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### Author Topic: Rigid Tapping  (Read 25381 times)

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#### Hood

• 25,849
• Carnoustie, Scotland
##### Re: Rigid Tapping
« Reply #30 on: January 08, 2014, 02:12:15 PM »
Very gracious of you Terry, much obliged that you have given me your blessings
Hood

#### simpson36

• 1,374
##### Re: Rigid Tapping
« Reply #31 on: January 13, 2014, 08:44:23 AM »
Hopefully without jumping into the frey, I just wanted to point out a detail that is missing on re-tapping the same hole.

In addition to having the spindle at the same starting point (not necessarily azimuth 0 or index, but the actual starting point from which that particular hole was tapped). you need to have the Z at the starting point for that particular hole also.

This is not too bad if you have only a few holes at the same elevation, but can get complicated with a lot of holes at different elevations or a string of holes done with relative moves.

In my 'rigid' tapping macro, the retract amount is also a consideration because I allow any retract value (to clear obstacles in moving to the next hole) as a parameter passed to the macro. Since the retract can be any value, the macro has to determine if the current  Z is at the retract height and accommodate that space in the next Z move prior to starting the next tapping motion.

In my setup, I have the choice of homing the spindle before each hole, but I found the cost in time to do this is too great in contrast to the benefit of being able to re-tap old holes. holes tapped too shallow are typically discovered at assembly and in practical terms it is much cheaper to hand tap the hole deeper or spec a shorter fastener (if possible) than to run the part back thru the CNC process.

Whether there is a tool changer and whether the tool holders have drive dog also plays into the 'sync' equation. Assuming the spindle homes before each tool change and the tapping routine returning to the starting azimuth for each tap sequence, the spindle will retain it zero reference during any tapping run regardless of how many holes are tapped.  With dogless systems like Tormach, your options would be greatly reduced and you could only reliably re-tap (assuming a spindle home is done after picking up the tool) until the next tool change. When that tool is retrieved again, there is no guarantee the azimuth will be the same , so homing the spindle again will NOT necessarily return the tap to the same starting point.

I think everyone agrees that blind holes are the Gorilla in hard tapping. In practical terms it is often doable to simply drill the hole an amount deeper than the required threads to accommodate the variation posed by tapping heads, VFD's and perhaps even the occasional programmer screw-up, but it is undeniable that exact control of the Z depth is the holy grail and the only method (other  than thread milling) to provide safe 'bottoming' type tapping. My tapping macro stores the current acceleration parameters and then maxes them out for the tap operation and then restores them to the original settings. My experience has shown that normal acceleration settings (and mine are already much higher than most) are not adequate for rigid tapping with small taps. 'Stop-on-a-dime' performance has been mentioned. Taps smaller than 1/4" or M6 need 'Stop-on-an Insect' performance . . .  and a small insect at that.

Love the clutch idea, but it seems that would defeat any ability to go back to that hole for a re-tap. To prevent broken taps (my nemesis, incidentally), my plan is to use the torque control I/O on the servo drives to set the max torque below a certain threshold depending on the tap size. In this way, the drive would stop prior to snapping off the tap and would fault the system allowing for a change to a new tap without having to then dig the old tap out of a hole. This method would also protect against a spiral tap from becoming a permanent part of the workpiece if it snaps off due to being bottomed in a blind hole.

A lot of the above comments would not apply to the high end machines being debated, but considering the typical machine running MACH software, I'm hoping the information may be useful to the few who have servo powered spindles.

#### Hood

• 25,849
• Carnoustie, Scotland
##### Re: Rigid Tapping
« Reply #32 on: January 18, 2014, 05:34:09 AM »
Well I got a reply from CS-Lab regarding using a VFD for tapping, the depth will not be so precise as using a servo and may overshoot a bit. That will depend of the acceleration/deceleration of the VFD, so the faster the better obviously.
Regarding peck tapping, CS-Lab say it is no problem with a VFD and works in the same way as I can do with the servo, and it works exceptionally well. So looks like the only thing with VFD tapping and the CSMIO is you will have to learn your overshoot, obviously this will change slightly  with different materials and how sharp/dull the tap is but it should work well enough for most that do not have a servo.

Hood

#### simpson36

• 1,374
##### Re: Rigid Tapping
« Reply #33 on: January 18, 2014, 09:30:30 AM »
Just as an FYI, I found a significant improvement in thread quality (and less broken taps) by adding a pause at the end (bottom) of the tap cut.

Some Systems have this and some don't, but if you have the option, try putting dome delay to let the torque settle down before the reverse juice kicks in. Even 1/4 of a second can make a significant difference even with an AC servo.

Also, in my experience, even with ball screws, it takes longer for the spindle to accelerate than for the Z axis to start moving. Also the spindle accell is effected by tap size and material being cut, sharpness of the tap, etc.) while the Z movement is not.  Compensation for this can be found in putting a delta between the Spindle and Z axis to give the spindle some bias in acceleration.

#### Hood

• 25,849
• Carnoustie, Scotland
##### Re: Rigid Tapping
« Reply #34 on: January 18, 2014, 12:23:21 PM »
With my setup it seems to work exceptionally well and so far I have had no problems with snapped taps, have only gone as small as M3 taps in 316.
I did some videos a while back when rigid tapping was first implemented in the IP-A and IP-S.

Here they are.

As you will see, in some of them I have deliberately set the spindle  accel very slow for them, others I have my normal acceleration.

Hood
« Last Edit: January 18, 2014, 12:33:16 PM by Hood »

#### simpson36

• 1,374
##### Re: Rigid Tapping
« Reply #35 on: January 19, 2014, 08:58:56 AM »
I do enjoy the 'Hoodisms'. You phrase " . . . .  'ONLY' gone as small as M3 in 316 . . . ." as if this is still in the realm of easily doable. If I had to hard tap M3 in 316, I'd need a couple shots of Jack Daniels first . .    Plus I would stick to the 50% thread hole size as a minimum. Just thinking about 316 makes my eye start twitchin'.

And that tool changer . . . man, I'll never get tired of watching that thing! Hopeless. Deer in the headlights     Anyway, did you find any advantage to using the slow accel on the spindle or were you just testing? The mill seemed to do fine both slow and fast, but if you had 100 holes to tap, there would be a massive difference in the part cycle time.

Theoretically, if you have the Z accel higher than the spindle and a fast computer, Mach should be able to coordinate the Z movement to the spindle regardless of the spindle acceleration, slow or fast. What I would be wary of is the tapping speeds you are going to transition thru and the period of time spent there.

Tapping tends to be SFM critical (in my experience) so I prefer the tap to cut at the correct speed and stop quickly to minimize the possibility of binding if and when it passes thru an SFM that is nasty bad for the particular size/material being cut. Also, I am a speed freak when it comes to CNC and I am not one to wait around while a spindle (or anything else) comes up to speed. I tend to think in terms of MRR for general machining and Seconds Per Hole for drill/ream/tap and anything else to do with holes.

Speaking of seconds per hole, I can't tell from the videos if you have a pause in the tap sequence between directions.

Considering that the accuracy of the Z stop is the Gorilla for VFD rigid tapping, did you find and difference in accuracy or consistency between fast and slow spindle accel? It would be interesting to see the results of tapping a row of blind holes with fast accel and the next row with slow accel and compare the depth and consistency and quality of the resulting threads. Then do the same with and without a pause between directions.

No hurry on that. You know, whenever you have some free time . .

#### Hood

• 25,849
• Carnoustie, Scotland
##### Re: Rigid Tapping
« Reply #36 on: January 19, 2014, 09:28:42 AM »
I was only slowing down the spindles accel  in response to questions asked by Terry in the thread we were discussing this in at the time.

There is no pause at the end and actually Mach itself is not controlling the tapping, it is done totally in the CSMIO and is called via M84 instead of G84.

The spindle in these vids is dual speed, ie feed out is set to double the in-feed rpm, can be set to what you want but double seems to work well for me.

Just to avoid confusion, my spindle is a servo and is controlled by the CSMIO as a servo ie +/- 10v control to the drive and it adheres to the acel I have set in motor tuning, the Z is synchronised to the actual encoders counts and it seems to work very well, the spindles encoder is 32,768 pulses per rev so the CSMIO is getting plenty of positional data to keep things on track.

Hood

#### Hood

• 25,849
• Carnoustie, Scotland
##### Re: Rigid Tapping
« Reply #37 on: January 19, 2014, 09:30:51 AM »
Oh and regarding 316, I think nothing of it as it is what I use day in day out being that the vast majority of my work is done for fishing boats. When I do 5083 Alu I think of it as cutting air. Just wish I could get my hands on a faster spindle motor

Hood

#### simpson36

• 1,374
##### Re: Rigid Tapping
« Reply #38 on: January 20, 2014, 02:00:18 AM »
I was only slowing down the spindles accel  in response to questions asked by Terry in the thread we were discussing this in at the time.

I was reading that stuff and I thought it was actual and not theoretical.

Quote

There is no pause at the end and actually Mach itself is not controlling the tapping, it is done totally in the CSMIO and is called via M84 instead of G84.

It was my impression that Mach did not implement rigid tapping at all.

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The spindle in these vids is dual speed, ie feed out is set to double the in-feed rpm, can be set to what you want but double seems to work well for me.

Now there is an interesting idea. Why didn't I think of that?

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Just to avoid confusion, my spindle is a servo and is controlled by the CSMIO as a servo ie +/- 10v control to the drive and it adheres to the acel I have set in motor tuning, the Z is synchronized to the actual encoders counts and it seems to work very well, the spindles encoder is 32,768 pulses per rev so the CSMIO is getting plenty of positional data to keep things on track.

I guess I better revisit that site. It was my impression the CSMIO was a motion controller. From this description, it is also a servo drive? I may give this device a close look for the new mill I will start work on in a couple weeks. The Kflop is a powerful tool, but getting it to do things takes a huge investment in time.

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Oh and regarding 316, I think nothing of it as it is what I use day in day out being that the vast majority of my work is done for fishing boats.

Yes, I remember. And you work on big stuff with big machines. That's why I was surprised to see M3. Figured the only use you'd have for that might be as a toothpick

#### Hood

• 25,849
• Carnoustie, Scotland
##### Re: Rigid Tapping
« Reply #39 on: January 20, 2014, 03:08:47 AM »
Quote
It was my impression that Mach did not implement rigid tapping at all.

Mach will go through the motions of rigid tapping if you call a G84 but it requires that you have a floating holder and that your spindle speeds are close to that commanded as it is just going through the motions and not looking at what is happening. Mach4 will be different but ..... well enough said

.

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I guess I better revisit that site. It was my impression the CSMIO was a motion controller. From this description, it is also a servo drive? I may give this device a close look for the new mill I will start work on in a couple weeks.

It is just a controller, it is controlling the servo drive/amplifier with +/- 10v and takes in feedback from the encoders so that it can do the PID. The position in Mach is also updated from the encoder position by the IP-A controller that  I am using, the IP-S is the Step/Dir one and it does not take axes feedback other than spindle encoder for use in tapping or threading.

Quote
The Kflop is a powerful tool, but getting it to do things takes a huge investment in time.
That has always been the drawback of the kflop for me, it is also however the big selling point to others. If you are fluent in C or whatever it uses then you can likely do most things, if you are not then you are relying on others for even basic things or you need to learn programming quick.
The CSMIO products are not so open in this regard but they have thought of nearly everything that is considered normal for machine tool control and implemented it for you.

Quote

Yes, I remember. And you work on big stuff with big machines. That's why I was surprised to see M3. Figured the only use you'd have for that might be as a toothpick

I do all sorts, do work for a couple of companies that supply products to oil companies some of which are tiny electronic devices
Big machines can do tiny work where wee machine usually cant do big work

Hood
« Last Edit: January 20, 2014, 03:11:17 AM by Hood »