Crashing a micrometer stop?

[graham-xrf]

It seems obvious to me, but I take it that "micrometer carriage stop" and "threading" are not concepts that should ever feature in the same sentence. My search does not yield anything much about power feed crashes involving hard stops.

Of course, the survival sense in the bones is yelling that, if you are threading, you pay attention, and let go with the lead screw half-nuts pretty darn smartly, or start from the left end!

So how often do we hear of a lathe driving the carriage into a micrometer stop - or for that matter, into any other part of the lathe? Has anyone ever simultaneously reworked their chuck and toolpost?

This is just background to me thinking that a limit switch arrangement might be used to stop things if the saddle gets to places it should not go. I also am imagining that a micrometer carriage stop might reasonably have a 2-stage feature which would push on some arrangement on the apron to automatically take out the power feed (stage#1) while still allowing the carriage to be manually moved a little further up to the intended mechanical stop (stage#2).

OK - so I am somewhat a beginner. Maybe the fancier lathes have something that does this?

 

[Chuck K]

As you said, I wouldn't use a micrometer stop while threading, but I use them regularly while turning. It's really not hard to disengage the feed before crashing. After you've done it for a while you find that you're comfortable enough to almost kiss the stop before disengaging the feed. I do use feed screw stops while threading, but I have an old Hendey that has feed screw stops.

 

[GL]

You are correct in many places of your thread. Carriage stops limit travel to a shoulder, manually. Some lathes have cams that disengage travel. Some VFD systems have a proximity switch built in to shut the lathe down at the stop and can be very accurate. Skill can get you close, but driving into a stop should be avoided. Some have clutches that will ratchet on normal feed, but would go out of time on a thread - but you have to set the tension on the clutch and there is a balance between carriage drive for a cut and the stop that causes damage to gears ans such.

 

[ttabbal]

If you can thread away from the chuck, the stop is extremely useful. Makes setting up the next pass much faster.

 

[mmcmdl]

Over time you won't need a stop . It just becomes easier with experience .

 

[mksj]

You would not use a micrometer stop for threading, the leadscrew has no clutch mechanism so crashing it would = damage. Leadscrew has a shear pin that should break with a hard crash, but you could bend the rod, break the half nut, or other damage. Even if it had a clutch, you would loose the registration with the thread if its position changed relative to the spindle. If turning to a shoulder using the feed then some people will use the feed rod clutch to stop the carriage. Not routinely done and I recall someone discussing the forces exerted and they were high. Higher level/cost machines have disengagement stops for the carriage feed that operate within the carriage. I set the micrometer carriage stop as a hard stop more so to prevent the carriage/cutter crashing into the chuck which often results in significant damage. I also use electronic stopping which has its merits/accuracy but is more difficult to implement and is very different then a mechanical stop system.

The micrometer stop is not used as a hard stop for threading. So plan on disengaging manually and only use the micrometer stop more as a safety device, with the exception of using the feed shaft that has a clutch, which is not the de-facto. Even in the with a feed clutch, I do not know of anybody that routinely uses it in this manner. On the higher end lathes the feed shaft clutch tension can be adjusted with a knob, and some have different forms of limit switches.

 

[mikey]

A "micrometer stop" is actually intended as a precision end point when boring or cutting to a shoulder in a turning operation. You feed until you get close and then disengage power feed and feed manually until you gently contact the carriage stop. This gets you your precision depth.

Stops are not really intended as a substitute for a brain. Likewise, on lathes that have clutches at the end of feed rods, they are not a substitute for proper technique; they are there in case of a crash so you don't destroy your gear train. You might destroy your chuck, tool, tool post and maybe your compound but the rest of the lathe should survive, hopefully.

The guys are right. We either learn to disengage half nuts on time when threading or we learn to thread out, away from the chuck. Limit switches are another option but with just a bit of practice you brain is actually quite good enough, I think.

 

[graham-xrf]

My thanks to all of you. It helps a lot to get the words of experience!
mksj - thanks for the pictures.
Chuck K - I get distracted. Curious about what a Hendy looked like, I ended up reading their history, which comes with lots of links to great photographs, from 1871 through to 1950s when Hendy were taken apart by a corporate raider.

I have acqured two 9" South Bends for bargain silly ££, a 9C and a 9A, both in reasonable underlying condition. The 9A is in a total stripped down state, the present activity being to give it the kind of cleanup and maintenance it now deserves.

I came across a YouTube video of a micrometer carriage stop being made, and also I spotted one for sale on eBay. For the present, there is much more important stuff to get right, but one day, I will likely add a micrometer stop to at least one of them.

 

[eeler1]

What they said. Undercuts or threading away from the chuck work well if you can do it.

If you must, for some reason, use a carriage stop while threading, you can shut the machine off as you approach the end of the thread, then crank the spindle by hand till you get to the end, leaving the half nuts engaged. I’ve done it this way when I couldn’t undercut the end of the thread. Had to make a crank handle for the outbound end of the spindle.

 

[graham-xrf]

If you must, for some reason, use a carriage stop while threading, you can shut the machine off as you approach the end of the thread, then crank the spindle by hand till you get to the end, leaving the half nuts engaged. I’ve done it this way when I couldn’t undercut the end of the thread. Had to make a crank handle for the outbound end of the spindle.

eeler1: Thanks for the handy work-around. At my level of experience, I was just looking at a (micrometer stop), and my mind was playing out the scenario. I get it that it takes serious mis-attention for a crash to happen. The actual need should be minimal. Threads do not usually need to get made right up to a shoulder. There is usually at least one turn's worth of inner diameter cut before one is truly out of space, and if it needs to be accurate and repeatable, then sure, let us use a stop - and approach it manually.

I am guessing that bad stuff happening from inadvertently having the leadscrew drive the carriage into an unfortunate situation must happen sometimes, and not only into a micrometer stop - but maybe not so often!