Screwless vise screw stripped, why?

[WobblyHand]

So I've been using a 3" toolmakers screwless vise for a while. Every so often, I'd say once every 2 months or so the screw strips. This has happened 3 times now. I've been using grade 12.9 screws, sourced from McMaster, so they shouldn't be junk. (But they are all from the same lot.) The nut in the vise seems to be fine. Is this just what happens? I'd rather the screw strip than the nut, but what's going on? Using a short allen wrench (under 3") on the M8x40 screw, so it's really not possible to reef down on the screw. When it happens, the vise appears to tighten then it gets loose. That's when the threads yield. Any ideas? What would happen if the screw was slightly too long? The strip point is near the top of the nut, I think.

I don't know where the first screw is, but it failed the same way.

And why on earth is this kind of vise called a screwless vise! Those things above look like they were screws once!

 

[KyleG]

What do the threads in the nut look like? Maybe the minor diameter was drilled too large. It doesn’t look like those threads had to deform much to strip. It’s also interesting that only one thread is damaged.

 

[WobblyHand]

I'll have to dig out the vise later tonight to get a picture. The nut threads look fine - or at least they did the last time I looked. I will measure the minor diameter.

As for stripping one thread - after that moment, I know the screw is done, so I back it out, since there is zero clamping force.

 

[darkzero]

I've only stripped the screw once on my smaller toolmaker's vise that I use for the bandsaw but I figured it was just a low grade SHCS. I don't remember if it had markings showing the grade. I just replaced it with a SHCS from the hard store & hasn't happened since. I don't remember if it was grade 10 or 12 (metric).

Mine also gets damaged on the threads near the head like the ones in your pic. That's caused by spherical washer/spacer but it doesn't affect tightening. But as you said, I rather the screw strip out than the nut.

 

[benmychree]

You could make a new nut that takes a USA screw.

 

[WobblyHand]

You could make a new nut that takes a USA screw.

I was thinking about that a few minutes ago. Might be possible to upsize the screw slightly. From 8mm to 3/8". Do you recommend a coarse or fine pitch for this application?

 

[benmychree]

Fine thread, I should think.

 

[WobblyHand]

I've only stripped the screw once on my smaller toolmaker's vise that I use for the bandsaw but I figured it was just a low grade SHCS. I don't remember if it had markings showing the grade. I just replaced it with a SHCS from the hard store & hasn't happened since. I don't remember if it was grade 10 or 12 (metric).

Mine also gets damaged on the threads near the head like the ones in your pic. That's caused by spherical washer/spacer but it doesn't affect tightening. But as you said, I rather the screw strip out than the nut.

In the picture one can see the shiny threads near the head, that comes from the spherical washer, the stripping is below it. I suspect the stripped thread is coincident with the first few threads of the nut.

 

[hman]

View attachment 373176

Something looks very strange here. I have that kind of vise myself, and it's the threads furthest away from the screw head that engage the nut ... not just the one or two threads 1/3 of the way down from the head which are stripped on your screws. Can't figure out why it's just those few that are gone. The threads further toward the head do look a bit boogered ... but I assume that's where the screw goes through the washer on the movable jaw, and sometimes experiences side forces.

My best guess is that the manufacturer did a poor job threading the nut ... but how they got threads near the surface without forming threads deeper inside is a mystery to me.

Please post a photo or two, showing the rest of your vise, especially the nut.

... meanwhile, here's a mod I've done on my own "screwless" vise that has served me well for a goodly number of years. Mine is threaded 1/4-20, but the principle can be adapted to other thread sizes.
Post #15 at https://www.hobby-machinist.com/threads/mill-vise-opinions.37171/page-3#post-319154
-or-
Post #10 at https://www.hobby-machinist.com/threads/screwless-precision-vises.55048/#post-455018

And why on earth is this kind of vise called a screwless vise! Those things above look like they were screws once!

Maybe they're just trying to "screw" with your head

 

[WobblyHand]

Bolts are marked 12.9 on the head. Package from McMaster states they are 12.9 and sourced from Taiwan.

I think it is the vise design. Here is a picture with the screw bottomed. The spring clip was removed to take the picture. If the nut is tilted up the screw will hit the edge of the hole and grind away the screw. When that happens the vise slips. The screw is hanging on the hole - and it shouldn't! It's very hard to prevent which cog on is in. Most of the time it's ok. Once in a blue moon, or once every 2 months or so, the peg is on the closer cog. Under some conditions - the screw is damaged.

The nut is fine, it fits very well. The stripping of the screw is above the nut and where it contacts the body of the jaw.