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Electronic Leadscrew for the New Lathe

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vtsteam:
Russell I'm going with 10 tpi leadscrew because I still think of this as a manual lathe, and there will be a leadscrew crank with a thousandths dial at the tailstock end, Just like the Gingery. On a 12" leadscrew, 1/2" x 10" acme allthread is available and just right. I'll use the same on the cross slide, etc. I don't care about handedness so it will all be RH thread. Cheap and easy.

I'm thinking a 3x reduction on the spindle motor, which is a treadmill type rated at 6000 RPM. I'll probably put the encoder on the motor. That will give a theoretical max spindle speed above 1000 RPM probably. I'll probably go with a 3 to 1 reducton on the leadscrew.

This combo should give enough resolution for threads down to 4 tpi, and yet keep the encoder disk printing relatively coarse. That could be adjusted either way on both ratios to hit whatever works out to a happy medium. I think there will be a sweet spot balancing thse for this specific lathe/moto/leadscrew -- but might take some experimentation.

BTW the motor comes with a good sized flat faced flywheel, which could serve as an encoder mount. I wasn't going to leave it on, but maybe it would also smooth out cutting as well as hold the encoder, so it's a possibility.

I could use help on choosing a suitable reflective pickup (ideally working with paper and ink, one face) and how to buffer or condition the signal (if necessary) to the step pin of a typical stepper driver.

vtsteam:
I spent a lot of time today with a spreadsheet I made up to try what-if scenarios for different ratios on the leadscrew/stepper and the encoder/spindle. I used a thread range of 4 to 40 tpi.

My conclusion is, with that large a range your encoder disk needs either too many lines at the low pitches to make easily, or too few lines at the higher pitches for thread accuracy.

You could manage it by changing the reduction ratios for the leadscrew/stepper and spindle/encoder into 3 ranges with three sets of encoder disks, but this starts getting cumbersome.  It's a little better than change gears but not enough, in my opinion to justfy it on my own lathe.

I was only working with simple straight encoding, not quadrature, and that is pretty limited in range for a computer disk printable on a computer printer. That's likely the problem.

The alternative I came up with which would definitely work, and is more electronically conventional, is just use  a single encoder of reasonable resolution, and instead of swapping disks for different pitches, use a divider circuit to generate the pulses to the stepper driver.

Some dip switches to set the pitch would work -- all the thread pitches between 4 and 64 could be set in 5 bits (5 switches) for input Make it 6 switches to include direction. And no screen necessary, since the switches are indicators themselves.

This is approaching what a computer does, but you don't really need a computer, a screen, or keyboard.

 If you do use a single board computer for that, not much of one. It doesn't need a graphical OS, keyboard or even a LED dsplay driver. Basically it just needs to poll the switch state and do the dividing and output the stepper pulse. So I need 6 input lines and two output. Actually, the direction output could just be a switch -- no need to process. So okay, 5 input lines.

Because the pulse triggering is the encoder on the spindle, there is still the protection in case of spindle stall.

You could either make up the divider by discrete IC's or use something like a pic or arduino board.

The one question I have is the ability to do the input, dividing, and output fast enough. The throughput required for say 100 rpm spindle speed and a 1000 pulse per revolution encoder would be about 1.7 khz. Seems like integer math could be used for this to keep it fast.

I might try this -- I don't have the know-how to do it in discrete components, but maybe I could learn enough about Arduino programming to do it with the Uno board I've got.

Basically I'd need a dip switch and an encoder.

From my spreadsheet it looks like a 1000 line encoder run at twice spindle speed, in conjunction with a stepper with 4 to 1 reduction to the leadscrew would give reasonable threading accuracy for the full range of pitches I mentioned.

sparky961:

--- Quote from: vtsteam on March 30, 2015, 11:14:48 PM ---The one question I have is the ability to do the input, dividing, and output fast enough. The throughput required for say 100 rpm spindle speed and a 1000 pulse per revolution encoder would be about 1.7 khz. Seems like integer math could be used for this to keep it fast.

--- End quote ---

I'm running software quadrature decoding of a 4000 count servo encoder using a state machine on a "Teensy 3.0" board ( http://www.pjrc.com/teensy/teensy31.html#specs ).  It uses 2 pins as interrupts to trigger state changes and it's astoundingly fast for what it is.  I've implemented error checking for bad state transitions, so I'm fairly sure I'd know if I were missing pulses.

I added a quick conversion to show RPM in my code and it looks like I'm doing 1750 RPM and still maintaining an accurate count.  This is all in development and yet to be verified, so take everything here with a grain of salt.

Short answer: I think you're alright

awemawson:
High count Chinese quadrature encoders are amazingly cheap on eBay

PekkaNF:
You probably know this one allready:
http://www.ni.com/tutorial/7109/en/

Short discussion about creeping count...
http://www.dynapar.com/Technology/Encoder_Basics/Quadrature_Encoders/

What is important here is to note that two channel encoder (quadrature encoder) used as two channel (bidirectional information) and coupled right to counter (hardware or software) produces reliable counting. Too simple signaling could produce few (or sometimes many) erroneous counts when signals are not kept stable, even if they would work in ideal world.

To get big enough disc to lathe spindle and guarding it well (if optical encoder is used) is bit of a problem.

As mentioned before cheapest high count solution would be to use timing belt to "upgear" the encoder, it is used a whole lot in industry. Only few things to check is that encoder axial bearing loading is not exceeded, rpm is not exceeded and pulse count is not exceeded. You even could use "index" pulse for rpm-display.

Pekka

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