Repairing a Myford 4 jaw chuck.

Just took a look at my Myford's 4 jaw. I can see no reason on earth why those screws would be made of cast iron. Steel would be my bet with above 99 per cent confidence.

The wall thickness around the square hole is risibly thin. Would not take much to fracture it at all. Probably why the replacements have a hex hole.

make four new screws from hex head allen grub screws and then make a new key from a cut off allen key. I have done the same on a myford four jaw.

Brian

Simple but effective:

Edited By Hopper on 31/03/2022 08:20:18

I did think of using outsize grub screws to make new jaw screws. That went down the pan when I took the offending screws out of the chuck. They were ACME threads, so not much chance of finding ACME grub screws.

Donkey, as a matter of interest, what were the threads on your Myford 4 jaw? Seems as though there were ACME and Old Mart added square threads and by the sound of it, yours appear to have some sort "normal" standard thread.

I have made the missing pieces for repairing the two damaged screws. Waiting for it to warm up a little, before I braze them in place.

Andrew.

I've never seen grub screws that long in that diameter range either. Would be handy to find some though.

The chuck screws are a bit of an awkward thing to make/repair from what I can see in a quick look. The square hole goes right down inside the Acme thread but the corners of the square hole are very close to the root diameter of the thread, so there is no room there to play about boring the original screw out and adding a spigot made from a grub screw turned down etc.

At the moment I think you are on the best route: braze or weld some repair pieces in and hope for the best. Put those two jaws ins subsequent slots in chuck and use them as your initial setting jaws then do the real tightening with the other two that are still intact. .

Are you an optimist or a pessimist Rob? Life's disappointments have taught me to expect the worst! This is not the same as defeatism.

Thing is, neither of us knows how hard Andrew's chuck was walloped or how resistant to damage a Pratt-Burnerd 4-jaw independent chuck is. Optimists will notice it's a heavy block of metal, probably cast-steel, with a brand-name that magically protects it forever against everything. Pessimists see a expensive precision chuck that's been seriously abused by a previous owner. Someone hit it hard enough to break two jaw-screws, probably by pounding on the chuck key to get a stuck chuck off the spindle. I guess someone broke the first screw and decided the chuck was done for, proceeded to break a second, and finally got the chuck off by hitting a third.)

If the jaw-screws were made of easily broken cast-iron as Andrew suspected, then damage would be limited to the screws. Unfortunately cast-iron isn't used to make screws because it's weak in tension, so the screws are at least mild-steel. But this is a well-made Pratt-Burnerd, where the jaw-screws are almost certainly made of a tougher steel, taking considerable force to break.

Best hope is Old Mart's suggestion that a ham-fisted operator caused fatigue cracks, and they eventually broke due to repeated over-tightening rather than a single violent event. I fear Old Mart is wrong, because ham-fistedness would break one screw, and stop. Andrew's chuck has two broken screws, which suggests aggressive hammering.

I think Andrew's justified in mending his chuck, but no-one should be surprised if the repair doesn't make the chuck 'as good as new'. As a pessimist I advise checking the surviving screws with a lens for cracks, dings and bending : there's no reason to believe they're OK.

Dave

At no time did I mention "fatigue cracks" being responsible for the corners of the square sockets being cracked. The cracks are caused by overstressing the thin corner sections, usually by using the key to unscrew the chuck by hammer merchants. I also mentioned, just in case somebody was going to get replacemant screws that they were made either in square OR ACME thread forms which are not interchangable as they have to fit the existing jaws.

 I like Davids C spanner method, it is better for 4 jaw independents than my strap wrench method. Unfortunately, we have chucks from 100mm to 6 3/4" in three and four jaw plus collet expanders and er collets on backplates, not to mention three faceplates, and my shop made strap wrench covers the range quite well.

Edited By old mart on 31/03/2022 16:54:27

SOD - in this case heavily optimistic, as a) I've got a Myford 4 jaw chuck (wouldn't want to drop it on my foot), and b) I don't regard a 4 jaw as anything approaching an inherently precision device, more like a spinning clamp adjustable in two axes.

But still no reason to abuse them

And to hijack the thread, as is my wont, I've just cleaned up a Taylor 3 jaw which has a dished front face and strangely angled jaws which move closer to the back of the chuck as they close. Not seen anything like it before.

Rob

Edit - just found this video on how to repair/replace four jaw chuck screws

Links to https://www.youtube.com/watch?v=u9zcgoJJ_6A

Edited By Grindstone Cowboy on 31/03/2022 17:34:12

I repaired a cracked chuck screw with my cheap stick welder. To avoid a problem with penetration into the square socket I placed a piece of square aluminium in it acting on my theory that I can't weld the aluminium. It worked perfectly and I'm certainly no expert at welding. The chuck has been OK for at least ten years. I can't believe that a chuck screw would be made of cast iron. Colin

Hello again,

Just to report that SIF bronzing a replacement piece into the screw worked a treat. A little filing and a quick skim in the lathe and it looked perfect. I started to reassemble the screws and jaws and noticed that there was room for a sleeve to go around the weak area. I turned a couple up and loctited them in place. So that won't break again!

Repair was better than new despite SOD's pessimism. I actually took heed of SOD's tongue in cheek suggestion that I buy £15,000 worth of kit from Ebay to analyse the material of the screw. I remembered that one of my ex collegues had access to a scanning electron microscope with EDX analysis (SEM-EDX). I gave him a small piece of the screw, which I had to remove to square up the joint. Results were that it was steel and not cast iron. It seems that the fatigue crack produced a surface that looked like cast iron, as SOD said (with picture) in one of his replies.

Regards,

Andrew.

Andrew, can you please post some pictures of the repair that you have made please.

Thanks,

Neil

Glad that you have got it sorted!

Presumably, the other screws have also been sleeved, as a precautionary measure?

Howard

Glad that works well for you Chris, but seeing you asked, and FWIW;

I only have a little Myford ML10, and only since 2016. For 30 years before that I had a Hobbymat MD65 with 3 studs, so the issue only recently arose for me.

My method is to use a 3/8" OD rod in the tailstock chuck, with a centrally drilled sliding fit 3/4" AF hex MS bar approx 2" long for the 3 jaw chuck, and a similarly prepared 20mm square bar for the 4 jaw. This is primarily for mounting the chucks because the ML10's spindle register is an extremely close fit, and engages before the thread, so if I present the chuck up by hand even slightly cocked, it jams on the register, and the pinpoint loads incurred and the bruising...ughh, makes me wince to even think about it.

For the independant 4 jaw chuck, I just leave the block in the jaws on removal so it's already aligned for next installation, but I've turned the corners off one end of the square block to match the ID of the chuck body, so it can be located centrally and the jaws snugged up when the chuck isn't mounted.

But to the point in question; this tooling also provides spanner flats to untighten an overtorqued chuck, sending the torque back through exactly the same path it was designed to go from the motor to the workpiece, so it can't be wrong. I wouldn't risk using a chuck key to undo a tight chuck, I mean....you'd never consider plugging two 12" long 3/8" drive socket extensions together to use as a 2 foot crowbar!!! And a block of wood between the jaws as a lever; I considered that, but that's a hell of a sideways bending load on the jaws, worsened because they won't be clamped tight like when in operation.

This tooling works well for fitting & removing chucks, and works to remove a high torqued chuck ( perhaps from heavy cuts at large diameter) leaving one's mind free to worry about what's suffering further upstream,such as the woodruff key on the bull gear to spindle, or the little aluminium baulking post stood under the backgear disengagement bolthead, etc.

To cover this I was planning to make a manual turning handle (mainly for short single point threading, and tapping etc), and thought this might be a useful tool to keep the torque of undoing a stuck chuck, wholly within the spindle, but was wondering if a 5/8" diameter expanding bush type arrangement would be able to grip hard enough without slipping. I've never used expanding sleeves so any thoughts anyone?