CNC – Easy as pressing a button – Not

[JasonBModerator @jasonb]

Fails & Crashes

[JasonBModerator @jasonb]

Roger over on MECH forum posted a link to this video Which those that think CNC is as easy as pressing a couple of buttons may find enlightening.

There are also a couple of examples of why you should not have excess work length sticking out the back of the headstock of any type of lathe.

There is no blood (just tears) so OK for those of a delicate disposition to watch

[Brian HParticipant @brianh50089]

Very interesting Jason although some of the incidents could just as likely have happened on conventional machinery, overlong overhangs etc.

The most frightening ones are where the work and tooling are driven into each other, often at high speed. Surely most programs would be run in a safe manner before thee first production use?

Brian

[Ady1Participant @ady1]

Whats Bob Ross doing there??

The joy of painting. BBC4

https://www.tasteofhome.com/wp-content/uploads/2020/04/bob-ross-painting-social.jpg

Edited By Ady1 on 21/03/2021 08:22:26

[David ColwillParticipant @davidcolwill19261]

Although I have seen most of those clips before I still wince every time I see them. I really feel for the poor sod who has to see the tool changer from his VMC lying on the bed or his shiny new CNC lathe lying on its side. That must be a terrible feeling.

I have had my share of CNC mishaps but nothing too serious. The worst of it is that once you have seen the cutter spinning at 3000 rpm and unexpectedly heading at rapid feed towards the table, the fear never quite goes away! Fortunately for me I was able to estop before it hit.

David.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ady1 on 21/03/2021 08:21:40:

Whats Bob Ross doing there??

The joy of painting. BBC4

.

I suppose it’s regarded as ‘omage rather than intellectual property theft

MichaelG.

[Ady1Participant @ady1]

The first one is hilarious

Edited By Ady1 on 21/03/2021 08:37:49

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ady1 on 21/03/2021 08:37:35:

The first one is hilarious

 

.

That first one reminded me of the ridiculous little/Lidl wood-turning lathe that was recently discussed on these pages.

MichaelG.

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Edit: __ I wonder anyone here actually bought one 

https://www.model-engineer.co.uk/forums/postings.asp?th=169442&p=3

Edited By Michael Gilligan on 21/03/2021 10:12:40

[Bob RodgersonParticipant @bobrodgerson97362]

I can vouch for its, CNC is not as simple as pressing a button. I've had two or three crashes and they definitely moments that cause you to take a few pictures with your backside.

[robjon44Participant @robjon44]

I would say that I have witnessed more than my share of devastating pile ups in the 45 years of CNC machining I racked up, thankfully not of my own creation, all caused by folk who will not prove out new programs, especially those written by people who might easily be replaced by a pencil with a rubber on the end! Back in the days of progams loaded by punched paper tapes (yes that long ago) any tape proven by me had my signature at the beginning of it & in the immortal words of the the Young Giant trained by me who worked on the machine in front of mine "If it ain't got your signature on it I treat it with suspicion". My definition of a short period of time was how long from pressing the green button to having a finger in each ear. The most devastating & costly one I ever witnessed occurred when someone asked our foreman for some advice about the program of a rather large component (read dustbin lid but bigger) written & proven by me, he took a big snatch on his pipe & proceeded to scribble on a piece of paper & handed it over before walking away, as this was before Noise Cancelling Headphones for fighter pilots were invented I was relying on everyday ear defenders, tappy tappy tap tap press green button, launch turning tool turret in rapid traverse (unusual as it was supposed to be an alteration to a boring pass) which smashes into component ejecting it where there was no place for it to go, machine stops dead but not before the two foot wide (vertical) slide that the turret was mounted on breaks in two down the middle causing £10,000 of damage plus cost of machine being out of action for 2 weeks, probably around the time my mantra became "no wonder my nerves are bad"

BobH

[Ex contributorParticipant @mgnbuk]

Lathes seem to have bigger "bangs" than mills – more energy in the rotating bits at a guess.

I was working one bay over from a medium sized CNC chucking lathe in an oil tool plant where the programmer had failed to put a spindle speed limit in a new program. The part was a long steel tube 8" or so diameter, extended well beyond the headstock and supported on a two point roller steady in the aisle. Being an "oil country" lathe it had chucks at each end of the spindle & the exposed tube was covered with a tubular guard to prevent contact with it. While facing the tube in CSS mode, with no spindle speed limit the revs went very high as the tool approached centre, the tube bent 90 degrees throwing the tubular guard down the shop & levering the 12 tonne machine of the floor & turning it 90 degrees to the aisle. Fortunately no injuries, but a very shaken operator.

At the firm I started my apprenticeship with they had a 4 axis (twin saddle) Churchill vertical bed lathe (one of the machines I was set on to learn how to maintain). One of the older apprentices was running the lathe & was on a piece work bonus – I was giving him a hand loading it while he checked the part just removed to get the parts count up. The parts were stainless castings for ball valves & the two saddles were programmed to cut on both the inside and outside of a flange at the same time. Every so often there would be a clatter as the loads on the component from dulled inserts got too much & the flange twisted off to rattle down into the swarf conveyor. After one of these "clatters" there was also a loud hissing noise – the valve component was fine, but all the top slide keep strip screws had stripped & the whole top slide and turret assembly was just hanging on the bent ballscrew – the hissing was from the ruptured hydraulic and coolant flexible hoses. Took a couple of weeks to sort that out !

A former colleague – who I found out after he left in a hurry was referrd to as "Clanger" by the others – left scars on most of the milling machine tables, scrapped a Renishaw probe and (the cause of his "urgent return to Poland on personal business" ) a crash involving a Gildemeister lathe turret running hard into the tailstock barrel. The next user of the machine reported that the turret wouldn't index – which didn't prove too difficult to sort out – but then the hydraulically operated tailstock barrel seized. Long story short, the 100 mm diameter hardened steel barrel was bent. It had to be ground undersize to get the bend out, hard chromed oversize then reground to size & finally have the 5MT taper reground. All because he would not check his programs before hitting the "Go" button & didn't have the feed and rapid overrides set low while proving out a new program.

Nigel B.

[Tony Pratt 1Participant @tonypratt1]

The feed override dial & distance to go data is your best friend when proving programmers, I could never understand why the 'cowboys' didn't use them? I like to think I was a careful & conscientious CNC machinist but even I had a few 'mishaps', one I like to forget about was on a Haas Toolroom lathe with a manual tail stock, after drilling a 25mm hole with a brand new drill I forgot to move the tail stock out of the way & when I pressed cycle start the tool post came straight across & snapped the drill like a carrot, this was on a lightly guarded machine so the consequences could have been fatal for me, luckily I lived to tell the tale, I know of others who haven't been so lucky.

Tony

[egaParticipant @ega]

Thanks for the link.

As an amateur manual machinist, I couldn't follow the precise nature of every collision catastrophe but I often found myself wondering whether these wonderful modern machines incorporate clutches or other fail-safe provision.

The last excessive overhang incident showed that a simple under-support is just not good enough.

[Ex contributorParticipant @mgnbuk]

I often found myself wondering whether these wonderful modern machines incorporate clutches or other fail-safe provision.

I have come across very few mechanical overload devices – one that comes to mind was on Herbert AL series slant bed lathes that were designed to allow the Z axis ballscrew to move axially in the event of a collision. There was a limit switch on the end of the screw which stopped the machine if the screw was displaced.

Modern (meaning most makes from the last 20 years, maybe more) controls have multiple monitoring functions which will stop the machine if a). they have been correctly configured and b). the event occurs slowly enough that the monitoring has time to intervene before too much damage is done. But such monitoring is far from infallable & it is often a difficult balance between setting up for maximum protection and setting trip thresholds high enough to prevent nuisanace tripping in normal operation.

Nigel B.

[Neil WyattModerator @neilwyatt]

Ouch. That massive lathe with the bed in half… must have been quite a bang!

I've had the tail end of a vice pop off like that Fixed with four M6 cap heads and now does light duty on my Adept hand shaper.

One did make me laugh I'm afraid – the machine that looked normal, except the enclosure contained noting but orange flame…

Neil

[Ex contributorParticipant @mgnbuk]

One did make me laugh I'm afraid – the machine that looked normal, except the enclosure contained noting but orange flame…

Not sure that the operator would have been laughing along with you, Neil. Quite possibly a metal fire – very difficult to extinguish.

At my previous employment we got the repair work for the aftermath of a metal fire on a CNC milling machine. The machine was drilling titanium hubs, using neat cutting oil & there was a fume extracation hose above the working area. A bit of swarf got trapped between the drill body & the part that got glowing hot – no probem whilst submerged in oil an no air present, but when the drill retracted it set the titanium swarf and cutting oil alight. The flames were drawn into the fume ducting, which also caught fire & dropped, burning, onto the machine hydraulic pack. This also caught fire. The main electrical trunking carrying the wiring between machine and control cabinets ran behind the hydraulic tank – this burned through. Apparently the fire was very difficult to extinguish due to the combination of oil and burning metal. The machine ended up being repainted, rewired, had to have a new hydraulic pack and pipework and a replacement cabinet, control system and drives (electronic units damaged due to the wiring harness burning through & shorting while under power) .

There are specialist automatic fire suppression systems available for machines that are at risk of metal fires.

Nigel B.

[Bill PudneyParticipant @billpudney37759]

Not a CNC failure, butttt…..Many, many years ago, there was a large grinder, it had a vertical axis cylindrical wheel about 24" diameter, the bed of the machine was about 20 feet long. The machine had a rapid traverse that was pretty quick. I was in an adjacent workshop, and wasn't involved, other than as a witness, fortunately. The experienced operator was setting the machine up and as a result had the guards off or down, the workpiece (apparently) wasn't clamped down, it was a piece of steel about 20" diameter and 2 or three inches thick. Anyway, the operator was just checking the coolant flow, with the bed at an extreme of travel, i.e. right uo one end, when the bed traversed at high speed, flinging the workpiece the full length of the workshop, about 50 or 60 feet, it started at about 5' off the ground, and finished about 2'. It flew down the walkway, between workbenches and missed the nine or ten people working at those benches, and impaled itself in the brick wall. Several people lost their lunch. Rapidly made me realise how dangerous working for a living could be!!

cheers

Bill

[Ron LadenParticipant @ronladen17547]

A lot of those looked very expensive but what amazed me was the too long a piece of bar/tube sticking out of the machine. It wasn't that heavy size wise but the force it generated once bent out of shape and the way it shook that fair size machine about was scary.

[HopperParticipant @hopper]

Posted by Ron Laden on 22/03/2021 07:20:07:

A lot of those looked very expensive but what amazed me was the too long a piece of bar/tube sticking out of the machine. It wasn't that heavy size wise but the force it generated once bent out of shape and the way it shook that fair size machine about was scary.

Yes I wondered about that and had to wonder if the machine was not bolted to the floor. As an apprentice I saw one of my fellows do almost the exact same thing with a lenght of 5/16" steel bar in a Hercus 9" South Bend clone. The bar bent at the headstock spindle and flailed about like that at quite a rate of knots until someone shouted at the oblivious apprentice and he hit the stop switch. But the lathe never moved. It was bolted to the usual stand like a Myford or Boxford , which was bolted to the floor. But maybe that was some bigger bar in the video. I suppose a piece of 1" or even 3/4" bar would have some heft to it. Plus it looked in the video like it maybe had a large piece on the far end so the perfect storm. But still, pretty amazing the way it just shook that lathe around like a cat with a mouse in its mouth.

The really dangerous thing, in both cases, was the bar was sticking right out into a walkway. Could kill someone quite easily as they walk by. In my incident, it was actually caused by another apprentice walking by who saw the bar 5/16" bar spinning around in his path and thought "Hey that's interesting" and gave the end just a gentle little pat with his hand, setting it off centre and starting it spiraling out out of control. Somehow he did not get smacked in the face by it. It must have bent at the lathe spindle and whipped right past him before it came around the next revolution. Lucky.

Edited By Hopper on 22/03/2021 09:54:37

Edited By Hopper on 22/03/2021 09:56:14

[robjon44Participant @robjon44]

At our local technical college where the workshop was overseen by a man I knew who had worked in my dads office, I could see the attraction as the college paid him £29.75 an hour!, when I had to attend night school there he set me on as his chargehand in a bid to curb the worst excesses of folk who believe you can turn metal with the spindle rotating in reverse, anyroad up they had a small CNC lathe & milling machine to teach the black art on, almost immediately the head of the computer department decreed that he would be the one to do the programming on them, the next time I was there I enquired how the fledgeling CNC department was faring he said that as the 'expert' believed that all movements would be done in rapid traverse he had decreed that the only material it would ever machine would be that nice plastic that is made for beginners to to practice on that turns to powder. In a similar vein another young man just out of his apprenticeship was given to to me to train in the dark arts with a view to becoming my shift partner regaled me with a similar tale, another 'expert' while speaking to him as if he was mentally subnormal demonstrated setting up the aforesaid mill, vice in centre of table holding a square of white nylon, your mission Mr Phelps (cue original theme tune of Mission Impossible by Lalo Shiffrin) rapid traverse from park position in rear right hand corner to start position in 3 axis + spindle start, select fine feed, machine slot with slot drill & return to park position by safe detour, not rocket science then? Well tippy tippy tap tap, press green button & with spindle standing still rapid traverse from RH rear corner to LH rear corner & stop because you have run out places to go, my granny could do better ( she was a turner in an armament factory in 1917 )

BobH

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