Author Topic: Leadscrew Clutch Options?  (Read 4078 times)

Offline sparky961

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Leadscrew Clutch Options?
« on: October 12, 2015, 12:53:25 AM »
My lathe/mill has a parallel-face dog clutch between the spindle-driven gear train and the lead screw.  Anyone that has used a dog clutch in this application knows that under even fairly moderate loads the clutch does not want to disengage easily.  This certainly leads to some sphincter tightening moments when you're cutting up to a shoulder at a pretty aggressive rate.

My question is this: what's a simple way the parts can be modified or re-designed so that it engages/disengages smoothly and easily under load?

I've used many lathes where the action is smooth and easy regardless of load, speed, or whatever.  What mechanisms are they using?  To ensure accurate threading it wouldn't be able to slip.  I was thinking maybe making new "gears" (dogs? what's the correct term?)  with teeth at something like a 45 or 60 degree angle, likely requiring spring pressure to keep them engaged.  Any obvious flaws to this idea, or better ones?

Online chipenter

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Re: Leadscrew Clutch Options?
« Reply #1 on: October 12, 2015, 02:37:06 AM »
My Faircut lathe had an auto knock off that worked verry well , the clutch parts were hardened and slid off smoothly , the more teeth the less presure mine had four .
Jeff

Offline velocette

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Re: Leadscrew Clutch Options?
« Reply #2 on: October 12, 2015, 03:01:41 AM »
Hi Sparky
You can use a lot less steep angle than 45 degrees to  about 5 to 10 degrees no springs and use an eccentric type mechanism that is locked engaged by going just over center.
Thrust bearings will be needed on each side if the clutch dog mechanism.
Do a search for "Over Center Clutch"
Then instead of friction clutch substitute a dog clutch.
Screw cutting and and using dog clutch not be possible with all thread pitches only a very few.
Personal preference is to drive the lead screw with a 24 volt variable speed gear motor and a poly vee belt drive and use a carriage stop when cutting up to a shoulder.

Eric

Offline Manxmodder

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Re: Leadscrew Clutch Options?
« Reply #3 on: October 12, 2015, 07:36:33 AM »
Eric, I agree with you that the 45 deg angle on the drive dog is excessive and would make disengagement difficult.

On your second point about dog clutching restricting the range of thread pitches I don't see why that would be the case.

If the clutch is placed between the spindle and drive gears then engagement with a single point engagement clutch will always be accurately repeatable no matter what thread pitch is being generated.

Also I can't see how a carriage stop could be used with the leadscrew half nuts engaged as when the carriage meets the stop the leadscew/half nuts would come under destructive strain.....OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline sparky961

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Re: Leadscrew Clutch Options?
« Reply #4 on: October 12, 2015, 11:44:17 AM »
Ok, a picture's worth at least a few words, right?  A saving grace of this machine is that it did come with a full set of exploded drawings.  It has helped many times to see how things come apart and where I can modify things without having unanticipated problems.

I was hoping to be able to improve the action without massive changes.  Looking at the attached exploded view, I was wondering about maybe replacing the "dogs" (142, 143 - lower centre) with custom versions having a different contact face angle, and also very smooth, tough and hardened (4140 HT?).  I'm definitely concerned in doing this that the clutch will have a tendency to disengage on it's own due to the pressure forcing the dogs apart.  To counteract this I was thinking to add a spring or lock of some sort to the actuating lever (135) or associated parts.

Some of you have probably read posts of mine in the past dealing with this machine and its limitations.  It is for this reason why I don't want to spend a ton of time on this.  If I had more space, I'd almost certainly have a separate full sized lathe and mill.  Who knows, maybe it just turns out to be an interesting discussion on clutches and machine design.

Regarding the angle of dog engagement, can you be more clear on whether this angle is from horizontal or vertical?  Maybe a sketch or link?

Offline BillTodd

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Re: Leadscrew Clutch Options?
« Reply #5 on: October 12, 2015, 12:33:45 PM »
There's not a lot of space to play with, is there?

Improving the general fit and hardness of the parts will, I'm  sure,  improve it.

Pay attention to any part that forces another to  i.e. are the dogs exactly 90 degrees apart ? (if you remake it, i suggest two dogs 180 apart would be easier ) does the slide lever try to push the dog to one side (causing it to jam) .?

I have a very similar mechanism on my Haughton Cadets they can get stiff but are usually ok under normal threading loads.

Btw. The dogclutch is not usually much use when threading as there is not  normally a simple relationship between the clutch engagement points and the screwpitch. For instance the Cadet's 10tpi lead screw and four tooth clutch, means that is is only useful on 0.025" pitch (40 tpi - where is can be engaged at any point) .

You may find it easier to fit an electrical stop (and reverse ) or thread backwards away from the shoulder (there's an explanation somewhere on the forum of this) .

Bill

Bill

Offline velocette

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Re: Leadscrew Clutch Options?
« Reply #6 on: October 12, 2015, 03:39:28 PM »
Hi Sparky
The pictures tell the story not a lot of room to play around in is there.
Bill Todds suggestion that two points of contact to separate the clutch will help.

Eric

Offline velocette

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Re: Leadscrew Clutch Options?
« Reply #7 on: October 12, 2015, 05:08:02 PM »
Hi Again
Pondering on this over breakfast Came up with this to get two points to operate the dog clutch
Bore the bottom of the gearcase and fabricate two shafts with a yoke on the outside of the gearcase.
Attached a rough sketch

Eric

Offline sparky961

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Re: Leadscrew Clutch Options?
« Reply #8 on: October 12, 2015, 10:30:59 PM »
Thanks for your thoughtful input so far. I shouldn't have mentioned threading in relation to the dog clutch. I don't use it for threading. In fact, I thread very rarely with this lathe because it takes way too long to set up.

It brings to light an important omission on my part though. I have been using the dog clutch instead of the half nut because a number of years ago I installed a fixed full nut. This was due to a lot of backlash from wear. It wasn't simple at the time to fix the half nut.

Now that I'm looking at the bigger picture, it seems using the half nut would work better. I may have even advanced enough since then rhat I can correct the backlash to a minimum.

Online chipenter

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Re: Leadscrew Clutch Options?
« Reply #9 on: October 13, 2015, 02:51:24 AM »
Case hardening the exsisting my be the quickest and cheepest option for you .
Jeff

Offline velocette

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Re: Leadscrew Clutch Options?
« Reply #10 on: October 15, 2015, 12:53:25 PM »
Hi Sparky
The angle on the dogs is from the shaft centre line with negative rake of 5 degrees tending to disengage the clutch then an over centre locking is required or a spring and  a lockout latch.

manxmodder
""Also I can't see how a carriage stop could be used with the lead screw half nuts engaged as when the carriage meets the stop the lead screw half nuts would come under destructive strain.....OZ"".

It doesn't a 1/20 hp gearmotor with poly vee belt with just enough tension to move the carriage hits the stop and the belt slips.
 
Eric

Online chipenter

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Re: Leadscrew Clutch Options?
« Reply #11 on: October 15, 2015, 01:37:48 PM »
Hi Sparky
The angle on the dogs is from the shaft centre line with negative rake of 5 degrees tending to disengage the clutch then an over centre locking is required or a spring and  a lockout latch.

manxmodder
""Also I can't see how a carriage stop could be used with the lead screw half nuts engaged as when the carriage meets the stop the lead screw half nuts would come under destructive strain.....OZ"".

It doesn't a 1/20 hp gearmotor with poly vee belt with just enough tension to move the carriage hits the stop and the belt slips.
 
Eric
My carrige stop disengaded the dog clutch stopping the lead screw , and could be adjusted to 0.001" every time .
Jeff