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.

 

 

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