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hopefuldave:
I'm going to assume the half-nuts stay closed?
Quote:
...It doesn't make any difference to the repeatability of re-engagement if the dog clutch is located directly on the drive end of the leadscew(like the harrison lathes) or if it is located on the input end of the drive shaft for the screwcutting gearbox. ,
I'm going to disagree, lathes with single-tooth clutches (Holbrook, Hendey, Hardiinge, Pratt & Whitney - toolroom lathes) run them at spindle speed before driving the QCGB, not leadscrew speed after - at spindle speed it preserves the relationship between spindle and leadscrew (it can only engage once per spindle rotation so once per threading pitch as the half-nuts stay closed), a single-tooth clutch on the leadscrew can engage at any *leadscrew* pitch, with the spindle in an arbitrary position depending on the QCGB ratio, not the same thing! It MIGHT work for "native" threads, as suggested, as you'd effectively be opening the halfnuts and closing them on a random point on the leadscrew, definitely doesn't for anything else, e.g. metric on an Imperial lathe and vice-versa, which the spindle-speed dog clutch will do.
Another quote:
a single point dog clutch will always re-engage the leadscrew at exactly the same relevant index as it was disengaged from and there will be no variation in the relationship of gear engagement because they haven't been disengaged or had their relative timing disrupted.
Except that the spindle can rotate though an angle dictated by the QCGB setting and align the dog clutch without coming to the same angular relationship between spindle and leadscrew - e.g. for the 8tpi leadscrew, 11 tpi thread to cut, the spindle will have rotated 11/8ths of a turn (or multiples of) before it realigns the dogs. Assume the carriage is exactly where the dogs disengaged, the spindle will have rotated an extra 3/8ths of a turn and you'll be cutting a multi-start thread by accident...
I've looked into this a bit, as my lathe would very much like a single-tooth clutch, leadscrew/feedscrew reverse, threading stop and extra control rod, for cutting metric threads among others - added to the complication on my lathe, although the forward/reverse (R/H Vs L/H) selection runs at "spindle gear" speed is that the back-gear drives the spindle at 1/8th the speed of the "spindle gear" (on a concentric sleeve with its own bearings) that operates the QCGB etc. to get the "coarse" threads that really could use a threading stop disengaging a single-tooth clutch... Perfect if you need to cut 8-start 2tpi or 16mm pitch threads. :palm:
Plans are being made, slowly, fitting it all in is challenging! :bang:
There are a couple of single-tooth setups published, one is in Martin Cleeve's "screwcutting in the Lathe" (Workshop Practice series No.3), another in one of the model engineering magazines a couple of years ago, and all run the dog clutch at spindle speed to maintain synchronisation, there are versions for a good selection of hobby lathes.
Probably not what you want to hear, a leadscrew dog clutch would be MUCH simpler!
mattinker:
If I remember correctly it's a metric lathe. Unlike the imperial lathe, the lead screw must be kept engaged as the dial thread indicator doesn't work for metric threads. I seem to remember reading somewhere that three different interchangeable D.T.I's would be needed for metric lead screws. I'd be interested in a dog clutch for my metric Emco compact 8! I'm jealous of your lathe John!
Regards, Matthew
Manxmodder:
Dave,yes I forgot about the gears in the QCGB relative to the back gears etc if the clutch is located before the QCGB input shaft.
However,with my original idea it won't disrupt the synchronicity if the dog clutch is on the output side of the QCGB because it is simply disengaging the final drive line to the leadscrew in a manner that it can only re-engage at exactly the same single relative index to the final drive shaft output. With this arrangement no gears or half nut indexes have been altered at all....OZ.
Manxmodder:
Matt, just for interest here is a metric thread indicator for a Harrison lathe. This would only be workable on a dedicated metric leadscrew and thread cutting gearbox...OZ.
LINK: http://www.ebay.co.uk/itm/Spare-Metric-Thread-Indicator-Dial-for-Harrison-Lathe-etc-Part-No-L16-4-14-/121264384998
hopefuldave:
Hi Oz,
I have to disagree - only if the thread you're cutting is a multiple of your leadscrew pitch will it sync' up correctly by having a dog clutch on the leadscrew itself - believe me, over the last couple of years I've spent several weeks in free time working on this! I've even spreadsheeted it and knocked up a lash-up physical model as my maths isn't so practiced nowadays, and it gives multi-start threads if you're not really, *incredibly*, EXTREMELY lucky (once in several tens for most threads, far more rarely for the oddballs like B.A., D.P. and module which have irrational numbers like Pi as factors) - the only way it works for ALL thread pitches is if the clutch runs at spindle speed! Honest!
This is probably why proper toolroom lathes run the dog clutch at spindle speed...
My solution (not yet built, CADded, simulated and appears to be likely to work, some of the commercial-off-the-shelf parts like gears and bearings already purchased) is using the existing forward/reverse selector but adding a single-tooth clutch with a sliding dog in place of a sliding gear pair (the gears with their dogs will be fixed and free on needle-rollers, the sliding dogs keyed to the shaft), and as I have to deal with the 8xspindle-speed drive to the screwcutting / feed geartrain, I've had to come up with a synchroniser very similar to those for the firing pin on machine guns back in WW1 which fired through the propellor without trimming its blades...
There's a VERY long thread on single-tooth clutches at
http://www.model-engineer.co.uk/forums/postings.asp?th=49358&p=1
Where a lot of quite bright, practical engineers describe the mods necessary on a number of different lathes. It's worth reading, but everything in there is also in Martin Cleeve's book, if you dig a bit!
Matt, take a look at that thread - the principles apply to a lot of lathes (the Emco shares a lot of features with them) and could put you at square 3 or 4 rather than square 1?
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