Mike's P.M. Research No. 6 Steam Engine

[macardoso]

Here is the frame if you guys remember it. It is nearly done with the exception of the bolt hole circle on the mounting face and the oiler port on the top.



And the associated print. Since the head and the cylinder are nearly done, I wanted to get these bolt holes drilled so I could start mating the parts together.



Here is my fixture setup.



I have a big problem that whenever I walk away from machining for a while, I make a ton of mistakes as I get back into the rhythm. I should never start with really important non-replaceable parts. Here I just drilled the spot holes for the bolt hole circle. A smart machinist would have double checked their measurements, but nope! Not me!



And here are the holes drilled 7/32. This was the exact moment I realized I screwed up. The print is a touch confusing and leads you to believe the holes are equally spaced on the bolt hole circle. Unfortunately the are not and the holes on the left and right side of the image below need to be spaced closer together. This is to avoid getting too close to the casting walls below this feature. Since this part cannot be remade, I have to repair it.



Here was the spot on the print where I made my mistake. 2.312 B.C. does not mean equally spaced. The 1.000 measurement controls the spacing. Unfortunately the correct hole position overlaps the drilled hole position by 50%.



My solution was to purchase cast iron bar stock and turn stitching pins out of it. The incorrect holes are tapped 1/4-28 as it is pretty close in minor diameter to the drilled hole size, but not much larger in major diameter (as to not weaken the casting).



Here are some practice stitching pins. I was making these quickly and using the lathe DRO Sub-Datum Memory to allow me to change tools and directly read out pitch diameter. I made 2 scrap pins, 2 good pins, and I have 2 more to make. These will be glued in place with Loctite Green and machined flush. Then the holes will be drilled again, with part of the hole machining away some of these pins. The finished repair should be quite strong, and hopefully not too visually obvious.

 

[hman]

Mike -
This thread and your SCARA thread showed up right next to each other in my "watched treads" list. All I can say is WOW ... Your projects span the gamut from steam engines to SCARA robots!!

Now all you need to do is build a generator. Then you can use the steam engine to power the generator to supply electrizicals for the robot to stoke the boiler.

 

[macardoso]

Mike -
This thread and your SCARA thread showed up right next to each other in my "watched treads" list. All I can say is WOW ... Your projects span the gamut from steam engines to SCARA robots!!

Now all you need to do is build a generator. Then you can use the steam engine to power the generator to supply electrizicals for the robot to stoke the boiler.

I think I might have too many hobbies lol.

 

[macardoso]

Finished all the stitching pins last night. Two of them are 0.56" long and two are 1.38" long. I turned and threaded them in one operation and hit the middle of the 2A tolerance. The DRO made this fairly easy, but I found the threading less easy to trust the DRO numbers, probably due to the higher deflection and cutting forces.



The 1/4-28 tap I had could not thread the full depth for the 1.38" pin, so I used carbide inserts to turn the tap into a reduced shank tap. The steel was very hard, but machined nicely with carbide. Probably was cutting a bit fast and the chips were coming off deep blue.



I tapped the back holes full depth. I think I should have left the reduced shank right at the minor diameter of the thread. The tap seemed to want to walk a little once it was deep in the hole.



Here are the stitching pins installed. The shorter ones in front can be threaded in all the way with finger pressure. The ones in the rear get tight at 50-75% of full depth. I think there are two problems... First I dropped the pins and I think there might be some very tiny dings on the threads. Nothing obvious, but a magnifying glass might help. Second, I am worried either the drilled hole or the thread are crooked (or both). The pins are not at all flexible so they bind.

I will try to run the tap in and out of the hole to try to get the fit better. Alternatively I can add wrench flats to the pins and insert them with additional torque beyond finger tight. They will be glued in place with bearing retaining compound when installed for real.

 

[macardoso]

The stitching pins are installed. Tried a couples things to get the longer ones to fit better, but I just ended up slitting the face with a hacksaw and using a flat head screw driver to drive them in. They all seated to the proper depth so there is no step or divot on the bottom face of the flange.

Loctite 620 was applied to 3 of the 4 screws before installation. I was running the longer pins in and out of the threaded holes to loosen them up, but I got the back left long stitching pin stuck and cracked the head off of it trying to remove it. It is very secure, but I may drill the hole to half depth and add some loctite. It should wick into the crevices in the thread. I am not worried about it coming loose even without the added Loctite.



I am waiting the 24 hours for the glue to cure before facing them close to the original surface and machining the holes. The surface will be lapped to blend the pins into the surface after machining. Hopefully they will be mostly invisible.

 

[macardoso]

Repair is complete!

The Loctite 609 set overnight and I drilled out the holes with a carbide drill. The HSS drill really wanted to walk so I subbed in the carbide. Cut beautifully. Prior to drilling, the stitching pins were machined within a thou and a half of the flange surface.



All that is left is to lap the surface of the stitching pins flat with the flange face and drill the oiler hole on the spine of the frame near the top edge of this picture. I am waiting on my buddy to pick out his oiler so I can match the thread.



Here are two shots of the backside of the flange showing the repair. You can see how close the old holes were to the casting surface which prevented installation of the screw from this side.





I am very pleased with the outcome. This part is pretty much finished until my buddy picks out that oiler. On to the next (easier) part

Couldn't get a good picture of it, but there is a really cool zig-zag pattern down the wall of the hole where the thread was machined away.

 

[tq60]

You may consider filling the holes with solder.

Flux and low heat and a mess but it will fill in the holes and color will be close enough if leaving bare metal.

If painting Bondo will do.

Sent from my SAMSUNG-SM-G930A using Tapatalk

 

[Boswell]

Nice Save

 

[macardoso]

You may consider filling the holes with solder.

Flux and low heat and a mess but it will fill in the holes and color will be close enough if leaving bare metal.

If painting Bondo will do.

Sent from my SAMSUNG-SM-G930A using Tapatalk

Didn't think of that! I'd be concerned about the structural integrity of the repair under load. This isn't anything crazy, but the flange does support the weight of the entire cylinder assembly plus the pneumatic force applied when the cylinder is in the return stroke position. I also expect this part to heat up quite a bit, perhaps as much as 200*F in continuous operation. The Loctite 609 is temperature stable and will retain 75% of full strength at 100*C - although I am not relying on the Loctite for mechanical stability, but rather the thread engagement.

This part will we painted at some point, not sure if my buddy will use bondo or just gloss paint over the rough casting.

 

[RWanke]

Really got my interest up on this. I am looking at PM Researches #3 steam engine kit. I am wondering if the machining on this kit can be done with only a SB 9C lathe and a drill press. I don't have a mill.