Broken topslide base casting Myford S7

[HillclimberParticipant @hillclimber]

[HillclimberParticipant @hillclimber]

There I was battling a bit of chatter while parting off 50mm diameter EN3 in my S7, rotated the topslide to produce a couple of chamfers 'et voila'. I have severed the tapered locating stub from the bottom of my topslide base casting.

Oddly, this was not the end of the world as I had another topslide assembly on my bench, recently acquired from the Bay of Fleas to allow me to make some GH Thomas mods. So I just swapped out the base castings and carried on. The chatter improved anyway.

But my question is whether I am the only guy in the world to manage this? I can certainly ascribe it to 'overtightening', and I have had a couple of other parts shear since I began to really explore this machine – including the saddle-lock bolt and the idler-wheel retaining bolt/oiler. Was the previous owner heavy-handed? Couldn't possibly be me……?

Cheers, Colin

[HopperParticipant @hopper]

Parting off 50mm steel: I think I would be using an inverted parting tool in a rear toolpost mounted solid to the cross slide for that. Lot of strain for that poor little topslide to bear, as you can see.

You have to remember too that these lathes are how old? My Myford is 60, as old as me. Not sure how old your S7 is but probably at least 30. Repeated load cycles over that long time will fatigue metal, like your saddle lock bolt and your top slide. Imagine driving to work every day in a 1975 Morris Mini. Would you be surprised if something broke one day? They are good old machines, but they are good OLD machines. Anyting you can do to take a bit of stress off, like use a rear tool post, helps keep the old girls going.

[Harry WilkesParticipant @harrywilkes58467]

Couldn't possibly be me……? who am I to comment ​ but as Hopper suggests consider a rear tool post, I used to be one of the guy's that at times struggled with parting off but not since using the rear tool post method

H

[MuzzerParticipant @muzzer]

Probably a bit of bad luck. I expect if you looked closely at the fracture with a suitably expert eye you'd probably be able to identify an inclusion or other stress concentrator in the base casting that led to the failure. Age shouldn't really come into it. If anything, age seems to help CI.

If you minimise the tool overhang and use the topslide with the toolpost over the base casting (which is simply good general practice), there should be no issue with parting off, at least not of this nature.

Murray

[HillclimberParticipant @hillclimber]

I thought the parting off was going quite well, actually! The problem was only apparent when I loosened, then retightened, the topslide to cut a 10 degree chamfer. Then that fixing bolt just kind of went on and on….

The topslide had been very firmly, slap-bang on top of the base casting, precisely to keep it as solid as possible.

What I wondered was what kind of load overtightening would produce from the wedge retainers, and whether enough to crack the locating stub?

Anyway, now tempted to cut off the locating stub form this base, fix it to a plate, and produce something resembling the Radford topslide arrangement of bolting it directly to the tee slots.

[Martin KyteParticipant @martinkyte99762]

How big a spanner are you using on the square headed bolts ? I use one that is about 2 1/2 inches long, you only need to nip them up. Maybe the "previous" owner only possesed an 18 inch pipe wrench.!

regards Martin

[HillclimberParticipant @hillclimber]

Hang on. I'll just go check what the 'previous' owner was using…..

I confess that my spanner is a little larger than that (I mean your spanner, not the pipe wrench). But doubt if I could manage to deliver excess torque given the location of these – particularly the RH one that sits under the topslide.

But there is nonetheless a good crack that has run right round the narrow end of the tapered stub, extending to the piece that had not yet sheared.

[KWILParticipant @kwil]

Looking at the fracture surfaces on your picture at high magnification, I would say the only recent break you have in at the ends of the "horseshoe".

The surface where that meets the flat underside is relatively smooth. You have been nursing a broken topslide for some time, hence your chatter.

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