77 Degree Centers

It may have been a question of purchasing tooling in regular production, since the rest of the world (or at least in my opinion) seems to be fairly standardized on 60 degree included angle for centers. Possibly, if enough railroads were using the 77 degree center angle, the cutting tool and lathe center manufacturers might have offered tooling with this center angle. In the heydays of steam locomotives, most railroads had large backshops for repairing, modifying, and sometimes building locomotives. There was probably enough demand from the railroads to make it worthwhile for tooling manufacturers to include 77 degree center angle tooling in their product lines. In addition, any good sized locomotive backshop had its own toolroom, so jobs like making special cutting tools, grinding lathe centers and similar would often be done in-house. With an in-house toolroom, special tooling with that 77 degree included angle could be made up as needed.

My guess is the 77 degree included angle was something a convention of railroad master mechanics decided upon way back in the earlier days of railroads. Back then, there were a lot more "proprietary" types of tooling dimensions on things like spindle tapers, and screw threads were not entirely standardized either. Once steam locomotive really developed and there was a lot more of them and much more machine tools int he backshops, the railroads likely went over to the "standard" 60 degree center angle. The railroads had a history of doing some "interesting" or different things in the design and construction of steam locomotives. A notable example, here in the USA, was the way some railroad mechanical departments specified Whitworth threads on boiler staybolts. Staybolt threads are typically a fairly fine pitch in relation to diameter, and special staybolt taps (made with extra long thread cutting sections to tap thru both sheets of a boiler mudleg) were an example of this.
Steam dome and other studs tapped into the boilers were made up with tapered threads to seal the tapping as well as carry load. Again, this required special taps.
On a steam locomotive, there were numerous large bolts which had tapered bodies for a body-bound fit. One railroad might specify a different taper than the next railroad, and a third railroad or locomotive builder might opt for bolts with straight cylindrical shanks and a body-bound fit. It was a kind of wide open field, even with standardizations on pipe threads and machine threads, pipe sizes, and similar.

I have no idea why the railroads would have opted for a 77 degree included angle. It is a "flatter" cone than the 60 degree center, so possibly getting more bearing area on lathe centers might have been a reason. Putting a steam locomotive wheelset into a wheel lathe requires that two drive wheels and the axle, as a unit, be supported and turned between centers. The cuts are taken on the wheel centers (the spoked wheel) or on the steel tires once they have been shrunk onto the wheel centers. Taking cuts on hard steel tires on the outer circumference of the drive wheels requires quite a bit of torque. Supporting the wheel set on the centers puts quite a "reaction" ( a force opposing the force created by the cutting happening at the tire circumference) in the centers of the wheel lathe. Maybe the railroads were experiencing failures of the 60 degree centers on the wheel lathes and went to a center angle that had more load carrying area.

If we roll the clock back to the earlier days of steam locomotives and look at how machine work was done, we would see the master mechanics made a lot of decisions as to working practice, and the railroad mechanical department had engineers also setting design standards and practices. There was a railroad master mechanics' association which used to convene and set standards for things like staybolt threads, rivet heads, and all sorts of details. My guess is the master mechanics' group put their heads together to address problems with turning drive wheel sets in the wheel lathes and came up with the 77 degree angle. Another thought, again, rolling the clock back, is that dead centers were in common use for lathe work. Lathes up until very recent times, had a "well" on the side of the tailstock with a "dauber" to apply lubricant to the tailstock dead center. This lubricant was usually white lead and oil. Centers were made of hardened carbon steel, as high speed steel had not come along. The result was under heavy loads, a carbon steel dead center would run hot and could seize and have the point twist off in the work. The 77 degree center may have been an idea to address this by making a much beefier center point. Whether there was some explanation based on strength of materials or other engineering is another one of those things lost to the ages. It's like asking lathe manufacturers why some went with the Jarno taper vs the Morse Taper.

Brian,

Interesting question.

I don't know why they used 77 degrees, but I discovered something new (to me) recently. I have a "Rohm" pipe centre and needed it for a job. I checked the included angle and was surprised to find it was 75 degrees. I looked up a Rohm catalogue and sure enough they make them in three options - 60, 75 and 90 degrees, 60 for light turning, 75 for heavier turning etc. As far as I can see however, they only offer 60 degrees for their non-pipe centres, even the heavy duty type.

Peter:

Your response sheds some light on the reason why the railroads had sometimes used 77 degree centers. With your information about a pipe center or "bull nose" center, a much larger diameter is required on the center "point", while the actual "point" is not on this type of center. A pipe or bull nose center (as we sometimes call it in the USA) is a truncated cone. The larger included angle gives a larger diameter for a given "depth" of the center. If a larger center were made for something like locomotive wheel centers, in order to support the wheel set in the wheel lathe, the center would have to be quite large in diameter to get enough bearing area and resulting shear area in the center. If a 60 degree included angle were used, the center hole would be quite a bit deeper vs the 77 degree included angle. At the same time, a lathe center hole in the work has a pilot drilled hole, so the point is actually not doing anything either for locating the work or supporting it. By using the 77 degree included angle, the railroads probably figured they'd get a good bit more bearing area on the centers.

Recalling some basic trig from high school: the depth of the center and the length of the sloped side are used to get the cosine of half the included angle. Call the length of the sloped side "l" and the depth of the center "d"

d/l = cosine of half the included angle. Cosine of 30 degrees = 0.866, cosine of 38.5 degrees = 0.782 The ratio of cosines is 1: 1.107, which would be the ratio of long side lengths and depths. Not a very big difference.

Call the Hole Diameter at the larger ("mouth" or end of the work) "D", and its radius is "r". The diameter of the center hole varies with the sine of the included angle. Sine of 30 degrees = 0.500, and sine of 38.5 degrees = 0.622. The ration of the sines is 1: 1.244. This is the bigger ratio, so the likely explanation for the 77 degree included angle is it gets more hole diameter with less increase in depth.

If we get into computing the bearing area of the center and the shear area, this is where the bigger jumps happen for a relatively small change in the included angle.

Again, recalling basic high school geometry, a 60 degree included angle on a center point makes an equilateral triangle. All sides are equal, so to get more bearing are on the center requires an equal increase in depth. The 77 degree angle gets around the increased depth. Again, if we consider how a lathe center hole is made, the "vertex" or "tip" of the center point never sees any bearing contact and does no real work in supporting the job. The bearing area and shear area of the center which handle the "running" of the job on the center and the load carrying work through a truncated cone. Making a larger included angle increases the material in that truncated cone and increases the bearing area.

I am sure some long-ago master mechanics played with trig and came up with some "optimal" included angle which gave the best combination of supporting the loads and providing more bearing area while still maintaining the centering of the jobs in the lathes. The downside to increasing the included angle is it starts to "flatten" out the truncated cone. While the advantages to decreased depth and increased bearing and shear area are there, the less acute the included angle means the work has more of a tendency to want to "slide off" the center due to its own weight and the cutting forces and torque applied to it. Chances are the master mechanics determined the "optimal" angle through trial-and-error experimentation. I.e.- make a set of test centers with different included angles, cut those same centers into the ends of railroad axles with drivewheel sets, set them up in a lathe and take heavier and heavier hogging cuts on the wheel centers and on the tires and see which one allowed the heaviest cuts, and which one heated up the least if a dead center were used.

Brian Albin said:

I have been reading Colvin & Stanley: "Turning and Boring Practice" from the year 1936. They say the railroad's practice of using 77 degrees for the included angle of lathe center points instead of the 60 degrees used by everybody else, had long been abandoned by the time they wrote their book.
I guess the RR had a good reason to use this angle, so I am wondering why they quit it. Does someone here know this answer?

Thanks, Brian

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why are whitworth threads 55 degree ? cause in the early days they were trying different angles saying some were better for various reasons
.
my guess is 77 degree came from the British

TomB

In the USA, railroads, back in steam locomotive days, often specified Whitworth threads on boiler staybolts. Possibly the belief is the 55 degree angle makes a slightly stronger thread, and possibly, the slight radius' at the crests and roots of the Whitworth threads make for less stress risers. Boiler staybolt threads are loaded in shear, and the staybolts are usually tapped into boiler sheets that are a good bit thinner than the nominal diameter of the staybolts. A few threads engagement have a lot of work to do. Admittedly, screwed stays had the ends hammered over (peened like a rivet head). But the sheets saw thermal and pressure cycling, some flexing, and the stays had s lot going on in the way of stresses and cyclical stresses. For whatever reason, the railroads and locomotive builders in the USA sometimes spec'd Whitworth threads on boiler staybolts.

We can go back into the demographics of the railroads, and in the 1800's, into the 20th Century, a lot of the supervision and management were of English stock. It was not uncommon on US railroads in the 1800's to find English immigrants as shop foremen and as locomotive engineers. There was a certain heirarchy on the US railroads, at least in the Northeastern USA. If a person was of English, Scottish, or German stock, they could enter the shops or engine service and rise in the ranks. If a person was from Southern Europe (Italian), or Eastern Europe (Russians, Poles, Hungarians,) they were not likely to be allowed to advance too far and often limited in what departments they could work in. The English master mechanics or shop foremen stuck to what they knew, and it went beyond the shops and locomotives to the people they hired. I would not be surprised- not that we will know the answer- to find that some master mechanic who'd come from England brought the idea of the 77 degree center with him.

I know that there was considerable transfer and sharing of knowledge between US, European, and British railroads and locomotive builders. Old trade and engineering journals often have articles about the latest practice or new developments in England or Europe. Even before the internet and the ease with which we can access all sorts of information, people in the different professions and industries communicated by way of published papers, articles in journals, or by travelling and visiting examples of their particular industry or field in some other country. It could well be that the 77 degree center idea came from something ranging from a published paper or article, or from some US master mechanic or mechanical engineer visiting British "works" and discussing problems and what working practices they were using.

As to why it was abandoned, we come back to advances in a multitude of areas, and we come back to standardization. The US seemed to have settled on 60 degrees for the included angle of lathe centers. tooling manufacturers were offering centers with the 60 degree angle. High speed steels and then live centers came on the scene, so the need for a center with increased bearing and shear areas was not as critical as it had been when using carbon steel dead centers. Then, the steam locomotives passed from the scene and electric and diesel locomotives took over. Smaller driving wheels, lighter loads on the wheel lathe centers.

I suppose the 77 degree vs 60 degree lathe center question would come down to some trial-and-error experimentation with steam locomotive driving wheel sets (two drivers mounted on their axle) in a wheel lathe. Seeing which center angle gave the best support, and then experimenting with turning axles in a lathe with a tailstock dead center and seeing which center angle did not heat up as much and could withstand the heaviest cuts on the journals. Railroad machine shop work was not what you'd call fine work. In the steam locomotive days and when plain journal bearings were in use, it was often heavy machine work and not done to particularly tight tolerances. Another activity the railroad shops did was to "burnish" axle journals after turning. This consisted of forcing a hardened steel roller against the journal as it revolved in the lathe. Burnishing improved the surface finish and grain structure and produced a localized work-hardening. When a steam locomotive came in needing work on the "main brasses"- the plain bearings which the driving axles ran in had brass shells but were poured with babbitt- if things had gotten hot enough to melt the babbitt, chances are the journal was scored. A cut was taken on the journals and then the turned surface was burnished in the wheel lathe. This put a tremendous side load on the centers, probably more than was developed on hogging cuts on the journals or wheel tires. After the journals were turned and burnished, the brasses having been rebabbitted (or spares on the shelves) were bored to suit the new diameter.

A buddy who is retired as a freight engineer and sometime trainmaster has told me that old time railroad equipment was designed to be worked upon by "idiots" with minimal skills and minimal tools. The result is a lot of what I've seen on older rolling stock and locomotives fits this description. In the back shops in steam days, there was a schedule and work on the locomotives had to be done and gotten out in accordance with that schedule. Locomotives did not make the railroads any money when they were in the shops. It was a case of heavy duty machine tools and taking the heaviest cuts possible, rather than taking additional time with finer cuts to turn a journal or wheel tire. Maybe the 60 degree carbon steel centers were not holding up in this kind of service, so the 77 degree was tried and squeaked by (pardon the joke). This was the time of hardened carbon steel centers lubed with white lead and oil, when the "dauber" on the lathe tailstock actually was used.

Used those fat centers. Exactly ONCE. '63 or '64. Galis, Westover (Morgantown) WV.

We had a Niles and a Sheppard-Niles side by side. 50-inchers, "dual headstock" design, one with a "TS" / second HS - as was powered, the other was passive, just bearings. All I much remember is the powered unit was on the right of the machine hall bay, the passive one on the left!

As to the fat centre? Figured it was just the way the OEM part was prepped, so we followed suit.

The FOREMAN ...probably... knew why we had to turn in pairs and roller burnish. I had no klew at the time. Just did as asked.

If I had to guess on the origin of the turn-as-pairs bit? Roundhouses more likely had but large slip-joint calipers, had to transfer off good steel rules. They may NOT have had large micrometers or even vernier calipers as could measure wheels of any size. Ours were small, as wheel lathes go, BTW. Mine-haul RR didn't use "tall" drivers as fast passenger steam did.

G'Dad retired - 1950, IIRC - as a roundhouse foreman, 44 years, B&O, steam the whole way at his location (Weston, WV).

But.. I was but five years old at the time, hadn't understood very much of his proud-as-punch "guided tour" that same year to begin with!



We do have some legacy steam-age (and Diesel-Electric..) genuine rail experts here on PM as would know a great deal more though.

Brian Albin said:

With a broader point containing more metal for better heat sinking, I wonder if making a 75 or 80 degree point would be handy today when doing a point burning job such as turning a long narrow bar. But perhaps there is no place left for a Dead tail center, with ball bearing live centers being available.

Brian

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Brian,
Cylindrical grinding still uses dead centres (at both ends of the workpiece) because it gives better accuracy (not to mention their low profile).
Tungsten carbide-tipped dead centres are available to give better life.
I guess you won't find them in lathe tailstocks much now (occasionally for better access on small diameters) but they are still used in the lathe headstock spindle when turning between centres.

Brian Albin said:

.... a master machinist's forum.

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LOL