Ruling engines and lapping the ultimate screw

The story of the ruling engine is one of those bizarro incredibly important things that has slipped into obscurity, only really known by people still directly involved in this sort of thing. I was briefly involved in this area working at LBNL’s Advanced Light Source, measuring diffraction gratings, their efficiencies, and attempting to estimate how well they’d work in presence of error. I promptly forgot almost all of it in favor of learning how to pants goth girls or whatever I repurposed that set of brain cells for, but it’s still in there rattling around somewhere.

Diffraction gratings are those little rainbow thingees on your credit card. Or if you’re old, you remember the rainbow patterns on CDs, those were sort of ad-hoc diffraction gratings. Ultimately it is a set of very precise lines across a mirror substrate. There are all kinds of profiles and shapes of diffraction gratings for different purposes, but they all work roughly the same way. Different wavelengths of light are reflected into different angles via constructive interference. The simple grating equation is where is the diffracted order, is the angle of the diffracted order, is the periodicity of the grating, and is the wavelength of the light diffracted.

 

This is a long winded way of saying if you reflect light on a grating, it will make a nice rainbow pattern. If you make a slit out of razorblades (this is basically what people use) perpendicular to the first order diffraction angle, you get a monochromator or spectrograph, depending on how you use it. This means you can resolve narrow lines in the spectra of whatever it is you’re looking at. Of course, nothing is perfect, least of all diffraction gratings. There’s a figure of merit in spectroscopy called resolving power; where is the approximate wavelength of interest and is the narrowness of line you want to resolve. It’s easy to show that R is proportional to the number of coherently illuminated perfect grating lines, and that any error in grating line shape or tracking will cause R to be smaller. So  if you want to discover quantum mechanics, you need to make some nice lines otherwise you’re wasting your time. Oh yeah, and obviously if you want to resolve smaller wavelengths of light, say, in the UV, you need to rule your gratings with smaller lines.

Over complex representation of a monochromator or spectrograph

Now a days we have a number of ways of making gratings, but the first way (still important and used) is using a ruling engine, which is a very fine machine tool which mechanically draws lines on a substrate using a diamond anvil. The first important such tool was Rowland’s mentioned several times now; literally the machine that launched American physics and made quantum mechanics possible. There were gratings made before, but Rowland’s was the first to make useful gratings repeatedly. For decades it was the only one capable of making decent gratings; like a machine made by super intelligent alien beings that nobody else can figure out. For decades after this, all the subsequent ruling engines that worked were Rowland designs. The first successful ruling engine which wasn’t a Rowland design is the topic of the rest of this blog; that invented by the underappreciated experimental physicist John Donovan Strong (I’ve definitely been in the same room as him early in my career, but I can’t say I remember anything about him –his book is amazing BTW). This is the type of ruling engine still used today, more or less, with some additional complications of using feedback mechanisms made possible by electronics over the years. I’m following Strong’s article from 1951 as well as a couple of  Scientific American articles.

The original Rowland machine was a sort of overgrown and ultra precise metal shaper (or for a more familiar example; a grocery store meat slicer). Strong took his design cues from the much more uncommon metal planer. The difference, Rowland’s machine advanced the relatively heavy grating blank using the precision screw, making the screw subject to mechanical deformation and stick slip, while moving the diamond using ways that could wear out.  Remember, this thing is making long, straight lines, very precisely on the order of 1000/2000 lines per millimeter; a perfect line every 500-1000 nanometers. Real nanotechnology; not the imaginary kind done with Schroedinger’s equation and pixie dust. For contrast, an atom is around a tenth of a nanometer. While they call the latest semiconductor technology 14nm, it’s really more like 100nm, and diffraction gratings built with screws were doing that, over much larger areas than a defect free wafer more than 140 years ago using doodads such as these very precise screws. There were seven major sources of error with this design in absence of mechanical or manufacturing defects, to give an idea of the type of thing involved here; they were referred to as the “seven demons.”

1. Stick slip/lubrication forces of the various moving parts caused large irregularities.
2. Wear in the various parts of the engine were also hugely important; the carriage might travel miles in ruling a grating and the Rowland carriage was a big beefy object.
3. The metal parts also contain locked-up stresses from creation from raw ore to machining; as the machine ages, the stresses relieve and the perfect surfaces deform.
4. Creep also takes place from external forces; sag, motion, weight support.
5. Any vibration may cause bad gratings to be made; one worker correlated his grating defects to the swaying of trees outside the building (this is huge with optics in general, especially in current year with all kinds of machinery around and driving by).
6. Dust of course is a big problem; get dust under the diamond cutter or in the screw/nut interface and you’re, well, screwed.
7. Finally, the heat radiated by a human body can cause sufficient creep in the engine to ruin a grating.

Strong’s gizmo obviated the stick slip problem by moving the diamond rather than the grating blank, removing the ways for moving the diamond, and improving both the lubrication of the screwing mechanisms, and the alignment techniques.  His thing used two precision screws to advance the diamond, and as they’re pointing in opposite directions, they can cancel out pressure and sag errors as well as angular “fanning” errors in the grating ruling (Rowland’s machine had microradian misalignments that borked the resolving power via this fanning effect; a microradian across a few inches is easily a wavelength of green light). Downside; you need two nice screws instead of just one.

Strong’s exposition was fascinating. He points out that precision in his day was entirely “primitive methods.” Aka geometry, averaging and lapping compounds. The dividing heads on the screws for making microscopic motions were self lapped in place on an oil bath. Instead of a kinematic mounting system for moving the grating, he overconstrained it with multiple ways which averaged out to a nice straight line.

Strong was a great scientist who understood machinery and tooling in great detail. He also had a couple of helpers he credited with his success. One of them was Wilbur Perry, an engineer trained at WPI. Before he went to school he made a bunch of telescopes, and was a proud member of the Springfield Vermont telescope makers society, which still maintains a clubhouse. Let me emphasize the implications of this: a tiny town of a few thousand people had a telescope makers society at the turn of the century, when telescopes were still high technology, and they endowed it well enough it is still physically there. That’s sort of like a small town of a few thousand people having its own privately owned MEMS fab in the 1990s when this became a more common technology. Social capital is highly underappreciated and they had lots of it in those days. Strong himself got many of his ideas for the ruling engine from hanging out in a club he founded with John Anderson (the previous John Hopkins Rowland engine driver); the “100-to-1 shot club.” Some nice oral history before it fades away: an interview with Henry Victor Neher:

NEHER: This was a small group that was formed at Caltech in about 1934 or ’35. The
way it originated was this. John Anderson, who was at the Mount Wilson Observatory, had an office at Caltech when he was working on the 200-inch telescope, back in the thirties. One of the members of the staff was a young fellow by the name of John Strong [professor of physics and astrophysics, 1937-1942], who had his experimental equipment in the same room in Bridge as I did. John Strong was over talking to John Anderson one day. John Strong was always interested in ideas of one sort or another. He was an inventor if there ever was one. John thought that there ought to be a group that considered far-out ideas of one sort or another.

INTERVIEWER: For example.

NEHER: Primarily ideas connected with something scientific or mechanical, or something of that sort. And John Anderson said, “Well, what you are suggesting is to discuss things that have one chance in a hundred of working.” And so, this is the way the 100-to-1 Shot Club was formed. They got a group together which consisted of John Strong, John Anderson, Russell Porter, Roger Hayward—who did that picture up there above the fireplace—and then some others not connected with the Institute, like Byron Graves. And there were a couple of patent attorneys in the group.
Well, I didn’t get into it right away. I guess it was about 1936 or ’37 before I became associated with it. We met once a month at various members’ homes. It was mostly discussions of ideas in connection with astronomy or with physics. There may have been some mechanical things. One of the members was George Mitchell, who designed and made the Mitchell camera that was used in Hollywood for years. Another was George Beadle [professor of biology 1946-1961], who joined after World War II.

INTERVIEWER: Did anything ever come out of it?

NEHER: No. It wasn’t meant to be that. It was just a place where you could just discuss anything you wanted.

 

Or as Strong himself put it:

 

We called it the “100 to 1 Shot Club.” We met at various member’s houses at Palomar; in the Mohave Desert; etc. — about 6 or 7 times a year. It was called by the name mentioned to indicate that our considerations (like: Does the water spin in a contrary way in the Southern hemisphere when it runs out of the bath tub? — etc.) were restricted to topics that were fantastic by a factor of 100:1 over scientific. The dozen members included: Trim Barkelov — patent council for Paramount Pictures Roger Hayward — artist and architect Victor Neher laboratory roommate George Mitchell — millionaire manufacturer of the Mitchell camera; a former Hollywood camera man Byron Graves — an amateur astronomer and retired executive from Ford Co. in Detroit John Anderson — my boss Jack McMorris — a chemist (and disappointed concert pianist) George Worrell — successor to Mitchell at the Camera plant Milton Humason — astronomer I mention this because it was a group worthy to go down in history.

 

The importance of such clubs can’t be overestimated. They’re everywhere in the annals of technological history; from Wernher von Braun and company’s rocket club, to the famous Lunar society, to the X club even the Bohemian Club was responsible for the US nuclear weapons program. Most great human ventures have started in some sort of men’s club. And yes, they were/are men’s clubs, u mad? As my pal BAP put it, only the most depraved ancient Greek tyrants would ban men’s associations:

A brotherhood of men in this form is the foundation of all higher life in general: there is a certain madness, an enthusiasm that exists also in a community of true scientists or artists…. it is totally forbidden in our time…. the dedication, severity, focus and enthusiasm necessary to sustain true scientific enterprise are forbidden because they make women and weaklings uncomfortable.

Back to badass screws, Wilbur Perry of the Springfield Telescope Club eventually got a job running the Rowland engines at John Hopkins and was widely recognized as a genius and meticulous engineer with perfect hands. Strong hired him for this expertise. His fellow technical  coworker was Dave Broadhead, another optics hacker who made complicated telescopes in his spare time, and at one point made a living crafting roof prisms for the war effort, something he picked up in his spare time from reading magazines. He literally made them in his basement. His education, as far as I am able to determine, was reading popular science and popular mechanics magazines, going to the library and fiddling with things. Broadhead is the kind of guy I keep harping about; the careful working class machinist craftsman who basically no longer exists in American society. . Strong at one point asked him for a pair of 36″ parabolic mirrors, which he literally made in his basement 30 days ahead of a 90 day schedule. So, for the ruling engine project, he was a shoe in. He was working class to the bone; treating his employers to venison dinners from deer he shot himself when they’d come visit his basement workshop in upstate New York. I have to wonder what his descendants are up to these days. Hopefully not shooting heroin which seems to be the primary avocation in that part of the world.

Broadhead was a wonder, like many of this class of instrument builder machinist. More importantly though, we have a fairly good first hand view into how he did it; all the steps. Nobody really documented how Rowland and his guys built his doodads. He wrote some post-facto notes down, but nothing in detail. Broadhead’s adventures in fine screw craftsmanship was much better documented. Broadhead’s first step in building the thing was rebuilding his basement South Bend lathe. Scraping the ways and refitting all the parts until it could hold a 1 micron cut. That’s 1/1000 of a mm. As Broadhead put it. “It’s an old lathe, but instrument makers use such lathes for centuries, just scraping ’em over -which they’d have to do even with new ones, for this work.” Scraping of course was the manual technique used to make a flat surface back in 1800 when Maudslay invented the screw cutting lathe. Mind you a South Bend lathe is not considered a toolroom lathe; it was mostly used for light work and was popular with hobbyists for its relatively low cost. According to one account “I journeyed to Wellsville and found Broadhead peering downward through a 50-power toolmaker’s microscope attached to the lathe. The tool was smoothly peeling off a shaving only one micron thick. Without the microscope it seemed to be cutting nothing”

 

To remove stress in the screw blanks, he had two garbage cans with inner cells, one for heat the other for dry ice,  so he could stress relieve the screws before the finish cuts. He dipped them in what he called “tincture of skunk cabbage” (overheated Mazola corn oil at 400F, 100F) and “hobo cocktails” (dry ice and alcohol at 10, -60 and -100F). He did this stress relief cycle 50 times per screw.

When he moved on to lapping, he rigged up a tape recorder which kept a record of the torque  of the screw being lapped in its giant split nut. This way he could keep track of progress on the lapping process and the recorder would tell him when there was a burr or requirement for more lapping compound. Mind you this is a 1940s era tape recorder, so in addition to being a great machinist, he must have known a thing or two about electronics back in the vacuum tube era. He also rigged up a motor mechanism and ran the thing on his wall in his basement.

Apparently the whole new ruling engine worked the first time, which is a minor miracle. A hugely successful scientific breakthrough done with a sort of miniature Klein type-1 organization. More of a Klein type-1 A-team;  a common type of group for successful experimental physics ventures. The origins in a couple of men’s clubs and a couple of obscure working class geniuses makes it all the more sweet.

It’s also an object lesson in why current year can’t have nice things. No men’s clubs thanks to various vile and pathetic tyrannies. No working class craftsmen making things in matter. No physicists who understand how a fucking screw works (who worked on the Kansas wheat harvests).  And tens of thousands of nincompoops fiddling around on a computer instead of learning how matter works with the eyes and fingers. The very idea of using such mechanical creativity by talking to other artificers and hammer and tongs precision work is anathema to current year bugmen. I’m pretty sure they’d find a way to call the whole project sexist and racist because they don’t understand how a fucking screw lap works either.

That is the biggest advance in the grating art that I am responsible for. I also made an advance in the lapping of lead screws that is recognized in industry. I developed several techniques which are useful in precision machine tool practice. And that was a consequence of the work on ruling engines. But my work on ruling engines, in a sense, was supernumerary, because now the control of the relative position of the ruling engines components is accomplished by interferometry. Here Harrison was the pioneer.