Sherline Lathe Conversion ?

I hate to only show up with questions every few years, but what are you going to do? I have LinuxCNC on a converted Bridgeport size knee mill but tend to appear only when an upgrade is necessary. I should have more time for this hobby.

But I am now converting a Sherline Lathe to CNC. I've installed a Sherline CNC conversion kit and two NEMA 23 steppers. Also have in hand a Gecko G540 Driver board. I have the parts for a 48Vdc power supply. Finally, an Intel Atom Dual-Core D525 based computer for the mill and this lathe. I haven't hooked everything up but have high hopes. The purpose is to see if a CNC lathe is worth the trouble in a hobby shop. I'll find it really isn't for one off work, that the Sherline works great, or a lathe is so useful I'm forced to get a 8-10" lathe and convert it.

So, my first question (likely of many) regards the spindle encoder. The least expensive encoder seems to be a slotted disk with slotted optical switches. OK, the LinuxCNC "Set Up Wizard" seems to configure an encoder using one channel, say A, plus an index pulse. I think we can also set up a full quadrature encoder, A/B+index, but it seems less common and thus harder to accomplish. Will a quadrature encoder work for a lathe spindle? Does the quadrature encoder indicate direction to LinuxCNC? If so, that would give four state changes per slot, are all used? When using a single channel (A + index) does this decode to one or two indications per slot fed back to LinuxCNC?

Is there any pick between a quadrature encoder versus A+index encoding? A quadrature encoding should allow synchronization to be maintained if the spindle is moved backwards, but I'm not sure this is an advantage? Are there other considerations? The number of states per revolution can be controlled in making the disk. A quadrature encoder with say 50 slots should be equivalent to an A+index encoder with 100 slots. I'll ask about how many slots later.

Does anyone have suggestions for a slotted optical switch? Maybe a circuit diagram?

Thank you greatly for any assistance you can provide.

Hugh

Go for full quadrature, then you can rigid tap.

You need index too.

50 slots is enough. More is better. Too many is too much. You need to consider the frequency limit of the counter. But that's just arithmetic.

Andy:

Thank you for the reply. I likely can't do rigid tapping as the Sherline spindle has no reverse. But I'll go with the quadrature encoder. I think I can then turn the spindle backwards without loosing sync. I'll also learn a little about HAL which may let me put an encoder on the mill, but it needs a VFD before it has CNC spindle control.

I spent some time figuring out a Base Period for my computer when I initially set it up. This turned out to be 41666us. I may re-test but this should be OK for calcs. I think I can estimate the max number of slots with:
max # Slots = 60 / (Base Period * RPM * 4 * 2)
Base Period = 41666E-9 s
RPM = 2800
4 pulses per slot (full quadrature)
2 safety factor

If that's true I can use up to 64 slots. So 60 slots would work out well, 6 degrees per slot.

I just ordered several slotted optical switches, Optek OPB370T55. I'm weak on electronics but understand I need resistors on the LED and the phototransistor. The LED current is 50mA so, for a 5v supply, should require at least a 100 ohm resistor? The transistor "Collector DC Current" is 30mA. Using the same logic a pull up resistor 166 ohms (use 220)? A diagram of the circuit looks something like this: I finished installing the CNC conversion kit from Sherline and hooked up the Gecko 540 driver box to the steppers. Using Stepper Wizard set up LinuxCNC and have motion on both axes. I even have the correct amount of motion and backlash in the +/- 0.001" range. This might work.

I have a mess of wires all over the bench though. So I've decided to cage these in a box before continuing with the encoder (other than ordering some parts). When I get the box together I'll include a couple of pictures.

If anyone has input on the optical sensor circuit I'd like to hear it. Thank you.

Hugh
Klamath Falls, OR

Encoders can be purchased cheap on Ebay new. Check out this neat conversion Sherline Threading Encoder Mount

Hope this helps,

Iron-Man

The 50 mA LED current and 30 mA collector current are maximums, not reasonable design values

As a first try I would use 20 ma LED current and 4.7K RL

I would also put RL in the collector circuit so the transistor pulls
the output low when enabled, as this is more
likely to work with parallel port inputs that have built in pullup
resistors connected to 5V

Iron-man: Thanks. I'd run across this link before, a very nice and clean installation. I've read that the bearings on standard encoders aren't up to the side loading of a timing belt. Some are made for this but are more expensive. A lot of people seem to be doing it, so might not be a problem for light hobby use? The encoders I've run across on E-Bay are in the $200 range, a little pricey for this conversion. I suppose I'll go this way if I can't get a disk and optical switch to work. Thanks again for the info.

PCW: That makes sense. The 20mA on the LED side is near the max 30mA for high output, but not too close. And the transistor side is only showing an on/off voltage so higher current isn't useful? I'll move the output resistor. Thank you for the input, I suspect you've saved me many hours.

I'll post some pictures when the control box is together.

Thanks.

Hugh
Klamath Falls, OR, USA