Denford Software & Machines

So - finally I've had a moment to take a look at the threading auto knock-off arrangement.

A note first - I've not actually used this feature - or indeed this lathe - I bought it a couple of years ago, but a couple of job changes and a house move half way across the country have meant that I've not used it - I've only really started to look at rehabilitating it from the horribly dirty state it was when I got it. So please excuse the mess. Phew - now that's out of the way...

The arrangement is actually quite simple - there's a stop that clamps to the bed, much in the manner of the auto-feed clutch knock-off on a Boxford lathe, which then activates a cranked rod with a piece of rack gear welded to the other end.

This turns a gear that's attached to the half nut activating shaft, and the half nuts open at the appropriate time. Part of the rear casting of the apron is removed to make space for the activating rod.

I've attached a few pictures below that should make this a little clearer. The rack is the same pitch, it appears, as that along the front of the bed.

One immediate observation I have is that the precision of the stop may perhaps not be too good, as the half-nuts will only open 'as fast' as the rod is pushed in by the stop - some biased-open over-centre arrangement would have been better.

On that subject, has anyone though of - or even better actually made - the dog-clutch threading reverse attachment serialised in ME (or MEW perhaps) for Boxfords and some other machines for a Viceroy ?

Any measurements anyone would like me to make of these bits, now's the time !

Youra.

Hi
The knock-off is rubbish. Constant use would round off the edges of the half nut threads. The slow release would provide uncertain release.

I reckon the way to go is to use the principle of a bolt action sniper rifle trigger mechanism.
http://www.practicalmachinist.com/vb/gu ... the-248553

This is obviously not a completed design.
The hard part is to reset the mechanism after it has been triggered.
It needs some hysteresis to allow the half-nut lever to be used to reset.
I haven't had time to implement it.

Surely this isn't a Denford original fitting? To be useful, it would need to spring the half nuts open, not just gradually open them. How is it reset, does the operator have to manually pull the bar out each time? As a safety device, yes maybe it does have some merit, but as a device for releasing consistently when screwing up to a shoulder, then no, I don't think so.

Richard

Hello
Yes, it is factory original. Not the best designed feature. It is more of a push off than a knock off.
It is reset with the half-nut lever.

Yup - as Dazz says, it's original.

Dazz - reading your other thread - did you every get very far with the 'gun trigger' style knock-off?

Now I'm almost set up with my new workshop (the electrician finished off last week) I'm looking properly at the machines again, and was thinking in terms of a version of the Graham Meek clutch - has anyone made one?

Youra.

Hello

Afraid not. It needs some careful design to get around all the shafts, gears etc in that area.
If the trigger mechanism was sandwiched between two plates, it could be made to be about 5mm thick total.

To use the half-nut lever to reset the trigger would probably require an adjustable travel.

At present I am focusing on finishing a VSD conversion on the Viceroy and my "new" Nardini MS 350.

I'm curious about this. I've looked at the various items of Denford literature I've got, and theres no mention of this 'knock off'/push off' anywhere, nor can I spot it on the drawings. Presumably it was a factory option, but did they ever publicise it?

Richard

My manual (downloaded, not one that came with the lathe) simply says something along the lines of:

"Automatic threading knock-off is also available", but nothing else.

Dazz - did I see you had drawings for this feature somewhere?

Y.

Hello

The only drawings I have come from this site.
Chances are that a lathe without this feature could be fitted with the knock-off bar. This assumes that Denford used the same parts for all lathes, with or without the knock off bar.

The only tricky part of the bar is the tooth rack that engages with the gear teeth on the half-nut lever. Given the application, I would make this with a hacksaw and files if I needed to.

Hi

I looked at the problem of adding a dog-clutch for threading. The major issue is achieving a mechanical linkage between the gears running off the left end of the spindle, and the apron. Perhaps the easiest (but not easy) option is to run the linkage behind the threading gearbox.

The fundamental problem with making and adding these elegant and useful mechanical features is that they tend to be very specialised.
Doing a CNC retro-fit would provide a general upgrade that would replace a range of special purpose accessories including taper/tracer attachment, threading knock-off and lead screw/spindle synchronisation. The effort required to do a CNC upgrade is probably about the same as adding one mechanical accessory.

Adding CNC to a well equipped lathe makes all those extra accessories redundant. So the best candidate for a CNC retro fit is a very basic model lathe with a lead screw and change gears (no gearbox). For CNC, electronic control of the motor is essential so that mandates VSD and a new 3 phase motor. If I was to retro-fit a Viceroy, I would also discard the apron and carriage. Both are unsuitable for fitting of ball screws. That only leaves the base, bed, headstock and tailstock.

I came to the conclusion a long time ago that my own lathe is too well equipped to retro-fit. It is also easier for me to buy a more suitable lathe than retrofit a Viceroy. I now also have a Nardini MS350 lathe which is much better candidate for CNC retro-fit. It is also well equipped but there is a lot more room available to make the modifications.

Hi

Adding a threading dog clutch would be a seriously good accessory to add to any lathe that I have looked at in the past.
There is plenty of room to add one to a Viceroy because of the space made available by removing the tumbler gears.

I can see opportunity to improve on the designs I have seen specifically around controls.
Existing designs use a left-centre-right lever.
I would design the control lever with a 2-way action and self centering.
Moving the lever left would engage to the left, but releasing the lever would bring it back to centre without disengaging the dog clutch.
Moving the lever right would engage to the right, but releasing the lever would bring it back to centre without disengaging the dog clutch.
Pulling the lever would disengage the dog clutch. The dog-clutch would be spring loaded to snap to the centre position.

What's the point of this. It means that the linkage to knock-off the dog clutch only has to act in one direction. The linkage only has to pull (twist or push) in one direction regardless of what direction the leadscrew is engaged. In addition, because the control lever would already be in the centre position, the knock-off mechanism won't have to deal with all the inertia of a lever with a knob on the end. It will be much easier to achieve a fast and consistent snap action on the knock-off.

It opens the possibility of using a limit switch clamped on the lathe bed and a solenoid acting on the dog clutch to knock off the engagement. That would be a lot easier to design and install than any mechanical knock off linkage reaching around the headstock.

The disadvantage is the cost and complexity of design and manufacture.

Hi

Here is the best drawing I have seen for a threading dog clutch.
http://www.practicalmachinist.com/vb/ge ... ndex2.html

Where it is labeled "Lead Screw" should read "output shaft".
For the Viceroy, this arrangement would need to be fitted to replace the tumbler gear mechanism.
As drawn, it doesn't include controls or a knock-off.

The gears for a Viceroy need pressure angle 20 degree and 18dp.