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.