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| Lapping small 3/4 inch to 4 inch steam valves https://rypn.org/forums/viewtopic.php?f=1&t=32650 |
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| Author: | Tallow Pot [ Wed Jan 04, 2012 12:19 am ] |
| Post subject: | Lapping small 3/4 inch to 4 inch steam valves |
I have been lapping steam valves used in the cab for many years (did my first one in 1976) now but I was wondering how others do it! What size grit (lapping compound) do you use? Do you start with course (SP?), then a medium and finally a fine? Do you use a hand drill or do it manually? I appreciate any and all comments on how you do it. I hope we all can gain from this discussion. Thanks Jeff from the SVRR |
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| Author: | David Johnston [ Wed Jan 04, 2012 2:40 am ] |
| Post subject: | Re: Lapping small 3/4 inch to 4 inch steam valves |
Are the valve parts brass or iron? |
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| Author: | Dave [ Wed Jan 04, 2012 9:37 am ] |
| Post subject: | Re: Lapping small 3/4 inch to 4 inch steam valves |
After trying to seat bronze valves that had been hand lapped in so may times they had worn into odd curvy shapes, I started making form-specific tools and alignment bushings to guide them to reform the integral seats in the valve body, then made new stems with the correct form of seat on them. I could then only lightly lap the surfaces together for a very good seal. That, combined with the discipline to relap lightly at the earliest sign of a steam cut, worked very nicely. Some older valves were made with replaceable seats and stem ends - you can remove these and machine them to spec, then lightly lap, or simly replace them if they are very worn. There were professional "valve seating tool kits" with an assortment of hand-driven shaped seat reamers I wish I had, but have only seen them in beautifully refinished hardwood boxes at high priced antique stores. So, making the seats and stems back into the correct form, then stopping every leak while it's still small is the key in my experience. dave |
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| Author: | Frisco1522 [ Wed Jan 04, 2012 12:51 pm ] |
| Post subject: | Re: Lapping small 3/4 inch to 4 inch steam valves |
Lapping dome throttles was always interesting and a whole different game from valves. |
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| Author: | Mike Tillger [ Wed Jan 04, 2012 10:16 pm ] |
| Post subject: | Re: Lapping small 3/4 inch to 4 inch steam valves |
From Dave's post above: "So, making the seats and stems back into the correct form, then stopping every leak while it's still small is the key in my experience." Great advice!!!!!!! Typically one seat is a flat seat at an angle, the other is radiused, giving a very narrow band of contact, making a steam tight seal with very little leverage on the valve handle. If you have been lapping and relapping your valve seats over and over you may be experiencing valves that are very difficult to open once fully closed or it may take excessive leverage to close the valve steam tight. Two signs, you may have to reprofile your valve seats. I prefer the use of stones similar to automobile valve grinding stones for truing up the flat angle seats using a homemade stem and the valve bonnet as a guide, over the reamers found in reseating sets Dave mentioned. You can put any angle on them by chucking them in a lathe and using a stone dressing tool in the toolholder (I use sectioned diamond concrete hole saws, supplied by a friend in the cement testing industry. He sections them after they are to small to use as hole saws). I use this handy formula to place the seating band at the right location in the middle of the flat seat, Radius of the ball seat is determined by dividing the inside radius of the desired location (middle of the flat seat) of the contact by .70711 I have a picture which explains it better, PM me if you want a copy. Most, if not all of the valves built to the AAR specifications have interchangeable parts regardless of who manufactured them. I have a small booklet on AAR valves which gives most of the OEM dimensions. please PM me if you need specific info. Mike Tillger |
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| Author: | 060 Hogger [ Thu Jan 05, 2012 2:31 pm ] |
| Post subject: | Re: Lapping small 3/4 inch to 4 inch steam valves |
Going back to my Navy days, my first assignment, while waiting for Nuke School, was lapping valves on the destroyer tender USS Puget Sound AD 38. There is some good advice posted above. Some additional things are: Typically, a globe valve seat is 1 degree more open than the angle on the disc. For example, the seat would be lapped with a grey cast iron lapping tool with a 45 degree angle from the centerline. The disc would have a 44 degree angle from the centerline. Clean up the disc with a light skim in the lathe. Be sure to indicate for a zero or next-to-zero runout before cutting. If you have a hardened or stellite disc, the toolpost grinding wheel is the tool to use. I learned to use 240 grit ( medium ) to start the process. This will show any cuts or out-of-shape areas. You can make your decision to repair or replace after this, based upon relative costs, tooling capabilities and time required. If you have a heavy gouge to repair, and you are going to lap it, remember that a coarse grit will cut fastest using a soft tool, i.e. copper, brass or grey iron. Also, coarse grit wears out relatively quickly. Take a few ( 10 to 20 ) strokes by hand, and you can feel the coarseness and cutting. Once the coarseness and cutting feel stops, it is time to pull the tool out and smear the grit around to get some good grit in the fit. ( pardon the unintentional rhyme, but it is a good reminder ) Once the grit has worn down, you are just burnishing and running the risk of gouging. I don't use a power lap, for this reason, unless there is a flushing system to continuously circulate or replenish the grit. Use the Prussian blue often, to see how you are progressing. As you gain experience, you will learn to ' read ' the ' tooth ' pattern left by the grit. Do not let your lapping tool get out of shape. On a big job, stop and re-shape it with another skim cut. A hard faced valve seat and disc are very prone toward gouging and galling if the lapping grit is allowed to break down. Remember, lapping grit is half abrasive and half lubricant. That lube and cleaning out the hard metal debris is the only protection against making your valve seating surfaces look like the Grand Canyon. On softer valves, I finish up with 400 grit. On hard seated valves, I finish with 600 grit. Always, Always, clean the grit out of the works. |
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| Author: | Tallow Pot [ Sat Jan 07, 2012 10:59 pm ] |
| Post subject: | Re: Lapping small 3/4 inch to 4 inch steam valves |
Thanks for all of the information. Now down to the work of lapping those valves. I discovered that there are both brass and steel valve seats. It appears from the comments that my method follows closely to how others do it. Have a great day Jeff |
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