Repairing Cross Slide Screw, Need Suggestions

[David2011]

If your concerned with strength and holding surface for the tapered pin, you could do a shrink fit on something like a 3/8" dia. fit up. First chamfer the major diameter's of both pieces, assemble, and weld up the V after a few evenly spaced tacks. Then turn the weld down to the shaft o.d.. This is of course, provided you have the equipment for the task, or a freind to help you out. I've had good luck going this route. Mike

Thanks but this isn't a place I'm interested in welding. I want to keep it repairable should the need arise again.

 

[David2011]

After working with the deck guns on the battleship USS Texas I've really frown to appreciate the utility of taper pins. There are many of them in the gearboxes and hand cranks of the guns. They hold securely and without any movement but are usually reasonably easily removed for maintenance. After 80-100 years a few have been pretty stubborn but most have come out with a little heat and some solid taps. I just don't have much experience with smaller taper pins in the #0 - #3 range. Those on the 5" guns are usually #5 or #7 ant those that hold the GI sight on the AR-15/M-16/M-4 are a tiny 2/0.

 

[ddickey]

When I replaced the screw on my surface grinder I made the boss separately in a roughed in condition. Used a collet chuck and finished as one piece to limit any runout. You have any pics?

 

[gard]

I made one awhile ago and just did a normal fit but used the "permanent" threadlocker. The larger diameter hole and bore will provide the most torque if you are concerned. It can be removed with heat if needed.
Back to your original question, a 0.05 inch wall with a 0.1 inch pin would support around 75 inch lb of torque assuming mild steel of 30,000 psi strength.

 

[David2011]

When I replaced the screw on my surface grinder I made the boss separately in a roughed in condition. Used a collet chuck and finished as one piece to limit any runout. You have any pics?

No pictures yet. It’s still in use with the worn parts. I will post pics when I do the work. The Acme tap is probably on a slow boat from China. Should be here in about two weeks. I couldn’t see spending domestic prices for something that I might use twice in my lifetime.

 

[David2011]

I made one awhile ago and just did a normal fit but used the "permanent" threadlocker. The larger diameter hole and bore will provide the most torque if you are concerned. It can be removed with heat if needed.
Back to your original question, a 0.05 inch wall with a 0.1 inch pin would support around 75 inch lb of torque assuming mild steel of 30,000 psi strength.

Is that the green threadlocker? It is strong. I had to remove a sight mount from a 2011 pistol slide that was mounted with 6-48 flat socket head screws. Ended up drilling through the screws to weaken them. I tried heat but chickened out before the threadlocker gave in. Thanks for the torque info. I’ll probably do away with the pressure type knurler and get a scissor type. That’s my biggest source of concern.

 

[bill stupak]

I meant make it from the acme stick you bought, but I see your point with the odd threads.

 

[gard]

There are several companies that make a bunch of threadlockers, I do not think there is a consistency in colors to strength. Permatex and locktite have or had some decent technical information on strength, how much of a gap they will fill, shelf life, maximum temperature and cure time. Usually the tube will say something like "removeable" for the low or medium strength ones and high strength or permanent for the permanent ones. I did not do the calculation but I suspect the permanent one will be way stronger than any kind of pin.

 

[Smithdoor]

The cross slide screw on my lathe is so badly worn that the change in the width of the thread is easily seen so I'm replacing the screw and the cross slide nut. The machine is a 13x40 Victor. I've purchased a stick of 4140 5/8" 8TPI LH Acme threaded rod from Grainger and a block of bronze for the new nut. I'll be cutting the old threaded portion of the cross feed screw off, boring and reaming a hole and machining a matching stub on the replacement Acme screw stock. It will be pinned with a taper pin (first choice) or a tension pin (second choice).

My first question is concerning the diameters of the hole and stub. The OD of the boss that will become the socket on the original screw is 5/8". The minor diameter of the 5/8" Acme thread is 1/2". Based on the reamers I have on hand the options are .4375", .401" and .375". I'm concerned that at .4375" the wall thickness at .091" may not be enough to hold up over time and at the other extreme the pin will take too much metal out of the .375" stub; tied into the next question. Here's a graphic of the relative sizes based on the 5/8" OD.

View attachment 490888

The second question is the size of the taper or tension pin. Ordinarily there's so little torque on the cross screw that it wouldn't be much of a concern but I still use an old knurling tool that presses against the side of the workpiece. Is a pin with a nominal section of about 1/8" large enough?

Edit: Additional info: I have access to a hydraulic press with a large depth capacity so I intend to make an interference press fit.

It not hard to make one.
Use your all thread

The nut is always a problem.

McMaster-Carr

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You could make one but this is faster.

Dave

 

[Jake M]

I'll be cutting the old threaded portion of the cross feed screw off, boring and reaming a hole and machining a matching stub on the replacement Acme screw stock.

It's been done before. It'll be done again. If something goes south, you really aren't out much. I don't hear many failure stories....

It will be pinned with a taper pin (first choice) or a tension pin (second choice).

I do tend to agree that taper pins are quite stout. Depending on what size tooling and "stuff" you have, it's not "necessarily" going to be "that much" better. How hard can you yarn on that knob? That's a good metric. How hard is your power feed going to yarn on that knob? That's real when you're facing off a surface that was a little more irregular than you first guestimated. You figure it out right quick, but the power feed gears know exactly what's happening, instantly....

Personally, I would do a press fit and/or use a chemical retaining compound. 603 comes to mind, and here's why. It's NOT for securing press fits, but I'm gonna recommend it that way. Don't worry about reaming to a fit, instead, bore your pilot hole. There will be tool marks. That's good. Dial those in close, as you don't want "much" interference at that size. (I'm thinking half to 3/4 of a thousandth interference, leaning towards half), it's a small amount, but verify that as I just pulled that out of my I just think I remember that). It won't take a large press to a very good press fit here (although it wont' hurt). Anyhow, the "texture" from less than perfect "regular" pass on the boring (no finish pass) will give you tool marks that will make the press fit still press and take torque, and (in theory) hold the whole thing together as is. Although it is defnitely a compromise over a "perfect" finish on both parts, the "backwoods" way I'm describing still holds "most" of that. And the Loctite... The loctite retainer compound will still have somewhere to sit, so it doesn't get pushed to the bottom of the hole, where it's useless. It will give an excellent bond in this use case. Best of both worlds, and it makes strong joints. Not for NASA stuff of course, but for one off jobs that aren't getting the plans reviewed by a proper engineer... That's what I'd do. How strong? Better than either joint by it's self, but not the "sum" of the two possibilities if they could both be done properly on the same piece (which they can not, they're mutually exclusive if done correctly).

Dimensionally speaking, I'd probably shoot for a 3/32, or 0.100 inch wall thickness on your bore in the 5/8 section. (no those are not "the same", that's the ballpark figure area. There's no blueprints here). The larger diameter of the "outside part will have a much easier life than the "geometrically challenged" pin on the other part will. Save as much of the "pin" as you can without making the bored wall "too" thin.
Maybe one of your reamers comes out close to that? I'm not doing the math.... But like I say, if you can get a "OK toolmarked finish surface finish inside the bore though, overly pointy tool, or overly aggressive feed to "thread" it a little... Press plus a retainer compound that actually stays... There's some engineers screaming and hollering right now, but it's an option. Unquantifiable, but tested, stout, and functional.

FWIW. That's my thoughts on a subject with as many workable solutions as their people on the internet posting solutions....