Thursday, 11 June 2020

Rigid tapping vs tension compression vs Tapmatic??

What's with the tapping?
I've got a lot of tapped holes to make for the myriad fixings I've designed in the 4th axis assembly. They need to be placed and tapped accurately and square if it's to fit together properly.

I've got a tension compression head, a Chinesium Tapmatic clone (reversing) and ... well on The Shiz I have a Centroid Acorn controller that can't do rigid tapping without a spindle mounted encoder. So that rules out rigid tapping. It's a question of either getting the Tapmatic clone working or sticking with the tension compression head.

Fusion 360 seems to offer the option to control both the infeed and the retract feed separately, so that would appear to suit the Tapmatic / self-reversing tapping head. 

That's rather helpful, since the Tapmatic heads tend to reverse at twice the (rotational) speed of the spindle. Which means that the retract feedrate should be twice the infeed. One key advantage of the Tapmatic concept is that you don't need to start, stop and reverse the spindle - the spindle continues at constant speed throughout.

Connecting it up:
One drawback of the Tapmatic head is that it requires a reaction arm to enable it to reverse. That could be a PITA, as there's nothing easily accessible to mount said reaction arm on.

In fact I find there's enough of the quill nose poking out in the parked / homed position to get a pinch collar on there. I bought a spindle speeder from JohnS a few years back that fitted his Beaver mill. I haven't used it yet, as it's imperial (NMTB40) rather than the metric of my power drawbar (ISO40). However, being a speeder it also has a reaction arm and came with a pinch collar thing for his machine. Turns out his quill was a couple of mm smaller than my Shizuoka quill, so I was able to bore it out on the lathe and Bob's your auntie - I have a solution. John must be smiling on us from his workshop up there.

I bodged a short length of steel strip into a reaction arm and I screwed a short pillar into the collar to pick it up. So I can remove the pillar when not in use and the tapping head is otherwise self contained. The pillar is made of loominum as it was ready to go but I may change it for steel at a later date if this works out.


Installed on the machine:

Looking good so far?

The problem:
So it turns out that when you generate the g-code from within Fusion 360, it simply outputs the std G84 tapping macro. WTF??

O08002 (Tapping trial)
(T2  D=4. CR=0. - ZMIN=-4. - right hand tap)
N10 G90 G94 G17
N15 G21
N20 G28 G91 Z0.
N25 G90
(Tap right)
N30 T2 M6
N35 S600 M3
N40 G54
N50 G0 X10. Y10.
N55 G43 Z6. H2
N65 Z5.
N70 G98 G84 X10. Y10. Z-4. R5. F420. << (6000rpm x 0.7mm = 420mm/min)
N75 G80
N80 Z6.
N90 G28 G91 Z0.
N95 M30

That doesn't allow different in and out feedrates. There are other drilling / boring / threading macros available within the "Drilling" cycle menu in Fusion (and within the Centroid commands) but they don't offer any further choice. You have a choice - either retract at the same rate as the infeed or retract at the rapid speed. Bollocks.

The Tapmatic would work but the head has to soak up a fair bit of axial movement as the spindle withdraws at the wrong retract rate. May be fine in most cases but for longer holes and coarser pitches it could get a bit iffy. Other issue for my machine is that the reaction arm collar mounts on the threaded nose of my quill that holds the main bearing in place. During the retract, the reaction arm tries to unscrew it, which wouldn't be ideal if it succeeded.

Given these and the fact that I also had a tension compression head and a fair selection of collets, at this point I changed my focus from the tapmatic method. Although I got it working I've now put it back in the cupboard and may bring it out later for use with the Bridgeport.

Tension compression, then?
The tension compression approach is simple enough to implement, the main difference to the Tapmatic being that you have to stop the spindle and reverse it, ideally in synch with the infeed and retract moves. The std G84 works fine as long as you don't try to run it too fast. The main difference between this and rigid tapping is that you don't need a spindle encoder.

My tension compression head seems to have around 8-10mm axial movement either side of the unloaded position. But if you are running slow accel and decel times in the VFD, there can be a fair amount of overshoot, so that the tool is still going forward while the spindle has started retracting. I have a braking resistor in my Yaskawa VFD but even so, if I set the decel time to under 1 second, I can't run much more than 400-500rpm without overvolting the VFD. You don't want that to happen with the tap at the bottom of the hole.

Given the need to sharpen up the start and stop times for the spindle (increasing the risk of overvoltage), it's a good idea to tie the VFD error signal into the controller, so that the machine will stop if you cause an overvoltage and the spindle stops. It took a bit of fiddling to optimise the accel / decel rates against the max speed I could run. If you are going to be using a larger tap, there will be less chance of the VFD overvolting as the spindle will be running slower and there will be more torque required to drive the tap. So I did my tests on a thin aluminium section with a small (M4) tap.

Here's the proof:

This is baby stuff of course. Things will man up a bit when we get to tapping full depth holes in the loominum plate I'll be using for the various components - but it's a good start.

Carry on now...

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