A Couple Homemade TC Tappers (Tension Compression)

[Bob La Londe]

3/4 shank with a recess (trepan) in the bottom so they can register up against the spindle nose. I need one more preloaded and measured for 6-32. Those 3 are all pre measured for the Tormach 1100 and entered in the tool table. I run them with a drill routine, with some extra parameters set. so far I haven't seen more than 1.5-2 threads over or under run at 500 RPM. I've got all my preset styles for these set at 500 RPM. When I need to tap a hole I just pull up the style I have preconfigured in CamBam and set the depth.

Now I need to figure out how to make some in KWIK-200 shank.

[hammernanvil]

They look nice, I’m guessing they help to prevent
breaking taps.?

[Bob La Londe]

They look nice, I’m guessing they help to prevent
breaking taps.?

On a machine with an encoder on the spindle that is capable of "C axis" control of the spindle they do what is called rigid tapping. You can mount a tap firmly in the spindle and exactly coordinate movement in and out to tap a part. You may have heard people ask, "can it do rigid tapping?" when talking about a machine. I'd guess most CNC mills can't do rigid tapping. They have a VFD for frequency speed control for a 3ph AC motor or a DC motor with a pulse speed control. They may or may not have some form of feedback to check that RPM is "pretty close."

What you use these tappers for is a sort of fake simulated rigid tapping. You can usually calibrate the spindle to be pretty DARN close at one particular RPM. In my case I chose 500 RPM. Its slow enough that I can watch a bad outcome and cry in real time. Its fast enough that the spindle motor starts to develop some real torque. In low gear on the Tormach I can tap mild steel and I can tap aluminum in high gear. The springs which provide some vertical stiffness allow the tap to go forward or back a little if the feed rate and RPM do not match up perfectly. Like I said its pretty darn close though when the tap has come up to full speed. Sometimes I see a tiny bit when the tap starts in the hole, but usually when I notice some over run or under run is at the bottom of the hole when the spindle reverses. By having some travel under tension or compression it allows the spindle to slow down, stop, and reverse and retract from the hole without breaking the tap. Its a coordinated dance, but its not perfectly coordinated. The tension and compression nature of the tap holder allow some leeway while it does its thing.

You really should only tap through holes this way. Preferably with a spiral point tap or "gun tap." I have tapped blind holes, but I do not recommend it. I have only done it after making a few test runs and in holes where I could drill them excessively deep to allow for any over run during transition. Blind holes of course should be tapped with a spiral flute tap which draws the chips out of the hole. Spiral flute taps do take more tapping force than spiral point taps so its always a bit scary. Often I just do those parts on the drill press with an auto reversing tapping head that has a clutch in it. I only do blind holes on the machine if there are a bunch of them, and its worth my time to do careful testing first.

If you are interested I'll pull a sample line of code for the operation from a program where I use it, but basically you do this:

RPM/TPI = FEEDRATE
Pause at the bottom of the hole based on experimentation. Usually some fraction of a second.
Reverse out at the same FEED and RPM

Lower RPMs are more forgiving. 500 is good for me on the Tormach. I need to make some changes on the Retrofit Hurco before I would consider using them on it, but its on my todo list. The Hurco has a 5HP spindle so its got plenty of torque at low RPM.

This video shows TC tapping near the beginning and end of the video. It does have a company logo near the end so stop watching when I say "that's it" since advertising isn't allowed on this forum.

[skunkworks]

This is rigid tapping... Didn't time start... 8 mins in is the tapping.

We used floating tap holders on the k&t with the 60's controller...