Any ideas how to repair broken leadscrew

[Ian PParticipant @ianp]

[Ian PParticipant @ianp]

I have a high precision dovetail slide which I was going to use to hold a second quill on a vertical milling machine. The slide was sat on a shelf under the bench and the handle got hit when I dropped a small vee block. The leadcrew is glass hard and snapped like a carrot!

The leadscrew has a 12x1mm pitch thread and the point where it has broken is threaded M7x1 The plain parallel section is 9.80mm diameter and 20mm long. The 7mm dia ground portion is about 10mm long and fitted into the handwheel ballraces.

I am not trying to get the leadscrew back exactly as it was but rather modify or adapt the bearings/housing and handwheel so that I can make use of the leadscrew and nut. My first thought is to machine an adapter that I can loctite on to the 9.8 dia part but I am concerned about keeping it absolutely concentric because the bronze nut is a very close fit (over 50 threads!).

Is it possible to create threaded holes in very hard steel, say by spark eroding?

Edited By Ian Phillips on 08/04/2012 16:14:07

[Phil PParticipant @philp]

Yuur idea of loctiting an extension sounds OK, but I would leave it over size on its OD and machine it to size as a last operation.

Loctite 601 is what I have used for years, and its never failed me yet.

Phil

[Tony Pratt 1Participant @tonypratt1]

I can't remember if you can create threaded holes in hardened steel by spark eroding but if you can it will be extremely expensive!! Your best bet would be to drill a hole in the leadscrew with a carbide drill, loctite a shaft in and finish machine said shaft in situ to ensure concentricity.

Tony

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

All sorts of complicated and even re entrant holes can be spark machined but this is not the way to go even if you could find anybody to do it .

Make a very accurate threaded collet to to hold the screw by the screw threads . This needs to be a true collet which closes and which grips on the threads not on the screw od and which has a long engagement .

By grinding or turning with carbide tools reduce the whole of the remaining plain parts of the screw back to where the threads start to one common diameter and as large as possible .

High strength Loctite a sleeve in place with a solid metal extension to get adequate length for the handle . Measure up carefully and make a little longer than original to ensure solid metal everywhere after finish turning . Turn down to make the bearing steps and handle fixing . Make the sleeve in advance so that screw never leaves collet until job is done .

Michael Williams .

Edited By MICHAEL WILLIAMS on 08/04/2012 19:12:44

[Ian PParticipant @ianp]

Michael

Your method would be the ultimate way to go but a true, long, collet that grips on the threads would defeat me. As it happens the tops of the threads have been ground off (I thought it was an Acme thread at first) and the resultant OD is probably true to the axis of the screw. In theory at least, and with it being dead on 12mm diameter it should be possible to grip it in a high precision collet. Unfortunately I do not have anything to suit.

If I could find some way to hold it accurately I could still use you method, but if I make an adapter with a stepped bore I would not need to reduce the diameter with carbide tooling. If the adapter does leaves some of the 9.8mm part exposed after being fitted I could use that with a finger type dial gauge and create a new centre in the 4 jaw chuck.

Ian

There is a centre at the far end of the leadscrew and the 7mm diameter part is another original reference

[Les Jones 1Participant @lesjones1]

Hi Ian,

If you mounted it with a centre and drive dog at the headstock end and fitted a ball bearing to the ground part near the break with the bearing held in the fixed steady you should be able to drill the end. If you then made an extension piece slightly over size and loctited it in you could finish machine it in position. You would need to apply pressure to the end with a live centre. The live centre would bear against the flat end of the extension piece or in a centre hole drilled after the extension was fitted.

Les.

[Ian PParticipant @ianp]

Les

What you describe is close to my original thoughts, the only problem is that the hole would have to be smaller than 7mm. Actually 6mm would be OK but will only leave a 0.5mm wall thickness, also I'm doubting whether it will drill as the steel is really very hard.

I used to have some drills (bought after a demo at an exhibition) that are supposed to drill through files although they looked no different than normal masonry drills so thats what I eventually used them for.

Ian

[Les Jones 1Participant @lesjones1]

Hi Ian,

I too have seen those drills advertised. (On one of the shopping channels.) I would not trust them to drill an accurate hole. The only further suggestion I can think of is mount the leadscrew in the lathe as I suggested. ( With a live centre againnst the broken end.) You could then skim the 9.8 mm (0.386&quot part to take a 3/8" (0.375&quot bearing. You could then remove the 7 mm part and drill the 0.375" part and fit the extension from there. You would still have the drilling problem. You could try heating the broken end to soften it but that may not work. One last thought is turning down part of the 9.8 mm part to about 7 mm and making the extension to fit over that part.

Les.

[Richard ParsonsParticipant @richardparsons61721]

Know any one who has an electric butt welder?

If so try them you could get the two bits butt welded fairly accurately

Rdgs

Dick

[Ian PParticipant @ianp]

Richard

'Fairly accurately' would not be near enough for this part. I would imagine the manufacturers tolerance was measured in microns.

I could imagine a very high instantaneous current from a stud welding machine joining on blank bit of steel which could then be machined up.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Ian ,

There is always a way .

(1)

Instead of the collet prepare a chucking bush about 3/4 inch diameter and at least 1 1/2 inch long . Mild steel or quality brass but not common brass or aluminium .

Put in four jaw chuck , centralise and grip reasonably tightly . Drill and bore tapping size for your thread and then ideally screwcut but ok to tap if careful your feed screw thread . Aim for the best fit of feed screw in your resultant threaded hole .

Pre make a lock nut wth your feed screw thread in it .

Enter the feed screw into your threaded bush and lock in place with the locknut .

After that proceed as for collet method .

(2) If you think that feed screw O/D is accurate enough to grip by then make a plain chucking bush for three jaw chuck . This is a plain bush with about 3/32 inch wall and bored or reamed to take feed screw with a very close fit . Once in slight further tightening of jaws will distort bush and grip screw . This won't work properly if screw is initially a poor fit in bush .

After that proceed as for collet method .

Michael Williams .

[Richard ParsonsParticipant @richardparsons61721]

The break seems to be in the part where the handle fits. The bearing looks as if it is on the main part of the shaft.

If the bit of steel you had welded on then you will be able to machine it to size.

The thing is it is a lead screw its major element for precision is that it has little or no back lash.

If it is set up carefully a butt welder can position within the tolerance allowed by my Chinese inspector friend Wun Thou Wong. The butt welded area would need to be ground down.

Rdgs

Dick

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