Crosshead machining woes

[Dale Grice]

Hi Esteemed and Knowledgeable group,

My biggest problem with trying to construct a locomotive is order of operations in machining castings. Wish there was a novice procedure - 1, 2, 3 ..

Anyway, I backed myself into a corner. I squared up alligator crosshead castings, Drilled, bored and reamed the wrist pin hole. Setup to face, drill and tap with a center in the mill spindle, the piston rod hole. See photo. And the vise is flat to the mill table within 0.001 and indicated in on the X axis. I'll have to look at vise flatness to the table and see if I can get that better.

I made the piston rods. But mounting the rods in the castings and clamping the crossheads in the vise then checking with a dial indicator, the worst one is out 0.03 vertically, and 0.06 horizontally.

My plan is to mount these assemblies in V blocks. Then support the crosshead probably in a vise on parallels, clamp it the crosshead, and then clamp the vise down. Then machine the surfaces for the crosshead guides.

But my wrist pin hole will now be out of square. Since it is finished, I need to bore it out and bush it. How big? maybe an 1/8 diameter oversize. What material should I use to bush it? Brass, Silicon bronze, 1144, 1018? Wrist pin is drill rod.

Opinions?

Thanks,

Dale Grice

[rmac]

My biggest problem with trying to construct a locomotive is order of operations in machining castings. Wish there was a novice procedure - 1, 2, 3 ..

I don't have enough knowledge or experience to make suggestions related to your immediate problem.

As for the "order of operations" thing, though, I find it really helpful to carefully think through all the machining steps for a given part before I start, and to actually write down that 1-2-3 "novice procedure" that you wish you had. Then maybe even let it sit overnight so you can reread it over a cup of coffee with a fresh set of eyes. Quite often that exercise saves me from mistakes like you're talking about.

-- Russell Mac

[AnthonyDuarte]

Looking at your setup, I don't see anything too alarming that would cause that much error. Only thing I would say is it looks like the vise doesn't have very much bite on the crossheads. Even with the bar going across, it would not take much force to move them around. But still, .03-.06 (I assume that's over the entire length of the piston rod) is a lot more than I would expect.

Double check the tram of your spindle to make sure it's actually square to the table. If it's not, you'll be chasing your problem in circles.

[Dale Grice]

Didn't mention tram on the spindle. I do a lot of facing. It is trammed. And that is over the entire length of the piston rod.

[Wolfgang]

The way to determine the order of machining the various features of a part is to analyze the functions and the required accuracy of said features.
Then it is necessary to estimate the accuracy of the alignment of the machined features and the impact thereof on the assembly.
In the OP case with the cross head, it is imperative that the cross head slide faces be parallel with the piston rod in two planes, and that the wrist pin bore be at right angles to the piston rod.
The least predictable operation from an alignment perspective is the drilling and tapping of the piston rod hole in the cross head. The reason is that a .001" error in parallelism of this bore and its locating spot face, with the other features on the cross head, will be magnified by a very large factor over the length of that piston rod.
There are several methods of addressing this difficulty, depending on the size of the engine being built (this is a judgment call from my perspective).
With a largish engine I'd begin by machining the external surfaces of the cross head to provide some reference surfaces.
Then lay out the various features, and then drill and tap the piston rod bore, including the spot face for the shoulder on the piston rod or jam nut, as needed.
Assemble the piston rod to the cross head.
Clamp a precision Vee block on the milling machine table, aligning the Vee with the long axis of the table. Put a long straight bar in the Vee to get this alignment really accurate. Locate the piston rod in it, with the cross head attached. Use an angle plate to clamp the cross head independently to the table, first by clamping the cross head to the angle plate, followed by clamping the angle plate to the machine table.
Now you can mill one guide slot dead parallel to the piston rod.
Rotate the rod and cross head 180 degrees and clamp as described above. Be sure to put the angle plate on the same side of the cross head as the first time.
Mill the second guide slot. You can centre the piston rod between the guide slots by measuring from the piston rod to the horizontal surfaces of the guide slot, and making this dimension identical, more or less.
Turn the assembly 90 degrees, clamp as before, and drill and ream the wrist pin hole. You would need an edge finder to accurately locate this hole directly off the piston rod.
One caveat: If the piston rod is fixed in the cross head with a jam nut, do not use a commercial nut for this purpose. Make one yourself and tap the nut while still in the lathe, right after drilling. This ensures that the thread axis and face of the nut are at right angles to each other. The nut face on the cross head also needs to be at right angles to the thread axis in the cross head.
Post any queries you have here.
Perhaps others will chime in if they have a better way. To be sure, there is another way which I have used myself but, it only works for smaller engine sizes depending on the size and type of machine tools available.

[Dale Grice]

I did not think of the jam nut. Good thought.

But, the real question at this point, what would you use for a bushing in the wrist pin hole?
I need to bore it out and bush it. How big? maybe an 1/8 diameter oversize. What material should I use to bush it? Brass, Silicon bronze, 1144, 1018? Wrist pin is drill rod.

Dale

[Gra2472]

I have a dumb question! Since i am not a machinist, but I am learning. I have been wondering where you start with a casting too. What part of a casting do you trust as a starting point to square the part?

[Harold_V]

How big? maybe an 1/8 diameter oversize. What material should I use to bush it? Brass, Silicon bronze, 1144, 1018? Wrist pin is drill rod.

I would recommend against using a ferrous bushing unless it was gray or ductile iron. Brass wouldn't be a great choice, although 85-5-5-5 most likely would serve just fine. If I was concerned about wear, I'd probably make phosphor bronze my choice, even leaded phosphor bronze. A 1/16" wall would be adequate, although for such a thin wall you must use care in providing an acceptable fit. The thin wall will allow easy collapse of the inside diameter, which would best be sized after the bushing was pressed in place.

When you're working on parts that should be square and/or perpendicular, you're best served to not use a vise. I built a lot of tooling for the aero-space industry in my years in the shop and learned that an angle plate was always a better choice, especially when trying to machine edges that would determine the perpendicularity of vertical components. All too many times, even using a Kurt vise, I was not pleased with the outcome. I attribute that to the use of cold rolled steel, which may or may not be dead flat, yet more than adequate for tooling members. By holding with an angle plate, the machined edge is guaranteed to be square with the member as it relates to the base of the tool. You most likely can apply that logic to the parts you're machining, assuming they lend themselves to being held with C clamps. Even round objects can be so held with the aid of V blocks.

H

[Dick_Morris]

If the wrist pin is going to rotate in the crosshead, I'd go with bronze. If the wrist pin will be stationary and all of the movement will be between the pin and small end of the main rod a good bearing material shouldn't be needed for the bushing.

I'm not a fan of using threads for holding an accurate alignment. For my crossheads I bored and reamed about half the depth of the hole to be a close fit on the piston rod and threaded the second half.

Concerning planning machining steps - I've been working on expansion links for Walschaerts Valve Gear for the past few days. to avoid painting myself into a corner I drafted a list of the individual machining actions. I was surprised that I came up with about 25 discrete operations.

[Bill Shields]

Only 25?

[SteveR]

I have been wondering where you start with a casting too. What part of a casting do you trust as a starting point to square the part?

My advice here is to determine which features of the casting you are NOT going to machine and machine around these. For example, the steam chest ports on some cylinders were determined to be so close to one edge that I could only remove 0.050 and still keep the ports in the correct area. In another example, the outside/front surface of a spoked driver generally needs to be a fixed distance from the edge of the tread, start on the front and let the back surface be determined by the thickness since no one sees the back.

Still learning.
SteveR

[Pontiacguy1]

I had a crosshead on my 0-4-2 chloe locomotive that the main rod pin hole, and the threaded hole that held the pin in place, had egg-shaped badly on. I ran the thing hard and it finally egg-shaped where you could simply pull the pin out without unscrewing it.

Since the pin was stationary relative to the cross head, and the rod bushing pivoted around that, I didn't worry as much about the material being a bearing-type material, only the strength. I bored it oversize on the original center, then I made two bushings out of cold rolled steel, and then silver soldered them in place. I then did my final drill/ream/thread after everything had been soldered in place. It worked fine and I never had any problems with it again.

One issue is how do you want it to look. At the time I did my repair, I really wasn't too worried about that. I just wanted to get back out on the track. My thought was that steel would be less likely to egg shape in the future than the cast bronze. If you want it to not be noticeable, I'd use some bronze and try to be neat with the silver solder.