I'm new to all of this and just doing my retrofit right now.  I would like to add a sensor to the machine for spindle speed feedback to the VFD and to Mach for the readout and I got to searching and reading some tapping threads. 

Regarding tapping items on a milling machine, there seems to be a couple of ways to do it.

1) Clamp item in chuck and spin it (mount your cutter on the table) - only work for external threads

2) Clamp item to table and spin tap (feed tap into hole at precise feedrate) - only work for internal threads

For item # 2 if you were going to tap 100 holes, it would require you know spindle speed obviously and have to spin the tap, feed into hole, stop the spindle.  Reverse the spindle, retract from hole, stop the spindle.  Restart spindle in normal direction and repeat 100 times.  This seems like a lot of energy being used to start and stop the spindle over and over.

Would a tapmatic style of device (which I just learned of today) be a better solution as I don't believe the spindle has to stop spinning in the same direction?

Maybe it's not a big a deal to reverse the direction of a spindle that often (like I said I'm new to all this), seems like it would heat things up in a hurry?

Radio, even without spindle feedback you can tap on mills with Mach very effectively without even having to purchase a tapping head. I use a floating tap holder (www.maritool.com has them) to make up for the inaccuracy of the spindle control, which in my case is just my breakout board sending an analog signal to the VFD to control the RPM of the spindle, along with stop, start and reverse commands.

Below is the tapping macro I wrote to use on the machine. I use spiral point taps and it works great, threads from 4-40 up to 3/8"-24 on both 6061 Al and mild steel. Don't sweat the reversals on the spindles, VFD's are designed to do that and smoothly ramp the motor speed up and down, you can set how fast you want this to happen.
The underfeed parameter should be tweaked for your machine, 9% is what works good for mine.

Would a tapping head be better? Someways yes, someways no, probably faster but keep in mind you lose a fair amount of Z travel with the long tapping head.

Paul T.

(Filename: TapHole.tap)
(Taps the hole at the current X, Y location to the specified depth)
(Thread Pitch in Turns per Inch must be set)
(Z must be set at zero on top of workpiece)

G20 (Units: Inches)
G40 G90

(Set Editable Parameters)
#1=0.36      (Set Depth)
#2=32.0      (Set Thread Pitch in Turns per Inch)
#3=140      (Set RPM)
#4=0.1   (Set Rapid Height)

(Internal Parameters)
#103=0.91      (Underfeed, 0.91 -> 9 %)
#104=[#103 * 1.0/#2]   (feed rate adjusted for under feed)

G0 Z#4        (Move down to rapid height)

M98 P1025 L1 (Call Tap subroutine)

M30

O1025 (Subroutine to Tap)
G95 F#104   (Set to ipr mode and set feed rate)
M3 S#3 M8   (Start Spindle and coolant)
G1 Z[#103 * #1/-1]   (Tap down to compensated depth)
M5         (Stop Spindle)
M4         (Start Spindle in Reverse)
G1 Z#4      (Raise spindle at feed rate to rapid height)
M5 M9      (Turn off spindle and coolant)
G94      (Go back to ipm mode)
M99       (Subroutine Return)

Thanks Paul,

I looked at a floating tap holder and it appears as though it allows the tap to move in and out a little ways (0.3" inwards, 0.5" outwards for the one I looked at) so that the feed rate into and out of the material doesn't have to match the rpm perfectly?  Is this correct?

I plan to use my breakout board to control the VFD as well, rpm, forward reverse, etc.  Can you perform the same tapping function with out a floating tap holder?  Would this be called rigid tapping?  Or is this not possible...  Still learning...

Rigid tapping is not possible with Mach unless you have a servo or stepper spindle motor and do a workaround by swapping the spindle to an A axis. The SmoothStepper will hopefully at some point be able to sync axis to spindle but you will need an encoder on the spindle even for that.
Hood

Just to be sure, rigid tapping is defined as having the tap mounted rigidly in the spindle?

What affect does the underfeed paramenter have on it?  Does this cause the tap to pull the holder downward a small amount while entering the material?  Thanks

Yes Rigid tapping is where the tap is solid in the collet like an endmill would be.

Hood

Radio, regarding the underfeed, yes it allows the floating holder to slighty extend as the hole is tapped. Most floating holders are meant to work that way, with more extension than compression. You want to tweak the factor so that on your machine you see slight extension as the hole is tapped. When the spindle is stopped it will extend again as the spindle comes to a stop, on my machine that's about one additional revolution, so on critical depth holes you have to factor that in to your depth setting, but its very repeatable.

Even on full size VMC's with accurate rigid tapping some machinists use floating holders on smaller taps to minimize breakage issues.

Paul T.