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Electricuting the lead screw

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PK:

--- Quote from: eskoilola on April 18, 2018, 02:06:04 PM ---
--- Quote from: sparky961 on April 17, 2018, 10:19:52 PM ---If you're dreaming of rigid tapping in the future, a spindle encoder with a few thousand counts per revolution would be in order.

--- End quote ---
If the spindle is running at 1500 RPM there will be around 100.000 interrupts for the uC to be served each second. I know from experience that a PIC18F runniing at 64MhZ will be capable of serving around 30.000 intererupts per second if those interrupts are fast executing, simple things like advancing a counter. I know for sure that 100.000 is way too much.

--- End quote ---

This is the old chestnut that everyone struggles with. Rigid tapping aside (in that case you use a servo drive for a spindle motor) , You need 1 pulse per revolution to CNC thread on the lathe.
All the software and hardware based controllers are set up for it...

Re servo motors.  You buy a Stepper motor and a drive or you buy a servo motor and a drive. The interface is the same, the price is pretty close, the servo motor spins 3 times as fast as the stepper.   I spent many years building my own stepper and servo drives. Eventually I came up with some OK ones:
http://www.caswa.com/cncathome/drives.html
and
[ Invalid YouTube link ]

But there are now so many cheap options on the market that are actually pretty good that it's not making them yourself (and I say that owning an electronics company with an assembly line).

One piece of generic advice, DIY CNC is the best example I've come across of "The cheapest way to do it is properly". I can't tell you how many times I rebuilt that lathe around some new motor or drive that I got cheaply. I undoubtedly spent thousands... 

JHovel:
@eskoilola
if you haven't read the link I provided, there is provision for and a design for an "electronic half nut" for this ELS controller. It works just as you describe: you move it a certain amount in either direction and it moves the apron at a preset cutting speed. You can move it a bit further and it accelerates progressively to a defined 'jogging' speed. Pretty much like you would use the apron handwheel. He used the analogue input of the control chip to achieve this with a simple potentiometer. I used an electric wheelchair handle with pot to do this.
Finer control is provided by the manual pulse control on the panel. The whole thing is very functional.
It also has a port to connect it to a full CNC controller, should you want to use MachIII, G-code or whatever to operate the lathe.
The ELS by itself just gives you better control over feeds and cutting depth, and includes all common threads, all common tapers and taper threading in memory. It withdraws the cutting tool at the right time and allows nice consistent cutting and threading at the correct speeds for the tools used. Cutting threads a high speeds with nice fine depth of cuts automatically is very nice - and a bit exciting when they finish against a large shoulder!

eskoilola:

--- Quote from: JHovel on April 19, 2018, 05:38:37 AM ---@eskoilola
if you haven't read the link I provided, <snip>

--- End quote ---
Actually I DID read the article(s) and this seems to be somewhat what I want to do.
- I want to maintain the manual controls touch and feel as much as possible
- I want to have the thing to be a "fly by wire" - probably G-code compliant
- I want to do this as far as possible with the original controls
- I want to have sex with my lathe ....

What I would really hate is a big bunch of buttons and some reverse polish notation aka Sinclair scientific calculator from the 70's - it was a really sexy gadget at it's time. Had one and hated it all the way.

For example. At the moment there is a automatic feed on/off lever. That lever would be really easy to modify to be a forward/reverse feed control. On center position (positive lock) it would allow manual control with apron handwheel or the handwheel at end of the lead screw. When away from the center the feed would be equally faster to the desired direction.

Same kind of thinking would go with the cross slide. This time the feed would be controlled with the ex-half nut lever which would now be repurposed for that function.

That Slo Syn motor can do (per spesification) 10.000 steps / second (aka 2500 Hz) - quite amazing for a large stepper like that. I am pretty sure that the torque is an overkill - which is not bad. That 10.000 steps/second amazes me as I also have done my fair part of stepper drivers. The best ones had a switching power supply with constant current output. This means that the voltage rises with the drive frequency. These could easily do 2.000 steps/second but after that most of the steppers went berzerk. That sort of speeds also require acceleration time. Assuming that the ball-nut I have has a 3 mm pitch (same as the current lead screw) the max speed would be then 150mm / second. It is enough for most purposes. I have never measured the pitch of the ball-screw but if my memory serves me correctly - it is more.

When it comes to accuracy there is one unspoken element in the lathe, the compound. You figure it out from there. Auto-feed in the compound ... Hmmmm ..... Maybe a angle sensor and whatnot.

As PK very nicely put it - all this is really cheap - one just needs 10.000 worth of equipment to do it  :beer:

Further more - all this is purely academic as long as I am on my assignment here and my home is there. That being said - well thought is halfway done.

eskoilola:
Designed the schematics for the spindle controller.
This controller will take care of .... spinning the spindle  :loco:.
After a lot of thinking i ended up in the following configuration:
- there is a 60 step quadrature encoder in the spindle and an index
- there is another 60 step single phase encoder in the gearbox input shaft (this lathe has gears too). This encoder is used to detect the gear
- inputs for an emergency switch
- controls the VFD using MODBUS
- connects to the host with a proprietary protocol using RS485
- time critical signals have their own bus, for example the virtual index, sync marker and emergency stop

With this configuration a virtual Index can be moved at any location from the physical index with 3 degree steps. The virtual index is fully independent from spindle speed.

The Huanyang VFD has an inbuilt PID controller with configurable P, I and D factors. In order to make this work the controller must be aware of the gear ratio and thus the inertia of the rotating system. This will be fun to configure  :dremel:

There is a schematic of this controller HERE. In case someone is interested.

This, of course requires a program to work  :dremel:

PK:
One pulse per rev. It's all you need.

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