Steam Locomotive Throttle Valve Repair: Turning and Lapping
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- Опубліковано 13 жов 2016
- While we are having some repairs done to the boiler for the 1917 Vulcan Iron Works narrow gauge steam locomotive at the Georgia Museum of Agriculture in Tifton, GA, we are taking the opportunity to do some much needed repairs in other areas as well. One such repair that was needing to be done was to work on the throttle valve for the locomotive, which has been leaking a small amount of steam when closed. The valve was removed before sending the boiler off and in this video, we will go through the process to face the valves and then lap them in to the valve body.
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"Oh man, he's gonna need a power tool to lap that"....[Keith gets a power tool]. "Oh man, that's gonna take absolutely ages to lap cleanly"....[2 hours later Keith has it finished]. Really nice job Keith. Ugly task, great to get it done.
I was initially thinking that plugging the old center hole and then re-cutting it may have been better than moving the tailstock, but then seeing as how you had to taper turn more than .030" off both faces to get them to clean up, trying to re-cut the center hole would have probably been a waste of time anyway and so moving the tailstock as needed to get those faces running as close to true to begin with was the most expedient. Very nice to see Keith, especially knowing both the heritage steam locos within my vicinity, 2-8-2 SOU 4501 at the Tennessee Valley Railroad Museum in Chattanooga and 2-8-0 GSMR 1702 of the Great Smoky Mountains Railroad, both engines' throttles are of the same type as you worked on here, just quite a bit larger of course.
After reading some of the comments I figured what the heck I might as well put my 2 cents in. This one is pretty easy, the hole is not concentric to the seat.......fix one check runout and cut a piece of shim stock and shim the live center over....fix two indicate seat without tailstock and cut a small area for center to run in.....fix three place small plug in hole and indicate seat in, then center drill plug for a concentric center......fix four go ahead and run off of existing hole and turn seat concentric to seat(this is in effect what you did) Love your Videos BTW: it's really easy to be a backseat driver! I've used a center punch and moved existing centers over a bit to get an OD to run true when re centering wasn't and option. Usually when OD grinding between centers. It works.
My real name is Michael McCluskey. When I took a course in auto mechanics. I remember using a lapping compound when installing new intake and exhaust valves and a special tool to spin the valves with a drill to get them to sit well on the valve seats.
This train repair stuff is really cool, you don't get to see this type of detail repairs much
"Bell-crank" That's the word you were looking for to describe throttle linkage. You knew that, I'm just reminding you.
Good job, Mr Rucker.
Wow - you are right and I have learnt something !
Although it could merely be called a 'crank ' and even possibly 'crank arm'.
Nice job you have done there, but almost guaranteed to leak. Double beat valves require a trick that not many people know about. Do as you have done in your video, then apply fine compound to the top seat, and oil to the bottom seat. Give this a good lapping then clean and reassemble. This allows the steam pressure to push down on the top seat shortening the valve a very minute amount. Without doing this the valve will always hang on its top seat. I will send you a diagram on Facebook. Works first time every time and a steam tight regulator is a certainty.
Very interesting comment Jason.
voting this up for increased visibility ^
Keith. You will have trouble with that valve big time as others have said only lap one side and you mentioned a step in the seat ,that will cause the valve not to seal in quick order ,just my thoughts
Stuart
I never considered the difference in surface areas helping to hold the valve closed. very interesting.
The educational comments like these are one of my favorite parts of the video sometimes. I was actually thinking that it should be short as well only I was thinking to allow for expansion so that you wouldn't end up with the valve heating and lifting the upper seat by expansion. If the valve was short the throttle linkage could somewhat help to pull it closed but if the valve is long no chance to ever close it.
There aren't too many places where you can watch someone repair / renovate a 100-year-old steam locomotive. Thanks for posting this.
The part/function diagram was a great addition to the vid. Thanks for sharing.
John Strange j
A design that is a pain to work on, but I think I can see the reason behind it. Balancing of forces:
The top valve is opening against the pressure, and that alone would be very hard to open. The bottom one opens with the pressure, and would open by itself. The top one is larger, so it "wins" and keeps the valve closed, but the force needed to open it is just the difference in size between the two.
I'm not sure how it is for steam spool valves, but we were taught when surfacing poppet valves and seats to go for the middle of the taper for contact area, the desired contact area would be 15%-25% of the total width of the seat (depending on the medium being sealed). Because you had two seats to keep track of in this spool valve, it made this job far more demanding. It might not be a bad idea to catalog what you did here for the next time this job needs to be done to make sure the seats and valve faces get resurfaced together before lapping; the sharp edge that developed during lapping is indicative of a seat problem which will ~eventually~ need to be addressed. Great job on a difficult task!
Matthew Helton would it make sense to go back and remove a bit of material above the step so the valve doesn't hang up there?
Without looking at it, I cannot say. There is no reason to doubt it won't seal, but my thought is that the ridge is going be attacked by the steam flow and might cause abnormal wear. Keith mentioned, and I don't disagree, that the valve didn't look like it was serviced since the engine was new, so it will probably be a very long time before it will need to be serviced again.
Jason, I love seeing these 'secrets' get shared - they're the real magic.
Wonderfully informative video. Thank You Keith.
To get them cut even, do the top first like you did at 030, then take 10 off the bottom to get it to seat. Keep fitting with a 010 feeler gauge in the top gap until you've incrementally shaved enough off the bottom to get that measured 010 gap at the top. Then take the final 010 off the bottom.
Your valve reminds me of a valve that a friend of mine designed for the navy for fuel transfer between ships. It sealed by something that he called pressure difference sealing. I worked with him in his shop 45 years ago, and was amazed at his knowledge. We would always hand lap the prototypes and there were some that took 8 to 10 hours for the seal to work. When you finally get it to seal you feel a sense of satisfaction for a job well done. You must feel very proud to see your hard work actually function and work properly. I know how it made me feel. Great videos, keep them coming, and I have learned a lot from you. Love your rebuilds.
Fuel valves get a much easier time of it than steam valves. One gets lubricated the other gets corroded !
Keith, have enjoyed your channel for quite a while now. I especially like when your at the museum and working on the steam locomotive. As a retired locomotive engineer, who never ran steam, I like learning about the maintenance and mechanics of the locomotive. Makes me appreciate what the guys in the roundhouse and machine shop did. Hard work in those days.
Lapin' valves.. man it's the 1950 and 1960's all over again.. I remember doing this to my cars to get that valve job done.. what a long pain in the butt.. That was some great lathe work!
Great example of how to do this in the home shop. Within a couple thousands, I don't think so. Nice fix tho.
Nice job Keith.
Sir, you are a true inspiration and I am so happy that your videos will help keep some age old skills relevant forever. Thank you. I know little about machining or engineering, however I'm learning with every video. Once again, thank you so much.
I would imagine, 100 years ago, that would've been machined with a dual insert tool holder of some sort. Set up the distances apart and machine both faces at once.
Would hardly be worthwhile to make a setup like that for one individual valve though. I'm impressed you got it so close first go.
Keith, I just would like you to know that I appreciate the time you spend on producing these videos. It is a time consuming job, but its good to see you maintaining the quality of them. Well done.
I was always told that when lapping valves, you never make full circles. The thought being that if you somehow ground an imperfection in it, you wouldn't want it to encircle the entire valve and that is why valve lapping tools (that are awful to use and the suction cups never work) automatically reverse. However I couldn't imagine trying to lap this project without rotary tools like you did.
I suspect that's the wrong way around. You SHOULD make at least full circles. Whatever imperfections are put in with the lapping process will affect the valve and the seat equally so should result in a matched pair of surfaces.
If you don't do full circles at least, neither the valve nor seat may be perfectly round - and they have to be as in use the valve is free to rotate to any orientation - and wherever it ends up, the seat has to match its shape.
Valve grinding is always a long tedious process. No doubt their is a lot left to do before the engine goes back together. Keep on keeping on.
Nice job Keith, it is so satisfying watching that old metal come off and shiny new metal appear on an old part! Also what is the story of the chick-fil-a cup on the bench? Subliminal messaging or coincidence? 2nd or 3rd time seeing it with the logo nice a clear. Great video!
Nice job Keith!
Keith,
I don't know much about steam locos and I really enjoy learning. Great video! Thanks.
Bob
Always enjoy watching your videos
Wow, sent me back just over fifty years to learning how to lap valves in small engine shop in eighth grade. Was looking for that uniform gray on the seating surfaces. Well done!
Always interesting to see your restoration techniques of old machinery, learn a lot😀
Always enjoy the locomotive repair work you guys do there. Thanks for the video Keith!
Thank you very much for posting these videos. I really enjoy them and hope you continue to make them. I want to thank you also for posting high quality videos. I like the fact that you explain how what you're working on operates and what purpose it serves. I have learned something from every one of your videos I've watched. THUMBS UP MR. RUCKER!!!!!
Good job Keith if worst come to worst you can always fit a modern valve to it, but I thank it will be fine.
I've always wondered why they don't use gate valves that screw down as those would give a finer control of steam flow - hence better control of wheelspin.
Thanks Keith. Interesting process.
Thanks for a great video Mr. Rucker!
Hi Keith,
That was a recognizable job you had there. Fun to watch as always.
A looong time ago I had to do a similar job myself. With grease I "glued" a leadwire on the circumference of both seats in the housing. I than placed the valve in the housing, applied some pressure and removed the valve. Measuring the (remaining) thickness of each leadwire was an easy job, so I knew were to take of the most. Saved me a considerable time to lap the seats in.
Very nice repair, was glad to see you lapped them in, even though it is tedious.
Thanks Keith, nice videos restorations are always super
good jos like always. it will be like a new train when you get it back together
Hello Keith,
Very interesting that you show inside the locomotive and explained how things works, thing I never have seen. Also interesting to see your repairs and not only just replacing parts but repair the old parts.
Looking forward to the next video and many greetings from Roel !
That was really neat, I never knew how a steam engine throttle worked. Thanks for sharing.
First part was funny to watch, we all have our moments. Surely you didn't think moving the tail stock would help.
Looks sweet Keith , Good job man !
Adding to what others have said about the center and correcting run-out, early machining texts say to scrape the center to correct run-out of the outer diameter.
The problem shown here is why you never, never smack the end of a part with a center in it with a hammer directly...if you try to do repair work you will have exactly this problem.
Great job Keith
always very interesting thanks for sharing your great skills
Your way or a hundred other ways,,,, eh,,, I just enjoy watching you do that machine magic. Thanks.
G'day Keith great video thank you John
THANK YOU...for sharing.
another job well done !
Very nice taper turning, which is not easy at the best of times.
Good job for another 100 yrs.
I always get a kick out of the desire to be dead nuts on vs. good enough. The linkage didn't care if you were off by a bit or a lot, what matters is the seats. It seems nigh on impossible for us to get our heads around "good enough".... especially with our ability to machine to .0001" with a CNC, or see 10 places east of the decimal point. That was a good fix.
Especially keeping in mind that the valve's going to be in an extremely hot and harsh environment within the boiler, conditions that will cause both the housing and the valve itself to expand while at operating temperature. You practically cannot duplicate those conditions on the lathe while doing the machining, so 'good enough' is what you have to settle for.
That is a floating valve, it is more important that the seats be parallel and concentric to each other, then to be centered between the chuck and tailstock. the reason you're having trouble centering the valve seats, is that the face is at an angle, to be more precise curved, when you move the center line, you're actually riding up and down the face. And that face is worn out. It is no longer parallel to the other face. There is absolutely no way that you could truly get it centered using that method. You would need a reference surface on both ends that was unaltered from when it was first made, This is as good as it gets. At least without making a new valve.
Great job, and keep the videos coming. They are very interesting.
Joe.
amen. SOMETHING has to be a good reference in order to get it lined up.
Hi Keith, it,s been a full time can,t catch a break type of summer so now I can catch up on your videos.
Very good explanation of how you do what you do I've seen my dad do it and it's so long process
keith nice work.
You are going to get a hundred comments all disagreeing with each other Keith .... 8-)
That was a decent method, and it will work as well as most. I would have expected the top and bottom stem seats to have a small amount of play between them - not one solid casting. In that configuration you could have the stem seat angle slightly less than the body seat angle, giving a sharp annular contact ring for both parts of the valve.
Thanks for sharing Keith. Good job on a difficult task. When you said you were going to lap the valve, I wondered how. Talk about tedious. Admire your patience.
TBH, there not really any other ways or better ways to do it.
Don't you just love how after 100 years of getting hot and then cold cast iron will distort.
nice job I enjoy your vids
That air wrench made the valve lapping pretty nice. I wish I could do that when I lap valves on a cylinder head :)
Keith, not sure why YT suggested this old vid but there we go ! So it's 5-6 years old but at 6:00 you talk about offsetting the tailstock to deal with run-out of the part. Unfortunately, offsetting the tailstock won't achieve that - as the run-out is still rotating and is not a fixed offset that the tailstock could adjust for.
Good old valve grinding compound !
don't know if anyone's said this before...the valve design is sort of self-servo - the top port, upon which boiler pressure acts to keep it closed ( when shut ) is somewhat larger than the lower, upon which end the boiler pressure is acting to open the valve, so that overall, less than full boiler pressure has to be overcome by the driver to open the valve...
which is all jolly good, so far...
but, once fully open with steam flowing under the top part & bottom, you are looking at full boiler pressure acting to keep it open...therefore, I suppose, the operating linkage geometry is probably set up to make more closing leverage than opening...but that is conjecture
And I forgot to mention that a love these videos.
nice job btw. cheers.
I really do appreciate your efforts on keeping old machinery functioning and enjoy the videos you produce, so, please don't take offense at my comments. Keith, I'm a arm chair supervisor. Your the guy with the turnin' machinery. Having said that, would there be enough material in the valve body to machine out to install new seat inserts? Automotive engines get new valve seat inserts frequently. I would have like to seen a Prushen Blue pattern without spinning the valve stem to see what kind of seal you were going to get. Maybe some kind of water pressure test would be of value to see if more work would be required on the assembly. bjr
Who! Dude! 4:13 100 years inside the boiler exposed to the steam, and those nuts just pop off like it was assembled yesterday? Man, you must be doing some clean living to get rewarded like that!
That seems to happen a lot for him. I take of a bolt installed a year ago.... snap! :o/
@@MrUbiquitousTech Ahh but in this case, it was correctly assembled with the intention of it coming apart later. The threads were probably well oiled prior to assembly - that oil being trapped in the threads but more importantly, the top nut was NOT fully on the thread - meaning there was no exposed thread to rust and jam on the nuts !
just a hint.. in case a viewer ever has a situation like this come up. could have used different grades of lapping compound top and bottom face.. super fine on the top. and course on the bottom to remove more metal from the bottom.. could also just spread compound on the lower seat to work that one away.
i know you are in a rush.. i would have loved to have see it tested.. using a shop vacuum hose duct taped to fit in the bottom opening and a little soapy water sprayed on. at least the steam pressure holds that valve closed.. or so it looks..
Thanks for the video. Very interesting. You have a lot of patience. Two hours to lap the valves is a lot of time and effort. Well done.
I hope you provide an update when the train is back together as to how well the new lapped valves seal.
Hey dude sorry just having a bit of a laugh to myself. Not being rude to you but two hours is pretty fast. Try spending a week or more hand lapping valves for a large reciprocating air compressor. Each valve being 6" in diameter, all 16 of them. Those were the days.
LV, with respect, what Keith is working on should only be called a locomotive. Non-railroad people will typically call anything on the rails a train, however: a train is a power unit, with or without cars, having a headlight in front and marker lights (traditionally, not today where we have EOT devices) at the rear. This was in the rule book in my day and I doubt it has changed. There were additional sub-rules but that's the basic description of a train.
@@fredericnystrom9045 That doesn't make sense - as a loco is a power unit !
A train is of two or more vehicles.
A loco + tender = a train.
Moving the tailstock will not center it. Fingers x'ed that you find a solution while watching. You'll just move it away from or towards the indicator. You need to "ex-center" your live end the same amount as the hole in the part is off center.
But now you're actually centering the lapping surface to the existing hole, and that should also work.
Sorry for being a couch machinist. I love your videos, and they are all "must see" for me.
Good observation.
Absolutely right, a better way would be to plug the hole, bump the larger seat true and drill a new centre, would have saved about two hundred years worth of wear, but if it works, it works and that is all that really matters.
Also the opposing seats only get the lapping treatment, not the "turning" part. I can't imagine which contraption could do that cutting.
RG is correct about moving the tailstock. I use narrow strips of shim stock between the center and the hole in the part. What you're trying to do it move the part off center, from the lathe center.
That was the first thing I thought, I thought he was going to say "So what I'm going to do is...stick some 7 thou shim material on my center." or even "Use the 4 jaw to machine an insert, and drill a new center into that insert with a 7 thou offset and temporarily friction-fit press into the part." and then he mentioned moving the tailstock and I was like "Oh, no no no, no! The hole is still concentric to the tailstock regardless of tailstock alignment, the error in concentricity is in the material, not the tailstock!"
Good fix under the circumstances. Think I would have blued it up to find my heavy contact points then worked from there. It has lasted this long and the way it looks will last a long time to come. Wouldn't you just love to cast one of those things!
I've never seen a steam control valve like this before. So interesting
If you don't have anything to indicate off of and nothing is straight then you do the best you can.You did a good job.If you had tried to true it up without moving the taistock either you would have ran out of area on the valve to lap or you would be there till next Wednesday lapping lol..
9:30 I noticed it first time you showed it, I'm good, very satisfied with how good I'm, lol.
nice nice video.
Really I wish you had film crew with that locomotive disassemble, to film every and each aspect of the process. I wish it badly.
As others have stated, you cannot correct runout with tailstock offset, only taper.
You could have bumped both valve faces in then lightly cut your hole to 60 deg for the center. Use the 60 for tailstock support. end. Wouldn't surprise me that the valve is worn off center from hole with use..
A point on lapping valves, lift the valve off the seat while spinning. The lubricant in the compound pulls unused compound inward. Drop the valve back down on fresher compound and the lap goes quicker. I've lapped hundreds valves on internal combustion motors.
Nice job on the valve. Lapping valves is one of my money maker jobs. On small engines. Is good the steam engine was taken down , I wonder if that not sealing caused it to burn the curve in the upper one. Thanks for the vid. Gary
Keith I am far from being an expert machinist but I am pretty sure adjusting the tail stock would only compensate for taper not run out. I am more used to run out involving center type grinders. Neat to see the actual working parts of an old steam engine.
It's comments like this that I'm browsing the comments for - how many spotted this error :)
Keith, Love your videos and enjoy the great. way you think out solutions in unusual projects! Are you going to test the valve before attaching to the boiler?
Nice....!!!
Keith,
Now you need to do a video on ways to get the tailstock back into alignment.
I probably will - was thinking of that the other night!
One of Adam Booth's sponsors, Edge Technologies, sent him a nice tailstock alignment tool that looked slick.
Lapping to the extreme. Can you imagine doing this by hand.
Is there a video that shows how well the "new" valve worked, once it got installed ?
Keith, what are you doing ??? You're not cutting a taper, you're compensating for run out ! Next time just tap the part around until It's true, then recut the center. Thanks for a great vid, Cliff
I was thinking the same thing
Second that. I had an idea of shimming the hole to make it run true but you're probably right, truing and then cutting a small seat for the center would be the right way to go.
I wonder if adjustable live centers exist, kind of a fourjaw for the tailstock.
Vintage projects shows plans to make an adjustable center. Perhaps you can modify it to a live center.
Very tricky task machining both surfaces to contact at the same time. You can't check it for fit while turning it, either can you? Thanks Keith, I appreciate your efforts. Take care on your travels.
He couldn't check it with the valve body with it mounted in the lathe that way round. Had he had it the other way around, he could have done.
Steam Engines are awesome!
Enjoyed the machining. Don't know if moving the tail stock will fix problem. In carpentry, I kinda think of it being a jack rafter cut at 0:12....square....and then doing the same cut on a 7:12 , something I do understand when it comes to angles. Just think of the hip rafter as being....the valve body....with jacks connecting to hip at cuts. The hip rafter can't run out of alignment in any plane, must be straight. Otherwise plumb cuts of jack rafters must change angles to fit. Thanks for the video!
....13
Now, how about wood working messing with your mind? lol
Hi Keith. Very interesting vid as I'm a steam buff from way back. It's nice to see an original valve in pieces as I need to design one in 2.5" scale for my K36. I've got lots of books with drawings, but nothing beats the real thing. One thing I noticed but is that you didn't dial in from the Flyghts which are what hold the valve central and the seats must be concentric with. Then again it is a hundred years old so maybe never made concentric in the first place.
Concentricity will come from the lapping process and as for the valve being correctly aligned, again, the seats will perform that function as the valve is not constrained by the central rod that operates it. A little slop there will allow both halves of the valve to find their best seating position.
Hi Keith
sure agree with you that getting both of those seats to match is a heck of a challenge!
I wonder if later models of that basic valve design were more adjustable with respect to simultaneous seat contact?
My other question is whether you are thinking of a test of the valve with water or compressed air for sealing effectiveness before putting the valve back in service?
enjoyed this video- I am still amazed at how well this equipment was designed and built-close to 100 years old and still working AND serviceable!!. How many of the things we are making today will last that long :-)
regards
vic
I've had the pleasure of doing lapping of a throttle valve before. Not an easy job. Only thing was you didn't mention that this was a balanced throttle valve body.
Semi balanced !
i might be wrong here but i dont think you can adjust runout by shifting your tailstock over like that, your runout is always going to follow the live center, what you should have done is left the big end dangeling, indicate it and carefully cut a new center with your compound, then put your live center on and cut the faces.
I was thinking the same thing. Moving the center over won't change the point in relation to the outer diameter around which the part is revolving, and that's really what needs to happen. You could either cut a new center as Georg suggested or put a piece of shim stock on the appropriate side of the center to nudge it over.
I am thinking the same, only my brain just melted...
What's happened is the centre bore is of centre from the outer valves. It was probably machined like that it's probably a complete taper. What's happened is you've now Nearly emulated how the first machine to turn it was set up.
Georg Andersson From what I can see, this is a cast piece and the only machined surfaces are so worn and uneven that there is nowhere to get any accuracy in indicating. I suspect that in the day this was made, hand fitting was the norm so what Keith did was about as accurate as the original if not more. If there is any criticism of his technique it may be his lapping of the 2 stage valve seats making them both even. This may cause problems.
looks round to me. im not trying to be negative but you still cant adjust runout with your tailstock like that. And lathes existed back in 1917
You guys should take measurements for a future casting. Better have them already to spec in mind, if you need them, you can send the part to be cast and make it whilst the old one is in the boiler still.
That nag aside, great work, and neat "footwork" with the tail stock. I'm a bit reticent in touching mine, but, this really brings in that, it's a feature/possibility, that i'm not using.
One of the things that I think that people get hung up on is that they don't want to mess with the adjustments on their machines. They were made to be adjusted and should be used when needed. It is not a big thing to adjust a tailstock, tram a mill, set up a vise, etc - all things people are afraid of doing.
You probably heard me hollaring!...Don't move the TS! ....the runout had nothing to do with the TS, and as you found out, you could have adjusted it til you were blue in the face, and not made any dif.....the runout was with the center and the valve face, so either shim the TS nose, or cut a new center concentric with the face....the heavy duty lapping likely moved things to true....great vid!
Nuts, actually
Moving tailstock will not solve radial runout--need to cut small area of part contacted by tail stock cente at 60 degreesr--- OR----run a bolt through the part and put a nut on the end to hold in place, the centerdrill bolt end-- install center and your are ready to go.
Strong work, Keith. That was a tricky problem. I wonder how long it would have taken to program that work in G code for a CNC turning mill. I'm guessing months.
hi keith, I think you should have kissed the broad-end hole with a boring bar to make it run true. frustrating probably all this comments on hindsight.
There is no way that I could have held it to true it up.
ok, that looked different in the four jaw on the video. nothing said.cheers.
I'm pretty sure that if the hole on the center isn't concentric with the outer diameter its not going to matter what you do with the tailstock because the axis of rotation isn't changing that's probably why it wasn't doing what you wanted.
Keith, after watching a great many of your videos I have noticed that in regard to removing tight nuts you will more often than not start with the open end of the wrench. This is a mystery to me. A 6 point of even 12 point box end is much better. You only have to round a flat off a nut once to screw it up.
If the center is not concentric with the seat, it doesn't matter where you pit the tailstock.
Hi Keith,
It seems to me that if the hole in the casting is offset you could move the tail stock all day and get nowhere. You would need an offset live centre. Like a fly cutter with a centre instead of a cutter. I don’t know if such a tool exists.
I very much enjoy your videos. Thankyou.
Yes, he's mentally lost it on this one ;)
Keith, I don't think you can adjust the runout with the tailstock offset, runout will stay the same. Challenging task though to match the distance between the valve seats, looks like you got them to lap in evenly, cool repair.
It's does actually work for type of work, if you consider how it was originally machined
Matthew Trott It doesn't, you need an offset center not to offset the whole tailstock, it's the same idea to the 4 jaw chuck at the headstock, you don't change the center of rotation by moving the spindle but by creating an offset between the center of the spindle and the center of the workpiece, same thing at the tailstock - you move the center away you don't change the center of rotation in relation to the tailstock center.
what you need assume is that the whole part is a taper so you are offsetting it to that taper. given the part machined pretty evenly when set I'd say it did work.
Matthew Trott he's not adjusting for taper but runout, if you have a lathe try to adjust runout with the tailstock and you will see you get the exact same amount once you start rotating the workpiece, geez....
if I'm honest I didn't watch much bad habit of skipping chunks. I assumed it was running concentric