Q: CNC 7x14 with replacement motor and tach

[Deviant]

Hello,

I'd like to hear some suggestions on what I can do, to remove all the gears and replace the equiped motor with something that has a tach and variable speed.

Minimally, I'd want to be able to read the turning speed from within the software. I.e. Mach3.

If possible, I'd like for the software to be able to control the speed.

Perhaps, I'm being to optmistic.

Is this possible.

I'm still a cnc newbie and life keeps getting in the way. Finished my workshop, but got a new job so now I have to move to a new location.

*anyway*

From searching the forums and my own observations. I'd assume that you'd need some time of dc or ac motor. Then you need a speed controller.

I've read alot of different comments that I'm not following 100%.

It would appear that my options is a obtain 3 phase ac motor and a vfd. I'm not sure exactly how this would work. But I'm assuming that it allows me to plug the vfd directly into the wall current.

Second option would be to pick up some type of DC motor, whether it's a servo or a trademill motor. Then I need some type of speed controller that varies the voltage..

Which option would be better.... easier.... cost?

Now, once I have the motor turning the chuck. I need to be able to read the speed. I'm assuming that I attach some type of optical wheel sensor to count the revolutions and sends pulses back to mach3 over a certain pin(s). I'm unsure of how this would allow me to control the speed of the motor. Is there something else I need?

Any info/links would be great.

Again I'd like to ditch the gears, do some single point threading (internal/external) as well as control the turning speed from within the gcode.

Which I hope would allow me to make cuts at one speed, then do another finish cut at a faster speed. Perhaps this isn't needed.

Is it 100% needed to replace the motor? Or is there another option.

Comments are welcome....and encouraged. *grins*

Thanks

[fyffe555]

I've probably misunderstood here but;

The 7x lathes have variable speed as standard? No easy way to have the spindle speed set by controller software without a new motor and motor driver as you suggest, though for just single point threading its not necessary.

You can add a spindle tach. Littlemachineshop.com has one for about $60 for these lathes. This just shows the current spindle speed and doesn't allow you to feed the speed back to controller software.

Dave Kowalczyk of turbocnc fame has some articles on cnc'ing these lathes including CNC single point threading at www.dakeng.com under the articles section. Includes a schematic for a spindle speed encoder which can feed the necessary speed signal to turbocnc or mach3. Same schematic is available as a made up bready to go board from CNC4PC.com. With that signal you can just manually set the spindle speed as appropriate and start the cnc threading operation which will take its position from the spindle speed signal..

[Deviant]

Link for the mini lathe tach.

http://littlemachineshop.com/product...ProductID=1684

I've considered this option. Just wasn't sure how well it would work. And how this will effect the removal of the threading/power feed gears.

I need to take the cover off the lathe and look at it a little closer. I was concerned that the motor might not engage the gears correctly when I take off the gears to add the servo/stepper to the headstock. Of course, I don't really like the gears themselves, as they seem to be a little loud and possibly fragile.

And yes the mini lathe has a variable speed built in. I just wasn't sure if it was worth while to remove that part of the lathe. Perhaps, I'm trying to make this more difficult that it needs to be.

On the single point threading, do I just hook up the tach and manually type in the rpm that is displayed. What is the downside of having the spindle slow down when the cutting starts. I'm assuming there is some slow down, if small. Will this be detrimental to the threading? Since I'm new to the cnc world, I hope these questions are relevant to the discussion at hand.

Thanks for the follow up.

[fyffe555]

The stock setup is the motor drives the spindle which drives the geartrain and the leadscrew. The tach disk can go on the end of the spindle or like on the micromark on the spindle inside the headstock. Doesn't affect the gear train.

To cnc it you'd usually remove the gear train entirely and put a motor on the leadscrew and one on the cross slide. Jfettig, a contributor here has a kit for the 7x lathes to mount the motors in that way.

When i had single threading working the software keeps track of the spindle speed via the signal and made very clean, consistant and accurate threads.

[mark c]

I have a 7 X 14 lathe with the CNC4PC spindle index card and TurboCNC. Works great. Just set the speed (manually with the knob) and the pickup signals TurboCNC the speed to sync the speed & feed with the M50 command.
Probably work just as well with Mach3

[wizard]

Originally Posted by Deviant Hello, I'd like to hear some suggestions on what I can do, to remove all the gears and replace the equiped motor with something that has a tach and variable speed.

You have several choices.

The oldest being a mechanically variable belt drive and a tach or encoder mounted on the spindle.

Not far behind this would be a DC motor with a variable speed drive. Again we would have a tach or Encoder for speed feedback. This has alot of potential if you can find the components at the right price.

Though newer technology, variable frequency AC drives are about the same as the DC drive in capabilities.

The next step up is DC or AC drives with feedback. These have better speed regulation and other features to increase performance. There is a bit of a gulf between the low end DC or AC controller, described above and what I consider to be a drive. A simple controller can do the job for a manual lathe like this.

The next step up are servos, be the AC, DC or Brushless DC. These have extended capabilities beyond velocity control. You do have to worry about stability when using servos and closed loop drives.

Minimally, I'd want to be able to read the turning speed from within the software. I.e. Mach3.

I'm not sure how Mach3 does this so I can't help there! But is sounds like you want Mach3 to control the spindles speed. If so the easy way is through an analog out port driving a suitable input on the drive, atleast from the standpoint of the electronics. If you are just worried about being up to speed some drives have the ability to set an output when velocity has been reached. A Mach user might chime in here but Mach3 might have the ability to read a stream of pulses and convert that to a velocity, that would be nice.

If you just want a human readable speed display then I'd opt for a digital gage hooked up to a counter.

If possible, I'd like for the software to be able to control the speed.

Oh Now you say so

Repeating the above I'm not up to speed on MAch3 but spindle speed control is pretty much a standard feature of a CNC software package. You might want to check in the Mach3 thread for response from heavy users. In any event I'd geuss that the easy way to do this is through an analog ouput. If Mach3 supports user programming maybe it would be possible to communicate via RS232 or Ethernet to a drive. That is not however trivial programming.

Perhaps, I'm being to optmistic.

No I don't think so. Before going to far though you might want to bone up a bit on the different CNC packages out there and learn how they work. You especially will want to look at the EMC web site. Even if you don't go with EMC there is a great deal of information there related to RS274 and controller hardware.

Is this possible.

Nothing is impossible. Some things do require a bit of hacking.

I'm still a cnc newbie and life keeps getting in the way. Finished my workshop, but got a new job so now I have to move to a new location.

I can understand your situation.

*anyway* From searching the forums and my own observations. I'd assume that you'd need some time of dc or ac motor. Then you need a speed controller.

If you really need to have the CNC controller control the spindle speed then you will need a drive that can be controlled externally. The available solutions all have various trade offs. You will have to decide what fits your application best. Some here like AC drives but an old fashion DC motor can offer really good performance also.

As you add more requirements things get to be more expensive. You have to realize though that you are working with a 7x10 lathe that is of limited capability. It would be easy to go overboard.

I've read alot of different comments that I'm not following 100%.

Well if you are new to this there is a lot to digest. Don't let that get you down. All the different options exist for a reason. Just realize that there is much in the way of overlap in capability.

It would appear that my options is a obtain 3 phase ac motor and a vfd. I'm not sure exactly how this would work. But I'm assuming that it allows me to plug the vfd directly into the wall current.

As to plugiging in yes after you have wired up the unit and adjusted all the required parameters. Not a big deal really.

The problem is that I might not suggest VFD right off the bat, especially if you want to integrate the control into your CNC software.

Second option would be to pick up some type of DC motor, whether it's a servo or a trademill motor. Then I need some type of speed controller that varies the voltage.. Which option would be better.... easier.... cost?

Properly implemented a three phase motor is about as reliable as a brick. Brushless DC motors are similar in this respect. DC motors require maintenance.

In all respects, any of the motor options is pretty easy to implement for manual control. Many times it is just a matter of hooking up the motor correctly and suppling power. Your needs change though with control from the CNC software. Here you need to know what is easy for you CNC software.

Now, once I have the motor turning the chuck. I need to be able to read the speed. I'm assuming that I attach some type of optical wheel sensor to count the revolutions and sends pulses back to mach3 over a certain pin(s). I'm unsure of how this would allow me to control the speed of the motor. Is there something else I need?

Feedback doesn't control anything. In any event you might want to pursue this with Mach3 users.

Any info/links would be great. Again I'd like to ditch the gears, do some single point threading (internal/external) as well as control the turning speed from within the gcode.

Ditching the gears for what? It is pretty easy to justify two or three steps on the pulleys even if you have a nice AC drive. It helps with torque and low speed control.

Which I hope would allow me to make cuts at one speed, then do another finish cut at a faster speed. Perhaps this isn't needed.

Depends on what your intentions are. You can get buy with extremely crude speed control with a couple of digital I/O's if it fits your application. I know of past implemenations that did just that in a production environment.

Is it 100% needed to replace the motor? Or is there another option.

On the 7x14 I don't think it is as big a problem as on others because I do believe that you have a DC motor in there already. As mentioned above you can get crude speed control over the motor if you have a couple I/O bits that can be driven with M codes.

Comments are welcome....and encouraged. *grins* Thanks

I get the impression that you want to do full CNC control including threading. Your choosen software defines what is needed to get that done. The lathe and its motor might be fine, I would not be surprised to find that the drive is wanting. Niether the motor nor the drive could pass for industrial quality.

Think about how important threading is for you and see if the extra expenses make it worth while. It might not be all that bad with Mach3 $$$ wise. Obviously a larger motor should help some with variable speed control but there is a limit to what the machine can handle. Also you might not have the performance at the lower end for things like threading depending on the drive and motor combo.

Dave