Faster Screw-Cutting

[Chris CrewParticipant @chriscrew66644]

I am fortunate enough to have an Ainjest rapid threading attachment fitted to my Colchester Student lathe which makes screw-cutting as easy as feeding buns to elephants, as a dear old colleague of mine used to say. Being an admirer of the late Jack Radford, I have recently been considering making his Six-Position Saddle Stop for my Myford ML7-R which works in a similar way but disengages the lathe's own half-nuts, rather than an auxiliary half-nut as on the Ainjest.

Could I enquire as to whether anyone has made and used this particular attachment and how effective it is? The design calls for a modification to be made on the apron and I don't particularly want to 'butcher' the machine if the overall end result turns out to be hardly worth the effort. Radford himself admits that there is small delay in the tripping of the mechanism equating to about 0.125" of saddle travel but I think this could be eliminated in practice when setting up for arresting the movement as with the Ainjest. Something in the back of my mind tells me that Martin Cleeve also designed a similar 'gizmo'.

[Chris CrewParticipant @chriscrew66644]

Radford Saddle Stop.

[DC31kParticipant @dc31k]

Posted by Chris Crew on 24/04/2021 08:41:44:

Something in the back of my mind tells me that Martin Cleeve also designed a similar 'gizmo'.

Martin Cleeve is generally remembered for his single tooth leadscrew dog clutch, detailed in his book and designed for many other machines by Graham Meek. It may be this that you are thinking of when Cleeve's name comes into your mind.

It achieves the same effect (stops the carriage moving under the action of the leadscrew) but in a different manner. Rather than opening the half nuts, it disengages the dog clutch using a spring action trip that has no significant delay.

The advantage of Cleeve's design is that the half nuts stay engaged so it can eliminate the need for a thread dial indicator and overcome language (imperial//metric) difficulties. A consequence of that advantage is that you have to reverse the leadscrew (maybe by a handwheel at the tailstock end) to reposition the carriage on threads where resynchronisation after opening the half nuts would be difficult.

[egaParticipant @ega]

It may help to show the JAR attachment:

and a brief extract from his description:

[Clive FosterParticipant @clivefoster55965]

Chris

Multi-position stops are very useful in their own right, my own six position one gets a lot of use, but when it comes to screw-cutting the dog clutch is far better than any half-nut opening device.

Grahams version is well engineered and known to be effective. It just works. Time to roll up your sleeve and build one.

Radfords design has something of a Michael Mouse Esq. air about it which I find unappealing and suggests a certain degree of faffing will be needed to get consistent results.

You don't have to leave the half nuts engaged with a dog clutch system necessitating reversing the the machine or a leadscrew handle to wind the saddle back to the start position.

My P&W model B has a dog clutch as standard fitting and its quite practical to open the half nuts and wind the saddle back by hand if desired. I never bother as it has proper third rod control set-up and reversing is so easy.

With a dog clutch system the leadscrew is stationary at the end of cut so the half nuts will always re-engage in the correct timing. Best to use a bed stop to define the engagement position. Theoretically it can be done via the threading dial but, even with the rather arcane P&W markings provided to facilitate this, its not the easiest of jobs. A start position stop is of course essential if working metric on an imperial machine or vice versa.

Clive

[egaParticipant @ega]

PS There have been a number of designs for this sort of thing in ME and elsewhere; I recall that one ME correspondent claimed to have copied the JAR design exactly and completely failed to get it to work!

And by the way, just about visible in JAR's photo is his quick change tailstock tooling with interchangeable tools sitting on the end of the (long) bed.

All this and much more is described in his book Improvements and Accessories for your Lathe.

[Nigel Graham 2Participant @nigelgraham2]

Jack Radford's modifications to the machine itself appear from the above, simply some drilled and tapped holes, and possibly a spot-faced area, in the apron and lever boss. So if it proved disappointing you won't have spoilt the lathe too much.

It uses a simple lever and over-centre spring, operated by a tappet locked into place on a bar mounted below the saddle in some way (brackets on the lathe's holding-down studs?); but the radius ratio that presumably gives fairly rapid action also produces a large mechanical disadvantage working for half its stroke against the spring.

That 1/8" delay is quite a lot, but accounted for by placing the tappet appropriately early.

'

A possibly better though more complicated arrangement (and which may be the Ainjest principle) would use a sprung lever or plunger to open the half-nuts, with the spring triggered by the lever.

This attachment on the Myford would allow repetition plain-turning passes to a shoulder, though I think you'd need use the thread-indicator for that as well as screw-cutting, and probably to the same number on each pass. All extra wear on the indicator that is not built for heavy repeated use.

(The Ainjest has this repeatability built-in, but is for threading only, on lathes with separate feed-shafts.)

'

Drive dog-clutches should give repeatable stop-points useful for plain-turning, but also having to wind the lathe back by the lead-screw for each pass, manually or by revering the motor, rather diminishes that advantage for screw-cutting. In such cases it may be easier to disengage the drive-belts and use a mandrel handle, with a simple solid stop-block on the bed. 

'

There is an alternative – George Thomas designed a Retracting Top-slide for the Myford Super 7, also for rapid screw-cutting, but is a far more complicated item, and you still need disengage the half-nuts in the usual way.

'

Incidentally, Millhill Supplies stock Ainjest units for Colchester and Harrison (only) lathes, but judging by MS' web-site photos, whether suitable for early L5s like mine is another matter. Probably at prices that make the virtues of patience and care in manual operation rather more valuable – and virtues are free! The publicity gives as example, large BSP internal and external threads on an aluminium-alloy fitting; 5 passes each, >1000rpm.

However, their illustrations usefully show that the Ainjest uses only the lower half-nut, of bronze, apparently controlled by vertical guides and snapped clear of the lead-screw by a pair of springs. So making an attachment on those lines, but simpler and designed to fit a Myford, may be feasible. Though I'd make the half-nut move upwards, to help avoid it clogging with swarf.

 

Edited By Nigel Graham 2 on 24/04/2021 11:16:12

[egaParticipant @ega]

Nigel Graham 2:

Thanks for answering my unspoken question, namely, as to the working principle of the Ainjest. Doesn't having one of these devices render other solutions unnecessary?

I agree that the JAR design looks a bit Heath Robinson but if JAR said it works then I expect it does.

I cherish the ambition of making the Graham Meek version.

[Anonymous]

Posted by Nigel Graham 2 on 24/04/2021 11:01:40:

The Ainjest has this repeatability built-in, but is for threading only, on lathes with separate feed-shafts.

As it happens my lathe has a separate feedshaft, but I don't see why an Ainjest unit wouldn't work as a fine feed with trip on a lathe with only a leadscrew.

This shows the internals of an imperial Ainjest unit, looking from the back and with the unit upside down:

As stated it uses one half nut, working against a plain guide to counteract the radial forces inherent in Acme threads. Being an imperial unit it has a single gear meshing with the leadscrew – metric units have a cluster of gears which need to be changed according to the thread pitch. Here is the unit installed, with the trip bar annotated:

The unit was dead easy to fit as all the holes in the bed and saddle were pre-drilled and tapped at manufacture. A little known advantage of secondhand industrial over new far eastern.

The trip mechanism is very repeatable, for internal threading I let the unit trip, move the tool into the hole to be threaded until it bottoms, only using the topslide, and then back off 4-5 thou and lock the topslide. Fingers crossed I haven't had a crash yet.

Andrew

[Clive FosterParticipant @clivefoster55965]

Posted by Nigel Graham 2 on 24/04/2021 11:01:40:

Drive dog-clutches should give repeatable stop-points useful for plain-turning, but also having to wind the lathe back by the lead-screw for each pass, manually or by revering the motor, rather diminishes that advantage for screw-cutting. In such cases it may be easier to disengage the drive-belts and use a mandrel handle, with a simple solid stop-block on the bed.

Nigel

No need to wind the saddle back via the leadscrew, whether by either reversing the lathe or using a leadscrew handle, when using a single tooth dog clutch. As the screw is stationary its perfectly practical to disengage the half nuts and crank the saddle back in the usual way. The single tooth dog clutch defines the relative rotational angle of the spindle and screw when picking up so as the screw is stationary the relative timing is maintained. Still need to use the thread dial or a stop to define the pick up position tho'. Reversing is generally easier, especially with a machine like my P&W B which was pretty much designed around the single tooth clutch.

This is Holbrooks take on the advantages of a single tooth dog clutch when threading

Clive

[Chris CrewParticipant @chriscrew66644]

May I just add, because 'ega' has mentioned the Radford quick-change tooling system on the tool-post shown in one of the photographs, that I made this accessory and found it to be the veritable answer to a maiden's prayer as my late colleague also used to say.

It worked so well and is so very convenient to use that I went on to make sixteen tool-holders for it and I have actually been meaning to make half a dozen more. It very rarely leaves the Myford and, in my personal opinion, it is far superior to anything that is available commercially if carefully made with the jigs that Radford also describes. It holds the tools securely and repeatably at centre-height and can be made for next to no outlay compared to the prices of commercially available models. I can't recommend it highly enough!

BTW, I am very grateful for all the responses so far received which I have found both interesting and informative.

Edited By Chris Crew on 24/04/2021 13:44:35

[Clive FosterParticipant @clivefoster55965]

Concerning the reported inconsistency of the opening delay from various builders of the Radford type I wonder if the frictional loads on the half nut and opening mechanisms may be both higher and more variable than those in a dog clutch system.

The engaged area in a half nut thread is considerable and lubrication limited. Also lubrication arrangements for the various slides and pivots in the apron are both sketch and intermittent. No great issues when hand operated as not only is there plenty of power available to shift things but also any stiffness will be immediately apparent to the operator and oil applied to remedy matters.

A spring operated system is, of necessity, dumb with no feedback and there are practical limitations as to how strong a spring can be used so some drag sensitivity is inevitable. Especially as the apron is open so oil immersion is impossible. I notice that the perceived stiffness of half nut operation on my Smart & Brown 1024 which does have a closed apron with oil reservoir and decent distribution arrangements for all components, including the leadscrew, is a little variable. Nothing of consequence but sometimes it feels "a bit easy" sometimes "a tadge stiffer". Its many years since I used a basic home shop size machine with open apron but memory says that half nut operation on such could be quite stiff.

I wonder if frictional forces were one reason why Ainjest used single sided half nuts as such would seem to make rapid operation easier.

Engaged area on a single tooth dog clutch is much less so, all other things being equal, frictional resistance to movement should also be less helping fast positive operation.

Clive

PS Many years ago I looked into doing a modified Radford for my SouthBend 9" Model A but the idea died when I changed it for a Heavy 10.

Edited By Clive Foster on 24/04/2021 14:20:02

[Anonymous]

Does a dog clutch have to go on the leadscrew or can it go before a QC gearbox?

Andrew

[DC31kParticipant @dc31k]

As a description of its function, 'leadscrew dog clutch' is OK. As a description of its location, it is maybe a little misleading. It needs to go immediately on the end of the spindle, before any gearing changes the angular relationship between spindle and leadscrew.

In some sense, it might be better described as a 'spindle dog clutch'; it is all about point of view – are you disconnecting the spindle from the leadscrew or the leadscrew (and all its associated gearing) from the spindle?

So, yes, it can go before a QC gearbox. It must go before the QC gearbox. It must go a considerable way before the QC gearbox, on the first place the machine sees the 'output' of the spindle.

Until I understood its operation, I often peered at the six-tooth dog clutch that disconnects my leadscrew from the gearbox, so the screw does not rotate when not required, and wondered if I could replace it with a single tooth one. I understand now why that is not possible.

[Clive FosterParticipant @clivefoster55965]

Andrew

The single tooth dog clutch is normally driven directly from the spindle so it lies at the start of the drive train. Most commercial implementations have separate forward and reverse clutch units that also substitute for the tumbler (or other) drive reverse arrangements.

Being at the beginning of the drive train means lower torque passing through it so lower loads which should help consistent release. But running faster does limit maximum rpm when threading lest engagement shocks become too large. Hardinge permits up to 1000 rpm but most other makes seem to advise around 200 – 250 rpm maximum.

Clive

[egaParticipant @ega]

Chris Crew:

Re Radford QC tooling, I think you are entitled to go off topic in your own thread!

However, I actually had in mind his tailstock tooling and commend this to you if you have not already tried it.

[KWILParticipant @kwil]

I have a Meek designed dog clutch fitted to my S7, I also use a GHT designed Retracting top slide.

To lower the impact on the dog clutch pin I also momentarily use the drive clutch whilst the leadscrew goes into reverse.

Screw cut at around 200-400 rpm according to the material being cut. Fast enough for me. Very rarely use a die.

[Anonymous]

Posted by DC31k on 24/04/2021 14:47:52:

It needs to go immediately on the end of the spindle, before any gearing changes the angular relationship between spindle and leadscrew.

Thanks for the elucidation. Given that 99% of the threads I screwcut are imperial I'll stick with the Ainjest.

Andrew

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