|Mick B1||31/01/2019 17:10:14|
|1242 forum posts|
Even if you left the halfnuts engaged, you'd break the rotational relationship between spindle and leadscrew. Implications of that don't seem to me to make life any easier.
|Michael Gilligan||31/01/2019 18:41:05|
14251 forum posts
"Single tooth dog clutch" is the important point of definition Mick
This short thread contains some useful references:
Edited By Michael Gilligan on 31/01/2019 18:47:08
|Keith Long||31/01/2019 18:47:35|
|794 forum posts|
Yes but the clutch needs to be at the input end of the screw-cutting gear chain not at the output end to the lead screw.
|Michael Gilligan||31/01/2019 19:05:24|
14251 forum posts
This is worth a look: **LINK**
|Brian G||31/01/2019 19:53:48|
|617 forum posts|
A dog clutch on the leadscrew could still be useful as I suspect for most of us turning up to a shoulder is carried out far more frequently than screwcutting (and of course it does allow the use of a leadscrew handwheel).
|435 forum posts|
Exactly my thoughts Tim, with an auto-stop like you mention a lot of the hassle of threading to a shoulder would be eliminated.
As for the guard on the lead screw - I don't mind loosing it (as a matter of fact I took it off already). I don't think I would need a separate wheel on the lead screw if my thinking is correct : Let's say I'm threading to a shoulder, dog clutch disengages at the preset point. Retract the tool and turn of the lathe. Switch it to reverse and turn on the lathe engage dog clutch and return to starting point. Repeat for next pass.
Is my reasoning correct or I'm missing something?
|Mick B1||01/02/2019 10:11:47|
|1242 forum posts|
If it is, you'll get a root diameter undercut from the delay between the clutch decoupling and you stopping the spindle. In many cases this is OK, but in pressure-bearing components often not - the thread is supposed to taper out to full diameter without an undercut.
But if the spindle does more than one rev to produce one rev of the leadscrew driven gear, even if you have a single-tooth dogclutch, I'm struggling to see that you'll automatically pick up the thread correctly on the next pass, because only the rotational relationship to the final drive is fixed, not to the spindle.
And back to you and any others: Is my reasoning correct or I'm missing something?
Someone earlier suggested putting the clutch between spindle and geartrain. I think that'd work, but it's a helluva different proposition to achieve.
Edited By Mick B1 on 01/02/2019 10:14:47
|Chris Trice||01/02/2019 10:27:50|
1362 forum posts
Graham Meek's Screw Cutting Clutch design is situated between the spindle and the gears. It has to be to work as intended with all combinations of gears. It is a single point pick up which means the spindle will always be in the same rotational position as it was in the previous pass if the half nuts are kept in engagement. A dog clutch on the lead screw is not the same. It will make no difference if cutting threads that are a multiple of the lead screw but if that's the case, you might just as well disengage the half nuts.
|Brian G||01/02/2019 10:50:49|
|617 forum posts|
To quote from the manual for the Cowells 90 which has a leadscrew clutch and trip bar as standard: "When screwcutting, the clutch is NOT disengaged at the end of each cut. The machine MUST BE STOPPED, THE TOOL WITHDRAWN AND TRAVERSED BACK TO THE START OF THE THREAD USING THE MOTOR REVERSE" (Their capitals - I'm not getting shouty). This is for the same reason that we use a thread indicator dial when screwcutting, there are (except when cutting a thread of the same pitch of the leadscrew or a whole multiple of that pitch) multiple possible engagements.
Edit: Sorry Chris, I took a while and didn't refresh before posting so have duplicated a lot of what was in your post.
Edited By Brian G on 01/02/2019 11:07:10
3776 forum posts
Which would bring your ML7 up to the standard of the pre-war Drummond M-type with its dog clutch and trip bar. Very handy for turning up to a shoulder or repetition work etc but not overly useful for screwcutting.
The leadscrew dog clutch only works for screwcutting thread TPIs that are multiples of the leadscrew TPI. For all others, the dog clutch must be left engaged unless you are crafty and quick enough to chalk mark the last two gears in the change gear train and the leadscrew itself and engage the dog clutch as the marks come into alignment. Easier said than done. There are M-Type owners who do this but I have fiddled around with it a couple of times and found it all a bit much for me.
Graham Meek's and Martin Cleeves' screwcutting dog clutch designs are both at the other end of the gear train, ie directly on the headstock spindle where one position is one position, relative to the job.
But fitting a leadscrew dog clutch to our ML7 is on the "to do" list so the graduated leadscrew handwheel can be used to fine position the carriage, a habit I have become accustomed to on the Drummond and really miss when using the ML7.
Edited By Hopper on 01/02/2019 12:00:53
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