Questions about cutting large transmission case with a plasma torch

[winchman]

Another AgTech project involves cutting a window in a large cast-iron transmission case. I read that plasma cutting does a good job on CI, and I had good results cutting up some scrap.

This case probably weighs around 400 pounds, and it's about 36" tall by 24" wide and 24" long. One end is open. The thickest part of the case is about 2", but the area where I need to cut is about 3/4" with some thicker ribs. I'm going to leave the heavy mounting flange intact around the open end.

Should I be worried that the heat from the plasma cutting will cause the casting to crack? What can I do to reduce the chances of cracking it?

[tdkkart]

How big a window?? Cast iron cuts easily, I would think a Sawzall with a good blade would do it very well. Not super fast but should be reasonable. Plasma is gonna be messy.

[Forrest Addy]

Yeah, If this is for an educational display so the people can see working guts of a geared transamission I really think straight cuts by a sawzall into drilled corners will do the neatest job with the least metallurgical harm to the case. Metallurgical harm may seem like a small potatoes until you consider a future change where machining plays a part. Plasma cut cast iron is almost diamond hard..

Do a good job with cutting windows and you could almost skeletonize the case maximizing visibility while reducing display weight.

I once set up a couple of fully assembled but otherwise junk Jimmy 6-71 diesel marine engines on a horizontal boring mills and machined windows in the block so the crankcase, pistons, induction, cooling system, lube, exhaust etc were all visible and accessible including the shifting and gearing of their attached marine transmissions. Hundreds of these were used in the Brown Water Navy in Vietnam in the '60's and '70's so vivid classroom aids were necessary for training the engineers and mechancs.

Planned the job in the EM Club bar on Friday night. Doing the whole job on both engines took that Saturday. Vacuumed up chips as I made them. Took longer for the instructors to tidy and paint the butchery. Machine work goes quick when you don't have to sweat sizes or tolerances. "Take more off there" "OK - is that enough?" etc. Working with the instructors and a couple of their students. Had a charcoal barbecure right there by the machine. Good food. Comradship. I remember eating a half dinky burgers on Base Exchange buns with sliced dill pickle on them very slightly sauced with cutting oil. NEVER put canned orkra on a burger no matter what a redneck diesel mechanic tells you. What a great day 44 years ago.

[winchman]

The window will be about 18" square. That'll leave several inches at both ends where the thick mounting bosses are located and miss the areas on the side which are really thick.

I'd like to make the corners rounded with at least a 1" radius, which should be easy with the plasma. Unfortunately, we can't get it into any of the machines, so the starting holes will have to be drilled by hand.

My current plan is to drill some 1/4" starting holes at the ends of the straight cuts, then make lots of short cuts with the plasma to keep from getting it too hot in one place. I'll work from the inside to keep the inside clear of slag, and I'll clean up any irregularities in the cut with an angle grinder.

[wierdscience]

I scrapped some 100hp cast iron frame electric motors by cutting the cast frame on each side and splitting them.
Cast was about 1" thick and our Thermodyne 102 cut it with relative ease.Actually could have done a neat job of it if I had wanted to.

If it were me I would try making a plywood template to clamp to it as a cutting guide,start about an inch inside the guide and walk it out and you wouldn't even need the start holes.