# Can something be TOO flat ?

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Can two sliding surfaces be too flat to slide nicely ?

Here's the story. I bought a little ( 4x7 ) Sanford Surface Grinder off eBay. Got it home, did some checking and found the ways pretty worn in the middle of the travel. ( The ways are a V way and flat way )

So I spent all of yesterday with my good friend Marcus, a tool and die maker of what is, to me, extraordinary skill and knowledge.

I learned a WHOLE bunch about surface grinding and even more about the use of prussian blue.

After form grinding the V way and flat of the saddle, we mounted the little Sanford table on the great big table of Marcus' grinder. ( His mag chuck was almost the size of the Sanford's table ! )

The saddle was ground first, as it had some ground surfaces obviously used as refereneces in the initial construction.

We form ground the V of the table and then proceeded to grind the last flat.

The problem here is that the relative heights of the V and the flat must absolutely correct or the flats will not sit parallel to each other, but will instead sit an at angle. The contact will then be along two lines, as opposed to three planes ( one side of the V, the other side of the V and the flat ).

The first cleanup pass got us, by measurement and calculation, about .002" high.

We spent the next 2 hours getting rid of that two thou, about .000,2" at a time. That is, remove a tenth or so, blue, look, measure, grind another tenth and a bit, blue look measure, and so on. We finally got to a point where the blueing matched the measurements matched the initial calculations.

At this point we removed the table, mounted it on the saddle and gave it a slide. BEAUTIFUL. Absolutely fabulous.

I'd spent a week wondering if I had bought two hundred pounds of scrap cast iron and left wondering how I could be so lucky.

Then I got home.

First thing I did was to oil the freshly ground surfaces, something we hadn't done. I then slid the table along, expecting to almost glide off the end, only to find it .. sticking .. kind of a hydraulic sort of stick. I kept sliding it back and forth, getting stickier and stickier until it stuck solidly.

It took a LOT of force to break it free .. in fact, they were stuck so firmly together that that lifting the table also lifted the saddle.

Once apart, I examined things and found nothing but clean, nicely oiled surfaces. The oil I used was nearly clear and it was still nearly clear, allowing a good look at things. Nothing. Finger test showed .. nothing.

Tried again and found the same thing. Cleaned all the (light) oil off and tried some heavier oil. Same thing. Cleaned the heavier oil off and tried the lightest oil I could find. Same effect.

It's like the back and forth slide acts like a pump, and a hydraulic "lock" is generated.

All of this is absolutely foreign to me. Two flat things with oil between them has always slid smoothly, but I've never had any two things THIS flat.

Can something be TOO flat to slide properly ?

Thanks for any thoughts.

Alan

• posted

Yes

• posted

Yes, although it's pretty rare. Ground surfaces on machine tool ways are best used with pressure oil systems. When you don't have oil pressure, it is possible, with an exceptional grinding job, to get high friction.

The cure used in most machine tools is to "frost" the way surfaces, which is a sort of after-the fact scraping job, usually done with a power scraper to produce a decorative surface effect. You want it to be very shallow and you aren't scraping to a standard here. You're just trying to reduce the bearing area and leave some low areas (by millionths, not by tenths) to hold oil and to reduce the contact area.

A good scraped surface has something like 60% bearing. Some machines, like older Moore jig borers and jig grinders, allegedly ran around 80% bearing. Anything more could be trouble.

Now you need a hands-on expert to explain what to do. All I'm doing is passing on what experts have told me over 30 years of asking and interviewing them -- including people at Moore.

Ed Huntress

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The phenomenon you describe is well known to amatuer telescope makers who discover a marked increase in difficulty when glass surfaces being ground get close to actual optical levels of flatness. Mirrors have been destroyed by the measures taken to separate them from grinding tools, and lapping tools are normally provided with grooves to prevent binding of this sort.

Suggestion: use a lower viscosity oil. It will flow more freely into the space between the facing surfaces. If things are really stuck as they are, use a little heat as well as adding lighter oil...

Al Moore

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Yes

See

Jim

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Yes. You've generated a huge contact area and the surfaces are now binding to each other with the stiction of the oil which is sufficient to make the contact airtight. You are generating something similar to what happens when you wring two slip guages together. You need some surface irregularities to reduce the contact area and let air get into the gaps. I think if you try the ways bone dry they will perform much better. You might also try paraffin (kerosene) instead of oil but I suspect any liquid will now make them stick until you rough the surfaces up a bit.

Dave Baker - Puma Race Engines

I'm not at all sure why women like men. We're argumentative, childish, unsociable and extremely unappealing naked. I'm quite grateful they do though.

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"Ed Huntress" snipped-for-privacy@optonline.net

Final scraping called "frosting," or "flaking" is used for hand scraping of ways for decoration (allegedly to hold oil), but would you want to use it after final grinding (other than for decoration)?

If the surfaces here related "stuck" together dry (as Jo blocks) on wringing, that would be one thing, but that's not the case so far related. FM

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Yeah, that's how it was used a half-century ago. Real top-quality hand-scraping jobs, like on Moores, weren't finished with frosting. BTW, "flaking" was a term usually applied to hand-scraping of little points; "frosting" usually was reserved for the more decorate, half-moon or other patterns.

Even back in the '40s, it usually was done by power after grinding, on mass-produced lathes and mills.

Whether it was purely decorative or not depended on the manufacturer and how they did it.

I won't try to analyze it, but my understanding is that it's simple adhesion from too much area with too-thin a layer of oil between the surfaces. The tribologists here can argue that one out.

Ed Huntress

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The jig borers had hardened steel prismatic ways, ground to size. They were seated in hand scraped female v-ways, scraped for alignment. The ground surfaces were then lapped for alignment. Because they were hard steel, they could not be scraped or frosted.

The female v-ways that rode on the hardened prismatic ways were hand scraped though. So even in this case the degree of bearing was controlled, to be below a certain percentage.

I'm not sure if moore's book mentiones what that number was that they considered optimal.

As a side note, a lot of the fancy photos of old machine tools, which were covered with decorative engine turning or frosting on the non-function surfaces were done as one-offs for the catalog photos, I bet. Or, maybe there was a bit of old-timey photoshopping going on! The cataract lathe I have came with a nearly unused cross slide:

Even though the ways and lead screws were pristine, and showed zero wear, it still did not look much like what hardinge said it should:

A bit of a difference, eh?

Jim

Jim

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• posted

snipped-for-privacy@aol.com (Fdmorrison) wrote in news: snipped-for-privacy@mb-m11.aol.com:

too flat of a surface can create a hydraulic lock between parts. The ways need to be scraped. The oil gets squeezed out of two really flat surfaces and the parts adhere just like wringing jo blocks. Which is why it occured after moving the slide back and forth a few times. Scraping keeps pockets of oil between the metal surfaces.

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Huntress

Oh, right, I was thinking of their gages and so on. They were pointed to a high bearing percentage -- 'looked like speckling on a trout.

Ed Huntress

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Yes, super flat surfaces don't have any "pockets" to retain oil between them. In the same way as you demonstrated, good gauge blocks stick together when you slide them on to each other. Scraping to leave that "frosting" on flat bearing surfaces is more than decorative. It provides pockets for oil retention and therefore low friction sliding.

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Yes, they slide quite smoothly when perfectly dry.

Alan

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One of my most fond rememberances at work will be a swiss EDM operator (since retired) who also was in charge of a moore jig borer.

One morning he arrived to find that the painters had been in over the weekend and had painted the jig borer. Including the ways!! You should have heard the cursing and hollering.

He had actually met Moore at one point in CT.

Jim

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"Ed Huntress" snipped-for-privacy@optonline.net

wrt Moore jig borer scraping

were pointed to a

My own "final" scraping looks more like spackling on a trough. FM "[S]ome machine builders still cling to the practice of handscraping the ways--an admission that the machine work is not quite so accurate as it should be. In these days of precision machine tools, it is indeed surprising to find tool engineers who believe that a man with a scraper can produce a surface more nearly flat than can be planed with a single-point tool on a modern planer [let alone, grinding machine]." Fred Colvin, 1947

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And here Moore's smarts really show - his idea is not to create a bearing surface with the scraper, but rather to prepare and align a seat for the real bearing (the hard steel prism, ground for dimension and surface finish) and then bolted into the way whos (whoms? ) alignment has been prepared by hand scraping. Sort of the best of both.

In any event, moore felt that machine tool accuracy, and that includes the planer mentioned above, depended ultimately on hand scraped gages that were developed fundamentally from master reference flats - hand scraped master reference flats.

This was not really a new concept, moore simply carried it to extremes. But even in the mid 40s I suspect that it was cheaper to hire an experience scraper mechanic to finsh, say, lathe beds. Because the large machines to do them cost a lot, and the men could be paid a fairly low wage in spite of their skill.

Jim

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Old Fred (my former boss's boss's predecessor) didn't always get it right.

Ed Huntress

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Huntress

I assume you mean the original, Richard (Dick). I used to have lunch with him once a year. It was something I really looked forward to.

That was at the American Machinist Annual Award Winner luncheon. I actually got to sit next to Dick Moore and two places from Isaac Asimov at one of those. I tried to remember every word they said. Maybe I did...I don't remember.

Ed Huntress

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Lots of affirmative comments, but not a lot of help on what to do. Are you using way oil? If not, get some. It is designed to not squish out of large, low PSI slideways. If this doesn't help, they you are most likely going to have to frost it a bit to reduce the gage-block effect. Having difficulty separating the surfaces is not a problem, you don't WANT the surfaces to separate. But, you do want to retain a thin oil film on the ways. If it has an oil feed system, get that working so it applies a slow feed of oil to the ways. That will prevent the film thinning down to nearly zero.

Marcus must have a REALLY fine surface grinder to make surfaces this true!

Jon

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some machine ways have a "S" shaped groove in them to help retain the oil. I don't know if you could create a groove simply enough. the groove seems to help the way float on each other like a hydrodynamic spindle (I think that the term). But some how some small groove parallel to the ways would work, I would think if you could scrape them yourself but maybe there's a retired person in your area who could do this for some beer and time to chat about the old days.

calculations.

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