Have broken a cast bit of my lathe – any hope of a repair?

[_Paul_Participant @_paul_]

Posted by Nicholas Farr on 13/02/2011 16:07:16:

Hi, the Magna rods mentioned should be suitable for the job if they do what they claim, however a true data/spec sheet seems to be elusive.

 

BOC do an electrode that is sutable and a data/spec sheet is avalable on the link below, page 123

 

http://www.boconline.co.uk/industrial_catalogue/

 

Regards Nick.

Edited By Nicholas Farr on 13/02/2011 16:09:05

Hi Nick,

 

Always interested in alternatives how much were the BOC ones?

 

Regards

 

Paul

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Paul, sorry I haven’t checked that out. My real point was to show good suppliers give you the technical data for welders to make an imformed choice and tell you what metals ect. are in there products.

 

I’m not trying to put Magna down, as I’ve not used any of there products, but specs to substanciate what they claim goes a long way. In my professional role I’ve used many different makes of welding electrodes, and some which claim to be good without giving very much technical info, have been a waste of time to use.

 

I used BOC in my example as I know they do good products. I have no connection with them whatsoever. As always it’s best to shop around.

 

Regards Nick.

[John CoatesParticipant @johncoates48577]

As Guy Lautard”s books are pretty expensive just for the split cotter design can anyone describe it to me or sketch it please?

[TerrydParticipant @terryd72465]

Hi john,

 

I don’t have Guy’s book but you could use G H Thomas’ design for a pad clamp, It uses a brass pad shaped to the spindle/bar with a bolt to tension it to clamp the spindle. I’ve scanned a couple of drawings from his ‘Workshop Technoques’ book and added a drawing of my own.

 

My drawing shows the clamp lugs on your tailstock removed and the slot welded. It would then need another shaped piece of round bar to be welded on at 90 degrees which would be drilled to take the clamp pad and pad bolt. I hope the pictures make it clear.

 

Pad Bolt Assembly from G H T

 

 

Brass Pad

 

 

 

My Drawing

 

Hope this helps

 

Regards

 

Terry

[John CoatesParticipant @johncoates48577]

Found a site describing split cotters

 

Machining split cotters

 

Strangely enough the locking mechanism for the cross slide uses a brass version of this principle, a brass barrel with the end scalloped out which presses against the bar the cross slide runs in when the locking bolt is screwed in

 

[John CoatesParticipant @johncoates48577]

Terry

 

Thanks very much. Very similar to the split cotter principle

 

John

[TerrydParticipant @terryd72465]

Hi John,

 

The slot in the side of G H T’s pad locates on a pin in the boss in order to prevent it rotating when being located.

 

Best regards

 

Terry

[John OlsenParticipant @johnolsen79199]

Hi Terry,

 

I made a GHT boring head which uses a double version of your fig 1.6 to hold the boring bar. The cotter is bored in place with the casting and then cut in two, either taking a bit extra off or using a wide enough saw, so that when the two halves are pulled together the ends don’t meet before you have a good grip. One half is threaded, the other half is clearance, and it all just relies on the bored part to stop it turning. Very successful, the boring bar never seems to move, and it doesn’t seem to need the screw tightening excessively. So I offer that as a possible varioation on the general theme.

 

regards

John

[TerrydParticipant @terryd72465]

hi John,

 

I assume that is what is called a split cotter?

 

Regards

 

Terry

[DMBParticipant @dmb]

Hi John,

Dont feel too guilty about your hamfistedness as you call it. I think that, (1) This is Cast Iron over 1/2 a century old and getting very brittle, (2) poor clamp design has abused the metal, renowned for its inherent weakness. The other posters wfo have described the split cotter method have described what I think is probably “best practice” with this metal. Good luck in repairing. Could you post pics of finished result?

John.

[mgjParticipant @mgj]

This weld repar – was it your intention to preheat before welding. That is fairly normal with cast.

 

Having said that, it’s not always necessary and you can get away without sometimes. In the same way as you can use a mild steel rutile rod perfectly well on alot of repairs. Not ideal but it depends on the criticality of the job.

 

At the risk of telling my granny. With any weld, like any other “casting” process the liquid metal goes onto a solid substrate. It then solidifies and contracts and sets up stresses. The problem with chunky bits of cast, or where you have changes in section, is that the tensile loads created by cooling often/can exceed the tensile strength of the cast and it either cracks as it cools, or the residual stress causes a break in service at a very low applied load. (Cast is very prone to this because it is not very ductile, so its not very accomodating in that respect.)

 

Peening is an option but clouting cast in some circumstances is either not a good idea or not practical anyway. you could shot peen of course, or something similar.

 

With a rather thin section and a bigger body, it might be wise to look at the business of prehating, and see if you cant get a decent wedge of heat in there before welding. If this were an easily replaceable casting one could afford to be less “by the book” perhaps?

 

The actual detail welding of cast with MMA is not difficult of course – it welds quite easily. Its the other bits that go with it that deserve a look.

Edited By mgj on 14/02/2011 09:53:06

[Keith LongParticipant @keithlong89920]

Yes – the split cotter is in effect a double ended pad bolt, with the cut-aways facing each other. One of the pads is threaded, the other is made a clearance on the actuating thread so that in clamping the two pads are pressed together, gripping the shaft and wedging in the bore. The anti rotation groove that GHT shows isn’t really necessary in most cases, such as your tail-stock, as it is something GHT put in on the design for the “pillar tool”. In that the arms are meant to be removed from the pillar to swap accessories and so the groove and pin keeps the pad aligned. With your tail-stock hopefully once it is repaired and together it won’t have to come apart again, so the pads wont get out of line. The pads will easily align for initial assembly with a prod from a finger.

Your comment about the cotter / pad bolt being used on the carriage really does make you ask “what on earth we’re Barker’s thinking of when they designed the tail-stock?” – do you think it was a Friday afternoon?

Keith

[John CoatesParticipant @johncoates48577]

Must admit I really like the idea of the split cotter because I think we can weld the casting back on to give an original look but convert the locking mechanism to a split cotter which will be much more effective and we can put one on each side to equalise the forces. Because the force applied is pushing against the bar rather than pulling the casting it hopefully won’t fracture again in the future.

 

mgj: haven’t spoken to my brother in law yet but I think we’ll take on board your suggestion of pre-heating (he’d probably suggest it anyway)

 

I’m feeling a lot better about this now as I think the split cotter will increase the locking strength but not put the casting at future risk as the forces applied will be in the new steel welded onto the casting (barrels for the split cotters for want of better terminology)

 

This place really is fantastic!

 

John

[mgjParticipant @mgj]

I’m sure he will- preheating is pretty standard. It also stops the “hard” effect referred to earlier. That may have been an inclusion or somesuch, but most probably its to do with cooling. Molten high carbon material cooled too fast and you get martensitic sctructures locked in because its cooling fater than it can transform. Hard and brittle – also known as chill cast iron which we have all met on thin castings. and it forms for the same reason.

 

Get it hot first, cool it slowly afterwards and it should be fine.

 

Wish you all the best.

Edited By mgj on 14/02/2011 17:48:35

Edited By mgj on 14/02/2011 17:50:28

[TerrydParticipant @terryd72465]

Hi John,

 

Let us have some pictures of the finished article (and perhaps the process of repair as well). I think that we’re all interested.

 

Best regards and fingers crossed,

 

Terry

[John CoatesParticipant @johncoates48577]

Well I’ve found an L shaped bit of metal to use a brace (from an old TV stand) so I can “lock” the tailstock

 

I think I’ll plan to fix the break and add the split collet as part of a total overhaul. As John Coleman says it’s over 60 years old and the paint and the rust tells that story so the old girl deserves a bit of a refresh (bit like a bike restoration). That means taking her out of use so it’ll be a while yet as I have things to make!

 

Total thorough clean all over, new grease and oil, fresh paint and the break repaired. Ooh I’m getting excited already

 

And Terryd – I found diagram 1.6 in my GHT book on the UPT. Top class reading mate

 

John

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