no load and under load. Everything I've turned so far has been with no speed related drama at all. Just quiet, reliable, steady speed & torque no matter what I've thrown at it.

Dunno if I'll need the extra accuracy of the tacho input once I try some threading but it's nice to know the option's there if needed.

[Al_The_Man]

On a true closed loop system, the Z axis can be geared off of the spindle encoder, Mach uses a 1/rev pulse to synchronize the Z, and assumes the variation between revolutions will not affect threading to any extent.
The Tach is just a method of stabilizing the rpm of motor to avoid variation under load/no load.
Al.

[DICKEYBIRD]

Hopefully, Mach4 will work with a multislot disc on the spindle and threading will be much better. We may never see it in our lifetime though.

[Bob La Londe]

Al, Its my understanding that Mach does NOT do anything to adjust the speed of the motor. It just adjusts the feed rate based on the time of the last sync pulse. Or where you referring to the tach used with the KB control.... Speaking of such. I wondered why Mach couldn't use more sync pulses and you just adjust the pully ratio in Mach setup to match.

[Bob La Londe]

Hopefully, Mach4 will work with a multislot disc on the spindle and threading will be much better. We may never see it in our lifetime though.

I believe I have seen a dedicated machine 3 axis version of Mach 4 listed as the controller for a machine somewhere.

http://www.machsupport.com/forum/ind...html#msg158571

[Al_The_Man]

Al, Its my understanding that Mach does NOT do anything to adjust the speed of the motor. It just adjusts the feed rate based on the time of the last sync pulse. .

Correct. Many have successfully used this method for threading.
Al.