Ho hum it's a cracker, but not in a good way!

Looking for advice from my peers

This is closer to the handle rather than under where the tool post fits, it doesn't go through to the top or extend into the sides (so far).

What to do please?

Roger

Top slide ? How old? What was the other weld and who did it and why. Can you get it replaced under guarantee? If nothing else I would drill a couple of 3mm holes, one at each end of the crack to try and stop it going further.

If its as you found it, I would guess that the weld hides the first repair from the manufacturer and the crack has developed since. Even if you weld up that crack, there will be troubles ahead. It looks like the casting chilled too quickly.

It may be stronger left alone.

Whoa steady on speedy , the lathe was built in 1938 so is 90 years old, bit out of warranty I think . The other weld pre-exists my ownership, I agree with drilling a couple of holes to try and stop it spreading.

It doesn't appear to be in a stress area so I might leave it at that but be prepared to make a replacement if it does disintegrate. .

This is the top

and the bottom.

It's strange the crack is in the middle of the casting but does not show on top (I had stripped it back to metal when I restored it), so I guess it is a chill stress crack that appeared at a later date.

Roger

Roger, its days are numbered - probably another 50 years ! But definitely drill those holes.

I would plan long term to replace it. A few years ago I replaced the cross slide on my 14" x 40" lathe.

As a one off I just made a polystyrene patter to be burnt out of the sand box prior to pouring the iron. I had it cast locally and did the machine work on my Bridgeport mill. Total cost was around £50.

If you have a mill, you could machine the painted surface flat between the toolpost and the leadscrew nut end and drill and tap to hold a piece of 12mm ground steel plate on top. Using about 5 off 5mm SHCS with their heads counterbored flush on each side of the casting. If it is possible to accommodate a lower toolpost, the plate could extend the whole length of the cross slide.

Maybe it doesn't matter?

My guess is someone messed up and cracked the top-slide while replacing the tool-post screw. The crack was fixed by running a weld along it front to back, and the heat concentrated an expansion zone across the top of the T. As cast-iron is weak in tension the top-slide went ping. At that point the welder may have decided not to push his luck by trying to mend the new crack.

If going 'ping' relieved the stress, with luck the top-slide has cracked as much as it ever will. However, the risk with cracks is they create boundary stress concentrators that can grow the crack every time force is applied to the metal. Well worth putting holes at both ends, but they won't stop the crack deepening if it's going to.

But with me in optimist mode, I suggest the crack caused by heating isn't deep, and the ordinary working forces applied to the top-slide won't stress it further. Be good to know how deep the crack is, and to check if it grows deeper over time. Even if it's still moving it might be possible to slow it down by forcing metal-filler epoxy into the crack, much as dinged car windscreens are protected.

Dave

I have always wondered how one knows where to actually drill the holes to prevent the crack spreading?

Yes, each end of the crack, but the visible end of crack on the surface might not be the whole story as the crack may continue under the surface at an angle. A hole drilled with the best intentions might actually weaken the casting. In an ideal word (not that a crack is ideal) a hole drilled just past the end of the crack will allow the crack to continue only as far as the hole and no further.

As to the OP's top slide, I would carry on using it. As long as underside of the toolpost and the top face of the topslide is too then there should be little stress or strain on the crack.

Ian P

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Judging by that picture, it could be that the weld is ‘original’

Boggles the mind a little, but there was a war on the horizon.

MichaelG.

It can be very difficult to figure out where the cracks end, and even then, only the surface part shows. I wonder if there is room to skim the underside and screw on a plate / plates that side?                                                                     That observation that the weld might be a bodge done by the manufacturer seems plausible.

There are some signs of bodges done by Senior to blowholes in the castings of the Light Vertical mill we are doing up.

Edited By old mart on 02/12/2019 15:58:47

Just for fun here's some close-ups:

That's at about 50x, doesn't look deep or recent so I'll leave it alone and just be prepared to make a new one if necessary.

I wouldn't worry about the crack but that slide certainly could do with a bit of scraping.