Boxford STS Screwcutting Clutch

[OldironParticipant @oldiron]

I would be very interested in the drawings for the SB/Boxford AUD clutch. Are they available any where ?

regards

[Graham MeekParticipant @grahammeek88282]

Posted by Oldiron on 18/02/2021 17:58:13:

I would be very interested in the drawings for the SB/Boxford AUD clutch. Are they available any where ?

regards

About the time as I was designing this particular clutch, I had enquires about the Boxford AUD. Volunteers came forward, but I think the work involved frightened them away. As requests for further information went un-answered.

In the interim, due to the similarities between the South Bend and the Boxford lathes I have always pointed prospective Boxford builders to the South Bend Post I made on the Model Engine Maker some years ago. As I have never had anyone come back to say the South Bend version has been completed, I cannot give any guarantees. However based on my past record I think it is a good one.

Maybe here is a chance for you to adapt the design to the Boxford and be the first.

As I said in the opening paragraph of this post I have deleted any drawings I made, which now seems a pity due to the rekindled interest.

Regards

Gray,

[OldironParticipant @oldiron]

Thanks for that Gray. I need to dig out my MEM pw & see if I can find the article.

regards

[Graham MeekParticipant @grahammeek88282]

Posted by Oldiron on 19/02/2021 15:46:03:

Thanks for that Gray. I need to dig out my MEM pw & see if I can find the article.

regards

A Google search for "South Bend Screwcutting clutch Graham Meek" took me straight there.

Regards

Gray,

[OldironParticipant @oldiron]

Thaks for the link Gray. I had a quick look but will need to study it further when I get a spare half hour.

regards

[Trevor SteeleParticipant @trevorsteele20547]

Just to let everyone know how I have been getting on with the tests on the Boxford clutch. I replaced the long operating arm with a shorter one to help improve the sensitivity of the end stop as can be seen in the photo. The final result was a bit inconclusive. The original system was a bit insensitive due to the long operating arm, while the modified design has a bit more 'spring' in the system due to the long extension to reach the shorter arm. In the end I believe either setup will work. I will stick with this latest version to see how it works out in practice.

[Pete.Participant @pete-2]

Thanks for the update, I'd be interested to know how you get on with this in practice when you've had a chance to use it.

Are there drawings available for the STS Boxford?

[James G 1Participant @jamesg1]

Just to say that I am also following this with interest and would be interested in drawings if available.

Is there any advantage in having matching slight tapers on the engaging surfaces of the dog clutch i.e. on the drive dog and the semi-circular end of the recess with which it engages ? The taper would be such as to have the dog more narrow at it's end than at it's base, such that there would be a slight tendency (depending on taper angle) for the clutch to disengage unless held by a detent.

The only dog clutch I have had a chance to examine was a multi-tooth clutch in the feed gearbox of a toolroom type mill and this had a very definite taper on the dog faces, hence my enquiry.

J.

[Clive FosterParticipant @clivefoster55965]

James

Taper on the dogs won't work. Basically you are starting off with it half worn out.

Multi tooth dog clutches are different. Taper is basically there so the things will engage without needing excessive backlash for clearance. A single tooth clutch doesn't have the engagement issues beacue it inherently has most of a circles worth of backlash.

Taper form wear is a known failure issue in the P&W Model B system. Basically leads to the clutch not staying engaged at higher speeds and higher cutting loads. Which isn't good because not only does the single tooth clutch drive pretty much everything except the spindle on the B the machine itself is capable of removing metal at a very serious rate putting high loads on the dogs. Mine is OK but not in perfect health.

Taper also leads to inconsistent release position as the actual release drag loads vary more with change in cutting load. Ideally the drag on the dogs as the clutch triggers out of engagement should be independent of drive load. The P&W system uses short, stiff actuation levers and hefty spring loads for very positive actuation. Its also deep inside the headstock where it receives continuous lubrication so its operating under best possible conditions.

Being external Grahams (excellent) design will, relatively speaking, suffer more from dog drag variation because you cannot arrange continuous lubrication. The longer levers inevitably mean more spring in the system which reduces the sharpness of the disengage. Flip side is that Grahams system is engineered for lighter machines so loads will be lighter and its not so fundamental to machine operation as it is on my P&W where the third rod control operates the dog clutch so it gets used for everything.

Ideally you'd briefly pause the drive as the clutch actuates so the drive train briefly overruns the spindle by a fraction of a degree minimising drag at the point of shift. Which is impractical without serious electronics.

Grahams design is what it is and works well. Trying to guild the lily leads you straight down a rabbit hole of needless difficulties which, in the real world, make little if any difference. Might be interesting to see the effect of a serious anti friction coating on the dogs would be, eg DLC, but I'm betting you'd hardly notice.

About the only realistic way to improve on Grahams concept is to use an externally powered actuator, motorcycle quick shifter perhaps, and operate it electronically by picking off a control feed from a DRO system. Unfortunately nobody makes a DRO with a suitable control output. Which is a pity because using a trigger at zero output in conjunction with the offset memories, mine has 99 of the things, would make a very flexible bed stop system.

Clive

[Trevor SteeleParticipant @trevorsteele20547]

Pete,

There are drawings for the Boxford clutch available, although they may require further explanation for some of the parts. I'm happy to supply these to anyone who is interested. If you PM me your details I will email you a copy. They are in AutoCAD format.

Trevor

[James G 1Participant @jamesg1]

Hi Clive,

Thanks for your comprehensive reply – it's amazing how subtle can be the details of a simple mechanism. Certainly as you say consistent lubrication should help – the latest version as discusssed on this thread is enclosed and may effectively have it's own oil bath. Hopefully I'll be able to build it and enjoy the luxury of using it.

Thanks again,

J.

[Graham MeekParticipant @grahammeek88282]

Hi Clive,

Thanks for picking up the gauntlet and explaining the flaws in the tapered clutch teeth, and thanks for the kind words.

Being an Analogue Guy in a Digital World I tend to steer clear of the Electronics. Barring metal failure, mechanical solutions tend to go on for ever. (There is no software to get out of date),

Gray,

Generally,

Any adaptation to a machine tool is going to beset with compromises. Usually one compromise leads to another and the desired goals or features of the adaptation start to fall below the initial design standard.

It is a totally different thing to design a machine from scratch which has a screwcutting clutch incorporated in the basic design. Trip rods and pivot points can be placed where they are best suited mechanically.

Sometimes, like with my Emco Maximat clutch. My initial design was to be fitted inside the headstock, but for other reasons I had to make a compromise and fit the clutch externally as here with the Boxford. This in itself led to there being more gears in set-up than I would have liked. Which means the noise level has gone up slightly, but as I am going deaf it does no bother me that much. Thus we have 2 compromises in this design, 3 if you count the external mounting.

Initially repeatability straight after fitting this clutch was not as good as I had hoped for, but after using the clutch for sometime now the trip points are predictable, and there is no variation. The point here is, things need time to bed in and the sharp edges to round over a little.

Regards

Gray,

 

 

Edited By Graham Meek on 23/02/2021 17:09:49

[Tony RayParticipant @tonyray65007]

Hi Graham’s & Trevor,

As previous poster commented the STS solution may well be a good start for other machines which have the tumbler reverse in the headstock. I have a Harrison M250. At first glance it looks like the challenge on the STS I was to get the unit in the space between the headstock casting and the first gear in the external train so as to maintain the alignment of this gear with the next one located on the banjo. On the M250 that first gear is commonly changed to obtain two ranges for screw cutting in my case 22 & 44T being metric. I must admit even with Grays book I struggled a bit to understand where this all fits until I read Clive’s helpful explanation which is worth repeating here:

“Screwcutting clutches are single tooth dog clutches running at spindle speed, located before the screwcutting drive gear train, operated by a fast operating knock out trip”

I will contact Trevor to ask for the CAD files as Inthinknthey will be a good starting point for the M250.

Tony

[Graham MeekParticipant @grahammeek88282]

Hi Tony,

Clive's statement does generally hold true, but there is, "the exception that proves the rule". I know in one instance, and it may well be the STS above. There was an internal reduction with-in the headstock of 3:1. In other words the output shaft where the clutch is fitted is running at 1/3 spindle speed.

Conversations at the time were stating that the clutch would not work. At first glance this would seem to be the case, however the single dog will only engage at one point in this reduction train each time, because the gears inside the headstock have remained in mesh throughout.

Please keep us informed of your progress and if you get stuck we are only an email away. (A photograph of the space available might not go amiss).

Regards

Gray,

[Trevor SteeleParticipant @trevorsteele20547]

Hi Tony,

Gray is correct in saying that the STS drive to the gearbox is 1/3 of the spindle speed. So long as the ration between spindle and gearbox is a whole number of turns, the dog clutch will work. If the ratio was for example 1:2.5 then it would not. The disadvantage of a slower gearbox speed is that engagement of the clutch may be a bit slower as it only comes around every 3 revolutions of the spindle in the case of the STS. In practice is is not really noticeable. The advantage is that it is kinder on the dog clutch as the speed of the clutch is lower, which reduces the impact on the drive dog.

Best regards

Trevor

[Tony RayParticipant @tonyray65007]

Hi Trevor & Graham,

Thanks for your replies. Firstly I can confirm that the output shaft on the M250 rotates exactly at the same rate as the spindle. Looking at the output shaft side on there is a boss approx 10mm deep that is bolted to the main casting, This supports a 38mm spigot on which the banjo hangs. All in all there is 40mm between the back face of the driven gear and the main casting which is a flat face. This space is occupied by the 10mm thick boss 14mm thick banjo and various spacers.

**LINK**

There is a good image of the drive set up on Lathes.co.uk

I will try to post some images in a few days.

[Tony RayParticipant @tonyray65007]

I can’t immediately see how the Boxford solution is going to be work with the banjo in its current location.

The driving gear sits on the shaft on a splined adaptor ( shear pinned to the shaft) that has been removed for clarity. All dimensions are approximate. Two possible routes come to mind, develop something that sits outboard of the driven gear or look at a new banjo arrangement pivoting on the lower input shaft to the feed and screw cutting gearbox. It looks like the oil filler elbow would have to be replaced, that wouldn’be a bad thing as it looks like it came out of a plumbers merchants! Sorry for the poor image the machine is right up against something els so I just have room to remove the cover & do change wheel adjustments.

[Graham MeekParticipant @grahammeek88282]

Hi Tony,

Harrison have surely made this one difficult with the banjo pivoting about the output gear. Relocating the banjo to the more conventional position about the input to the screwcutting box might need a whole new banjo design.

My inclination suggests taking the drive from the output shaft, 1:1, to a gearbox containing the clutch directly above the output shaft, there is plenty of room here, but part of this space will be taken up by the banjo movement.

The drive is then brought back to this output shaft either by gearing or by a timing belt, 1:1.

Failing that the obvious place to fit the clutch is inside the headstock, but that will be major work and not for the faint hearted.

Regards

Gray,

[Clive FosterParticipant @clivefoster55965]

Tony

If the banjo is re-made to swing off the gearbox in the conventional manner there seems to seem to be about 30 mm behind the current drive gear position. Maybe a bit more if the actual gearbox input gear can move outwards a little.

Probably be enough room for a concentric arrangement, as used by P&W, with the single tooth clutch unit keyed to the shaft behind a free running drive gear.

Basic idea would be to fit a hollow shaft over the existing drive shaft engaging in the existing key. The drive gear runs free on this hollow shaft. The clutch device is keyed or splined to the hollow shaft and pushed in or out of engagement with the dog on the gear by the actuator forks. It would be best if the engagement operation were done via a proper thrust bearing so the forks and carrier don't rotate. With a thrust bearing a simple Y fork, pin and ring actuator would suffice.

The hollow shaft needs to be very firmly fixed to the drive shaft lest engagement forces wallow out the existing key.

When trying to scheme out something for my 1024, before Graham showed us all how to do it, I sketched a concentric system using a rotating "balls on ramps" thrust actuator rather like that used for clutch operation on some British motorcycles. Probably not enough room for that sort of system here. Big attraction is that positive in or out hold can be done with a simple tension spring direct on the operating arm arranged to move in a over centre arc.

The 1024 was always going to have electrical knock off as there is no way through for mechanical levers. Have a husky solenoid unit out of a pre-engaged car engine starter tucked away in the useful box for that job.

Clive

[Tony RayParticipant @tonyray65007]

Its going to take me a while to process all of that useful info, I spent a bit of time this afternoon looking at the possibilities. This is the banjo in its upper most position, this reduces the space envelope to approx 40 x 90mm. I think I will need a more detailed explanation a new a sketch to understand Gray’s suggestion.

[Tony RayParticipant @tonyray65007]

I don’t think I would want to attempt an internal mod without a spare box to work on so here is a preliminary look at moving the banjo. The boss supporting the input shaft to the gearbox is cylindrical but smaller in dia that the current banjo and has protruding cap heads but they can be changed. A sleeve could be made to make up the difference. I hung the banjo on the boss with the two sets used to cut metric pitches.

22T on the output

44T on the output, the compound gear are just swapped round between the two setups. The wire is just to hold it in position

so my initial conclusion is that this could work , the banjo is not a complicated part to make and I can 3 d print mock-ups. I think I need to make an accurate model in Fusion360 then model the various gear trains needed to cut imperial, module and DP. I am fortunate to all have a full set of change wheels to do Imperial threads although recent experiments have yielded useable 3D printed items. Moving the banjo then gives a large space in which to implement the dog clutch. That will of course need to be figured out but it should be possible to use Trevors excellent solution as a starting point.

[Tony RayParticipant @tonyray65007]

Just noticed that I failed to swap the compound gears for the 44T set up.

[Clive FosterParticipant @clivefoster55965]

Tony

This is how P&W do it but they slide the gear. I see no reason why the clutch unit rather than the gear couldn't do the sliding.

With a decent helping of low cunning I think it might well be possible to squeeze  something similar onto the existing drive output shaft.

 

Dog is a pin (J -723-C) pushed into the gear (G 10-42-'37) clutch is on the right (J -742 B). Clutch is made as part of a sleeve sliding on and keted to the drive shaft. Gear and actuator slide on the outside.

Sorry for the poor quality but the scan I have is what it is.

Clive

Edited By Clive Foster on 05/03/2021 19:37:32

[Graham MeekParticipant @grahammeek88282]

Hi Clive,

I am not familiar with this lathe, but from what I can make out from the scan. The P&W clutch drives in one direction only. If this is correct then the leadscrew clasp nuts need to be disengaged to return the carriage to the start position.

The Hardinge HLV clutch, which the original Myford, and all my subsequent designs are based on, drive in both directions. There is no need with these designs to disengage the leadscrew clasp nuts.

This makes cutting any Thread, DP or Module dead simple. With no reversal of the motor needed either.

Regards

Gray,

Hi Tony,

As I said earlier, this is not going to be easy, and somewhere compromises have to be made. If you look back up this post to the clutch fitted to my Maximat. You will see the clutch proper is in front of the original output shaft from the headstock and below it. The final output from this clutch goes via 3 gears to the output shaft. which is on the same radius from the screwcutting box as the original output shaft was.

Turn this type of assembly through 90 degrees CW and you will have a similar unit to fit your lathe. More detailed shots of the Maximat unit can be found in My Album.

Regards

Gray,

 

Edited By Graham Meek on 06/03/2021 11:56:04

[Tony RayParticipant @tonyray65007]

Some great ideas here, not being blessed with a mechanically minded brain I have a question about the P&W clutch Clive: Am I correct in thinking that the output shaft direction of rotation is handled by another means not shown in the diagram? I like the idea of electromechanical actuation, that could solve the issues around sensitivity that Trevor is working on, I would think that suitable micro switches on a stop bar with an E stop as a fail safe would work. Your idea Clive of a DRO output stop signal is a good one and would not I think be difficult for someone with the right skill set but perhaps a full implementation of Clough42’s electronic leadscrew would be another solution.

Trevor has kindly sent me the cad files and introduced me to Nanocad so I can view them – many thanks Trevor.

Anyway off to the Workshop to get a better idea of what I have to work with.

[Author]

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