Curious if any others out there are doing lapping of rotary and slide valves with a "Lapmaster" machine and if have any best practices to share?

I used to do this by hand either on a "waffle plate" type cast iron lapping plate, or a granite surface plate with abrasive paper. The Lapmaster is much faster and produces far better results.

I use a 15" machine. The work normally is held inside the conditioning rings which are 5.5" ID. Some of the larger rotary valve seat castings are just a little bit more than this. Here are some photos of an M19A automatic brake valve that I worked on very recently


This is what one looked like before starting. Here is another unit, with a very odd pattern on the rotary valve seat:



It almost looks like someone in the past took a hand scraper to the thing!

As is typical with valves that saw very little / very poor maintenance near the end of service, the rotary valve and seat were very worn. Over time and especially with bad lubrication and dirt in the system, valves tend to grind themselves into a concave/convex pattern. In my experience, the brass rotary valve is always convex because it has the greater range of motion. The two surfaces may actually be very conformal and might even have acceptable leakage. That was not the case here! This valve had audible leakage out the exhaust pipe (which runs down through the floor of the car).

I put a straightedge across the surface and it was "out" by at least 6 thou. That's really too much to take by lapping so I had to resort to cruder methods:

The rotary valve casting is not normally machined on the outside. It is just the rough casting including the two "lugs". So that's very hard to get to run true in the lathe chuck. I made a quick fixture out of a cut off piece of steel disc stock, turned to fit inside the diameter of the bottom part of the casting and faced on both sides, then drill/tapped to line up with two of the bolt holes. We then faced it taking very light cuts with a sharp facing tool until it had been cut all the way across. Then back to the lapmaster:



The casting is about 5.75" across the outside of the "lugs" and rather than modifying the conditioning ring which comes with the lapmaster (well, there are 3 of them) I made a new ring out of cheap plastic, 6" ID and the same 7" OD. Now I could run the casting with the freshly faced seat in the ring. Rather than make a special workholder phenolic disc with a 6" OD, I just shoved some rags between the casting and the wall, so that it was firm but not tight (it has to "float" down) and off-center enough to create rotation.

It took about 15 minutes on the lapmaster to complete the lapping job. This is with 9 micron aluminum oxide abrasive. I know some shops prefer 12 micron which will cut a bit faster. Not shown in the photo above, I used the lathe chucking fixture as an additional weight. You want to be at about 1.5 - 3 lbs/square inch of surface in contact with the lapping plate for best results.

Here is the final result:


notice the even color and surface texture throughout. The lapmaster produces a dull "frosted" surface which when using the 9 micro abrasive will give a surface finish of 5-6 microinches. A polished surface is not necessary, but one does have to clean off all abrasive lapmaster slurry residue from the surfaces before assembling.

This particular valve, when assembled, was positively bubble tight.

The Western Railway Museum uses a Lap Master for some of the air brake maintenance. Iron parts in bad shape usually go on the surface grinder first. We have found once a valve is in good condition, if routine maintenance is preformed on a regular bases, little additional lapping is needed. What lapping is needed is done by hand lapping on an iron lap. We have a 15” round lap for use with brass parts.

We do separate iron and brass parts. For brass parts we use garnet lapping compound to avoid the harder material from imbedding in the brass. The last time I bought lapping compound it was from US Products Company in Pittsburgh. On their web site they have a lot of information on lapping techniques.

The best way to go. When working at BHM shop in Denver, the Grande guys went a little further after using the Lapmaster. They would lap the two faces together with oil and then lightly relieve the out circumference. They found through the years the out (roughly) 1/16" of circumference would get the most dirt (probably from carry over from main reservoir air being up top of the rotary valve). A brass ring was used as a guide when lapping rotary valve to seat when cleaning up the outer surface. While its been many years ago I believe they also for that part of the job a very small amount o 600 grit compound at the edge for the relieving. I doubt much more than maybe a tenth or two came off the outer surface. As what you are finding they would go over test rack in flying colors.

They also used them for the brass slide valves. Gustin-Bacon used to make lapping stones for all the different width slide valve seats in the bodies and using those instead of filing saved much time when lapping a slide valve to a seat. Once they went out of business don't know if anyone else produced the stones. One could probably get them from Norton (or equal) but at what cost.

There were limits that are out there for the height of the rotary valve seats.

Also, I have seen an attachment for the Lapmasters that provided a reciprocating motion to lap in a slide valve to a body. This required special lapping blocks used with lapping liquid. These must have been real nice to use when lapping service portion of UC slide valves their body seats. This attachment was driven off the top if the Lapmaster with the valve body mounted to the side.

Chris A

Greetings:

Chris A. (and "The Grande") are quite correct about relieving the outer edge of the rotary valves on H-6, K-14 and similar brake valves.

However, John Eames, Steamtown's (Bellows Falls) airbrake man (who had retired from the Maine Central Waterville Shops) said that the main reason it was good practice to relieve the edges was to extend the time the valve was cleaned and when it would start to become "stiff" and hard to move the valve handle. He also taught us to use the rectangular air brake hones for the seats of the slide valves.

When I first arrived at The Valley Railroad it was SOP for the engineer to remove the little plug it the top section of the H-6 first thing in the morning to pour in transmission fluid and work the brake valve handle back and forth until it worked freely. Then it would be "fixed" for the day. Of course, the transmission fluid worked it's way back through the brake pipe and fouled valve after valve with whatever dirt it picked up along the way.

Relieving the rotary valve solved the problem with the brake valve handle needing two hands to move it, but getting all the fouled valves cleaned and all the oil out of the brake pipes and reservoirs in all of the cars was a task that took all summer. Fortunately, Steve Zuiderveen was Car Foreman at Steamtown (Scranton) and lent me enough brake valves for us to get an initial terminal test each day so we could safely (and legally) operate. We ended up taking a couple of pickup truck loads up to John Eames' basement shop for cleaning, yes, our valves AND Steamtown's.

Be well & Be safe,
J.David

PS: The last place that made the rectangular air brake hones for me was Niagara Grinding Co., unfortunately now out of business. However, I have the Gustin-Bacon catalog sheets which have all the dimensions. I'll be happy to share. JDC

Regarding Chris' and J. David's note of:

"They also used them for the brass slide valves. Gustin-Bacon used to make lapping stones for all the different width slide valve seats in the bodies and using those instead of filing saved much time when lapping a slide valve to a seat. Once they went out of business don't know if anyone else produced the stones. One could probably get them from Norton (or equal) but at what cost."

In conversation with Dave Garcia, I recall him singing the praises of some sort of abrasive block for slide valve cavities. While I haven't as yet attempted much research on this specific subject, I wouldn't mind seeing some literature and seeing these blocks in action. Thus far, we've done all our lapping with the plates/blocks as shown in the picture.

"There were limits that are out there for the height of the rotary valve seats."

Timely, as I was just looking at a book yesterday that called out these limits for a few brake valves. First time I recall seeing something like this.

-Erich

I've heard of this practice from several different sources. J. David in this thread also talks about this. I personally have never done this unless there was a palpable burr on the edge of the rotary valve or seat. I don't have any strong theory as to why this "breaking the edges" would help if the valves came off a lapmaster and were dead flat to begin with. Maybe it delays the formation of ridges at the outer edges of the rotary valve.



I'm going to post up some stuff about lapping stones.


Do you have any photos or references to articles about this? While I have heard of automatic lapping machines for slide valves, I can not actually find any clear photo or drawing of one.

At Orange Empire, they have a machine for lapping tapered key plug "valves" -- of course "valves" is not really the correct word to use here, but this board is searched by Google and I'd hate to be seen writing about "c*** lapping machines"

Yes, "good condition" is an important X-factor. Very often, especially in the traction world where air brake maintenance standards could be very iffy, these valves come in with absurd amounts of wear. I started the post with a photo of a rotary valve worn .009" out of flat. Valves in that condition need some preliminary roughing before the lapmaster or hand-lapping can be considered. This is particularly annoying with slide valve seats. Unless you are going to invest in a broach, there is no practical way to get down into the slide valve chamber with a machining process. I'm going to post some more thoughts about that and lapping stones, below.



I use the garnet paste that you mentioned, also from US Products. I stock 220, 500, 700 and 1000 grit. The 1000 has almost never been used. It is more of a cosmetic thing. The surface finish at 700 is more than adequate for air brake devices.

On the lapmaster, I have not had any problem with the 9 micron aluminum oxide abrasive slurry embedding in the brass. The residue needs to be cleaned quickly with paint thinner or similar solvent and wiped clean. On the other hand, silicon carbide paste such as "Clover" must be avoided at all costs with brass parts!

The topic of lapping stones has been brought up. I've had mixed results with these. Here is a photo essay of a recent project with a #6 distributing valve. This isn't a valve that I normally work on and I don't have much tooling made up for it. The valve was probably last serviced in the 1960s. You might recognize the application valve seat somewhere under all this sludge and rust:

This valve was a disaster. The seat was "hill and dale" visible with the naked eye. It was a close call. Take it over to the milling machine for resurfacing? (a rare opportunity, since slide valve seats on air brake equipment are usually inaccessible) Well, the valve is pretty heavy, and I didn't feel like spending 30 minutes or more figuring out how to set up machinst's jacks or otherwise to get the casting parallel to the machine, so two of us took turns going at it with a file! Probably would have been quicker the other way as it turns out. About 20 minutes of filing. This is the result:

The seat was very low at the leading and trailing edges to begin with. As you can see, we still haven't touched these areas with the file, but I felt they were far enough away from the port to move on to the next step.

I have a few silicon carbide lapping stones. The late Dave G. got a bunch of these as samples from one of the major air brake repair vendors. I'm not 100% certain who the manufacturer is. One of them carried the "Sticks and Stones" label. It's possible the others came from Kansas City Abrasives. These companies carry 6" stones in a variety of widths and grits. The stones are not easy to cut. If you want to make a narrower one, you'll have to invest in a diamond bandsaw blade or slitting saw.

I used a 1-1/16 stone (which I have primarily for working on U4/UE-5 emergency portions) on the 1" wide seat. Normally this would be impossible since the stone won't fit into a narrower seat, but with this "open air" arrangement, I just made two crude lapping guides as you can see


I drilled a 1/4" hole dead center on the top face of the stone for use with various hook-type tools for working the stone back and forth inside a slide valve cavity. important note the stones are received from the factory are not very flat. So, how to you get stones dead flat? One suggested method is to use abrasive paper and a surface plate. This idea was DOA for the coarser stones. I tried 100-grit paper with this 150 grit stone and it just destroyed the paper.

What did work, but unfortunately I don't have a photo, is a very nice diamond impregnated stainless steel plate made by Atoma, a Japanese company. It comes complete with a little manual, totally in Japanese! The Atoma products are available in the US from a variety of knife foamer places. Placing this thin plate down on the granite surface plate, and using soapy water as a lubricant, I'm able to get the stone dead flat in a short amount of time. Secondary truing can be done on the Lapmaster, but doing so dulls the cutting edges of the coarse stones.

I use the same soapy lubricant when stoning the slide valve seat. When I used this with brass seats, loading of the stone was a problem. I was able to clean the brass gunk out with a brass wire brush. For the cast iron seat, I had no loading issue. If the stone glazes and stops cutting effectively, I am able to rejuvenate it with a few strokes on the Atoma plate.

The stones are not particularly quick. Here is the surface of the cast iron seat after about 30 minutes with the 150 grit stone:

You see the front edge we haven't touched, as expected. Also a bad low spot in the back left and a smaller low streak at back right. Sometimes as you are going along, because things move so slowly, it is hard to keep track of what's high and what's low. So I'll stop, clean everything, and make some hash marks with a marker. I was at it another 10 minutes or so before switching to a 300 grit stone (no photos of that). 300 grit for at least 15 minutes. The 300 stone I rough initially on the 120 (equivalent) grit Atoma plate and then finish on the lapmaster. The lapmaster 9 micron is approximately equivalent to 700 grit.

I have 600 grit stone but I didn't use it here. Instead, I finished up with my serrated cast iron lap. I see Erich's laps are double-serrated. I always make them single, because I find the double pattern leads to weak spots, plus it takes twice as long to make. This lap is made on a Bridgeport, touched up with a surface grinder, and then lapped dead flat on the lapmaster. If I'm working exclusively with the cast iron lap, it will start with a heavy abrasive, such as 220 grit garnet paste, and then work up the grit scale. When I get to the 600-ish range, I just use the "lapmaster juice" that is already left behind on the lap. The lap needs to be re-trued on the lapmaster periodically. This is more the case when working with the coarser abrasive and early on in the process, when high spots in the work lead to ruts in the lap. At this point, near the end of the job, I just keep the lapmaster spinning, do about 30 seconds with the lap on the seat, and about 10 seconds on the lapmaster held by hand.

Looking for a uniform, frosted apperance. On this valve, I stopped short, chosing to ignore the low areas on the ends.

Btw, you may see there is considerably pitting on the cast iron surface. The end result was far from perfect, but it was acceptable. In retrospect, with the depth of pitting involved, I should have milled the seat and then milled the surrounding casting which would otherwise wind up below the gasket

Additional Lapmaster info

Chris, thanks for posting that 1957 ad. Erich sent me a similar one but it didn't show the attachment. So it was a crank affair. It doesn't look like it had any way of returning the lap to the lapping plate. One must have had to disconnect the linkage and put it back on the plate by hand to re-true.

How many parts can we identify in that photo?

In the photo showing the internal lapping, it does not show any method of getting the liquid lapping compound into the lap. I wonder if they are using paste in that application. The closet ring is lapping laps. I have never seen notches at the end of the laps like that. Maybe it is a ball attachment to prevent any side or down forces.

I have some notes from Dave G. on cast iron lap construction. Unfortunately my scanner is not nearby at the moment. He sketched the "lapmaster" style lap. If I recall correctly, at one end of the lap there is a rectangular slot (clearly visible in the ad) and then a hemispherical recess. The crank from the lapmaster must have snapped in there with a ball-and-socket type arrangement. At the top of the lap, there are three tapped holes suggested that another piece could be bolted on. My intuition is that this piece contained a spring-loaded roller for applying the necessary downward force. Perhaps there was a reservoir for the lapping slurry too? You can see the top piece in that photo.

That arrangement with the spring would address the suckiest part of slide valve seat lapping: generating enough downward force, and doing so without "rocking" the lap.