TurboCNC G76 confusion

[jihe]

I have just finished the spindle encoder for my newly cnc converted mini lathe. It seems to work fine and I can thread with G33 without problems.

G76 is giving me a headache though.

This is the example from the manual thats supposed to thread a 1/2 20 tpi (first the original, then my metrified):
G76 X-0.210 Z-1.25 K.040 D0.003 F0.050 A60
G76 X-5.334 Z-7 K1.016 D0.0762 F1.27 A60

What happens on my lathe though is that it seems to turn an ID thread. It retracts the tool in the direction of the _workpiece_ when Z-1.25 is reached. If I remove the minus from X, it adds X to the feed for the first pass :eek:

I have tried many other X values but I don't really see a pattern in what it does, except for occasionaly crashing my tool really deep in the material :(

I think the main problem is that I don't really grasp what the X value is supposed to do. It isn't even mentioned in the manual.

In this thread (http://tech.dir.groups.yahoo.com/group/turbocnc/messages/10089?threaded=1&m=e&var=1&tidx=1) Dave K writes:

X is the root or minor radius (or diameter) of the thread
That sounds reasonable, but doesn't explain my weird results.

The lathe is setup so that X+ is away from the operator and my tool post is front mounted. I THINK the example from the manual would work for an external thread if I had a rear mounted tool post. So if TurboCNC had a setting where I could tell it where my tool is located, I guess it would work. But I can't find such a setting.

When I read my post I realize it's not the clearest. But I would really appreciate any pointers to information. Maybe someone has examples of G76 lines actually used with TurboCNC?

[jihe]

The lathe is setup so that X+ is away from the operatorWhen rereading the TurboCNC manual I now see that I have this set up wrong. The convention for X is apparently that MINUS should be away from the operator. I'll correct that in my setup and try again.

[MuellerNick]

The convention for X is apparently that MINUS should be away from the operator.

It might be a little confusing but it is not. :)
The direction of X depends on the type of lathe. Slanted (Swiss type) or conventional bed. Tool behind or in front of work.
Bot it is very simple. X+ goes away from the center line. Bigger diameter, bigger X.
That's all.

Nick

[jihe]

I have now successfully, with G76, turned a male M8 thread that mated with a nut!

But not by following the directions in the manual, or the developers instruction in the yahoo group.

I had to enter a value for X that was identical to the K value.

Perhaps, or very likely, there is something I'm doing wrong with the initial position of the tool. I zero on the outer diameter and start my G76 cycle with X in that position.

But at least now I have confirmed that everyting is working right, so I can go on with a more permanent way of attaching the encoder :D

http://fotografi.se/forumbilder/spindle-encoder.jpg

[MuellerNick]

I zero on the outer diameter and start my G76 cycle with X in that position.

But X shouldn't read zero then. If you touch it off at say 20 mm diameter, X should read 20. Be shure to have selected the right tool offset.

Also note, that X is the diameter, not the radius.


Nick

[John Stevenson]

Long time since I have used TurboCNC as a lathe program but i can half remembering having to alter the coding for the X axis as it's reversed from the standard mill version which is downloaded.

Best guy to answer this question is Eddy at motorworks as he uses TCNC for threading all the While.

EDDY YOU THERE

[jihe]

But X shouldn't read zero then. If you touch it off at say 20 mm diameter, X should read 20.Ok, sounds reasonable. That way the DROs actually shows something useful even when you have departed from zero :-) I really need to get a book or website where I can read about the fundamentals of CNC, but I haven't really found anything good.


Also note, that X is the diameter, not the radius.But, isn't that depending on if the lathe is in "diameter mode" vs "radius mode"? The TurboCNC manual specifically says that the examples assume that the lathe is in radius mode. And when I tell X to move 1 mm it shaves off 2 mm on the diameter - that means I'm working in radius mode, right? Are there any good arguments for the choice between them?

[MuellerNick]

Are there any good arguments for the choice between them?

Your drawings are in diameter, the caliper is in "diameter mode", the mike doesn't know how to divide by two, you will go nuts when you go back to your manual lathe*).

Can't help you with a good tutorial. I have been looking for one too. No luck.

*) Except you have one, where the X-dial is in radius. I had one, that's too stupid to work with.

Nick

[jihe]

Ok. The developer seems to have a similar lathe as mine, and they have graduations that give 1x infeed, so I guess it was natural for him to threat radial infeed as the default choice. But if the rest of the world is using 0.5x graduations I'll probably relearn to make the cnc learning curve less steep. Right now my manual lathe has no graduations at all since it has become my cnc lathe, so that won't be a problem :-)

As to TurboCNC I'm still evaluating it. I really like the idea to be able to use my old HP 800ct laptop with it's tiny footprint. With Mach 3 I'd have to allocate space for a regular pc with separate keyboard and display.

[macona]

But with mach you have wizards which make simple operations very easy, especially threading. No messing with Gcode. You can even pile the sections on top of each other to make longer routines.

Mine running code made mostly with the wizards:

http://www.youtube.com/watch?v=YKb2g5xalEk