Wire EDM Question

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Dick_Morris
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Wire EDM Question

Post by Dick_Morris » Wed Apr 28, 2021 7:01 am

A question for the wire EDM experts -

There was excessive play in the Walschaerts valve gear on full-sized Alaska Railroad #557. We are replacing all the bushings and any pins that have excessive wear. There is also play in the reverse link. The overhaul crew believes it can be corrected by building up the surfaces of the link block and having it ground back to the 64-5/16" radius for the proper clearance. We have a local firm that can do the build up, but not the grinding. We have a good heat treater. However, the firm that does the spray welding says that if the part is built up by spray welding that it can't be heat treated without the possibility of the new surface delaminating.

The attached drawing is from 1942 (during wartime material shortages) with the part made from an obscure grade of carbon steel which was case hardened and ground all over. We also have a later drawing of the same part using another obscure grade of carbon steel. A note on the later drawing originally specified case hardening, but this was stricken through and changed to flame hardened. The dimensions of the later part is identical except for a change in the oil grooves. That makes me wonder if would be more practical to make a new part from a modern, hardenable grade of steel with all of the machining done except for the radii, having it heat treated, and then finishing the radii with a wire EDM.

The comments I have seen here over the years have lead me to believe that wire EDM is a viable replacement for grinding in this type of application.

My questions are -

If we built up the surface by spray welding, is wire EDM an appropriate and cost effective process, in lieu of grinding, to finish the radii?

Or would it be more practical and cost effective to make a new part from a modern, heat treatable alloy, heat treat it, and use wire edm to machine the radii on the hardened part?
Attachments
reverse link block.jpg

thej611
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Re: Wire EDM Question

Post by thej611 » Wed Apr 28, 2021 10:21 am

I shared this with my Dad who has done extensive work ON Steam locomotives. He Was CMO for Mid continent for a number a years and is an engineer and a machinist . Here is what he said

Well for my 10 cents I would make a new part for both sides. Additionally I would kiss grind the the link surface and hopefully not go through the hard surface.

I would not spray coat the surface and then heat treat. One grade on top of another has differential expansion and to me will come loose. Make a new one and grind the link.

AnthonyDuarte
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Re: Wire EDM Question

Post by AnthonyDuarte » Wed Apr 28, 2021 10:35 am

I worked in a production EDM shop and not a job shop, but if EDM is only finishing the radii in either situation (built up or made new), I don't see any reason why the price would differ. If anything a new block might be a little easier to work with for setup. Be sure to specify a surface finish tolerance for the working surfaces.

In my opinion, making new blocks is the better route.

hoppercar
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Re: Wire EDM Question

Post by hoppercar » Wed Apr 28, 2021 11:52 am

That's a pretty hefty piece of steel, it's not going to move during heat treat, there's no reason to cutt the radius after heat treat, do it before, heat treat it, and polish it.......you will be fine

little giant
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Re: Wire EDM Question

Post by little giant » Wed Apr 28, 2021 1:16 pm

I wired a link and block both out of harden steel for an Allen ten wheeler. A little smaller than yours, they mated fantastic. My friend that I gave them to loved them so much never put them on just enjoyed the sliding fit. Machine the block square and do all the mill work first then send it out to heat treat. Wire the outside of the block last and any burrs that were left on the block would be wired off.

Harold_V
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Re: Wire EDM Question

Post by Harold_V » Wed Apr 28, 2021 3:05 pm

hoppercar wrote:
Wed Apr 28, 2021 11:52 am
That's a pretty hefty piece of steel, it's not going to move during heat treat, there's no reason to cutt the radius after heat treat, do it before, heat treat it, and polish it.......you will be fine
I don't agree. Heating and quenching material is almost a guarantee of change. The question, here, would be if the change is significant, or not. It may result in pieces that will not fit together after heat treat.
H
Wise people talk because they have something to say. Fools talk because they have to say something.

hoppercar
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Re: Wire EDM Question

Post by hoppercar » Wed Apr 28, 2021 4:20 pm

Not something this significant in size......were not talking about splitting hairs and watch making,. It's just a link block.....it can stand a little play

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Bill Shields
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Re: Wire EDM Question

Post by Bill Shields » Wed Apr 28, 2021 5:12 pm

Going to be an expensive part to wire...since write time is charged by the square inch...and dimensions will be questionable since the wire cuts differently as the make up of the material changes...for any given set of electrical settings the depth of cut (kerf) will vary depending on if in virgin material or in sprayed material.

Wires like really hard consistent material to cut.

Cheaper and easier to start from scratch with a known block of heat treated steel that had a hole in it.
Too many things going on to bother listing them.

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Dick_Morris
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Re: Wire EDM Question

Post by Dick_Morris » Wed Apr 28, 2021 10:58 pm

Thanks to all for the comments. They give us some food for thought.

Harold_V
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Re: Wire EDM Question

Post by Harold_V » Thu Apr 29, 2021 1:54 am

hoppercar wrote:
Wed Apr 28, 2021 4:20 pm
Not something this significant in size......were not talking about splitting hairs and watch making,. It's just a link block.....it can stand a little play
Chuckle!

Well, yeah, but they already have one with a "little play". That's what they're trying to eliminate. Makes no sense to start out with something that already has issues, and it's virtually sure to have them if it's finish machined before heat treat. That was my point. In support of your idea, though, they sure could use air hardening tool steel, which is quite stable, and they'd be able to avoid a rapid quench. I'd still not bet on it, though.

Size has little to do with it. Metals have stresses, and heat causes them to relax. Doesn't matter if they're large or small, and, in fact, one can expect greater movement in large pieces than in small pieces. All depends on what is removed.

There's a right way to do things. If one chooses to ignore procedures that are known to work, they must also be prepared to pay for failures.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

super7b
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Re: Wire EDM Question

Post by super7b » Thu Apr 29, 2021 3:04 am

Finish all the other straightforward remedial work on the valve gear and then measure the valve events.
Then make a 3.500" diameter die block from any suitable material, for each cylinder, fit them and remeasure the valve events.
Should there be no significant difference it would be possible to reuse the "worn" blocks as they are. Obviously if there is still an issue, blame me for wasting your time!!

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Dick_Morris
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Re: Wire EDM Question

Post by Dick_Morris » Thu Apr 29, 2021 4:10 am

My thoughts after reading all the suggestions (although I won't be making the decision) - If we make new parts from 1018, machine and polish the sliding surfaces and have them case hardened we will only be out about $200 and volunteer time - even with new material. It would take some creative fixturing on the mill. Given the likely cost of grinding or wire EDM it may be worth trying.

Harold - Good point on internal stresses. The parts are small enough that they could be stress relived in my electric burn out oven before they are taken to the final dimensions.

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