a dual use CNC and manual milling machine project - something learned

My Wells Index 520 CEN mill has now been retrofitted with a 3 axis step motor based CNC system and is fully operational as a 3 axis CNC mill. It has also had manual handles and indexed dials and table locks and etc installed as required for manual machining.

The step motor drivers are half step, the NEMA 42 step motors are 200 full steps per rev, the ball screws are 5 TPI or 0.200 advance per rev, and the cog belt drive ratio is 2:1. This yields a step increment of 0.00025 which is quite satisfactory for my needs.

A big problem now arises when attempting to use the machine in manual mode. These NEMA 42 step motors have considerable residual magnetism in their magnetic poles and generate considerable cogging torque when rotated manually. As a result, the axes can not be set to any points other than ones where the physical magnetic poles of the motor are in alignment. A 200 full step per rev motor has 50 physical poles/rev. With a 2:1 cog belt ratio and a 0.200 inch advance per turn of the lead screw, those 50 poles align every 0.002 inch. You can not reasonably set an axis to anywhere other than those points where the motor poles align, and that only occurs every 0.002 inch of travel. That is clearly not acceptable given that I expect to be able to set an axis to 0.0005 inch as I currently do with a DRO on an all manual machine.

So, my conclusion - you can not expect to build a dual use manual and CNC machine using reasonably large step motors for the axis drives!

As a result, I have removed the step motors and will start over again with DC servo amps, DC servo motors, tachs, and encoders.

Before you get too carried away with any criticisms - this is a retirement hobby project and while I have a need for such a machine I am doing this for the challenge of doing it as much as anything - not because I need a mill to make customer parts with. I enjoy manual machining and I do not want to give that up, but I sometimes need to make parts that require a CNC mill. Next year I hit 70 and I am seeing benefits to downsizing and I do not want to have two machines if I can get by just as well with just one. So this approach fits my particular needs. As an engineer I am also pretty embarrassed that I did not anticipate this problem much earlier.

I am still convinced I can make a no compromises dual use machine, but I will have to do it using DC servos.

aninventor said:

Before you get too carried away with any criticisms - this is a retirement hobby project and while I have a need for such a machine I am doing this for the challenge of doing it as much as anything - not because I need a mill to make customer parts with.

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Sir,
your post would be better over at "see and see zone dot com" (can't send actual link)

Get creative and figure out a way to disengage the timing belt. Shouldn't be that hard to design a device that will loosen the belt and lift the "cogs" off the drive pulley.

Why would you ever need a manual machine anyhow? Get good enough at CAD CAM and you will have a need for a manual machine so rarely that it won't be worth the effort.

One other consideration is if you have replaced your lead screws with ball screws. Ball screws have very little rolling friction and are not very user friendly in a manual capacity. I imagine that is why you put the locks in place but to me having to lock the axis every time I took a cut is a compromise.

The Wells Index 520 CEN is a CNC machine and has ball screws as manufactured. This one was bought without any CNC motors or electronics as they had all been removed by the former owner who used them to retrofit a much larger mill.

Setting the table locks lightly gives enough friction to simulate a normal manual mill with the friction of normal screws and nuts, so that has not been a problem. I could have provided a screw with a brass tip pressing against the lead screw shaft itself to provide friction but that has not been necessary. If I see any indication of increased wear on the ways/gibs, that solution would eliminate it and would be easy to implement.

So far as disengaging the cog belts - not much room in the motor mount castings for that. The periphery of the cog belt pulleys is up against the sides of the castings. Doable? Perhaps, but not easy at all. I considered it but rejected it. Besides, I think there are some benefits to DC servos and I have most of the parts required.

So far as why would anyone want a dual use machine: I actually enjoy turning the cranks and doing manual machining the way I was taught to do it 50 years ago. It is not because I can not use a computer or a CNC machine - I was designing computers and computer controlled machines before most of you were working - it is because I am retired and I want to do what I enjoy the most and do it in the manner that is most pleasurable to me. Manual machining is a very different experience than programming and running a CNC. It is a bit like hunting with a bare recurved bow and broad head arrows instead of a scoped semi-auto rifle and mag ammo. I have done both. Both have their time and place. They are very different experiences.

Also, retirement and living on a fixed income has certain realities. A manual machine is much less expensive to own and to operate and that can be important in retirement. There is also a certain security in knowing that my manual machine will probably still be fully functional longer than I will be. CNCs are much more failure prone. Getting one fixed can set you back quite a bit. So, my CNC must be something I can troubleshoot and maintain myself and something I can expect to get cheap parts for when parts are required. Commercial CNCs with proprietary electronics and software do not fit that description very well. So, it is do it yourself time - and I am learning a great deal in the process.

I think you are going to find that you may need AC servos, as DC servos may have cogging as well.

post this at the Artsoft Mach forum, you will get help. also, what G00 Proto said. when i bought my cnc turret mill, i searched hard for one with hand wheels because i thought them a necessity. i can do anything i want to quickly with either a quick conversational program (mach calls them wizards) or a single or multi line mdi command. i dont think i have used the hand wheels for machining in 10 years.

aninventor said:

My Wells Index 520 CEN mill has now been retrofitted with a 3 axis step motor based CNC system and is fully operational as a 3 axis CNC mill. It has also had manual handles and indexed dials and table locks and etc installed as required for manual machining.

The step motor drivers are half step, the NEMA 42 step motors are 200 full steps per rev, the ball screws are 5 TPI or 0.200 advance per rev, and the cog belt drive ratio is 2:1. This yields a step increment of 0.00025 which is quite satisfactory for my needs.

A big problem now arises when attempting to use the machine in manual mode. These NEMA 42 step motors have considerable residual magnetism in their magnetic poles and generate considerable cogging torque when rotated manually. As a result, the axes can not be set to any points other than ones where the physical magnetic poles of the motor are in alignment. A 200 full step per rev motor has 50 physical poles/rev. With a 2:1 cog belt ratio and a 0.200 inch advance per turn of the lead screw, those 50 poles align every 0.002 inch. You can not reasonably set an axis to anywhere other than those points where the motor poles align, and that only occurs every 0.002 inch of travel. That is clearly not acceptable given that I expect to be able to set an axis to 0.0005 inch as I currently do with a DRO on an all manual machine.

So, my conclusion - you can not expect to build a dual use manual and CNC machine using reasonably large step motors for the axis drives!

As a result, I have removed the step motors and will start over again with DC servo amps, DC servo motors, tachs, and encoders.

Before you get too carried away with any criticisms - this is a retirement hobby project and while I have a need for such a machine I am doing this for the challenge of doing it as much as anything - not because I need a mill to make customer parts with. I enjoy manual machining and I do not want to give that up, but I sometimes need to make parts that require a CNC mill. Next year I hit 70 and I am seeing benefits to downsizing and I do not want to have two machines if I can get by just as well with just one. So this approach fits my particular needs. As an engineer I am also pretty embarrassed that I did not anticipate this problem much earlier.

I am still convinced I can make a no compromises dual use machine, but I will have to do it using DC servos.

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1) every cnc i have used you could do manual machining in handle more
.
2) on stepper machine if i cut power to steppers they turn easier.
.
3) i have seen a timing belt drive from stepper or servo to ball screw so you get a better ratio like .001 resolution. backlash compensation most systems have a way to measure and put in backlash compensation amount
.
4) biggest problem with open loop stepper motor is if cutting forces too high you can loose steps or not be in position you think you are cause rather than move stepper just went to next step pulse. better closed loop system have a way to monitor actual position and put it where it should be
.
for example on a prototrak mill in position drill mode i can turn X til it moves .002 but when i let go it returns to position it is suppose to be at like magnetic springs

garyhlucas said:

I think you are going to find that you may need AC servos, as DC servos may have cogging as well.

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I have not noticed any cogging on dc motors, they tend to spin pretty freely. That's what most dc servos are universal brushed motors with encoders.

And to OPs original post, you shouldn't spin the steppers by hand while connected to the drivers, not all drives are protected for the current created by the hand spinning the motor.
You can see this easy when drivers are off, spin the motors by hand and all the led's light up in the drives.

Marko

Some stepper motor theory:

There are three cases to consider when forcing a step motor to rotate by mecahnically turning it. They are:

Case 1) the stepper motor leads are not connected to anything ---- in this case the winding circuits are open and there is no path for current to flow. Since there can be no current flow,ther can be no power generated and no electrical power dissipated and no net work is done in turning the shaft to generate electrical power. The windings do cut through the residual magnetic fields of the step motor physical poles as it is turned and a high voltage is generated across the open leads. Since there is no current flow in the windings there is no magnetic field generated in teh windings and no torque produced by the windings. There are magnetized pieces of iron in the poles of the rotor and stator and these do attract each other and this attraction does produce a cogging torque which you can feel as you rotate the motor shaft. Each pole produces an attractive torque pulse and a cogging action as it attempts to align. This is the source of my problem. The unconnected step motor prefers to stop in certain positions and stay there. I will turn away from intermediate positions and return to a position where the internal poses are in alignment. You can not position it anywhere you want it and let go - it will snap back to where it wants to be. That is a problem.

Case 2) The stepper motor leads are connected to electronics but the electronics is not powered ---- in this case the motor windings are cutting through the residual magnetic fields and a voltage is being generated across the open leads. The magnitude of this voltage depends upon the strength of the residual magnetic fields, the speed of rotation, and the impedance of the electronics it is attached to. The higher the impedance the higher the voltage and this voltage can be many thousands of volts, enough to damage electronics. It is relatively easy to design the electronics to limit these generated voltages to safe levels and avoid this problem by giving the induced current a safe path to flow through. The generated power is safely dissipated in the electronics itself. It takes torque to generate this power and this torque is added to that required to over come the cogging action discussed in #1 above. When the motor is stopped, there is no torque generated by the windings as they are no longer moving through a magnetic field and we are back to case 1.

Case 3) The stepper motor leads are connected to electronics and the electronics is powered ----- What happens in this case depends on how the circuits are designed but generally the driver reduces the step motor drive voltage and current to holding levels when the axis is not being moved. The holding voltage/current values are chosen to produce a certain design break over torque that the motor can resist while holding position and still not over heat. If this torque is exceeded, the motor will turn through a full set of steps to the next aligning position, typically 4 full steps. The torque required to make the motor rotate is much greater than in case 1 above because the magnetic fields generated by the motor drive current are much higher than the residual fields of an unpowered motor.

So yes, removing the power or the electrical connection from the motor to the driver will allow a step motor to be rotated with much less effort. Doing so does not eliminate or even reduce the cogging torque discussed in 1 above, and that is the source of my problem. The bigger the step motor is the greater the amount of iron in it and the greater the residual magnetic fields in all that iron and the greater the cogging torque generated when trying to force it to turn when it is unconnected. There is no solution other than to use another type of motor.

I have both Baldor and Siemens DC servo motors. DC servo motors are designed very differently as compared to step motors and they rotate very smoothly. They are the solution to my problem.

Because DC servo motors do not have any preferred position and will not hold any position on their own, the control system must have feedback loops that measure actual position and control the drive voltage/current to the motor to actively maintain the desired position. Most of these DC servo motion control systems also include a speed sensor (tachometer) and close a second feedback loop on speed as this gives smooth acceleration and more accurate position control.

So, if you want a dual use manual and CNC machine you had best use DC servo motors to build it.

Figure out a slick way to decouple the cog pulley instead of removing the belt. Like the manual dial lock, disc clutch or perhaps just a drive pin.