Repair or improve elevation screw for steel jig

dbooksta · Jul 20, 2017

I have been using 5/16-18 hardened UNC 3A screws to fine-tune the elevation of a steel jig. But they're bearing on a 3/4" flange of mild steel, which is developing a bit of slop – maybe because the tapping wasn't 3B to begin with, or maybe because the hardened screw threads are wearing on the softer steel threads.

I'm trying to figure out if there is a way I can fix or improve this. Here are three thoughts:

Restore the tightness to the hole threads? I tried using blue loc-tite for this, but that just makes it harder to turn the screws, and doesn't actually tighten the fit.
Drill out and replace the tapped holes with a hardened threaded steel insert. I haven't found the right one, but I'm suspect there are good options out there.
Is there a better approach for precise elevation adjustment on a roughly 50-pound jig than this flange-and-screw?

DDoug · Jul 20, 2017

Bigger threads, I would upsize it (based on your 3/4" thick ear), to 1/2".

While your in there drilling it out, put in a keensert, heavy duty series.

Or get into acme theads, buy a acme nut and threaded rod, weld the nut in there

WizardOfBoz · Jul 20, 2017

I agree with bigger thread. Perhaps finer pitch, if you want better adjustability.

Also, do people make bronze cup bushings for jack screws to bear on? That would get around the issue of the jack screw digging into the base. IF they don't sell them, not hard to make I think.

Illinoyance · Jul 20, 2017

Add a wing nut above the boss. Adjust the screw then snug the wing nut. Slack is gone.

Tony Quiring · Jul 20, 2017

How often and how much is it moved?

One could drill and tap a larger hole that would allow a replaceable unit to go in.

Example for clarity is drill to say 5/8 thread.

Take a 5/8 bolt and drill and tap a 5/16 through port.

Select correct material for bolt and make 3.

On head of 5/8 bolt from a flat drill and tap for a set screw and use a screw with nylon or soft tip to jamb the adjustment screw.

Add a pad to the bottom via drilled hole with a ball and grease or other process.

When it wears out grab another bolt..

Sent from my SAMSUNG-SGH-I337Z using Tapatalk

dbooksta · Jul 20, 2017

Tony Quiring said:
How often and how much is it moved?

One could drill and tap a larger hole that would allow a replaceable unit to go in.

Example for clarity is drill to say 5/8 thread.

Take a 5/8 bolt and drill and tap a 5/16 through port.

Select correct material for bolt and make 3.

On head of 5/8 bolt from a flat drill and tap for a set screw and use a screw with nylon or soft tip to jamb the adjustment screw.

Add a pad to the bottom via drilled hole with a ball and grease or other process.

When it wears out grab another bolt..

It's adjusted during use very frequently.

I'm not sure I follow what you're describing here. But if you mean to use a set-screw bearing perpendicularly on the elevation screw to tighten it up after every adjustment, it's adjusted too often for that to be convenient.

akajun · Jul 21, 2017

I've used lead shot to make a set screw to tighten up the fit of sloppy screws before. Drill and tap for the set screw, insert your adjustment screw, then drop one or two lead shot in the set screw hole and squish them down agAinst the adjustment screw. Do this a few times as the lead flattens out pretty small. The lead will conform to the thread , creating an almost zero tolerance nut and will force the screw to the other side of the hole. By playing with the set screw a little you can lock the screw or have it move with no slop.

michiganbuck · Jul 21, 2017

With wanting to save the fancy screw you could buy a high grade nut and test the screw..Then if good drill and ream a ½ hole in the fixture..Make a threaded tube of ½ “ crs drilled and tapped your 5/16-18 size.. loctite in the threaded bushing. Done and all with nothing more than a drill press and a half hour time.

Agree CRS wont last forever so 40 years down the road press it out and make another.

Or you could drill and tap 1/2-20 and then make a Dial head screw with graduations..

could tap 1/2-40 and attach the barrel of an old micrometer

Low ball 1/2 40 tap about $15 or so.

DDoug · Jul 21, 2017

dbooksta said:
It's adjusted during use very frequently.

I'm not sure I follow what you're describing here. But if you mean to use a set-screw bearing perpendicularly on the elevation screw to tighten it up after every adjustment, it's adjusted too often for that to be convenient.

50 lbs on that little screw is too much, especially with frequent usage under load.
Increasing the dia will reduce the wear, and keep the tighter fit longer.

And that skinny thread extending so far below the restraint looks wobbly in it's self.

Bigger thread, and switch to acme for wear.

McMaster Carr has what you need.

Bill D · Jul 21, 2017

You could trap a ball bearing under the point of the screw. just make sure the side down has the inner race below flush.
Bill D

michiganbuck · Jul 21, 2017

You could clarify what the machine, the expected travel and the tolerance just to make this more interesting..

I know Freud said everything is about a screw... but on occasion one is just to busy for that.

Mtndew · Jul 21, 2017

digger doug said:
Bigger threads, I would upsize it (based on your 3/4" thick ear), to 1/2".

While your in there drilling it out, put in a keensert, heavy duty series.

Or get into acme theads, buy a acme nut and threaded rod, weld the nut in there

This, use a threaded insert.

John Garner · Jul 21, 2017

If it were my fixture, I'd find a way to use larger-diameter screws with swiveling pads, ideally running in snug-fitting bronze bushings, and adding some form of self-wrenching jam nuts would probably be a fine idea also.

dbooksta · Jul 22, 2017

John Garner said:
If it were my fixture, I'd find a way to use larger-diameter screws with swiveling pads, ideally running in snug-fitting bronze bushings, and adding some form of self-wrenching jam nuts would probably be a fine idea also.

Yes, I'll definitely go larger diameter, finer threads, maybe Acme.

When you say swiveling pads with snug-fitting bronze bushings, do you mean for the "pointy" end of the screws to rest in? Do you happen to have any pictures of what you're describing?

mwilcox972 · Jul 22, 2017

dbooksta said:
Yes, I'll definitely go larger diameter, finer threads, maybe Acme.

When you say swiveling pads with snug-fitting bronze bushings, do you mean for the "pointy" end of the screws to rest in? Do you happen to have any pictures of what you're describing?

I would use a buttress thread. Machine it so the flat of the thread carries the load. Make a corresponding nut and weld it to the fixture.

DDoug · Jul 22, 2017

mwilcox972 said:
I would use a buttress thread. Machine it so the flat of the thread carries the load. Make a corresponding nut and weld it to the fixture.

Lot's of custom machining with this method, I don't think the OP
needs the ultimate in load handling (nor the price tag).

Plus the buttress thread form is really made for pushing in a vice, guided externally.

Simple off the shelf components should provide a simple, inexpensive, long lasting fix.

gbent · Jul 22, 2017

I agree with the screw being to small for the load. Increase and/or change thread form as desired.

After all that, I would split the foot vertically and add a cross bolt across the split. In this manner you will be able to adjust the tension required to turn the leveling screw.

Only you can decide how much effort is worthwhile for the fixture. If you live with the problem, the fixture will be in use for at least 5 more years. If you spend two days fixing it with split adjustable ampco bushings, hardened fine pitch screws, and swivel feet the fixture may become obsolete before the year end. DAMHIKT

extropic · Jul 22, 2017

For fine adjustment make a differential screw thread adjuster. Maybe 5/16-18 inside 1/2-20.

I'm wondering if the "bit of slop" issue is a symptom of the threaded adjuster(s) not being stiff under lateral loads. If the root problem is how to control lateral loads, think about a flexure foot design, stiff in the plane of the table and flexible enough to be driven by an adjuster.

Repair or improve elevation screw for steel jig

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