Subject: Can something be TOO flat ?
From: Alan Rothenbush
Date: 14/12/03 19:40 GMT Standard Time
Message-id:


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 ?

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 (www.pumaracing.co.uk)
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.