Tool Surface Plate Tool Contact

[PeterT]

I was thinking of making a sliding measurement device for use on granite plate using 3-point contact using bearing balls, as opposed to lapping / scraping / pocket relieving the plate bottom surface.
So, mill the ball cavities with ball EM, insert balls, secure with Loctite... something like that. Then I got thinking, balls are quite hard, it would be a tangent point contact, maybe that is not so great for the granite. Potential for scour lines as opposed to spreading out over larger area. Are bearing balls considered bad Ju-Ju for surface plates?

 

[thestelster]

I was thinking of making a sliding measurement device for use on granite plate using 3-point contact using bearing balls, as opposed to lapping / scraping / pocket relieving the plate bottom surface.
So, mill the ball cavities with ball EM, insert balls, secure with Loctite... something like that. Then I got thinking, balls are quite hard, it would be a tangent point contact, maybe that is not so great for the granite. Potential for scour lines as opposed to spreading out over larger area. Are bearing balls considered bad Ju-Ju for surface plates?

View attachment 36945

A lot of the guys on Youtube making squareness comparators will press fit the ball bearings, and use loc-tite, then flatten the balls with the surface grinder.

 

[Mcgyver]

I was thinking of making a sliding measurement device for use on granite plate using 3-point contact using bearing balls, as opposed to lapping / scraping / pocket relieving the plate bottom surface.
So, mill the ball cavities with ball EM, insert balls, secure with Loctite... something like that. Then I got thinking, balls are quite hard, it would be a tangent point contact, maybe that is not so great for the granite. Potential for scour lines as opposed to spreading out over larger area. Are bearing balls considered bad Ju-Ju for surface plates?

Can't say I've ever thought of that before, but now that you mention it, I do share our concern and wouldn't do it. My guess is granite would not be fantastic at point loads and I've not seen a device for use on a plate that wasn't flat bottomed.

 

[PeterT]

This must have been one of the video's in my cranial cobwebs, maybe a Stefan vid too. In any case they always grind a healthy flat before lapping in order to increase surface area. The carbides & similar would be harder than granite, but of no real consequence due to surface area. He did mention lopping the bearing balls down with a hand grinder but I suspect that was roughing work prior to surface grinding.

 

[Mcgyver]

The only you tube guy I've seen where I'd buy into just because he says so is Rob Renzetti.....others may be good or better than I but I take it with a grain a of salt unless it resonates with my own knowledge/experience. Its partialy because I don't watch youtub very much so don't really have a thorough sense of their credibility, I've seen enough crap that I'm a skeptic and from what little I've seen neither of those impresses me the way Robrenz does. iirc he's got a very build of a repeatometer ..... did you check it out? Can't remember how he did it.

A healthy sized flat then letting them find their position in the semi circular divot makes more sense. How Schuablin mounts their lathes - three point contact on the bottom of the bed with semi hemispherical washers in mating holes. The semi hemisphere is 1/4 of a sphere and flat is probable around 5/8" (going from memory)

Maybe try with some softer balls than ball bearings? brass or even mild steel? Minuscule wear over time wouldn't matter and better it wears than the plate.

Or maybe I'm fretting about angels dancing on pin heads lol

 

[RobinHood]

Here is Robin’s video of the repetometer where he uses carbide pins, surface ground & lapped, for feet to then use the instrument to assess surface plates.

I would agree with @Mcgyver to use mild steel (or harder) as feet. The problem with “soft feet” is that a harder chip (harder than the foot material) could embed into the foot and be very abrasive on the surface plate, potentially destroying it over time. (Brass /CI would be even worse).

With really hard materials, it is less likely that something will embed. If there is contamination on the plate surface, a hard foot with a sharp edge will act as a scraper and just push the debris out of the way thus saving the plate surface from damage.

If I were to use ball bearings, I would grind and lap a flat section on them.

 

[PeterT]

Thanks. I'll have to ponder this some more. It was supposed to be quick & easy, I should know better! LOL. The attraction to bearing balls was they are readily available & lapping is confined to relatively small area. I could probably rig up something to grind off a third or so, but I think it would best be done mounted in-situ. Or maybe bite the bullet & this becomes scraping project #1. Goal: make a flat surface. Sounds like bait to entering a deep rabbit hole haha.

 

[Susquatch]

The only you tube guy I've seen where I'd buy into just because he says so is Rob Renzetti.....others may be good or better than I but I take it with a grain a of salt unless it resonates with my own knowledge/experience.

@PeterT - I am on @Mcgyver's page on this - but even worse. I wouldn't fully trust Renzetti either. There is no such thing as absolute knowledge. Even what's right today is wrong tomorrow. One must always listen, assess, challenge, debate, digest, analyse, and then decide. Even then, a decision is what you have to make when there isn't enough information to make the answer obvious.

 

[Tom O]

The only problem I see is the fact that the 3 bearings will have a distinct footprint that may not work in a different setting.

 

[Rauce]

Maybe it wasn’t the best choice (In my defense I made it a while ago) but I relieved the bottom of my the squareness comparator I made to leave four pads about 3/4”-1” square. I then ground them on a SG and it works fine. With a tenths indicator at the maximum height (14”) up against something there is no detectable rocking.

 

[PeterT]

The only problem I see is the fact that the 3 bearings will have a distinct footprint that may not work in a different setting.

If I understand your comment - as a height comparison gage, it doesn't matter. The base or the column or anything else in between the pads & indicator ball can be whatever orientation. Visualize the exaggerated right sketch with un-equal balls under the base. When the DTI is registered to a datum & everything locked, it can move anywhere on the plate & read relative to that datum height, presuming the plate is flat & the feet contacts remain in identical contact. That's kind of the beauty of 3-tangent contact, 3 points fully defines a plane. Additional pads are fine as log as they are co-planar. But if one or more is off, the fixture will rock.

 

[Tom O]

I was thinking about the possibility of only having say 2 of the balls on a ledge ( one free) instead of a larger flat surface. Because it uses 3 areas of contact would it be possible to have optional drilled and tapped holes for a thread in optional feet ground at the same time?

 

[PeterT]

Not quite sure I follow you Tom. The typical way is the base plate is coincident to the surface plate. Could be minimum 3 pads, or more pads or a scraped entirely flay surface. Then the 'upstairs' things typically have some degree of adjustment & must allow positioning to any height within the range. There are lots of ways to adjust, simplest is only on the DTI head. But other devices have another deck over the base & a fine adjustment screw that tilts the whole assembly on a pivot or flexure. Tom Lipton did a series of videos, trouble is they are all spread out. I like the one he adapted a Noga stem arm, I have the same one. Not sure its the best but at this point I'm just thinking about the base.

 

[Dabbler]

Here is my take on the hardness/softness of the foot thing. Every higher end device, such as Brown and Sharpe surface gauges (not their cheaper variety) uses hardened feet. Renzetti uses carbide for a reason. (you could email him and prolly get a complete answer)

But here is my take on the matter. it is all about friction and how the friction wears the surface plate. The surface plate has a very soft surface compared to any steel or hard bronze, etc. The softer the material you drag on the surface the more friction you get, and that will result into wear. since the granite is softer, it will wear first in every event.

So you use the hardest material possible, ground to the smoothest surface, in order to get that 'silky smooth' action that results in low granite wear over time.

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An important thing about surface plates is that every use wears them a 'little bit' -- they are *supposed* to be accurate to variances measurable in small tens of millionths of an inch.... So any additional wear is to be avoided... But with every use they will still wear. No manufacturer uses Teflon feet for these reasons.

Personally I like the idea of hardened bearings ground to flat contact surfaces.

 

[Susquatch]

I don't know squat about this subject but I did do a bit of curiosity based research simply because it is my nature to challenge assumptions, but also because I have a granite surface plate that I really don't know much about.

According to what I found, granite is the hardest natural mineral (gems aside), and black and pink granite are the hardest of the granites coming in at 7+ on the mohs scale and 88 - 90 on the Rockwell scale. Pink is harder than black because it has higher quartz content. Just because something is hard though, doesn't mean it has high wear resistance. It's molecular bonding and fracture/chip resistance are also important factors. I couldn't find ratings for that and nothing was mentioned on the surface plate manufacturers websites.

For me, one of the other interesting aspects of this discussion was around its central theme - the flattened ball bearing idea. I think instinctively, we all know that point loads are harder on things than distributed loads. It's the very definition of psi after all. The formula for the force of friction is f=uN. In our case, u is the coefficient of Kinetic friction, and N is the objects Weight. The area doesn't factor in. On the other hand, Abrasive wear occurs as a result of the abrasion that happens when a hard and rough surface of one object slides across the surface of another object that is significantly softer. In this case, the higher psi associated with a smaller contact area for a given weight does matter. But nothing in my tribology sources and nothing I could find on the web provided any science or formulas for this wearing process. I was left to use only instinct and experience to guide me. In the end, my guts tell me that a big surface is better than a small one for a given material, and that for a given weight, a big softer steel surface will be better than a small harder steel surface when one is looking to protect THE GRANITE. This is somewhat contrary to what most others seem to think both here and elsewhere so I have to question my judgement. In general, I think we all instinctively like smaller harder wear surfaces like a flattened ball bearing. But I think that is because we are normally thinking about protecting the ball bearing surface from wear in order to achieve better measuring consistency, not protecting the surface it slides on.

I've read many times on here about the importance of cleaning a granite plate. This became abundently clear while researching granite plate wear. The suppliers, universally advocate covering the plate and keeping it clean. Air-borne abrasive dust was recognized everywhere I looked as the main culprit for wear on surface plates. Of course, I have a surface grinder now....... I do keep my plate covered with a sheet of hard plastic. But I have never cleaned it before or after use. So that habit will have to change effective immediately.

As far as minimizing surface plate wear goes, I think my preference is for a big flat surface (the bigger the better for a given weight) over the flattened Ball Bearings. But in terms of making a consistently reliable flat surface for the purpose of making consistent repeatable measurements, I think I'd prefer to go with 3 softer steel bars inserted like posts into the base and then ground flat to produce repeatable consistent measurements for a given measurement session.

A decision is what you have to make when you don't have enough information to make the answer obvious. If I were doing this, I'd shoot for a big flat surface first. If and only if I couldn't achieve that, I'd switch to inserted steel bars ground flat - not hard ball bearings ground flat.

Basically, for any given series of measurements on a given job, ALL of any wear no matter what approach is taken will be insignificant. But I'd like to maintain my surface plate as long as possible. I can always recondition my steel posts, but reconditioning my granite surface plate will probably involve replacing it, not resurfacing it.

I would change my mind in a heart beat in the presence of better tribology info.

 

[Mcgyver]

my guess is with a Starrett cyrstal pink plate and infrequent, sporadic careful home shop use it would be generations before wear was any an issue.

Still, it makes sense to use it with care, its a precision tool. A 3/4 piece of ply covers mine when its not being used; it makes for a little bit of extra bench space. Its not worked on, but a handy surface put stuff on. In use, mostly common sense. Keep everything clean and place items on it with care.

I've not used a cleaner but wipe it down with my hand, as well as what is going on it. I read somewhere how good the hand is for this and how sensitive it is at feeling any particles on the plate.

If I've got it all wrong, I'll let the great, great grand-kids bitch and moan about it

 

[Rauce]

I think the logical conclusion to this is just to build a surface gage with an air bearing!

A lot of the high end height masters and height gages have them.

 

[fixerup]

I think the logical conclusion to this is just to build a surface gage with an air bearing!

A lot of the high end height masters and height gages have them.

If I ever get the need to measure lots of stuff on my surface plate , i would be tempted to modify my spare 1-2-3 block. It would be easy to modify , just plug all the holes with some nylon stubs screws, maybe some sealant or epoxy to seal them. Drill a small orifice on all the bottom ones, one side port for the ait inlet and one top hole for my Noga arm.

 

[PeterT]

I considered that just as a quick & dirty solution. But the typical 123 block is swiss cheesed with holes in both dimensions of which maybe you want one remaining thread hole to mount an arm or post or whatever. So with all the work to plug, may as well just use the single hole block, they cost about the same & weigh more. But then maybe go with a solid block & just mount a mag base to it. Etc. Etc.

 

[Susquatch]

So with all the work to plug, may as well just use the single hole block, they cost about the same & weigh more. But then maybe go with a solid block & just mount a mag base to it. Etc. Etc.

I honestly don't know Peter. I think you might be better off with plugging all the holes that feeding just one hole. How do you prevent that block from lifting a cheek and farting out one side..... Course, I'm not sure how or why just one fart hole is any worse than 5 or 30.... What a fantastic experiment! You go first!