ridgid tapping with servos

[miljnor]

I figured this is a servo question so it should be in the electronics section

was wondering if I wired a servo as my spindle with full encoder and just gave it an driver to drive the axes. This should be capable of syncronizing two axis together just as it would to perform a xy move.

shouldn't it??

[Al_The_Man]

The way it works on most systems designed for rigid tapping, there is of course an encoder on the spindle, but normally the spindle is an imprecise positioner, i.e. can vary with load, so the Z axis will synchronize to what ever rate the spindle is running. If the spindle is created as an axis then in theory it should be like an interpolated move, I have never tried it however.
Al.

[JFettig]

from what I have found out, that you need to have a controller that supports it, Mach software doesnt yet. I went out and bought a tapping head.

Jon

[miljnor]

No controler need, other than a servo controler, becuase you would know the lead of the Thread, Lets say (for arguments sake) 1" 5tpi so the spindle would have to rotate 5 times as the Z moves down 1". This would have to be possible otherwise you couldn't machine anything.

My question here is if I use an axis as the spindle controler can that same controler just make the spindle spine at say 4000 rpm? and what command would mach3 or any other controler need to make this happen.

I know that some spindle controlers use step and direction but what makes the servo stop when it gets to the right spot? I assume the drive does becuase the driver boards have the encoders going to them. And if so how do you overide this because essentialy you have an axis with unlimited travel.

hope this is understandable.

[Al_The_Man]

The problem is that although the spindle is being controlled the Z axis is still the axis feeding with relation to the spindle, this means that for an interpolated move you would have to command the spindle to move an imaginary distance based on its Units set by pulses/rev and the Z would have to arrive at this imaginary distance at the end of the G01 move. It could be tricky to say the least. Normally in a full CNC system the encoder Marker pulse is used to sysnc the axis. control the Z axis accordingly.
Al.

[JFettig]

You could also set up the spindle motor as a c-axis and do something like this:
G0 Z0
G1 Z-1 C1800 F1000(assuming feed per degree and aproximately and 5tpi)
G1 Z0 C0


Jon

[miljnor]

I will have to try it but will need to actually finish the machine first. ITs sooooo close I can taste it.

[DareBee]

As Al said if you dont tie the Z onto the spindle rotate encoder and have it vary depending on what the encoder says you will probably have issues (unless you have insane HP).
My 10 hp motor slows enough on the start of a large tap that it is visible (untill it puts the nuts to her again) this would either break the tap or strip some threads if the Z feed didn't compensate.

[Geof]

So far you have been discussing running the tap in. Which is a significant enough problem in itself but at least you can give the machine space above the workpiece to get everything synched before the tap enters. At the bottom you have to stop and reverse in total synchrony and that could be the biggest challenge. I am always impressed by rigid tapping especially when you program a retract speed several times faster than the tapping speed; both the spindle and Z servo are accelerating throughout the length of the retraction.

[miljnor]

I guess it would realy depend on the control software as to the synchronization. So i wound how tight Mach3 is programed on axis sysc'ing?

Thats probably the limiting factor

[unterhaus]

It's not that hard as a control issue, but the problem is integrating it into a machine controller. It also requires feedback, and I wouldn't trust Mach under feedback conditions. This is nothing against the software, which is a wonderful piece of work, but I've watched Bill Gate's little treasure decide that something was more important than reading an A/D converter for a few seconds, resulting in $2500 of smashed equipment. Linux will do the same thing in it's stock configuration. That's why EMC uses Real Time Linux, which has very good latency and deterministic delays. There are real-time extensions of Windows, I'm running one right now and it works great. The only problem is that a license is considerably more than the price of Mach. When the new Gecko GRex is supported by Mach, then hard tapping seems doable.

Of course, tapping attachments come up for auction on EBay pretty frequenly.