repair,rebuild (replace)

[mick70Participant @mick70]

been given box of bits of what appears to be a portass clone screwcutting lathe.

headstock casting is broken.

i have 2 options.

1 repair it and get it bored so spindle ( which is missing ) runs true.

2 box it up until more skilled at machining and then mill full headstock

area flat and scratchbuild new headstock.

after reading post on building adept size lathe i like option 2.

there is no rush to do it as got several dozen projects on go.

your thoughts and suggestions please.

[mick70Participant @mick70]

[Ed DuffnerParticipant @edduffner79357]

Hi naughtyboy,

If it were me I would try to repair it so as to keep it as original as possible, that is if the broken off piece can be welded / brazed back on. Rebuilding a new headstock would be a much bigger project I think.

Ed.

[David Clark 13Participant @davidclark13]

I would bolt on an inverted u piece and rebore the bearings.

[OuBallieParticipant @ouballie]

Repair without question.

Go for it!

You will get a wonderful sense of achievement and satisfaction when done.

May take a while to build up experience and courage, but it will be one mental barrier broken down forever.

Geoff – We all started crawling at first.

[MuzzerParticipant @muzzer]

I have an almost identical Portass Model "S". And interestingly(?) it also has the same fracture. Unless I have a special Rolls-Royce version(??) the headstock spindle sits in split bronze bushes (single slit, similar to the headstock) and as they wear, you tighten up the clamp until it breaks….

The solution on my machine was simply to fit a steel strap across the fracture to hold the broken piece in place. The previous owner had done this very carefully using 4 (square headed!) bolts. I saw no need to try to improve on that.

Presumably you will be making brand new bushes for this. Obviously, as the bush wear was the root cause of the problem, you will need to. However, even having worn the original bush out, my spindle is still original and un-reground.

It would be possible to regrind the spindle but then the pulley and bull gear would need to be bushed out which would cause the scope of work to balloon. As it's a split bush, you can still persuade it onto the spindle with care and keep the work to a minimum.

The repair I described worked fine for my Portass. The clamp still adjusts the bearing adequately and there is sufficient strength in the bush itself to cope.

I painted mine a nice light blue.

Murray

Edited By Muzzer on 30/09/2015 17:07:11

[Brian WoodParticipant @brianwood45127]

Hello Muzzer and Naughty Boy,

Headstock casting cracking of just this sort was a common fate of Myford M1 to ML4 lathes which had headstock edjustment of similar type with a single split to the bearing housings at each end of the spindle..

The risk can be reduced a lot by closing the gap down onto a suitable thickness of washer, trapped in the slit, so that when the bolt is tightened it closes down onto a 'solid' surface.

Brazing the broken casting withh SIF Bronze is the way ahead and for a big lump like that, it will be best accomplished by two torches, one to provide a level of pre-heat to warm up the whole headstock end and the second, with a more intense flame to run the braze into the break. Bolt the broken piece in position with a suitable packer to hold it in place while the brazing goes ahead.

Slow cooling afterwards should help stress relieve it all and prevent cracking elsewhere.

Regards

Brian

[Nigel McBurney 1Participant @nigelmcburney1]

Provided the break in the cast iron is clean and not damaged it is possible to pull the two halves back together by using a pair of long socket screws positioned horizontally, the broken part has to be positioned accurately and the drilling carried out very accurately on a milling machine. This method is the same as the the big end of con rods in some modern engines are made and assembled, the con rod cap is deliberately broken ,during manufacture ,.the joint kept clean and undamaged the shell bearings are assembled the cap fitted and the big end bolts tightened. The "broken " joint between cap and conrod fits very closely and forms a very positive joint . I repaired a broken bearing cap on a vintage open crank engine by this method and it has held together and worked very well for 30 years.

[Phil WhitleyParticipant @philwhitley94135]

Dammit Nigel you beat me to it, thats exactly what I was thinking!

Phil

[Brian WoodParticipant @brianwood45127]

Hello Nigel,

That's a neat trick, not one I am familiar with and one to remember I think.

My only reservation in this salvage would be from flexing of the joint later across the two bolts since that section will be in cantilever.

Maybe a belt and braces approach might be better here with brazing into a vee groove on three sides around the break edges, using the two horizontal bolts to clamp everything together in register for the brazing. The joint faces will be newly made and any corrosion on the broken faces may be safely ignored

Brian

[MuzzerParticipant @muzzer]

Brian – to adjust the bearings on these machines, you nip them down until the spindle is almost binding. Putting a washer in the gap isn't going to change anything. You can either nip it up enough or you can't.

To avoid breakage you need to monitor the wear on the bush (= gap in the headstock) and at some point decide that it would be sensible to replace it for one with a marginally smaller bore. There weren't many rocket scientists involved in the "design" of these things but it's possible you could work out the minimum safe gap and then make up a suitably thick washer but I doubt if anyone has ever gone through this exercise. I'd love to see someone try, especially when trying to account for the presence of the bronze bush in there.

As I said, the bush is strong enough to cope by itself with the remaining 3/4 circumference support plus the support of a bodged up strap. The reason the casting is so heavy around the bush is simply down to the crude, rough and ready design, not the need for rigidity nor the result of precision design calculations.

Murray

[OuBallieParticipant @ouballie]

Nigel,

Headstock:

Which end would you drill in from on that headstock?

Big-ends:

Never knew that, and I worked on racing engines in the '70s.

Thinking about it, that 'break' makes a perfect join.

Could be simulated on 'normal' big-ends I wonder?

How do they/did you 'break' the big-end?

 

Geoff – Never stop learning

Edited By OuBallie on 01/10/2015 09:41:37

[Ady1Participant @ady1]

I presume you would drill in from the broken piece to the main casting horizontally or at an angle and use high tensile hex bolts

Don't forget that if you can find a decent welder, welding may be an option

[KWILParticipant @kwil]

I knew a Polish welder in Leicester who could weld anything in cast iron, high nickel weld rods spring to mind.

There is always stitch repairs and this http://www.castironweldingrepairs.co.uk may well help you.

[Brian WoodParticipant @brianwood45127]

Murray,

I agree with you in principle and the Portass case in point is much more massively built than the relatively slim bearing housing on the early Myford lathes I mentioned; I think that the temptation to 'give it just a trifle more' was what failed so many.

I've seen the method used elsewhere, notably on the overarm support on my Senior mill which has a fairly light section cast housing round the bronze bush for the arbour. It also has the merit of reducing any tendency for the clamp bolt to slacken with transmitted vibration working on the threads.

Brian

Edited By Brian Wood on 01/10/2015 10:55:33

[VicParticipant @vic]

I wonder if it's broken once what's to stop it breaking again. Sounds like a flawed design. If that is the case it may be worth using it as best you can in a new way?

[John McNamaraParticipant @johnmcnamara74883]

Hi NB

How about packing the adjustment gap to the correct height and pinning and epoxying both sides of the broken piece to the main casting. Instead of plain pins you could deep drill, tap and counterbore the holes for a number of small cap screws then epoxy or Loctite the screws in place. I would turn up a temporary plug (well waxed) to keep everything aligned while the epoxy and Loctite cured.

Clearly there is no means of adjustment after solid pinning, If you turned up a nice tapered bore flanged bronze bushing and set that in the hole together with your newly made spindle with a taper to match you would have a better setup than original and it would be hidden from view. I would fill the set screw counterbores with body filler and paint over them.

Regards
John

Edited By John McNamara on 02/10/2015 04:39:07

[Ian S CParticipant @iansc]

I'd go with Nigel McBurneys idea of two cap screws, properly placed the bearing housing will I think be stronger than the original. With some bearing housings like this, it is normal to pack the slot with shim brass sheets soldered together, and if you require to adjust the bearing, you take out the shims and tear one off, put the rest back in and tighten it up.

Ian S C

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