Best way to cut 1"+3/8" plate together

[ogorir]

Hey guys, I've got (4) 3/4" holes to cut in a truck crane mounting plate that is a piece of 1" plate welded to a piece of 3/8" plate. After 25 minutes of heating, I got the top plate hot enough to cut through the 1", but as soon as it hits the 3/8", it stops cold.

I'm going to try heating the 3/8 plate from the underside next, but I'm out of acetylene, so I figured I'd solicit opinions while I'm stopped.

Any suggestions would be appreciated.
Thanks!

[DaveO]

Hey, welcome back, haven't heard from you in a while.

What's your budget? :-) And what's your tolerance? I'm sure someone will come back with a mag drill suggestion... And there's Petrogen.com, a cutting system that uses gasoline instead of acetylene. The videos at their site are offline today, but cool to watch.

Why is the plate soaking off the heat so fast?

[performance]

You have to clamp it tightly so there is no air space whatsoever. I mean TIGHTLY. Oxy fuel can not cut discontinuities in metal. It would be best to grind or sand off the mill scale as well. A plasma ignores this and can cut layers fairly easily. Our 80 amp unit can handle this, though it is pushing it to the limits.

[zoama]

If you do much of that stuff you may want to get an oxy gasoline cutting torch.

[ogorir]

This is for work, so the budget is pretty much Nil. the plate is about 26x20, so it soaks up a lot of heat. I think the nearly empty fuel bottle may have been part of the problem heating the plate, though. I was using a #2 tip, which is the biggest we had, but I'm going to pick up a #3 tomorrow.

We have a 60a thermal dynamics at work... it will sever 1" if you move damn slow, but it certainly won't pierce it. (and sadly, until that one craps out... a new machine is not in the cards.)


(I didn't realize I never posted this response...)


So, what worked in the end was drilling a 1/4" pilot hole and the #3 tip. I pre-heated the first hole with the rosebud and by the time it was hot, the whole plate was hot, so I have no idea if I could've omitted the preheat all together.

Cleaning up the hole I made partially through on the first day was a PITA... I ended up cutting the 1/2" plate off the underside with the plasma and enlarging the hole with the torch. even after I drilled a pilot, the slag still wouldn't flow downward, it just bubbled up.

Interestingly enough, there was almost no issues going through 2 plates with oxyacetylene once it was pierced. I mean... it wasn't the cleanest hole you've ever seen, but I'm not the best torch cutter you've ever seen either


Anyway, thanks for the tips. I'll post a pic of the crane truck when we get it finished. the body and crane are all painted, but I've got to paint the boom cylinder (was being rebuilt) and get a few leafs put in the crane side, it's sitting about 3" low.

[performance]

Again, it's futile whatever you skills are because you have an insulative air gap, ever so small it may be between the two plates. It cannot cut through it, no matter the heat you put into it. Once you cut through the top, you have to essentially preheat the bottom plate all over again. It is nearly impossible to do because of the way the oxyacetylene burns the carbon out of the metal. Once thermal continuity is lost, it won't cut. That's why you can use a torch to "wash" a nut off an axle stud, or a stuck or gaulded bearing off a shaft and NEVER touch the piece underneath. Not so easily done with plasma because the arc is 10k degrees and if electrical continuity is there, its going to cut.

[sticks]

I'm coming in late to this, but if you are still fighting this, I'd spend the $40 on a Champion CT7 plug cutting drill bit. 1" max depth so you would have to probably end up coming up from the bottom, twist drill bit, or use a torch for the last 3/8". Don't need a mag drill, just a good 1/2" hand drill.

Crane mounting holes...you really want to have tight tolerance on those bolt holes unless this is only like a #3k crane.

ETA - I have done multiple layers of oxy cutting. It is a PITA, but if you have a drilled pilot hole that helps. Your top hole will be 3/4 but you will be closer to guesswork on the bottom of the hole due to the nature of the torch flame drift, angle of the torch head...just going to be ugly in general. Not helping that you are doing the 1" first then the 3/8".

[Rambozo]

Another unclean option is to use a cutting electrode with a stick welder. They burn through multiple layers pretty good. But like a torch, a lot depends on the skill of the operator.