"Weak link"

[danlb]

Calculations that would encompass all possible conditions across the full range of speeds and gear ratios would be quite an undertaking, And I doubt one sacrificial element could provide both useful service and comprehensive protection for most machine tools.

That's quite true, but begs the question of how to protect against the most common crashes. For instance, I'd like to protect castings and gear train. To do that my larger lathe has a limited slip clutch mounted between the motor and the pulleys that drive the chuck and change gears. The down side is that the motor has plenty of power, but the clutch slips when turning down an 8 inch CI backplate.

So the idea of a single shear pin might need to be modified. A series of ever smaller ones may be the answer. One for the lead screw, one for the gearbox, one for the power train...

Dan

[CalM]

Disagree completely.

No reason to sit helplessly in this matter, hands folded, unable to define everything, and so unable to start any action.

**What is the threat?

Running the carriage into something under power.

**What does it do?

Potentially break gears, and halfnuts, etc, parts of the lathe in the feed that are expensive to replace.

**What about my extra delicate indicator? It might get broken and nothing else is damaged. I wanna protect it.

If you want to protect an indicator from crashes, pay attention. Don't expect something else to protect you against being careful.

The goal is to do a reasonable job of protecting major structure and mechanical parts from breakage or damage from standard sorts of accidents, the major one being power feed goofups. A pop-out bearing CAN be made to do a decent job of that. It's totally unrealistic to expect protection from every hazard, without some sort of double-check robotic vision system watching over you like a shop class instructor.

You have a choice, if you are concerned about this in the first place.

1) sit with hands folded and do nothing because you don't know how to protect against every possible problem.

2) Take action to provide protection against the typical power feed related crash breaking feed mechanism parts. Accept that there may be other bad things that occur as the chuck jaws beat your tooling, etc. That can't be handlled the same way, and you are free to develop some other idea for that, if you wish. The power feed safety is a decent idea if you are worried about it (I have never bothered about it, but others may)

For that, however, a carriage stop comes to mind, set to trip the power feed safety if the carriage is going to power into the chuck. It's not that hard to make the carriage stop easily movable and lockable. And it can be set to allow all the legitimate work you need to do close to the chuck, but keep the carriage out of the chuck's way, for any particular setup.

See? That wasn't too hard, was it?

Tiers

You have completely disregarded the thread topic question.

i.e. "Don't know how to size it to be in safe shear/thrust load area."

No one is sitting with hands folded. Rather, constructive comments are offered regarding considerations that may be included.

[J Tiers]

I understood you and Brian R to suggest that it was just too complicated to come up with any figure for the force allowable.

I ALREADY suggested both means for protection, AND at least one way to come up with the release mechanism or shear pin setting. Plus I suggested a way to avoid the issue of a chuck and carriage collision.

Were those directly on topic responses not enough to qualify as "constructive" in your estimation?

Is saying "it's too complicated" MORE constructive?

Just what are you driving at, here?

As far as the various "releasing leadscrew bearing" schemes that I mentioned, the one that looked the most promising to me had a cylinder carrying the bearing, that had, as I recall, a ball and spring detent. The spring force, and a setting screw adjusted the release force. If exceeded, the cylinder would slide and remove the leadscrew drive, but was, obviously, easily put back with no change in setting.

Edit:

Now that I think about it, I don't recall if the one I referred to slid lengthwise, or if it simply spun in position. Either one would prevent additional travel, and limit forces applied, but if it just rotated in place, it would be trivial to reset. One that slid out of position might be more trouble, although it probably could be made so that it would release, but not slide out all the way and cause trouble with the detent mechanism.

A "no replaceable parts" type release, such as described, is obviously far better than a shear pin if you will use a stop to prevent chuck collisions. The article, which unfortunately I do not recall the source of, will give details of the construction and setup.

I THOUGHT it was in one of the magazines, but COULD have been just on someone's website. A google search may find it

[Jim Hubbell]

Many, many thanks to you all. Ive learned a lot. Maybe I should explain my case in a bit more detail. I was close to the chuck turning to a shoulder, using a carrage stop. Changed the tool and forgot the stop. Bang. Built a replacement and finished the job. Now have no "crash" protection. Am leaning toward thrust protection vs shear. Bearing assy pinned to solid mount? In the origional design I wonder what the outcome might have been had the thrust been in the other direction? As was mentioned, only so much may be protected against.
Again thanks to all.

[George Bulliss]

I THOUGHT it was in one of the magazines, but COULD have been just on someone's website. A google search may find it

"A Replacement Lead Screw Bearing," June/July 2011 Machinist's Workshop. Shipped to your door for $6.50. Go online
J/J 2011 back issue

or call 800-447-7367

[old mart]

It's a funny old world, having used the Smart & Brown model A for about 8 years at the museum without breaking the pin, and having posted on this thread the other day, I crashed and snapped the pin.
I was making new 8 tpi acme screws for a Pratt 6" four jaw chuck, left hand thread, so cutting away from the chuck, in the back gear. I was experimenting with reverse to avoid disengaging the leadscrew, stopped the lathe in the starting undercut and forgot to switch forward before starting. Fortunately I had set the saddle stop to avoid running the tool into the chuck, and hitting that snapped the pin. I had a spare and a sketch with dimensions so some more will be made next week. The thin centre is 0.092", not 1/16" as I mentioned earlier.

[J Tiers]

I looked up the article. It's a 2 page article, and not overly detailed, but it does give two different ideas, one of which is similar to what I described above. Unfortunately, the key information, the release force, is not given. But some other details regarding use with the Atlas, are given.

[J Tiers]

.... Am leaning toward thrust protection vs shear. Bearing assy pinned to solid mount? ....

As I think more about it (interesting design problem, I am not planning to make one), I like the direct thrust type better than the "one level removed" shear pin in the leadscrew drive idea. The thrust type directly controls the thrust that is allowed, which after all is what is the factor that is of interest.

The standard shear pin has the screw and halfnuts interposed, so friction etc will affect the actual thrust at which the pin shears. Of course, the shear pin in the leadscrew will also offer some protection to the crossfeed, but it seems unlikely that the same exact release/shear force would adequately protect both of these different motions in any case.

I gather that you are leaning also toward a shear pin based release. I had envisioned a detent type release, but I suppose there is no reason a shear pin would not work. A shear pin might be easy to replace, plus one could control the break point by how deep a groove is put in the middle. A spring/ball detent might be tricky to get right, and what happens to the ball and spring if the released bearing slides all the way out?

Thinking further, the leadscrew release could also be done with a detent, and would be the easiest to reset, it would rest itself in one turn. The major difficulty, aside from possible diameter limits, would be the tendency to have a "hammering" effect that might continue to turn the leadscrew. One assumes that power would be cut off soon after the release was apparent, announced by the clicking of the detent releasing.

[Jim Hubbell]

Been busy. Too many projects. Am now back to lead screw/machine protection. I am considering a long sleeve on screw at headstock end. Sleeve to be keyed at drive end. Screw cut so as to turn freely at output end of aleeve. A ball/detent with adjustable spring tension to drive the screw. Probably not the best for long life. If not tripped too too often should do fine. Again many thanks for all the considerations offerd.

[brian Rupnow]

Okay--Here is my suggested solution to the problem. Fits right onto the drive-shaft or lead screw. If your carriage is lead screw driven as on smaller lathes, your golden. If the drive shaft is separate from the lead-screw and this slips, it may throw off the synchronization between tool/carriage travel and rpm of material being turned, which would yield a funky thread. If anybody wants to build one and try it, contact me and I will provide you with details to suit the shaft diameter on your lathe.---Brian