Friday, 30 December 2022

Change of plan - bummer - Kitagawa chuck

Welcome back.

Much of the Xmas break has been taken up with endless shopping excursions, overconsumption, sitting on our arses etc etc. The actual number of workshop hours has been minimal, with most of that taken up with the tool touch-off issues I found within the Centroid software. Time to change that! 

My cunning scheme for using the DIYO touch probe on the Tree sort of worked out but it's a bit fraught when it comes to turrets spinning, boring bars and LH tools all moving in different directions. One of the main reasons for taking this unconventional approach was the difficulty mounting the probe in the spindle, mainly due to the fact that my the chuck that currently fits is the Kitagawa hydraulic 3-jaw, which in turn has a very limited (~2.4mm) movement on each jaw. 

I'm not planning on moving the jaws every time I want to touch off tools, hence a system that doesn't mount on the chuck. 

There are some useful videos on the Haas website and of those, the lathe "Tip Of The Day" (TOD) videos contain some useful content for the likes of me, learning the CNC lathe ropes. Here's one that took my fancy - a stepped conical mount for a chuck-mounted lathe tool probe. This accounts for the fact that many hydraulic chucks have very limited jaw movement by incorporating many small steps in the diameter. No matter what the current jaw position, this should fit.

My chuck seems to manage about 2.4mm movement between open and closed, so steps of 2mm might work. Furthermore, I have some 70mm diameter 6082T6 in the box. 

So let's model something up for my machine. The DIYO probe has a 6mm dia shaft and I need some meat for a grub screw as well as needing to avoid any deep grooving - typically the MGT tools I am using have a reach of around 18mm. This revolved section looks workable without getting overambitious. I used the Fusion 360 parameter table to drive the step length and radius increments:

I should be able to machine the stepped conical section using a VNMG insert and groove the necked down section using a KGMN200 (2mm) grooving / parting tool. I have the polished, uncoated (H01) flavours for use with loominum.


Stock prepared, tools ready for action:


This should work:

Will the chuck be up to the job? With the acetal I used for my pawn test piece, the runout is pretty reasonable - about 20um.

I need about 70mm from the front of the stock. This leaves some safety clearance.

But hold on. The runout is more than just visible. The fucker is a major imbalance issue, or would be if the Tree didn't weigh several tonnes. Try as I might, there is no consistent way to clamp down on this stock to give an acceptable level of runout. I can tap it with a mallet so that the runout is brought to an acceptable level. But alarmingly, this doesn't require much of an impact. Put it another way, the stock is far from secure in the chuck.


I've got a centre drill mounted in the turret, so I can find the centre of the stock when it is rotating and leave a small centre hole. I can then come out in the X+ direction and leave another witness mark. But hold on - the stock has moved in the meantime. Clearly the stock is held so loosely that creating that witness has dislodged it. That doesn't feel like a sensible basis for attempting the sort of machining I had in mind. Tits.


Bollocks.
Yes, I have to conclude that all the time and money I've expended on the Kitagawa chuck has been a complete waste. The runout is appalling (basically it's buggered and irrecoverable) and it doesn't have enough grip to safely hold the stock. Apart from that it's been a dream. The latter has always been a concern at the back of my mind but I've not managed to come up with a means of measuring the gripping force. In the meantime I've been hoping against hope that it would be fine. Well it isn't, idiot.

What now?
Well it's back to Plan A - machine up a spindle nose adaptor so that I can use my various D1-3 chucks on this A2-5 nose. Back in March, I modelled up an adaptor in Fusion. I guess it's time to resurrect that. 

The machining will have to be done on the Bantam, as clearly I have no sensible (safe!) means of holding a large (140mm dia) lump of 4140 (aka EN19T) steel in the Tree. This means coming to grips with LinuxCNC again. 

Last time round, I started machining the main body but I'd clearly got a bit carried away with the feeds and speeds, as it stalled part way through. I don't seem to have posted anything about that aborted attempt but the stock is still mounted in the 4-jaw, so is almost ready to go again once I change the CAM and/or machine setup to have another go at it.

It's rather too large a part for the Bantam to run in top ratio (2000rpm range). I'll probably have to reduce the feed rates and/or the spindle speed. In the latter case, I will probably have to change the gear ratio and rescale the VFD control voltage accordingly ie the rpm per volt it expects to see. If I understand correctly, it operates open loop speed control, so if I simply change the gear ratio I suspect it will either object to a big speed mismatch (it can see the spindle speed through the encoder if it choses to) or apply the specified feedrate, assuming the spindle speed is correct. Neither would be ideal.

Next steps:
Let's dig out the CAD, CAM, machine setup etc and see about machining up that spindle nose adaptor. No point crying over spilt milk etc.

Saturday, 3 December 2022

Centroid CNC12 lathe tool touch off for boring bars - WTF?

Been struggling to understand how to get boring bar tool length offsets correctly measured in CNC12 lathe. I've been finding this all very frustrating but I think I'm getting there now. I was planning to update from 4.80 to 4.82 but TBH I can't see any suggestion from the release notes that anything has been done that would affect lathe touch off. Besides, I now have a custom PLC on account of my ATC turret, which would further complicate matters.

Instead, I've done some experimentation / learning / calculations to better understand what is going on and I seem to have made some progress.

I started out by setting Xref to zero at the point of touch off of Tool #1, which I'm considering to be my reference tool. I'm not sure why I'd do this from what I've read in the manual but it seemed sort of in line with what Uwe said just now. Subsequently, the offsets are back around the -70mm for the external tools. But I still couldn't understand the boring bar offset.

On my machine, the boring bar tip is ~26.5mm closer to the turret axis (ie in radius units) than the external tools. That's equivalent to a difference of ~53mm in diameter. My probing "tip" is actually a piece of 3/8" square stock (so that I can probe it from all sides unlike a ball tip) ie 9.53mm across flats. The section in the CNC12 lathe manual dealing with boring bar offset measurements says "multiply the [boring bar] manufacturer’s offset by negative two (-2), and type the number into the X Offset field. The value you type should appear as being added to the measured X offset already measured." This seemed to be pretty relevant and indeed my "offset problem" distance is twice the width of the probe tip.

Quite simply, if I subtract twice the width of the probe "tip"(ie a total of 19.06mm) from the offset, I end up with the "correct" offset. By way of a check, when I change tools and approach a fixed DTI, the typical final "error" in the DRO position between my external tools and the the boring bar is 19mm before this correction is implemented.

So (and yes, I'm still new to CNC12 lathe):

  • Offsets are measured as diameter units, not radius.
  • As suggested by Uwe Mattern on the forum, the Xref should be entered only once. I can't say I'm quite clear how this works yet but it's a start.
  • The auto touchoff process doesn't seem to allow for the width of the probe tip when measuring boring bar / internal tools.
  • That's likely because the process outlined in the CNC12 lathe manual tells us to manually enter the boring bar offset. This is essentially the same process used for drills and taps but with the offset entered manually.
  • The manual covering lathe tool auto touch off is "commendably brief".
  • When I auto touch off boring bar tools with my probe, I will need to subtract twice the width of my probe from the measured tool offset.

Phew. Probably quite a simple problem but it took me some head scratching to figure out!

Shall I update Centroid CNC12 from 4.80 to 4.82?

What up?

I thought I'd got the tool touchoff process sussed out last weekend but now I come back to it this weekend, the offsets being reported during tool touchoff don't make sense. I seem to be getting -140mm instead of -70mm which is suspiciously exactly doubled. I'm also flummoxed by the boring bar touchoff procedure which has simply "compounded my confoundance".

Given this strange behaviour, it sounded as if it may be sensible to update to the latest version. I'm currently running 4.80, yet there is a 4.82 out now. There could be merit in the update. Or persisting uncertainty if I don't. What to do?

What's involved?

The link to the 4.82 update package is here and the update instructions are here.

On the face of it, the process is fairly simple. Rename the existing folder from c:\cnct to something else, such as c:\cnct_4.80. Then do a clean new install (which will be automatically named 4.82 in this case) and finally, "just" copy the macros, homing, library etc files and licence file across from the old folder. 

Finally, finally, go into the setup wizard and change the settings to reflect what you had before in terms of input and output settings etc. Ideally you'd have saved screenshots showing you what those were beforehand, otherwise you will have to rename the old folder back to c:\cnct again and run it up so you can check what they need to be.

And finally, finally, finally, you need to make any changes you made to the original PLC program to the new source (SRC) file and recompile it. Otherwise, the turret controls (in my case) will have been lost. This is a job for the Visual Studio Code PLC plug-in, to highlight differences between the new and old, particularly the changes I made for the turret, in my case.

Conclusion?

Not for now, Fatty. The hassle factor is too great given it will likely make little or no difference to the offset measurement issue.

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