Electronic leadscrew on a big boy lathe

[crickets]

Moving in this direction is making the problem much harder to solve. Its usually counterproductive to make instinctive changes to a system because they "ought to make it better". I've found its preferable to have the system generate a figure of merit- how well its performing- and then seek optimizations to improve it.

A 5k control loop would be difficult on some of our realtime systems mostly due to lack of cpu headroom but estimating based on cpu utilization we could hit it with a cpu upgrade; these systems move a lot of data which is a different problem than controlling I/O signals. In comparison we do have a 5k control loop system running I/O lines on a 32 bit 120mhz PPC which runs full-up at about 95% utilization; right on the ragged edge- thats a vxWorks system. Probably would be no problem in Linux on a 64 bit modern CPU.

As to single-user mode, the problem is not so much other tasks but selecting priorities and scheduling algorithms for the tasks that need to run realtime vs letting the others run at default priority. That is much easier to do with the system running full-up. We don't find readily measurable disturbances in realtime scheduling even when we have users coming in via remote desktop and running the gui control software- all that runs at default priority after the hard realtime processes execute.

On some of the fancy network I/O systems we do apply cpu affinity to some threads provided a measured increase of scheduling stability in some cases. But even those cases were out at the ragged edge in our stress-test system.

The Linux scheduler is really quite good, but application software architecture is probably the most important factor. Its very easy to have design assumptions serialize ops which could be parallel or inadvertently incorporate blocking/high-overhead calls in the critical realtime code- once you do that a fancy scheduler isn't going to help.

Those are all good real world observations. Sometimes people do ask theoretical questions though, so it's good to give them options they're comfortable with.

 

[crickets]

Thanks, this is pushing or exceeding my level of knowledge, especially about how the kernel behaves on a multi core system. Is it necessary to mask off some interrupts or can you force some cores to idle? I vaguely remember some people wanting to use certain cores as monitors for critical execution on a single core, but can’t recall how that came out.

It's possible to set cpu (core) affinity on the process and on the thread (when multi-threaded) level. I actually don't know what the execution would look like if we had a single single-thread process on a multi-core system without explicit call to pin it to the particular cpu, maybe it would always run on cpu0 ? No idea

 

[CalG]

Is LinuxCNC a thing anymore? The ELS has been hashed there for years.

For the past 20 years or more, ELS has been recognized as about as much work as going full numeric control.

A ball screw on the cross feed, and encoder on the spindle, along with unidirectional approach on the carriage takes care of it. Canned cycles and you are off!

 

[crickets]

Is LinuxCNC a thing anymore? The ELS has been hashed there for years.

For the past 20 years or more, ELS has been recognized as about as much work as going full numeric control.

A ball screw on the cross feed, and encoder on the spindle, along with unidirectional approach on the carriage takes care of it. Canned cycles and you are off!

Looks like there was a new release a few months ago, so it's still kicking. I've got a 3 axis system running the version from 2019.

 

[mattthemuppet]

So no ideas for simple features to incorporate?

not sure how feasible this is, but incorporating an adjustable proximity sensor (think bed mounted stop) and some way of doing a quick retract of the threading tool would allow you to program a stop feed + stop spindle -> retract -> rapid carriage reverse to start (2nd proximity stop) -> reverse retract (whatever word that would be).

The retract/ re-engage threading tool could be based along the lines of those quick retract threading tool holders + a small stepper motor.

Just spitballing, no idea how I would do that, but it would allow easy threading to a shoulder and ease the hassle of keeping the half nuts engaged for metric threading.

 

[crickets]

not sure how feasible this is, but incorporating an adjustable proximity sensor (think bed mounted stop) and some way of doing a quick retract of the threading tool would allow you to program a stop feed + stop spindle -> retract -> rapid carriage reverse to start (2nd proximity stop) -> reverse retract (whatever word that would be).

The retract/ re-engage threading tool could be based along the lines of those quick retract threading tool holders + a small stepper motor.

Just spitballing, no idea how I would do that, but it would allow easy threading to a shoulder and ease the hassle of keeping the half nuts engaged for metric threading.

Stopping the spindle adds quite a bit of complexity (i.e. need to cut power but also apply the brake). What you're describing can work relatively well with a sufficient relief without stopping the spindle. Also second stop isn't necessary, it would be easier to add the button to reverse the carriage until released and eyeball it.

 

[thermite]

THIS... is ALL ...too damned MUCH like "work". Of the "reinvent the wheel, all alone, and in the dark" distraction from payin' the bills and earning-a-crust class.

How many DIFFERENT metric threads.... and what run-lengths?

How HARD could it be to ignore the leadscrew, 'arf-nuts, QCB.. altogether...

And gin-up an Old Skewl, all in the metal, thread-CHASING rig.. that could be mounted on one, two, many, perhaps even ALL of the "Big Boy" lathes.... as needs dictate.

- potentially less work. And straight-ass metal work, not fragile 'lectron pushing and digital head-games.

- potentially good for all the lathes presently under-roof ...or any of many future replacement candidates.

I mean.. they did a variant on this theme for rock-crusher-grade turret/capstan lathes "back in the day" and it just MADE THREAD as if they were ordinary as pouring piss out of a rubber flip-flop.

Hardinge had a similar teeny-weeny rig that still has the chaser parts turning up in the remaindered world in all MANNER of pitches.

Tiny parts are no use. The CONCEPT might SCALE UP?

If all you gots is half a dozen pitches? The goods are not a huge investment.

And your existing CNC stable? Could was you already have what you need to MAKE yer own chasers?

One thing for sure. "Apprentice proof" ? Or just tired-eyed and bone-weary tryin' to catch-up to an overdue RDD proof.

A chaser do NEVER up and make some OTHER thread by accident.
The one it was cut for.
Period.

See-also geometric dies & box threading tooling. Lest we overlook the too-obvious.


Plan B:

If you have money to "invest" and f**k-around time to "play with"?

Make it work harder.

- a bolt-on independently powered thread milling rig.

Like unto a cross between a Versa-Mill on LSD, a CNC'ed machine-tool "slide".. and the Devil. Whole dam' on-lathe carritch can sit on its lazy ass. Lighter bolt-on topslide gets synced for the traverse.

It ain't but a thread. MANY light passes, fast, and you ain't the one getting tired.

Might even come out of the torture of it with a marketable "product"?

2CW .. but thread-milling capability is a sore USEFUL arrow in the quiver. "Modern" exotic alloy challenges most especially.

Any body already do'ed this?

Hacker's Second Law sez somebody already has done, somewhere, some time.

Universality of NEED thing..

We ain't hardly EVER the first poor, struggling bastid as HAD any given need, y'see.

Law sez find out how the most-recent SUCCESSFUL cannibal solved HIS problem.

Determine whether good enough to simply adapt it.

Or if'n it needs improved upon.

Oh.. BTW.. rumour has it all-Metric lathes exist.

Go figure they do not also need "inch" .. if you already got plenty of that.

 

[SeymourDumore]

THIS... is ALL ...too damned MUCH like "work". Of the "reinvent the wheel, all alone, and in the dark" distraction from payin' the bills and earning-a-crust class.

How many DIFFERENT metric threads.... and what run-lengths?

How HARD could it be to ignore the leadscrew, 'arf-nuts, QCB.. altogether...

And gin-up an Old Skewl, all in the metal, thread-CHASING rig.. that could be mounted on one, two, many, perhaps even ALL of the "Big Boy" lathes.... as needs dictate.

Was going to pipe up, but then you beat me to it......

5 years ago I only knew about them, owned none.
Now, I have 3 geometric and 2 tangential heads to save my ass when needed ( when needed = when the 5 CNC lathes at my disposal just cannot friggin' single point as required )

 

[thermite]

Was going to pipe up, but then you beat me to it......

5 years ago I only knew about them, owned none.
Now, I have 3 geometric and 2 tangential heads to save my ass when needed ( when needed = when the 5 CNC lathes at my disposal just cannot friggin' single point as required )

Yah.. well... I DID ask about SIZES?

There were good reason. I do NOT expect Garwood is a guy who single points on large all-manuals jest to prove to his Industrial Masochist self... that he ain't fergot HOW?

Prolly some sore challenging tasking in that equation?


Mining machinery...

Lotta threading is a feature on sore AWKWARD parts once into large-lathe territory. BIG diameters.. CUSTOM to the application, IOW .. not any "standard" thread size atall.

"Standard", "stock item", "commonly available" do not apply.
Whatever some clever designer was paid to do to solve a thousand-dollar problem on a two-million-dollar machine.

Picture a 2 TPI range.. or down-hole for repair to weld-up on a component yah cain't hardly even reach. A ten-inch ring to mate into a cavity.. only two inches deep.. or a lot less.

"Etc"

That-there is kinda.... "challenging" ...for dies & chaser?



Thread-milling, OTOH? Could be as ordinary as a chik'n-noodle soup lunch.

All-around useful extension to overall shop capability?
Wishn' I had had that .... "back in the day"

And had-never..

Time was.. over fifty year ago.. OEM had the luxury of MAKING a(ny) given component in a well-thought out "process" with specialized machinery, tools and tooling, fixturing etc.

Job/Repair shop had to do the dirty on an already EXISTING part that complex process had produced, annnd had nowt but a general-purpose engine lathe to-hand.

"Under which conditions..." I did enough damned-near impossible thread AWAY from the HS, large-JUNK lathe, blind-hole and external single point threads into corn-cob 400 AMP Linde Dee Cee dead IGNORANT-UGLY and slag-scabby "stick" weld build up to last me several lifetimes worth of - what was that term? - "Industrial Masochism".

Fuggid!

Half a century FOLLOWING sech aggravation, I have owned a LOT of US, Metric, and Brit-weird taps and DIES. And buy more, project bespoke if need be.

And PREFER buying already expertly made fasteners or threaded stock, even so.

Single-pointing, all-manually, in a well-supplied and CNC'ed out the ever-luvin arse of a try-sex-you-all-thread-tribe age?

Is a nuisance ... worth avoiding.

Disclosure:

Cazeneuve HBX 360 with the "BC" capstan TS and "special apron" (full of weird gearing!) option.... if I happen to get a wild hare on speed sneaking up my wrinkly ass.

Could was the best Inch/Metric single pointer ever built ~ 14" swing range, anyway. Large end of a "small" or lighter end of a "medium" lathe, not "Large" atall.

My world those are minimum 30 inch, 50 inch about the middle of that range?

Ever' lathe has to be expert at SOMETHING.

Even then, I ain't takin' the full day the complex bitch needs to re-train my too damned-infrequent ass to "French Weird" if I can avoid it.



"lectric leadscrew as walk up and JFDI?

Looks easy on You-Tube. Mebbe NOT, "real world"?

Could have just about the same "retraining curve" to git yer HEAD into the right "mode" to run it proper-not-scrap?

 

[EPAIII]

The problems with timing of PC serial ports (USB or otherwise) are one good reason for not using them for any time critical application. I worked with a software engineer on a system that needed +/- 8 ms accuracy. He spent the better part of a year trying to make a Windows OS achieve this about 99% of the time. He never did get it to the 100% point.

A PIC or micro controller has direct control over the output pins so this kind of timing accuracy is a lot easier to accomplish.

If I were designing an ELS I would use a PIC. PERIOD!

OK, an edit. I would use a PIC to control the screw. I would not use that same device for the user interface. There I would use something more "interface" friendly. The Raspberry Pi would be one candidate. Let the PIC worry about not skipping a beat and use the Pi for controlling it.

One of the challenges for realtime is I/O, like you pointed out. On modern PCs there are typically no built-in low latency I/O buses that can be used for machine controls - everything is USB, so it's an indirect challenge for using Linux in such applications. LinuxCNC requires either a parallel printer port or some specialized bus to run. On Pi GPIO should provide adequate low-latency I/O hookup.

 

[thermite]

If I were designing an ELS I would use a

Phone call.. wtth a "native" Metric lathe at the other end of it.

Horses for courses.

Not as if they never existed, is it?

Used Voest DATR Engine Lathe – Coast Machinery Group

I'm not rich enough to waste that much time solving a non-problem.

What next?

Convert ladies' stiletto heels or rubber flip-flops ... to steel toed BOOTS?

Need it. Acquire it. Somebody else already built it.

Sure HOPE we overly inventive lot never go short of p**sy!

The "design decisions" could become HIGHLY "contentious"?

Leave me OUT. Ain't broke. Don't fix it!


Welllll. Guess I AM "out".. but dam' if I can remember why it mattered?

 

[David_M]

Is there a K.I.S.S. way to connect the lead screw to the spindle? Nothing but a hollow pulse wheel mounted directly to the spindle, a fiber-optic cable, a servo motor and an analog frequency converter of some sort to create the needed ratios.

 

[thermite]

Is there a K.I.S.S. way to connect the lead screw to the spindle? Nothing but a hollow pulse wheel mounted directly to the spindle, a fiber-optic cable, a servo motor and an analog frequency converter of some sort to create the needed ratios.

Well.. guided missile control to deliver 12 foot CPE 90 miles out in mere seconds, 1950's ... off extended 1940's Ack Ack cannon technology?

Hell yes.

Is it a simple Digi-Key box-tick exercise where parts "self-identify", jump-off the screen, sit for a job interview, and explain to you which of many is the optimal to purchase? The do up a PCB layout, stuff parts, etc?

Hell NO!



Surely not to get the same sort of consistent fail-SAFE or fail-NOT reliability?

That needs more "players" on the team. Specialists, even. Several.

Even so.. the Big Bucks and greatest time need is always the testing, proving, STRESS testing, de-bugging, revisions, more testing..

Covered in the Acronym for one of those that paid the bills "back in the day".

MASSTER

Mobile Army Sensor System Test, Evaluation, & Review. [1]

The problem? Same one being wandering into HERE. Cost over-runs.

My next-higher, the Chairman, was a 40-year, and famous, Metrologist, atom bomb and such.. NBS retiree.

Annnd I could NOT get him to cease approving his VP of Engineering's underbidding DoD spooky-projects by a solid 25% and let me simply"mark up" the very CONSISTENT figure.

Because of the freakin' PRESTIGE he sought in his firm being seen to win the bid and do the bleeding edge tasking so very well.. all the while LOSING 25% of contract award value at EACH go!

"Marv? when a 24 year old barefoot-poor West Virginia Hillbilly of a seriously LAPSED Presbyterian... has to explain f**king grocery-store arithmetic to a wealthy sixty-five year old Florida JEW??

The whole f**king WORLD has turned upside down!

We shared a damned fine laugh.
But change?
He did never.

NEXT damned Executive job? I was in with two of Harold Geneen's former direct reports. And that was not by accident.

We made OUR numbers! With Out Fail. As did I , "forever-after"

Sharpen your time and materiel pencil.

These projects can eat you up.

Even so..??


Common enough general need that it is best bought-in as an already developed and tested "package".

Results come sooner and cheaper as well as better.

It isn't HARD for "mericans who have been dealing with Metric automobiles, if nowt else, for ages, already .... to mentally shift gears between inch and metric.

It is SAFER as well as EASIER if each lathe also has dial & c. in the same system "all over", and threads with 'conventional use' of half nuts and thread dial, not requiring h-n left engaged during successive power-backs, etc.

Not chit chat.
Money invested.

That Cazeneuve is HERE "primarily" because .....it was not going to be CHEAP to add one of the available Metric threading gear sets to a 10EE. even if I had the luxury of LEAVING one set-up for metrifryed all the time, the other left pure "inch".


[1] https://www.atec.army.mil/otc/history/history masster.html

 

[MrStretch]

If you want 100% reliable, glitch-free timing use an FPGA. If you want linux running a gui, use an mpsoc. Xilinx makes ones that have a cluster of 4 A53s, 2 R5 cores and and fpga. You can run linux on the A53s, bare metal or an rtos on the r5s and then use the fpga for the real timing critical stuff. Problem is it would take several lifetimes of Sundays to figure it all out.

 

[Greg Menke]

It's possible to set cpu (core) affinity on the process and on the thread (when multi-threaded) level. I actually don't know what the execution would look like if we had a single single-thread process on a multi-core system without explicit call to pin it to the particular cpu, maybe it would always run on cpu0 ? No idea

On a multi-cpu system the process will often settle on a single cpu but if there is other activity on the system (including kernel threads), then it might move to another. The cost of moving between cpu's is generally limited to L0 and/or L1 caches having to refill on the new cpu. Its a very small effect- we noticed it only in some maximum stress-test utilization of our test systems. I would be inclined to bet if your cpu utilization is < 90% then the timing jitter caused by moving between will be in the noise.

 

[Greg Menke]

Those are all good real world observations. Sometimes people do ask theoretical questions though, so it's good to give them options they're comfortable with.

Perhaps, but why work hard guessing at solutions instead of building in instrumentation from the get-go to reveal system performance and then working out the fixes?

I would argue you don't know what your realtime performance is, regardless of kernel and OS, without such metrics. Even something as simple as a control loop heartbeat brought out to a GPIO pin for a scope to watch is a big help. We accumulate min/max/avg/stddev metrics on control loop duration and headroom, reported as status data. Its very important to see what headroom is left in an algorithm, and how often the deadlines are missed.

 

[GregSY]

All those electronics sounds like a nightmare. Even the guys who understand it want to quibble over which way will work without disasters.

Again, it goes back to the willingness to scrap a part every so often. If a success rate of 90% is OK, and you have lots of time on your hands to tangle with black boxes made in China...go for it.

Otherwise...a pleasant solution awaits.... buy another lathe with gearing for metric threads. Much like women can never have too many cats or shoes, a man can never have too many lathes.

 

[Chip Chester]

(snip) A friend/biz partner already has a generic touchscreen GUI that he uses for industrial process control that could work for an electronic leadscrew controller.
(snip)

Is this GUI a product, a 'freeware' project, or something he wrote himself? Curious, that's all...

 

[jz79]

doing this properly makes that thing basically into a teach in cnc lathe, which sort of defeats the whole manual machine thing, and, if everything is done properly all will be good (no steppers though), BUT it what about when something goes wrong with it?

basically - it will be only as good as the support, I'd throw this idea away immediately if I couldn't do it all myself, or if you have a REALLY good friend doing it, anyone doing it for money will eventually try to fleece you or you'll end up with more or less ruined machine and lots of wasted time

 

[Greg Menke]

The problems with timing of PC serial ports (USB or otherwise) are one good reason for not using them for any time critical application. I worked with a software engineer on a system that needed +/- 8 ms accuracy. He spent the better part of a year trying to make a Windows OS achieve this about 99% of the time. He never did get it to the 100% point.

A PIC or micro controller has direct control over the output pins so this kind of timing accuracy is a lot easier to accomplish.

If I were designing an ELS I would use a PIC. PERIOD!

OK, an edit. I would use a PIC to control the screw. I would not use that same device for the user interface. There I would use something more "interface" friendly. The Raspberry Pi would be one candidate. Let the PIC worry about not skipping a beat and use the Pi for controlling it.

A pic is a very good choice for this kind of thing IF the numerical end of the algorithm can be adequately represented in the small program space ie if floating/fixed point math ends up being needed for averaging or filtering the PIC program space will fill up quickly. I/O-wise the pic's are very effective for a small investment in board space and they can be quite fast also. The win is the control loop runs in the pic w/ an rs232 connection to a PC or whatever for the click-and-drool interface.

I had the misfortune of trying to use Windows in a realtime application with 1 to 5ms deadlines and low cpu utilization, it was essentially hopeless. Even the realtime scheduler was subject to being blocked by things as innocuous as scrolling a window. It would work OK as long as nothing whatsoever happened on the GUI (including a screensaver)- this was on NT 4 with a dual 450mhz SMP system. I've not seen indications that Windows is any better nowadays.

I've also run into enough corner cases with USB that I wouldn't trust it for anything more than the usual sorts of peripherals- and even then once the device count on a bus is more than 6 or so things can still get sketchy. Partitioning the slow things onto separate USB buss'es so the fast stuff can be fast helps- but if theres a misbehaving driver or some device is weird in some way then reliability can be variable. I had one system where the combination of USB peripherals was somehow confusing the motherboard USB controller to the point it wouldn't initialize and prevented the computer from booting.