Arghh.....
I designed an airlock for a high tempereature furnace this past winter. One of the key components is a cooling flange that fits between the furnace and a vacuum gate valve.
The cooling flange is made of copper. It has a cooling channel cut into it with a copper plate welded over the channel. There are additional features for sweep gas connections, mechanical connections for actuating thermal shutters, and bolt holes for mounting the gate valve.
This piece has been one of the most trouble plagued parts I have ever tried to get fabricated.
The first problem was the cooling channel was cut too far. The machinist programming the machine didn’t pay close attention to the drawing. The channel ended up intruding into the area where the holes for the sweep gas were to go. Rather than forcing the fabricator to start over I redesigned the part rerouting the sweep gas porting.
The reasons for redesign instead of starting over were it maintains good will with the fabricator and the time required to get a new billet was not insignificant.
After the redesign the next step was to weld on the cooling channel cover plate. The cover plate warped leading to concerns about leakage. So it was machined off and the surfaces dressed for another attempt at welding.
I asked to be notified when the second attempt at welding was going to be attempted. I wanted to witness the welding. I don’t know what went wrong the first time but I thought that by being present it would discourage any attempts at shortcuts. The full welding procedure is as follows: clamp cover plate on billet over the cooling channel, preheat to 800 F, tack weld, reheat to 800 F, weld while pausing to reheat billet as needed. I watched the welding and it proceeded according to procedure.
A few days later I got another phone call: the weld had porosity in it which was uncovered when machining on the weld to form a sealing surface for an O-ring. In addition a counterbore on a large through hole formed oversize. The oversized counterbore leaves some components exposed to the furnace atmosphere and greatly increases their heat flux.
At this I finally threw up my hands and said we need to start over on this piece.
The cooling flange is made of copper. It has a cooling channel cut into it with a copper plate welded over the channel. There are additional features for sweep gas connections, mechanical connections for actuating thermal shutters, and bolt holes for mounting the gate valve.
This piece has been one of the most trouble plagued parts I have ever tried to get fabricated.
The first problem was the cooling channel was cut too far. The machinist programming the machine didn’t pay close attention to the drawing. The channel ended up intruding into the area where the holes for the sweep gas were to go. Rather than forcing the fabricator to start over I redesigned the part rerouting the sweep gas porting.
The reasons for redesign instead of starting over were it maintains good will with the fabricator and the time required to get a new billet was not insignificant.
After the redesign the next step was to weld on the cooling channel cover plate. The cover plate warped leading to concerns about leakage. So it was machined off and the surfaces dressed for another attempt at welding.
I asked to be notified when the second attempt at welding was going to be attempted. I wanted to witness the welding. I don’t know what went wrong the first time but I thought that by being present it would discourage any attempts at shortcuts. The full welding procedure is as follows: clamp cover plate on billet over the cooling channel, preheat to 800 F, tack weld, reheat to 800 F, weld while pausing to reheat billet as needed. I watched the welding and it proceeded according to procedure.
A few days later I got another phone call: the weld had porosity in it which was uncovered when machining on the weld to form a sealing surface for an O-ring. In addition a counterbore on a large through hole formed oversize. The oversized counterbore leaves some components exposed to the furnace atmosphere and greatly increases their heat flux.
At this I finally threw up my hands and said we need to start over on this piece.
posted by JBC at 3:35 PM
6 Comments:
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Thanks for all the posts! I am thinking about becoming an mechanical engineer in Calgary and I really appreciate the information.
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Thanks for this post. I'm about to graduate from a mechanical engineering program in Calgary, and I'm not looking forward to these issues ha. But it comes with the profession I suppose.
great post !! it's having valuable and professional information about mechanical engineering.
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