Lathe crash!

[not done it yetParticipant @notdoneityet]

Posted by ChrisB on 30/12/2018 16:13:51:

(………)

I am also taking Bayzle's suggestion regarding the keyed bush. I'll machine new bushes with a thicker wall, the originals are 14mm OD, 10mm ID, the new ones I'll machine will have the same 14mm OD, but a smaller 8mm ID and cut a 2mm slot for a key. I'll make the key from aluminium so hopefully if anything has to break it will not be the bush! The Bush I'll make from either steel or maybe titanium.

Are you changing the spindles, on which these run, to an even smaller size than design? Or securing the key, then boring out until there is no complete ring of original metal remaining?

[ChrisBParticipant @chrisb35596]

Changing the shafts on which the gears run as well. I'll use 8mm steel bolts. It's not going to make any difference in the rigidity or strength as the shafts are stepped down to what I believe is an M6 thread attached to the banjo.

Will it's from the original crash, don't have new bushes yet and with two out of the original three beyond usage I'm looking at a more durable solution, as although I ordered replacement bushes, I'm not seeing they'll survive for a long time (with me)

Edited By ChrisB on 30/12/2018 17:14:24

[Ian Skeldon 2Participant @ianskeldon2]

Hi Chris B,

I am sure your last sentence was tongue in cheek, however in order to help prevent another crash which may cause more serious damage or even worse hurt you, why not look at making a carriage stop, nothing elaborate. It will allow you to work with gaps of thous between your chuck and tool post knowing that it cannot creep into the chuck.

Please don't take this advice as criticism, we have all done or nearly done what you have done at some point.

ATB Ian

[JasonBModerator @jasonb]

Ian, how does running the carrage into a stop offer any more protection than running the carrage into the headstock? If the stop is solid then it will do similar damage to what Chris suffered by running into the headstock.

[ChrisBParticipant @chrisb35596]

Hi Ian, the lathe came equipped with a carriage stop, and I do use it, but only when using automatic feed as the feed shaft is equipped with a clutch. On the other hand the leadscrew is direct drive so if I had to run into a stop I would still end up with a crash. I'll just be more careful next time.

Btw – I always take advice on board and even criticism where it is due

[Ian Skeldon 2Participant @ianskeldon2]

Hi Jason,

On various websites there are several DIY projects showing various ways to knock the machine out of autofeed using a stop type bed clamp. I also remember being told as a lad that shearing the leadscrew pinion would cost me to have it replaced whereas running a saddle into a running chuck would end my apprenticeship. I can only assume that that was because either it was more costly or more dangerous, or maybe both.

Best regards, Ian

[ChrisBParticipant @chrisb35596]

Hi Ian, I have searched the internet for an automatic stop which could stop the lathe when thread cutting but all that I found seem to work with disengaging the half nuts. In my case, having an imperial lathe I have to keep the half nuts engaged when cutting metric threads (which was my case). The only option I could maybe attempt is to make an electrical stop which would effectively turn the lathe off when tripped – should work I think no?

[Michael GilliganParticipant @michaelgilligan61133]

Chris,

I suggest you have a look at this current thread: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=139901&p=1

… and especially the previous discussion, which I linked.

MichaelG.

[JasonBModerator @jasonb]

But the whole reason Chris had a crash was because he did not want to disengage the nuts, what use is a system to disengage them, he could have just flipped the lever and drilled his hole if he could disengage them.

I'm assuming the Meek clutch will mean you loose your position when cutting say metric threads on a machine with imperial leadscrew.

Edited By JasonB on 31/12/2018 20:34:02

[Chris TriceParticipant @christrice43267]

I have one of Graham's clutches fitted to my Super 7 and you don't disengage the half nuts from the lead screw or lose synchronisation. It's a design borrowed from Hardinge. It has a stop facility to automatically disengage the spindle from the drive train but since the engagement only has one position in relation to the spindle, it engages at the same point every time. As long as you retract the tool when reversing, you simply operate a forward and reverse lever (with a neutral position too obviously) to make the carriage travel either way. Basically, at the required point, the carriage contacts a stop on a rod connected to a lever which knocks the unit out of gear. The lathe motor remains running in the normal direction at all times even while reversing the carriage.

 

Edited By Chris Trice on 31/12/2018 20:54:14

[JasonBModerator @jasonb]

Just read some of the Cleeve article and it does stay engaged but hardly "nothing elaborate" as Ian suggested as you would also need to make a leadscrew handwheel to return the carrage.

[ChrisBParticipant @chrisb35596]

Oh wow, just had a look at the article describing the clutch, and while it looks like a good idea – I wouldn't know from where to start to implement it on my WM280 though!

 

Probably the reason I crashed the lathe got lost in the depths of this thread, so I'll explain as best I can again.

I was aware of the dangers and consequences of crashing the lathe while thread cutting so I was being careful, slow speed and finger on the off button. I was cutting an M16x1.5 thread 2.5" long, and half way through I realised I should have better drilled a centre hole and supported the shaft with the tailstock as I was having some chatter. Not wanting to disengage the half nuts, I backed off the tool post out of the chuck's way and with the half nuts still engaged reversed the carriage as far as the tailstock would permit in order to drill the centre hole.

Now to drill the centre hole I had to increase the speed, feed the centre drill and keep an eye on the carriage as the half nuts were still engaged….it was a recipe for disaster – did not manage to switch off the lathe on time and the carriage ran out of travel and slammed into the headstock with considerable speed

Lesson learnt…the hard way – but considering, the damage was minor…a pin and two bushes.

Edited By ChrisB on 31/12/2018 21:05:51

Edited By ChrisB on 31/12/2018 21:07:01

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 31/12/2018 20:50:39:

Just read some of the Cleeve article and it does stay engaged but hardly "nothing elaborate" as Ian suggested as you would also need to make a leadscrew handwheel to return the carrage.

.

It's a single-tooth dog-clutch' so it doesn't matter how you return the carriage.

Cutting and modifying the leadscrew is, of course, 'non-trivial'

MichaelG.

[Ian Skeldon 2Participant @ianskeldon2]

I read somewhere on line but not on this forum about a DIY adjustable stop that disengaged the tumbler gear, I wish I had book marked it as I can't find it now. IIRC it would allow pretty fine adjustment but until I can find it I can't say much more about it.

Ian

[JasonBModerator @jasonb]

Cleeve article seemed to suggest that if you used the tumbler reverse to return the carrage you had a 1 in 25 chance of picking up the thread in the same place.

Although Chris' machine does not have a tumbler reverse, use of the head gearbox reverse would have the same effect as above as the gears could remesh in any position.

 

Edited By JasonB on 01/01/2019 07:12:03

[ChrisBParticipant @chrisb35596]

Wouldn't it be more efficient and simpler to construct if I used a micro switch or proximity sensor as a stop for threading? (can be used for powerfeed as well I guess)

Something like these links here? **LINK** and this: **LINK**

Edited By ChrisB on 01/01/2019 08:01:33

[JasonBModerator @jasonb]

One problem with a switch is although it cuts the power you will have a certain amount of run on until the spindle comes to a stop and this will alter depending on speed, chuck size and work size. You could not work to thous consistantly

You then have to work out how to return the carrage with a lathe that has no power. You would need to bypass the cutout switch, select reverse (spindle) and then return to the start of the next cut, stop machine, select forwards, etc.

Stopping the lathe while the tool is still engaged when say turning to a shoulder would risk damage to the tool tip particularly with carbide.

[ChrisBParticipant @chrisb35596]

I get your point Jason, but this is what I have to do at the moment with my lathe, I stop the lathe with the tool engaged as I don’t want to risk a crash. That said I use HSS for threading so maybe its not such an issue as using carbide.
As we’re running similar lathes would you mind me asking how are you using your lathe for thread cutting Jason? Maybe I’m doing something wrong.

[Chris TriceParticipant @christrice43267]

Does it have a clutch?

[David Standing 1Participant @davidstanding1]

Posted by Chris Trice on 01/01/2019 16:20:00:

Does it have a clutch?

That question is already answered above, more than once

[JasonBModerator @jasonb]

No Clutch, I think Chris was asking about a motor to spindle clutch not a screwcutting clutch

ChrisB mine is imperial so keep the nuts engaged for metric though I tend to cut away from the chuck at the back so don't have to worry about stopping in a rush if going to a shoulder and can cut faster so can stop easily and run in other direction to take me back to the start.

Mine takes 2-3 turns to come to a hault from 300rpm unlike the one in the first video which stops almost immediately, that would mean you need a 5mm plus runout groove for your M16 thread.

Edited By JasonB on 01/01/2019 16:29:55

[Anonymous]

This thread suggests an alternative, essentially moving both the saddle and spindle back to the original starting point and relative orientation:

**LINK**

As background I have now used the Ainjest unit for many screwcutting jobs. I normally allow 4 thou clearance for the trip both internal and external. Thus far I've only cut imperial threads (on an imperial lathe) with the unit, but as and when I need to cut a metric thread I'll investigate the method in the link above.

Andrew

[Ian Skeldon 2Participant @ianskeldon2]

ChrisB I had a nightmare getting my lathe to cut the thread I wanted (internal and external imperial on my metric lathe).

Thankfully there was an answer and pretty much all of those involved in this thread helped me to sort it out, looks like your issue is going to be more difficult to resolve, maybe buy a load of bushes and hope you never need them

Ian

[Chris TriceParticipant @christrice43267]

Posted by JasonB on 01/01/2019 16:29:01:

No Clutch, I think Chris was asking about a motor to spindle clutch not a screwcutting clutch

 

Correct. Sorry if that wasn't clear. What about a retracting cutting tool or one that swings up out of the way? Good for external threads but no good for internal. 

 

 

Edited By Chris Trice on 01/01/2019 18:28:39

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Chris Trice on 01/01/2019 18:27:23:

 

What about a retracting cutting tool or one that swings up out of the way?

.

Such as designed by the aforementioned Martin Cleeve:

[quoting from a recent thread]

Here is the design for the one I like: **LINK**

https://archive.org/stream/PopularMechanics1976/Popular%20Mechanics-04-1976#page/n105/mode/2up

MichaelG.

.

Ref. Christmas Eve

https://www.model-engineer.co.uk/forums/postings.asp?th=139739&p=2

 

Edited By Michael Gilligan on 01/01/2019 19:05:19

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