The Steamboating Forum

I'm contemplating sending the expansion links for my York engine out to be CNC machined but have some questions I need to work out.

I could set up a jig to do the large radius on my mill but it would be as much machining to make the jigs than it will be for the links themselves. There also may be another advantage to having the links done on a big CNC and that is I can have them hard machined. So here is my question....For those that know (not speculating), Is there an real advantage to having the expantion links on a Stephenson valve gear hardened? I know that the wear should be decreased with the proper material selection and hardening but does it *need* to be made hard for any reasonable longevity? The material callout in the plans just says - steel. I'm considering 4142 pre hard, ground plate. This won't have the hardness and wear resistance that hardened A2 would have, but I don't run the risk of warpage during heat treat. The other unknown is that the suspension pivot gets silver brazed to the link and I'm a tad worried about warpage of the link during this operation regardless of material selection.

Thoughts?

Thanks

Ken

I have no experience with Stephenson Link gear but, it is well known that if you rub two metals together for long enough (even with bountiful lubrication) it will be the harder one that wears first, not the soft one as you would imagine. That therefore suggests that the thing that is the most trouble to machine should be the bit that should be softest, as the other bit is easier to replace when it does wear and everything will wear eventually.

Forgot to mention...

The die block is Alumuinum Bronze and will be done on the CNC as well (maybe will get a pair of extras made at the same time)

Ken

Ken:

41xx and aluminum bronze (C954 or equivalent) is a marriage that we utilize a lot at work where the scale is twelve inches to the foot. While I'm trying to recall if any of our link blocks are aluminum bronze, I can't imagine you would need to harden your links for the pressures your York links will encounter. The one who would know for sure is my colleague, Kelly Anderson, who occasionally posts on here. I'll ask him to weigh in, if he will.

I had my links laser cut. I had to do some finish machining and if I was to do it again I would go with water cutting as the finish is much better.

Can't comment on advantages of different steels.

The jig for machining the links was fairly simple because it was mainly a tidy up and light cuts were the order of the day. I just used 2 3/8 plates pivoted with a threaded bolt to move them relative to each other. I'll see if I can find a picture.

Cheers

Paul

"I could set up a jig to do the large radius on my mill but it would be as much machining to make the jigs than it will be for the links themselves."

I have setup milling of large radius pieces by just clamping a pivot point to the machine, clamping the work to a long piece of steel, and manually swinging the workpiece into the milling cutter. This worked OK for milling a 1/2 inch wide opening (slot) into 5/8 inch thick mild steel. The cut radius was 15 inches, and the manual lever arm was about 5 feet long.

"Is there an real advantage to having the expantion links on a Stephenson valve gear hardened? I know that the wear should be decreased with the proper material selection and hardening but does it *need* to be made hard for any reasonable longevity? The material callout in the plans just says - steel. I'm considering 4142 pre hard, ground plate. This won't have the hardness and wear resistance that hardened A2 would have, but I don't run the risk of warpage during heat treat."

Hard steel is more than you need for this type of application.

"The other unknown is that the suspension pivot gets silver brazed to the link and I'm a tad worried about warpage of the link during this operation regardless of material selection."

Warpage will probably occur unless you control even heating and cooling throughout the process, which would be difficult considering the shape of the part. Many of us have learned about heat warpage the hard way. Better to braze on the piece and machine afterwards, or alter the design to eliminate the brazing.

If you're going to machine steel into an asymmetric part, your best approach is to make it from annealed or hot-rolled material so that when stresses are relieved by removing material the part stays flat. Unless you have a lot of slop in the reversing lever, the amount of motion that the link will see is minimal. Make the aluminum bronze slider a close sliding fit and the assembly will last a long time. The original bronze link and sliding block still works just fine on my 115 year old steam engine in the Otter. It has a piston valve and a knob to friction lock the Stephenson link; I can recommend both as friendly to link motion. The most worn piece are the eccentric straps themselves, as it is hard to keep them properly lubricated on this design.

- Bart

Here's a picture of my setup when machining the links
Seemed to work OK. Used the same setup to machine the Al Bronze blocks

Cheers

Paul

Thanks for all of your answers.

I'm not going with hardened steel for the expansion links. If the bronze links & sliders on Bart's engine can hold for 115 years, I don't see any reason to harden the expansion link.

Thank you to Paul for sharing the setup photo.I'm going to look around the shop to see if I have the materials to build the jig. I still may send the links out the be done on the CNC as it may be a better use of my limited time to make progress on some of the other parts of the engine.

Ken

Thanks for all of your answers.

I'm not going with hardened steel for the expansion links. If the bronze links & sliders on Bart's engine can hold for 115 years, I don't see any reason to harden the expansion link.

Thank you to Paul for sharing the setup photo.I'm going to look around the shop to see if I have the materials to build the jig. I still may send the links out the be done on the CNC as it may be a better use of my limited time to make progress on some of the other parts of the engine.

Ken