Thread: Cracked chuck screw

First choice if possable would be to get a replacement scroll.

You dont have a real lot to work with so what ever method you use be careful not to get any thing on the threads.

Is it only one corner thats cracked?

If its only one it shouldnt be that greater concern.

I have an older 8" chuck that has one complete side of the square missing,the chuck still performs its job,I do take care when setting up to make sure that this jaw gets most of its loading from the opposite jaw pushing the work piece into it.

Have never done this but if all else fails and if the head of the scroll has enough meat in it you could attempt to drill and tap the centre and use a cap screw in the end of the scroll.

Originally Posted by pipeclay

You dont have a real lot to work with so what ever method you use be careful not to get any thing on the threads.
Is it only one corner thats cracked?

It's an old chuck (no brand) so spares will be rare at best. Three of the corners are cracked. The one in the picture is the worst.

You're right that there is very little meat to play with. If I try to weld it I'd try to hit the circumference at the end of the screw. Then I'd have to file the area back to the minor diameter of the thread. But even if this works I might wreck the heat treatment of the area and make the whole thing useless.

Thanks,

I would oxy weld it, tig might do it as well,but I have 0 experience with TIG.

I would make sure of what the parent material is before welding anything.I would suspect it is probably cast steel.

I assume that you have oxy as you mentioned it.If you indeed have an oxy plant, brazing will be the simplest option.

A nickel bronze brazing alloy would be strong enough given you have the skill to braze it properly.

If you do wish to braze and need a walk through ,say so and I'll post the steps for you.

Grahame

Originally Posted by Grahame Collins

A nickel bronze brazing alloy would be strong enough given you have the skill to braze it properly.

Hi Grahame,

Thanks for the brazing tip. I don't have gas nowadays (expensive luxury) but I will see someone tomorrow who has and hopefully they will assist this week.

I'm planning to use degreaser to clean the screw then I'll heat it in an oven to boil off any remaining oil. Does this sound reasonable ?

Yes please give me some hints about nickel bronze brazing.

Thanks,

What's the worst that could happen if it fails in use, is it likely to fling a jaw somehow if it's annealed by the heating and strips the thread?. I'd be inclined to buy another chuck off Ebay for $150 or so and use that one on a rotary table in a non rotating application .

Good point.
I failed to consider that.
I have seen a metal lathe spit anything as yet and I don't believe I want to.

Grahame

Wouldnt worry to much about damaging the thread by heat.

If you did or do soften the thread you would either feel the teeth break as you were tightening your job,or feel them stretch.

You would need to strip all the threads from the scroll to get the jaw to fly out.

It could possably happen but I think VERY unlikely.

What type of work do you use the chuck for,is there enough material at the root diameter to drill and tap.

Originally Posted by Graziano

What's the worst that could happen if it fails in use, is it likely to fling a jaw somehow if it's annealed by the heating and strips the thread?

I doubt it would fail while in use. At the moment the square hole will open up if I lean on the chuck key. If it's brazed and goes soft I think it's still the square hole that will fail rather than the thread for the jaw.

The chuck is a nice old chuck so I want to preserve it.

Cheers,

Originally Posted by pipeclay

Wouldnt worry to much about damaging the thread by heat.

What type of work do you use the chuck for,is there enough material at the root diameter to drill and tap.

The chuck is only for light work on a small lathe. I don't know who cracked the screws and how much force they used or if they used a badly fitting key.

The holes are about 10mm deep and the deepest cracks are 4mm deep. I was making chuck keys for another chuck today and was thinking that I could make a new key for this chuck and recess the key from 6mm back to 10mm back. This would stop any force on the cracked portion of the hole.

I like Grahame's idea of brazing. I could make a shield to stop the flame from hitting the threads.

I also like your idea of fitting an Allen bolt. I've done this on other fittings to get a strong thread on a mild steel head. But I could only fit an M5 bolt and it might be a bit small for the task.

Cheers,

You know it's ironic that today I was holding about a dozen of those chuck screws from a box marked "Metric Sheraton Lathe Spare Parts" that came as part of an auction lot for which I have no use for. I doubt they are the same size though and they have a hex hole instead of a square one.

Don't forget that there are significant forces on this part when you have it tightened down and then exert G forces when it's cranked up to 2000 rpm that would be many times the static tightening force on the thread. Anyway it's your part and your skull you are risking, why not just make a new part from 4140 bar?.

Originally Posted by Graziano

You know it's ironic that today I was holding about a dozen of those chuck screws from a box marked "Metric Sheraton Lathe Spare Parts" that came as part of an auction lot for which I have no use for. I doubt they are the same size though and they have a hex hole instead of a square one.

Do you have these screws ? Mine are 11/16" OD, 8 TPI Acme thread, and 1 5/8" long. The recessed portion for the locating lug is 3/8" dia, starts 3/4" from the inner end, and is 1/4" long. I'd gladly replace all four.

Cheers,

Originally Posted by phelum

Do you have these screws ? Mine are 11/16" OD, 8 TPI Acme thread, and 1 5/8" long. The recessed portion for the locating lug is 3/8" dia, starts 3/4" from the inner end, and is 1/4" long. I'd gladly replace all four.

Cheers,

I'll measure them up and take a pic but the odds are long

as requested here is some info on brazing.

Comweld Tobin bronze
Features
• • Ideal for braze welding joints in brass and bronze
• • Used for braze welding of mild steel in low stress applications such as the ‘filling’ of car body panels
• • Not suitable for cast irons

• Specifications
• Classification AS: R Cu Zn-A

Comweld nickel bronze rods
Features
• • Suitable for high strength braze welding of steels and cast or malleable irons
• • Fuse welding of copper based alloys of similar composition
• • High strength, wear resistant brazing alloy

• Specifications
• Classification AS: R Cu Zn-D
The above is the specs for Tobin bronze and Nickel bronze cut and pasted straight from a vendors site.
Again ,Nickel bronze is the the filler to use.

As with all oxy acetylene brazing processes the idea is to have the molten deposited pool melt the filler. This means heating the parent metal to a temperature which allows the filler wire to melt and bond with the parent metal.
In other words you DO NOT melt the end of the wire off with oxy acetylene flame as it will not bond to the parent metal being repaired.
It is NOT welding as at no stage will it fuse or mix with parent metal. The process is a bonding process meaning it is more akin to a mechanical bond of a glue which permeates the rough parent metal substrate-as viewed under a microscope and grips into the.

This is the reason that the surface of the repair where bonding is required has be cleaned up with a file or grinder.
The correct flux which is white needs to be used.

Grahame