Manual to CNC First Step Question

[Art99]

I am currently running a HF 7x10 in manual mode. I've removed the compound and added a manual crank to the z-axis lead screw. I like running that way BUT would also like to add a stepper motor drive to the z-axis as the first step toward CNC or ELS (Electronic Lead Screw).

I was thinking about an arrangement as described by "blades" in thread 82871 June 8, 2009 in which he had installed a dual shaft stepper on the lead screw and a manual crank on the other end of the stepper shaft.

Question! When the stepper is not in use is the "feel" of the crank on the stepper shaft ok OR do you feel the stepper cogging-detents and lose the feel of a manual lathe??

All inputs and opinions would be welcome.

Thanks

Art99

[doorknob]

My expectation, based on no experience whatsoever, is that you will feel the cogging when adjusting the Z screw rotation by hand, and the screw will want to move into specific detent positions rather than being "infinitely" adjustable to the precisely-desired location with the handwheel. But that may turn out to be a "don't care".

The Z screw on my 7x10 looks to be 16 tpi, so that would give a linear distance imparted to the tool of 0.0625" per revolution.

I have never counted them, but my guess is that there will be 200 cogs or detents per revolution on a standard stepper motor. If that is correct, then you would expect to have detents at a regular tool spacing of every .0003125" (plus or minus some slop, because they are not really detents, rather they represent peaks of magnetic attraction).

I suppose that if the screw has detents in its travel, then the workpiece surface might also evidence some associated imperfections, but there's the question of scale and also of other factors that may overshadow any such effect. So, is having detents every 3 tenths along the Z axis going to be noticeable in your finish as you're turning a workpiece? Is the rigidity of the system enough that you would be in danger of having tiny ridges in the surface, or will other mechanical factors make that unnoticeable? Isn't the typical width of the tool face as it makes contact with the surface of the workpiece going to be fatter than that?

My guess is that it's not going to be a problem, but YMMV. You might end up with a surface roughness on the order of a few hundred microinches. But I'm not even sure what kind of surface roughness figures you can expect to get from an unmodified mini lathe - maybe it's comparable to or worse than that figure. You might not even be able to measure that on a DTI. But then I only pretend to be a machinist, so someone who is an actual machinist may chime in and tell me that I'm all wet.

Using a greater diameter handwheel rather than a lesser diameter one might help you to more easily overcome the cogging action...

But as I said, that's just an uninformed guess...

[Art99]

Thanks!
Pretty INformed discussion from my perspective!
Art

[Fastest1]

Not to contradict anything said here but be careful of backfeeding your controller if Cnc. If you use a larger diameter handwheel chances are you will spin the stepper faster and generate too much. Also the larger mass of the handwheel might make directional changes more strenuous or slower. Just a thought.

[doorknob]

Backdriving the stepper motor is an interesting point to consider, thanks for bringing it up.

There was a good discussion of that issue here:

Back driving a stepper

[blades]

You do feel the stepper motor when cranking by hand. It's not silky-smooth like it would be without the stepper, but it also doesn't feel overly intrusive to doing manual work.

[Art99]

Blades

Thanks for your comment from direct experience.

One further question: I am thinking of hooking up the motor as you did BUT directly driving the existing z-axis lead screw. I'll remove the existing LH bearing and directly couple the shaft to the stepper motor. Since you had also changed the lead screw, I wonder if you have any comments as to why what I proposing to do seems ok or not.

Art

[blades]

Hey Art, right offhand I really can't think of a reason why this wouldn't work, and it would be the easiest approach, since you wouldn't have to build a whole new carriage block that's usually required. Most people don't want to go this route because they utilize ball screws.

Assuming you use the existing carriage apron, you will easily be able to engage/disengage the CNC control for manual turning, although Z-axis accuracy is liminted in manual because you'd be using the large handwheel for feeding. Actually not much different than how you would use the lathe in it's unconverted form.
Another consideration is you will have to have the X-axis motor sticking straight out from the X screw. You wouldn't have the option of tucking it under the carriage and belt driving the screw. I should note here that many, if not most conversions use this method of driving the X-axis screw. There are many, many ways of powering the screws of this lathe. None of them are wrong, although some methods are better than others, depending on your intended use of the lathe after it's conversion. It all depends on your level of ability, and what equipment you have available to do the conversion. It's extremely difficult, if not impossible to do a conversion if you don't have access to a mill, or at the very least a milling attachment for your lathe.

Generally speaking, the more competent the conversion, the better the end result will be. For example, someone who took the time & expense to fit their lathe with ball screws & appropriate thrust bearing supports will ultimately have a more capable, smoother operating lathe than mine. I'm basically just messing around on mine, and have no plans of turning anything other than plastics, acryllics, or soft metals, for which my conversion is well suited. I could not obtain the same level of quality or precision work on mine (not to mention sheer capability in turning steel) that other conversions that utilize ball screws and a more robust design could.

[Fastest1]

In my experience, (I do have 2 machines with both handwheels and steppers though little, a Sherline and A2ZCnc) about a week after you finish, you will never use manual again. In fact I have a 12 x 36 HF manual lathe that is yelling for a conversion. How simple it would be to control by a shuttle pro or an mpg. Anyway enjoy the process.

[cornbinder23]

In my experience, (I do have 2 machines with both handwheels and steppers though little, a Sherline and A2ZCnc) about a week after you finish, you will never use manual again. In fact I have a 12 x 36 HF manual lathe that is yelling for a conversion. How simple it would be to control by a shuttle pro or an mpg. Anyway enjoy the process.

Good point!!!
I look at it like this, do you get up to change the channel on the TV even though you have a remote?
And the back feeding issue is there, on my G540 I can see the led's light up on the driver when I rotate the stepper by hand so it is very apparent to me that it is back feeding the drive
I use a simple pushbutton interface for manual lathe operations

JTCUSTOMS

[blades]

Valid point. Years ago when I was machining for a living, I made the jump from a manual Bridgeport to a 2-axis CNC mill. I struggled with it for a week or 2, bemoaning the fact that I could do a simple job in 1/2 the time on a manual Bridgeport. Very soon though, I was at the point where I could do a given job twice as fast on the CNC mill that I previously could have on the manual mill.

[Fastest1]

We all are resistant to change. I still try to fight drawing a part in CAD. I am not sure why as it seems pretty easy to draw them. I am generally impressed with the result (I am not a machinist) and their accuracy. It doesnt mean the part is right just that it was made as drawn or designed. In the few years since I have been tinkering I cant tell you how many times I have read about people desiring to keep manual control. Plus the difference in finish. Cnc beats manual hands down for me. You can do anything quickly via MDI or a shuttle/mpg. If using the MDI box many times I will just keep editing the commands on the fly til I have acheived the dimensions I need. I am sure it isnt the only, right or proper way but it is what I have learned to do quickly and it works.