cracked platen on Pexto shear

I've brazed pretty heavy castings using a space heater (or two) to preheat.

John

Jon at this point its largely..IMHO a lost cause. Not that it cant be repaired with replacement parts from another shear.

Drop Leigh at snipped-for-privacy@aol.com with a question about replacement parts from carcasses. If he doesnt know where to find them...few others will either. The dude is good with Pexto stuff.

Gunner

"Considering the events of recent years, the world has a long way to go to regain its credibility and reputation with the US." unknown

Did you drill the end of the crack? You should do this! carefuly check that you drilled out the end of the crack (oil and chalk) or it will certainly crack again. Preheating by any means will reduce inner tension.

HTH, Nick

That's what I was thinking too, Nick. I've drilled out the end's of cracks in fiberglass panels to prevent them from cracking further before repairing them. Works like a charm.

How do you use oil and chalk to find the end of a crack?

It's much like a spray penetrant inspection. The crack holds some oil, which is wicked out by the chalk and is easily visible from the discoloration. It defines cracks quite well.

Harold

Pour some thin oil on the suspect place, let it penetrate, whipe it carefuly off, and dust some powdered chalk over the place. The chalk will suck the oil out of the crack and will get dark. voila! Well, you can also buy expensive super space science spray for that ...

Nick

The new crack is 3 inches away, and parallel to the old crack. It is at the end of the gusset patch, where stess may have been concentrated both by the application of the patch and by the patched area now being stiffer than the rest of the platen.

So, this has nothing to do with the end of the crack propagating farther.

Thanks,

Jon

Hello Jon,

Somebody posted this URL here a while ago:

BTW your EDM power supply worked exactly as I hoped on eroding a square socket into the hardened stud we were discussing a while back. So, thanks once again on that.

Regards,

Adam Smith Midland, >> J>>

This stuff seems to be about sealing leaks in the non-structural part of engine blocks. Maybe I was reading the wrong part of the site. The problem I have is this is in the highly stressed part of the platen, where it experiences great tension loading!

Hey, great! Glad it worked out for you. How long did it take?

Jon

OK, this seems to have to deal with thermal tension from welding. I am _not_ a welding instructor, so don't read my answers like a bible.

What could you do to reduce thermal tension?

- use MIG/MAG

- weld quick and not make the fillet in one pass (we call it "pilgrim's step", one back two forth)

- weld on both sides intermittent

- leave time between welding passes

- preheat

One last resort might be MAG-brazing*). But if that fails, your piece might be ruined. MAG-brazing is a "welding" with a CuAl-alloy wire (Or CuSi, about 95% Cu) in your MAG with Argon. The wire is quite expensive but will be more flexible. But I don't have enough experience to stand full behind this advice! Maybe someone else has something to say?

HTH, Nick

I've also heard of using pure nickel rod, and peening the weld before the part cools. I used some stainless steel filler wire, as it has SOME nickel in it. That weld has held! I did do the weld slowly, ie. in a small spot at a time, then working on the opposite end of the patch.

What's really driving me crazy is why this platen keeps cracking when I'm cutting thin aluminum sheet. I am operating at half or less the capacity the shear should be able to handle.

One "plan B" solution is to remove my patch and make a patch that runs the full length of the platen, welding it in just to the side of the two cracks. And, then jam in shims or weld on little blocks that support the platen along the full length. That ought to work, and should prevent any more cracking, but it will be a lot more work.

Thanks to everybody for the help with this.

Jon

Silly wild-ass guess non-expert off the cuff analysis from a hundred miles away comment... In other words, Feel free to ignore me, I'm just thinking out loud here. ;-P

(Oh, and after a quick bit of research... From here to your address in Kirkwood MO

If this area is cast iron that gets shock-loaded in tension during use, it won't take much to make it crack again. Since you tried a fishplate on the bottom side, and it just cracked where the loads were moved to one end of the fishplate, I'm thinking there may be an inherent design weakness in the platen. (Bet you when they changed over from "Peck Stowe and Wilcox" to "Pexto" they made it heavier...)

How about putting a tension rod through or under the platen, right below the shear throat edge, thread the ends and use some fine thread nuts to put some external compression on the platen? Maybe even a V-block (or two on the webs) at the center like the tensioning rod on your garage door, to counter any downward forces in the unsupported center of the platen? THEN you remove the old fishplate, drill the ends of the cracks, preheat and weld them up again.

You can check the finished side of the platen for bowing, and stop tightening the tensioning nuts when it gets a very slight positive arch from the preload.

Better than throwing it out.

-->--

Yes, that's the main problem. Since you tried a

Maybe, this is a 30" shear, I can't find a gauge rating on it, and it only weighs 150 Lbs. It has 18 (nameplate) 90 cast into the upper blade support, so it must be a very early design.

It seems to me that compressing the piece first and then welding is going to cause even worse problems (unless you never remove the preload again). If you do ever remove the preload, it might just crack right then. I arranged to leave the first crack slightly spread (maybe .020") before doing all the welding. After I was done, the crack had closed up quite nicely.

I'm not sure this is going to be stiff enough. I think having a thick piece of steel sheet that supports the platen along the entire length would be a lot stiffer than a rod. It might not even need to be welded to the platen at any point, just attached to the legs.

Jon

Ok, thanks. That almost sounds like my "dirty thumb" method of crack detection: Clean suspect area well, then wipe with dirty thumb.

As an retired farmer I used to keep a stream of water running on my chunks of cast iron , always able to hold it in bear hand. Can be very slow filling in.

I have a friend who uses nickel. I have also had engine blocks nickel welded.

Who knows why it keeps breaking, perhaps it is jus old!

If you fix it, try bolting the legs to each other, or bolting it to the floor really well. Stopping leg spread under load may reduce the cracking. MAybe

I have had good results welding cast iron with two different rods- the nickel tig rod, and silicon bronze- tig brazing the cast iron. With the nickel, a preheat really helps, but just using the rosebud on the oxy fuel torch will do it- 500 or 600 degrees is plenty, you dont have to get it red. The silly bronze rod is more forgiving, as you dont have to get the base metal as hot.

The small Pexk Stow and Wilcox 30" shears were 18ga max. And most of those weighed in at 400lbs. My guess is yours is a No. 30, or a No.