Meek style dog-clutch for a Colchester

[Taris JewellParticipant @tarisjewell61133]

so let me set some ground work

I am a hobby machinist at best so i apologize if i have missed something.

my machine is a Colchester master mk2 in a home shop. I only really cut metric threads and my lathe has a imperial lead screw

about a year ago i decided to make a version of Graham Meek famous dog clutch after finding and reading everything i could i began making my clutch. after a lot of work and re-design and re-making many parts to get everything to work properly, i have finally managed to get it working smoothly. how ever there is one major problem and this is why i am righting this asking for help. for some reason the clutch isn’t working, what i mean by this is when i am cutting a thread for example 1.5 pitch it cuts the first pass i disengage and run it in revers with the clutch to the start, when i restart cutting it cuts in between the original 1.5 pitch thread almost like i was cutting a twin start thread. now i have no idea how this can happen i thought that the clutch might be engaging early but that doesn’t seem to be the problem. so I’m out of ideas. the only other thing i can think is the gear ratios are wrong. from the pitchers I’ve seen of others clutches the use a smaller gear on the drive stack than the to gears with the pins in them but i don’t know why this is hens why i made mine all 40 tooth gears. (i would have to check but from memory the original gear set up was a 21t 127t 35t in that order from the spindle)

if any one has any ideas or questions please post them i really don’t know want to do now.

many thanks for reading

taris

 

[Clive FosterParticipant @clivefoster55965]

Taris

Sounds like you have the dog clutch running at the wrong speed.

To maintain synchronisation in both forward and reverse drive the dog clutch has to run at the same speed as the spindle.

Adjust the clutch drive gear sizes to get it running at spindle speed.

Easy way to check is to mark both spindle and clutch drive gear at some convenient point, eg at 12 o’clock on both, and verify that both marks remain in synchronisation as the spindle is turned by hand.

Clive

[Taris JewellParticipant @tarisjewell61133]

Thanks clive I’ll give this a go and see what happens. If that is the case can I just change the main gear coming off the lathe to get the correct speed? And if so I will then need to change the gear on the clutch drive shaft to be that 21t gear correct?

[DC31kParticipant @dc31k]

Have a look at the manual for the machine, particularly the exploded diagram of the headstock, to see the relationship between the spindle and the gear below it, that is providing the input to the leadscrew (and clutch).

If it is not 1:1, there’s your problem lady.

The historical precedent for leadscrew dog clutches is lightly discussed in Cleeve’s book.

[JasonBModerator @jasonb]

If it were just a case of the wrong gears that would only alter the pitch. The error seems to be with picking up the same starting point. The 40T gears are only idlers

 

From your description you don’t say if the “second start” thread is being cut when the tool is moving towards the headstock or away. You will need to back the tool out of the work to avoid any backlash messing up the thread.

Is it possible the dog clutch is engaging in more than one position which would be a bit like disengaging the half nuts when cutting metric on an imperial lead screw?

[JasonB]

On JasonB Said:

 

Is it possible the dog clutch is engaging in more than one position which would be a bit like disengaging the half nuts when cutting metric on an imperial lead screw?

Yep thats what happens if the dog clutch and spindle aren’t running at the same speed to remain in rotational phase synchronisation.

The point about the single tooth is that it defines the relative rotational positions between spindle and lead screw when engaged so it always picks up the thread in phase. If the speeds are different it effectively acts as a multi tooth clutch with several possible engagement positions so phase is lost. The screw cutting gears keep the spindle and lead screw in synchronisation at whatever difference in speed is necessary to cut the thread and the dog clutch ensures the relative phase is correct on engagement.

Mathematically it’s no different to how the numbers on a screw cutting dial helps you pick up the phase so successive cuts follow each other. But a single tooth clutch is right once per revolution. So you don’t have to wait

DC31K

The Colchester manual / parts books I have aren’t very helpful at identifying the drive gears between spindle and the shaft carrying the drive gear for the change wheel train. But it’s almost certainly not a 1:1 ratio. I imagine it’s easier to engineer things on the screw-cutting drive side if it isn’t 1:1. On my P&W model B, which has a dog clutch set up as standard, arranging the 1:1 gearing appears to make things significantly more complex.

Fortunately phase shift is easy to measure and, because a gear train is involved its all whole numbers so, in principle, compensation is easy.

Clive

[Graham MeekParticipant @grahammeek88282]

Hi Taris,

First of all I congratulate you on having a go at the design on a different lathe.

The Input gear out of the Headstock needs to be the same size as the dog-clutch gears. Currently there is a reduction and there-in lies the problem. You are not driving the Dog-clutch at 1:1.

If you want a solution send me a PM via the forum and I will let you have my email address.

Regards

Gray,

 

[MacolmParticipant @macolm]

To put it clearly (I hope), the dog clutch must be in a constant phase or orientation with respect to the spindle. The only place for the dog clutch is where it can remain thus synchronised. Allow that to change, and the pickup from cut to cut will be lost. It means in your initial picture that the steel and brass gears at the top necessarily need to be the same number of teeth, and further must never be allowed to loose mesh with the spindle. The Colchester lead screw reverser must never be disengaged or changed either.

It irritates me that the Colchester lead screw reverser could so easily have been no more complex overall, but worked like the Hardinge design! I have toyed with a conversion to achieve that, but I am too idle to have got on and done it. However, it might be a worthwhile project for someone.

[Taris JewellParticipant @tarisjewell61133]

So yes I am retracting the tool before running the carriage back to the start. I also am taking it back before the start so that it can take out the back lash. Initially I thought it was the clutch engaging early but in my testing it seems to be lining up every time so I don’t think that is the problem.

Does anyone know where I can find cleeve’s book so I can have a read through that?

as for the gearing the Colchester uses a thread cutting gearbox that sits after the clutch. If I understand what clive said correctly currently my clutch stack uses 3 40t gears and the spindle gear is a 21t so the clutch stack turns at slightly over half the speed. To correct that I can change the spindle gear to be a 40t and the 2 clutch pin gears to 40t and the final drive to a 21t that why the clutch turns 1:1 with the spindle and the final drive is correct to the gearbox input. 1:1.61 I think.

clive if you have any information on why it needs to be 1:1 I am interested in understanding I believe that you are correct I just don’t understand why it matters or why it can change the start point of the thread.

[cogdobblerParticipant @cogdobbler]

Martin Cleeve’s book is called Screwcutting in the Lathe. It is one of the Workshop Practice Series by Tee Publishing and is widely available. Well worth reading.

But you can’t do better than take up Graham Meek on his offer above of help. Straight from the horse’s mouth, so to speak.

[Graham MeekParticipant @grahammeek88282]

Hi Taris,

I have sent you a reply.

Regards Gray,

Generally,

As regards the 1:1 requirement. Each revolution of the lathe spindle or mandrel equates to one complete pitch of the thread selected.

With a 1:1 drive from the Mandrel to the output gear from the clutch you can only ever engage at one position or pitch length.

Introduce a reduction between the output from the mandrel and the output from the clutch. Then the pick-up points are a factor of this reduction. This will be made even worse if one gear has an Odd number of teeth. Coupled with the return cycle having a similar number of pick-up points.

The 19 teeth that are left over in this example on the return cycle is nearly half of any selected pitch, because of the 21/40 ratio. As such the leadscrew will not be returned to exactly the same start position by 19 teeth. This is what has produced the double thread.

Regards

Gray,

 

[MacolmParticipant @macolm]

The thing to grasp is that a single dog clutch at the spindle (or on a 1:1 synchronised gear chain) can be disengaged, then irrespective of how many revolutions the spindle makes, the synchronisation will be exactly the same when the clutch is eventually re-engaged.

However, the leadscrew must also be kept in synchronism, though this is usually achieved by moving it back to a reference start position (eg a stop) before each pass.

Any other configurations of gearing before the dog clutch spoils this. The reduction after the dog clutch merely sets the pitch of the thread, and can be any ratio and does not affect synchronisation.

[Taris JewellParticipant @tarisjewell61133]

Just increase their was any doubt I did check the timing between the spindle gear and the driver gear on the clutch shaft and remarkably 40 is indeed a bigger number than 21 so it doesn’t run 1:1 this is something that I have completely overlooked. So now I must make up a work around and as Gary has very nicely offered to have a look i must say I’m in capable hands.

there are marks on the gears but you might not be able to see them very well.

 

 

[Taris JewellParticipant @tarisjewell61133]

Afternoon all I thought I would share an update on the clutch. Over the weekend I finished up the new gears and got everything fixed up. I still need to make a couple more parts before I use it properly but I the first couple of practice cuts it is working perfectly.

once again thank you to everyone for the help and a thank you to gray for his help with modifying my backing plate to take the new gearing.

[Clive FosterParticipant @clivefoster55965]

Well done sir.

Very nice job.

Worthy of an article and drawings in Model Engineer & Workshop as an inspiration to those of us running larger, ex-industrial machinery proving that Graham Meeks design can be re-arranged to work with the typical end gear train layout.

Certainly made me wonder if I was premature in deciding there was no way of getting Grahams clutch onto my Smart & Brown 1024. Seeing yours I clearly missed an engineering nicety that should have been obvious. Concept is still a horribly “round the corner” drive but stealing your idea might just be enough to make it work. Another winter project!

Clive

[Graham MeekParticipant @grahammeek88282]

Well done Taris,

Thanks for sharing the outcome with us. You had done a lot of the hard work and my input was small by comparison. Given the number of Colchester lathes out there this might be a popular enhancement.

Regards

Gray,

[BazyleParticipant @bazyle]

So the outstanding mystery to non-Colchester owners is what the spindle to output ratio is and how you compensated to get 1:1 and then re-offset it to get the normal QCGB back on track. Not sure if these are same as the later squareheads that use some awkward numbers to get the metric approximations the best in class.

[MacolmParticipant @macolm]

For the Bantam Lathes, it would be easiest to implement a Hardinge style arrangement. It already has one to one gearing from the main spindle to the leadscrew drive spindle, but unfortunately implemented as a simple crash engagement, so can be in any tooth relationship.

All it would need is two permanently running gears, one of them via the already present reverser gear, each with a single tooth dog clutch to engage the leadscrew drive for either forward or reverse. It looks pretty easy to do, but obviously the knock-off design is critical for easy use.

[BazyleParticipant @bazyle]

Malcolm – re your second paragraph, that is what the Meek dog clutch design is. It has been adapted for several different lathes by GM himself and others. This thread is helpful in adding another to the stable.

[MacolmParticipant @macolm]

I appreciate Graham`s designs operate on the same overall principle as the Hardinge lathes (and others) in always having the same exact pickup point for the leadscrew. Because most lathes have a tumbler reverser immediately at the main spindle, his designs cunningly make good this less than ideal feature. However, the Hardinge design has the necessary reverse gears and dog clutches running immediately from the spindle, a more direct and simpler implementation.

It would be very easy to replicate this method on the Bantam as it already has a similar layout, but it fails to take advantage of the potential for easy threading.

[Graham MeekParticipant @grahammeek88282]

Hi Macolm,

I am sorry but I have to pull you up on your comments.

I regret to say you have the system I designed totally wrong. Although the system was initially designed for the Myford Super 7. The tumbler reverse in this design is dispensed with. After all the tumbler mechanism on a lathe is purely to reverse the feed conveniently nothing more. The screw cutting clutch is a stand alone item and works directly from the spindle, just like the Hardinge, and others. I have taken Hardinge HLV lathes apart in the past so I know how they work.

The design has been implemented and written up for the Maximat Super 11 in MEW. This lathe has tumbler gearing inside the Geared headstock, but it does not affect the functioning of the unit. It has in the past been an advantage to have the tumbler reverse on this lathe as some DP & Mod screw cutting gear trains to the gearbox make the leadscrew go the wrong way. Which in turn would make the clutch work the wrong way.

The Compact 5 my most recent addition to this system never had a tumbler reverse to start with but this unit still works as Hardinge intended.

 

 

The ease of threading is the whole idea of the screw cutting clutch. Stops to the front or rear of the lathe, which ever is the most convenient for the trip rod on that design of lathe, are set to determine thread length. After engaging the the clasp nuts, (which are never dis-engaged until the thread is finished), and setting the tool in contact with the work the screw cutting proceeds in the normal way retracting the tool at the end of each cut, (a retracting topslide, or tool holder is ideal for this), until the thread is finished.

I can, on the C5, set the gear train for a particular thread and finish the threaded item in around 15 minutes. When I had the Maximat Super 11 threads were much quicker than this because of the Leadscrew gearbox.

Above all the threads produced were truly concentric and the finish is always good.

Regards

Gray,

 

 

[Graham MeekParticipant @grahammeek88282]

I forgot to say the whole process is covered by the simple operation of one lever.

Sometimes I have the feeling that some think the clutch is just an over complex substitute for the tumbler reverse. The videos below I hope will show other wise.

Regards

Gray,

 

 

[MacolmParticipant @macolm]

Apologies if what I wrote suggests that a tumbler reverse is in any way part of your designs. It was merely to applaud you for dealing neatly with what at first sight is an awkward feature. Again, I have no doubt fitting one of your designs to a suitable lathe provides a valuable feature and great productivity boost. Thanks for the helpful videos.

I merely wanted to point out that things may be simplified if adding a similar feature to lathes with feed reversing like the Bantam. It is never easy to describe these things purely in words, so to clarify what I was suggesting, here are diagrams modified (fairly poorly) from a Bantam parts list.

In the first diagram, the feed reverser parts are within the blue rectangle. The first of the change wheels fits at B. Note that the left side of the inter reverser gear runs in constant mesh with the leftmost spindle gear, though shown separate for parts list purposes. The movable reverse shaft gear works in crash mode to couple either to this gear, or to the adjacent spindle gear. Obviously synchronisation is lost immediately it is disengaged.

The second diagram shows how it might be modified. The reverse shaft would almost certainly be replaced with a new purpose design, a simple item to make. Two new simple gears would be needed, shown green and red. Each would need a single tooth clutch dog, but be otherwise ordinary straight spur gears, and run on the shaft in constant mesh with the existing gears as shown. Making the new shaft with a suitable bore and cross slots would allow dog clutch control via the usual fork selector, three position with cental neutral.

The bad news with the Bantam (at least the Mk 1) is the gears appear to need to be 30 tooth, 10DP, and probably 14.5 pressure angle.

[Graham MeekParticipant @grahammeek88282]

Hi Macolm,

What you have shown for the original Bantam set-up is the same set-up as that in the headstock of the Maximat Super 11. The original design I had for the Maximat was to do exactly what you have suggested in your second set of drawings.  This involved stripping the spindle out, which at the time I was not keen on. Although Emco did offer me a spare headstock to get the ball rolling and a complete set of drawings.

Another problem with fitting the gears inside the headstock especially the Maximat is that the factory made gears are all Shaved for smooth running. Fitting standard gears in the machine means some loss of quietness.

It was not until some 30 years later that I decided to fit the unit externally. This took several attempts as the switch gear for the motor runs through this area of the machine. Given Taris’s solution I would be inclined to adapt this solution to the Bantam. The Colchester lathes have a strong family resemblance as regards layout. Fitting a unit externally means the gears can be of any DP or Mod you like to use as long as they are strong enough. Of course the output gear from the clutch has to be the same as the Quadrant gears.

Regards

Gray,

 

[Taris JewellParticipant @tarisjewell61133]

afternoon all.

I I’m thinking about re-doing my drawings to allow for the new gears and i was thinking about doing a proper right up about it, to help others that want to make it.

Question. Should i make a new topic or is it best to keep every thing in this one place.

As for the clutch i need to finish a few little parts like some spacers and the selector lockouts but the next real challenge is designing the auto stop. If any of you have any ideas i would be very interested to see/hear them. my main issue is the size of the lathe meaning everything as to be quite large and long but still very rigid and quick acting.

Taris

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