Smallest contact patch on a granite table?

jccaclimber · Jan 8, 2022

I’m in the process of making a few gauges which need to sit on a surface plate. Similar examples would be an indicator stand or a height master. I want them to be on 3 corner feet rather than one large flat because everything I have on large flats tends to develop a micron or two of play when the corners are pressed down. I’m wondering what the smallest practical foot size is?

I think the upper bound is a 3/8” circle because that is the diameter of the contact patch on a repeat-o-meter. In practice, while I can see occasional movement in a 2 mm indicator ball from an occasional chip, I suspect the full 3/8” isn’t needed in almost all cases. Has anyone else given this any thought, or are there any relevant standards?

Milland · Jan 8, 2022

Smaller contact areas will wear faster, and may be more upset by any local damage to the plate (small chips and such).

From a practical standpoint, three 1/4" round carbide inserts (like https://www.mscdirect.com/product/details/08442626) that I'd lapped flat on a diamond film sheet after mounting would be my limits for any gauge that weighed less than a pound or two, for something heavier I'd go 3/8" or larger. A slight edge break on the insert to insure it can't dig in would be wise.

plastikdreams · Jan 8, 2022

You may want to look into tfg's. They are precision made carbide inserts that come in various shapes and sizes. We use them as points when holding castings in fixtures.

John Garner · Jan 8, 2022

I've used 1/2 inch diameter punch-outs of Turcite B low friction bearing material for similar purposes. Bonded the Turcite to 4 inch to foot-long lengths of hot rolled bar, and "sheet lapped" the bonded pads, on 320-grit wet-or-dry, to coplanarity.

Several indicator-holder and scale holder bases that were "improvisatjon-engineered" in this way, on a weekend night 25+ years ago, are still being used.

winger · Jan 8, 2022

I have a digital height gage that had a lot of surface contact and wanted to chatter when moving it. I super glued 3 flat round carbide inserts (.5 dia) to the bottom. I doubt if there is enough weight to matter with the use it gets in my home shop.
I put the glue on the inserts and then sat the gage on all three at once. I haven't seen any marks showing they are not flat to the plate.

The B&S height stand where I worked had 3 rings about an 1" dia with a 3/4" hole. So I would trust B&S size for a lot of usage.

Dave

CalG · Jan 8, 2022

ONCE

I made a kinematic mount for an optics bench using three ball bearings bonded to the underside of the stage.

This "tool" made it's way on to the granite surface stone "optical bench" without unfavorable consequences.

The stage was moved around with every use, but the load was light. and if more than a nudge was needed, it was customary to pick it off the surface and place it carefully in the desired location.

Wear on the stone was of greater concern than wear on the ball contacts.

Good part, the stage was readily returned to it's role in the kinematic mounting system without issues.

Mark Rand · Jan 8, 2022

jccaclimber said:
I think the upper bound is a 3/8” circle because that is the diameter of the contact patch on a repeat-o-meter. In practice, while I can see occasional movement in a 2 mm indicator ball from an occasional chip, I suspect the full 3/8” isn’t needed in almost all cases. Has anyone else given this any thought, or are there any relevant standards?

The British Standard for surface plates (BS817 2008) shows a 'repeat reading gauge" that is much simpler than the Rahn Repeatometer. In their drawing, it shows 20mm 3/4" feet. it's six of one and half a dozen of the other...

For the feet of the mirrors I'm using to lap my surface tables, I've used slices cut from the shanks of worn 3/8" and 16mm-5/8" carbide endmills with a diamond slitting wheel on the surface grinder. My first attempts were with brass feet, then CCMT cast iron cutting (flat) inserts. In all cases, they're stuck on with Loctite 603 or 648, then ground coplanar with a diamond wheel on the SG and/or lapped with diamond powder on a lapping plate. These are being slid around a lot on the surface tables and the brass feet picked up dirt and grit, even from a recently cleaned surface. The CCMT inserts were a failure because un-set Loctite oozed out of the centre holes even days later and would make the feet slightly sticky.

If you can lap or grind the feet after installation, then the size does not matter too much providing it's big enough to bridge local imperfections/chips/scrape marks in the surface plate. A sharp 90° corner on the bottom of the feet with no chamfer is best, because it won't ride up over any dust or contamination on the surface.

plastikdreams · Jan 8, 2022

I'm telling ya, tfg's are the way to go. They are precision made pieces not hacked up endmills or inserts...do it right not half-assed.

jccaclimber · Jan 8, 2022

TFG inserts are an interesting option, although I'd want to braze them on which would take a bit of practice. I don't think I've brazed anything with a torch in well over a decade, and not much experience then either. I was planning 1/2" ball bearings, and then grinding them after installation so that they have 3/8" circles, but that's certainly an option.

We have both diamond and conventional grinding wheels for the surface grinder, so making them co-planar isn't an issue. It might not hurt to lap them in a bit after grinding, but that's also doable. If I grind a 1/2" sphere until I have a 3/8" flat, I'll end up with a 49° angle from the table, which should be an acceptable chamfer. I was debating about putting on a very mild radius, but I agree that sharp edges, while requiring care, do have a nice way of clearing grit. Despite cleaning my table before use and keeping it covered, the large height stand (Mahr 817 CLM) always has some dust and such built up on the edges of the pads when I (rarely) lift it off the table, but it never seems to have issues getting it under the pads. It does have air lift, but I don't always use that feature.

I was able to find the UK standard referenced above, and it specifies a 20 +0/-1 mm diameter pad, so in that case a 19 mm pad. A bit larger than 3/8". In the case of a granite table with no chips I suspect it wouldn't matter. I've never spent enough time around a scraped cast iron table to know what a common distance between points is, but I'm not likely to in the near future either.

The kinematic mount point is an interesting one. I have a few of those laying around, so I should spend some time moving one around on my plate and seeing if I can make it dip into local imperfections. My suspicion is that somewhere on the table is a dip that will move a 50 millionths indicator, but I haven't proven that. I would expect the shallow angle to make the kinetic mount ball ride up on any contaminants rather than clear them, although they might fall off just as fast. A convenient thing about kinetic mounts is that the part of the ball that touches a surface plate is not the same portion that interfaces to the mating side of the kinetic mount, so even if one did cause wear it wouldn't matter for the other application.

plastikdreams · Jan 8, 2022

jccaclimber said:
TFG inserts are an interesting option, although I'd want to braze them on which would take a bit of practice. I don't think I've brazed anything with a torch in well over a decade, and not much experience then either. I was planning 1/2" ball bearings, and then grinding them after installation so that they have 3/8" circles, but that's certainly an option.

We have both diamond and conventional grinding wheels for the surface grinder, so making them co-planar isn't an issue. It might not hurt to lap them in a bit after grinding, but that's also doable. If I grind a 1/2" sphere until I have a 3/8" flat, I'll end up with a 49° angle from the table, which should be an acceptable chamfer. I was debating about putting on a very mild radius, but I agree that sharp edges, while requiring care, do have a nice way of clearing grit. Despite cleaning my table before use and keeping it covered, the large height stand (Mahr 817 CLM) always has some dust and such built up on the edges of the pads when I (rarely) lift it off the table, but it never seems to have issues getting it under the pads. It does have air lift, but I don't always use that feature.

I was able to find the UK standard referenced above, and it specifies a 20 +0/-1 mm diameter pad, so in that case a 19 mm pad. A bit larger than 3/8". In the case of a granite table with no chips I suspect it wouldn't matter. I've never spent enough time around a scraped cast iron table to know what a common distance between points is, but I'm not likely to in the near future either.

The kinematic mount point is an interesting one. I have a few of those laying around, so I should spend some time moving one around on my plate and seeing if I can make it dip into local imperfections. My suspicion is that somewhere on the table is a dip that will move a 50 millionths indicator, but I haven't proven that. I would expect the shallow angle to make the kinetic mount ball ride up on any contaminants rather than clear them, although they might fall off just as fast. A convenient thing about kinetic mounts is that the part of the ball that touches a surface plate is not the same portion that interfaces to the mating side of the kinetic mount, so even if one did cause wear it wouldn't matter for the other application.

We make them press fit, they stay put well. We mostly use the pieces with .15625 shafts, we use a .1555 carbide reamer. They aren't cheap, like 25 bucks a piece (we get certain types in bags of 50), but they are quality, they can take a hit and not break or chip. I usually drive them in with a bronze punch and a hammer.

If you are worried about them coming out, loctite 680 retaining compound will hold them solid in a loose hole.

If you want more info I'll see what I can dig up tomorrow. I'll take some pics of the different styles I keep in my box...locked lol

jccaclimber · Jan 8, 2022

A related question, what is it that makes some things glide smoothly across a granite table and others not glide so nicely? Is it perfect planarity, a material property, or something else? I've always wondered if the excess carbon in say cast iron acts as a slight lubricant. I know there's enough to make a piece of paper dirty when rubbed, but haven't thought much past that. I'm used to seeing most surface plate tools made of a hardened steel.

On the argument for surface area, the above mentioned Mahr height stand doesn't glide effortlessly (when the air is off), but it does slide very well considering its 66 pound weight. Its 3 feet are just shy of 62 mm in diameter with an extremely fine edge break. They have a 16.4 mm relief in the center for fasteners, and a small relieved area for the air ports.
Contrast this with a Mitutoyo height master which is a PITA to slide on the table. It tips the scales at 49 pounds, but has 3 feet with 19 mm ODs and 12 mm IDs. The other interesting thing about the Mitutoyo feet is that they have a chamfer on the OD, but not on the ID. Surface area shouldn't impact friction, but something is at play. I've found the height master to also be much more sensitive to not wanting to slide if there's the slightest piece of dust under one of the feet. It's possible though that I notice this more because the height stand lives on the plate and thus doesn't have much opportunity to be placed on a piece of dust whereas the height master lives in its box and is placed on the table any time it is needed.

Milland · Jan 8, 2022

Center of gravity plays a role too - the Mits has a lot of weight up high (I have one too), that can make it want to rock more as it's moved around.

plastikdreams · Jan 8, 2022

I think it has to do with the cleanliness of the stone and the instrument surface. I clean my stone with acetone regularly and every few months I hit it with Starrett stone cleaner. I clean my height gage with acetone after stoning it with a fine stone. Stuff slides like it's on oil.

We get them calibrated yearly and they do some MOJO magic with these iron plates and abrasive.

MCritchley · Jan 8, 2022

I had a similar dilemma when building a surface gauge. I milled out a g-2 dura bar base with three 7/8” dia pads that I then scraped in to 40 points per inch. Slides great and it perfectly repeatable. The pads are laid out similar to a surface plates. I use mostly non hardened surface gauges that I scrape in to eliminate rock.
I would highly endorse cast iron. Don’t worry if you don’t scrape the three pads, it will wear in by the time you retire.

plastikdreams · Jan 9, 2022

Just to add, if it's really dirty I'll hit it with some green scotchbrite and a degreaser...then acetone...then Starrett plate cleaner.

jccaclimber · Jan 9, 2022

barbter said:
Not fully understanding here....
3 corner feet....but with 3, you'd have 2x in opposite corners at one end, and one middleish at the other end?
Otherwise you'd have 4x corner feet (and 1x in each corner....?)
So with 3x feet, the 1x foot end, you won't be able to press the corners down (the 1 or 2 micron flex) as the whole shebang will tilt as it's now unsupported?

Ref the granite - it's softish so I'd be worried about too small of foot area - I'd be concerned about the contact area ratio against the combined mass of fixture and part. And regarding wear, Milland is correct ref CofG as top heavy (height gauges) love to want to "dig in" (where air fixes that issue).

Ref brazing - if you want to go this way, I'd go silver solder or normal solder. Lot's less heat = easier to control and lots less distortion etc.

Glide factor as Plastik says, the answer is cleanliness.

I wasn't fully clear with the 3 vs. 4 foot explanation. In my case what I've been calling the 4 foot option is closer to 1 or 2 large flats like a Starrett 257. It seems like there's always a slight bulge or perhaps piece of dust somewhere in the middle. As a result a light pressure on one of the corners results in a shift to a new stable position that's different than before, and it isn't always obvious that it has happened. A slight shift in the another direction.
With 3 feet, so long as you don't press hard enough to topple the device there is only one stable position. I routinely run a precision ground stone over the bottom, so I'm confident I don't have any burrs or dings.

I don't have an optical flat, but digging in is an interesting point. It's possible that the feet on the height master aren't perfectly coplanar with each other. They are flat and I didn't get any shiny corners running a stone over the whole set, but it wouldn't take much of an error for it to feel like one is dragging.

Milland · Jan 9, 2022

jccaclimber said:
I don't have an optical flat, but digging in is an interesting point. It's possible that the feet on the height master aren't perfectly coplanar with each other. They are flat and I didn't get any shiny corners running a stone over the whole set, but it wouldn't take much of an error for it to feel like one is dragging.

Next time you move the HM, hold it at the base and near the middle (so, both hands) and try to move with equal force from both arms/hands. That should dampen the tendency to rocking. Try pulling as well as pushing to see if that helps.

jccaclimber · Jan 9, 2022

plastikdreams said:
Just to add, if it's really dirty I'll hit it with some green scotchbrite and a degreaser...then acetone...then Starrett plate cleaner.

I've been happy with the McMaster Carr cleaner (some sort of thin spray) as well as the Starrett cleaner and IPA, depending on what is closest. The main concern with the McMaster stuff is that while it seems great at lifting dirt, it's also water based. I'm close to the ocean and I have a couple gauges and fixtures that often live on the plate for several days at a time, right after I clean it. I'm only a few miles from the ocean, so it's hard enough to keep things from rusting without soaking water into the plate.
While cleaning solutions are abundant, I've had a harder time with locating a lint free rag as things like Kimwipes pill up and leave their own mess behind. I've been cleaning with them, then wiping the last dust off by hand.
Several jobs ago we had a Do-All surface plate cleaner that was a foaming product in an aerosol can. Honestly it looked a lot like scrubbing bubbles foaming cleaner my parents loved so much, but it smelled a bit different. Unfortunately I haven't been able to find any in nearly a decade.

jccaclimber · Jan 9, 2022

Milland said:
Next time you move the HM, hold it at the base and near the middle (so, both hands) and try to move with equal force from both arms/hands. That should dampen the tendency to rocking. Try pulling as well as pushing to see if that helps.

I've been wrapping my hands around it a couple inches up from the base and sliding. Now that you mention it, just slightly below the CG is probably best, thank you. I've should try pushing on the height stand higher up as well, just to see how it gets worse as I go above the CG.

plastikdreams · Jan 9, 2022

jccaclimber said:
I've been happy with the McMaster Carr cleaner (some sort of thin spray) as well as the Starrett cleaner and IPA, depending on what is closest. The main concern with the McMaster stuff is that while it seems great at lifting dirt, it's also water based. I'm close to the ocean and I have a couple gauges and fixtures that often live on the plate for several days at a time, right after I clean it. I'm only a few miles from the ocean, so it's hard enough to keep things from rusting without soaking water into the plate.
While cleaning solutions are abundant, I've had a harder time with locating a lint free rag as things like Kimwipes pill up and leave their own mess behind. I've been cleaning with them, then wiping the last dust off by hand.
Several jobs ago we had a Do-All surface plate cleaner that was a foaming product in an aerosol can. Honestly it looked a lot like scrubbing bubbles foaming cleaner my parents loved so much, but it smelled a bit different. Unfortunately I haven't been able to find any in nearly a decade.

Just like a magnetic vise the hand can be a very good instrument for cleaning.

Smallest contact patch on a granite table?

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