Lathe conversion from stock to CNC

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27 Jul 2012 10:59 #22512 by BigJohnT
Soundreflections wrote:

Firstly having a home and origin points are confusing me a bit, is the home switch the home position and the origin a virtual point where the tool will start / end?


The lathe diagram might shed some light on that.

John

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27 Jul 2012 14:47 #22525 by Soundreflections
Thanks, yes I did have my axis’s crossed! Not sure how 115Ncm compares with 3.5NM, I suppose it is 1.1NM, which surprises me, as it is about their largest stepper. I will check prices when they get back to me and decide from there, these motors look about 1/3 the price I can get them locally.
I will look at purchasing a joystick and see if it works, thinking of setting up the PC, etc... whilst I look for the hardware.
OK, so the origin point is where I wanted my home and the home switch is end of travel. That's fine, I suppose one just makes sure everything is clear on the end of the bed and let it home. Am I right that I can have a separate home switch, a bit closer to the head? I feel really silly asking these questions, but I probably will have all switches set up and running before I get any motors, drivers and breakout cards, so I want to be sure I am doing it right. It does look like the home position is any soft point I choose, so that helps, as long as I do not have to hit home switch every time I home in a session.
Thanks
Peet

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27 Jul 2012 15:03 #22528 by andypugh
Soundreflections wrote:

Thanks, yes I did have my axis’s crossed! Not sure how 115Ncm compares with 3.5NM, I suppose it is 1.1NM, which surprises me, as it is about their largest stepper. I will check prices when they get back to me and decide from there, these motors look about 1/3 the price I can get them locally.


Slightly closer than the US is: www.zappautomation.co.uk/stepper-motors-...tors-c-9_159_42.html
You could also try eBay. global.ebay.com/New-Nema-23-dual-shaft-s...19/140750726667/item (for example)

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30 Jul 2012 12:41 #22605 by Soundreflections
Thanks Andy, I will look into them, depending how other enquiries go. I have a question though on a viable startup options on a budget. What options / speed can I get using the parallel port directly? I am thinking of starting that way, and then expand with a break out board later.
I think I would need step / dir for X & Z axis, 3 pins for shaft encoder, I assume I set up a serial connection to my VFD? Then I just need limit / home switches and E-stop?
I can get some +-60 hole disks to use for shaft encoders, I am thinking of setting it up as a remote sensor, with a belt driving it, so I can choose the gearing as appropriate.
Is it better so shelve the whole project till I can afford it all, or do step for step, expanding capability as I go? Am I just going to get frustrated with parallel port speed limit?
Thanks
Peet

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30 Jul 2012 14:11 #22606 by andypugh
Soundreflections wrote:

What options / speed can I get using the parallel port directly?

A PC with good latency can produce steps at 50kHz, which is 3750rpm on a 200step stepper at 4x microstepping. In practice you will almost never get a stepper to spin anything like that fast.
You _might_ find that you have a problem with "granularity" in the step pulse speeds. You end switching from a step every 10 thread-periods (375rpm) to every 9 periods (417rpm) instantaneously and the stepper might not be able to follow that increase. I ran my lathe perfectly happily direct from the parport though. I bought a 7i43 because I wanted more IO pins not for any step-rate ssues.

, I assume I set up a serial connection to my VFD?

That's one way. Or you can use a PWM output from the parport into a capacitor/resistor to create an analogue control voltage (which is what I did). Ideally this should be opto-isolated s that the 5V p-port can control the isolated 10V on the VFD. You need a totem-pole opto for this (source/sink to discharge the cap as well as charge). uk.rs-online.com/web/p/products/734-4962/ is working nicely for me.

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19 Aug 2012 14:07 #23395 by Soundreflections
I now have some steppers (they look a tad small now I have them, but I will see once I get them working, then decide my next step). I have decided to build stepper drivers, rather than buy, as there is a budget, I am also quite well into electronics, so using the L297, with 2 L298s on the output I can get 4A per phase. If it all fails I think I paid about ZAR700 for the drivers, optocouplers, infrared sensors for encoder..., so not a huge loss if the drivers fail to work.

I assume a config of all X limits and home on one pin, Z limits and home on the other, then the last 3 inputs for the spindle is OK for a start, with X dir/step, z dir/step, PWM as output? Pity I don't need more outputs! I am scrounging the 5 inputs, yet sitting with unused outputs.

Will post as I get further.

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19 Aug 2012 14:21 #23396 by andypugh
Soundreflections wrote:

I assume a config of all X limits and home on one pin, Z limits and home on the other, then the last 3 inputs for the spindle is OK for a start, with X dir/step, z dir/step, PWM as output?

That sounds reasonable.

Pity I don't need more outputs! I am scrounging the 5 inputs, yet sitting with unused outputs..

Well... There might be a way:
Read the third paragraph of section 1.1 here
www.linuxcnc.org/docs/2.4/html/hal_paral...ort.html#sec:Parport
then carefully read footnote 1.
If you are lucky you can use some of the pins as both inputs and outputs. Or, more accurately, you can read the state of outputs and use them as inputs.

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21 Aug 2012 07:03 #23458 by Soundreflections
Thanks Andy, this makes me very optimistic! I may be able to invert the I/O, then have 4 outs, 13Inputs. I can then step/dir X & Z, get a serial or usb card for my VFD and have independent switches and encoders.

I can get www.logitech.com/en-us/gaming/joysticks/302 quite easily, I think. Would that be a good enough manual JOG input? Would I be able to program something like course on the joystick axis, fine step using the 4 top buttons? Hoping I can assign buttons in any configuration, not sure which buttons are really needed yet.

I have started building my stepper driver, though reading an article on CNCzone about stepper voltages it sounds like I may need much more than the 4A per coil I had thought I needed. I will start with 4A, see how much torque I get, and then see if I need to go with high current discreet drivers.

I wish I had a indexed rotary table, so that I can cut gears. At this stage I am planning to turn adaptors, then use my lathe's change gears between stepper and spindle. That has the advantage of allowing me to select different gear ratios, though I would have liked to cut gears specifically. I am still to figure out an easily adjustable motor mount, allowing spacing for different gear ratios, as well as an easy way to disengage, making manual operation easy.

Then I am yet to find a 3D CAD, and G code converter! Seems funny to do the CNC conversion, then find a way to generate G code!

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21 Aug 2012 07:34 #23461 by andypugh
Soundreflections wrote:

I can get www.logitech.com/en-us/gaming/joysticks/302 quite easily, I think. Would that be a good enough manual JOG input? Would I be able to program something like course on the joystick axis, fine step using the 4 top buttons? Hoping I can assign buttons in any configuration, not sure which buttons are really needed yet.

I use a Playstation-style USB joypad on my machine. It is possibly more convenient than a joystick as it is hand-held so you can stand anywhere he view if the tool/work is good to operate it.
I have it set up so that various combinations of the back buttons set the jog speed range, and the analogue joystick sets speed and direction.
See: wiki.linuxcnc.org/cgi-bin/wiki.pl?Simple_Remote_Pendant

I wish I had a indexed rotary table, so that I can cut gears. At this stage I am planning to turn adaptors, then use my lathe's change gears between stepper and spindle. That has the advantage of allowing me to select different gear ratios, though I would have liked to cut gears specifically. I am still to figure out an easily adjustable motor mount, allowing spacing for different gear ratios, as well as an easy way to disengage, making manual operation easy.


I am not sure what you are meaning here, but gear-cutting on a lathe isn't that easy, as you lack one of the important axes.
You seem to not be abstracting far enough into software-space though. The answer to what you want to do is likely to involve an encoder wheel on the lathe spindle (which could simply be some proximity sensors looking at a gear on the spindle) and then you can set the speed ratios in HAL.
This is my gear-cutting setup which uses a hme-made encoder on the spindle to synchronise a rotary axis.


And here are some pictures of how I added sensors triggered by an existing gear to add an encoder to my other milling machine:
picasaweb.google.com/lh/photo/50sU9lCdX6...pFm0?feat=directlink
picasaweb.google.com/lh/photo/wE4cF0uBYu...pFm0?feat=directlink
picasaweb.google.com/lh/photo/zH4KytVy-Q...pFm0?feat=directlink

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21 Aug 2012 09:54 #23465 by Soundreflections
I was referring to the "motor to thread" interface. My current intension is to remove the adjustable dial behind the handle, which I have not found too helpful (Probably not figured out exactly how to read them), then fit a gear in it's place. I then want a gear on the stepper to mesh with it. Making more sense, or not the confusing part?

I figure that if I can get an indexed head of some sort I can make a fitting to have my Dremmel cutting at 90deg. Probably much more complex than I envisage, but indexing accuracy seems critical on gear cutting. Would it be an idea to have a stepper, with a hectic gear ratio, driving the spindle shaft? I will make a shaft encoder, 3 sensors, on the spindle, so if I can give LinuxCNC accurate control of the head I could perhaps run gear cutting type applications on my lathe. Sometime I want to make a tool holder for my Dremmel. I suspect I can get very nice thread if I cut with the Dremmel and an appropriate cutter, rather than a chisel?

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