Thread: Testing a second hand surface plate

Here are a couple of drafts, still working through the rest.

-Josh

Edit Added some more

Originally Posted by Brobdingnagian

Ok, more simplicity or more protection?

And do you think we need a fine adjustment mechanism?

I think they are just left on for ease of use (why take them off if you don't need to)

Given that the supramess range is not much at all, I'd say that fine adjustment is moved from the "nice to have" column to the "almost essential" column.

Michael

A thought - as repeatometer is probably some one's trademark and doesn't really describe the thing well at all, what are these going to be called? B&H indicator (bump and hollow)? Josh-o-meter? Oh bugger meter? (as that's what you say when you see the results)

Originally Posted by Michael G

I think they are just left on for ease of use (why take them off if you don't need to)

Given that the supramess range is not much at all, I'd say that fine adjustment is moved from the "nice to have" column to the "almost essential" column.

Michael

Ok Will do, It will just mean an extra layer, a measurement pad to be mounted on the top of the arm for the indicator, and a few more holes and screws.

Stay tuned for version 2.

-J

Revision 2.0 Draft: A simpler build.

Looks good, when do we build it?

Regards
Ray

Originally Posted by RayG

Looks good, when do we build it?

Regards
Ray

Soon...

I tweaked the design a little more, for simpler build and better use of steel. I also made the calculations for the deflection of the double beam thingy and for the springs free length.

Spring Length is 2.17 mm with 0.030" thick that give ~70µm deflection at the measuring point under gravity.

The double beams thingy gives an adjustment range of about 10µm.

-J


PS. Hope that covers every thing that people wanted?

PPS the stock material is 16mmx65mm, milled/ground to 15mm x 60mm, you would need about 850mm of 65mmx16mm flat bar.

PPPS the finished weight is ~4.6kg

PPPPS. what would be good for the beam adjustment, just a cap screw, or something more elegant?

Edit: added pdf

Hi Josh,

Are you sure you dont need a plate under the cap screws on the hinge?(so its not hanging off the bolt heads)
As far as over travel protection. How about a bolt(couple of) through the hinged plate into the middle plate?

Stuart

Originally Posted by Stustoys

Hi Josh,

Are you sure you dont need a plate under the cap screws on the hinge?(so its not hanging off the bolt heads)
As far as over travel protection. How about a bolt(couple of) through the hinged plate into the middle plate?

Stuart

Both good points. I'm unclear if it is required or not, based on the FEA modeling it would suggest no it does not need an extra plate over the top. That being said there is 4mm clearance between the top of the spring and the bottom of the double beam so it would be a relative easy mod.

I like the screw idea, nice and simple.

So is everybody happy for me to do up the schematics for version 2.1? or are there more Great Ideas lurking about?

-Josh

Hi Josh,
Whilst on holidays here in good ol' South Australia, I popped in to see Michael. I am now the proud owner of the 16" x 16" plate in question (well when I pay for it I will). It is now on tour with us and will be in Victoria soon maaaaaate. I may be popping over for a visit.

Phil

Originally Posted by Steamwhisperer

Hi Josh,
Whilst on holidays here in good ol' South Australia, I popped in to see Michael. I am now the proud owner of the 16" x 16" plate in question (well when I pay for it I will). It is now on tour with us and will be in Victoria soon maaaaaate. I may be popping over for a visit.

Not sure if that plate had a date on it but if you are lucky you may be able to work out an approximate date from the exact name and address -
WING" Gauge & Instrument Co.
As granite came in around WW2 due to CI shortages, there is a good chance that it is pre 1950's. I'm guessing that the small Wing I have is a similar age - Wing gauge and instrument was in existence from say 1947 to 1965. The granite plate that replaces it was produced in 1968 so is only 45 years old.

(Josh, I'm sure that was a AAAA grade plate when I had it - Phil's probably used it as caravan ballast and trashed it. My excuse anyway)

Michael

Josh,
how much trouble and recalculation would it take to make the double/adjustable layers 10mm thick each and insert a 10mm rectangular layer between them and the base plate? I suspect we would get a fair bit more adjustment with a lot less force (= smaller knurled handle).
I would retain the 'coarse' adjustment of a bolt as the contact point with the gauge - and to suit different gauges.
What do you have in mind to use for the feet, given they will be sliding all over the surface plate? Should they be particularly hard and particularly flat and particularly smooth, or preferably self lubricating like cast iron or sintered bronze or just plain 'soft' like say brass?

Looking at the Rahn version, I think the adjustment is simply a slab of 25mm thick material that has been shaped and then had a saw cut through the middle. Like you suggest Joe, i was thinking of a slab of 25mm material and then rebating in some 8 or 10mm material say 25mm wide both sides (welding or brazing in). Cutting from a piece of flat and doing the saw thing may work, but seems very wasteful to me, although probably quicker and easier than rebating in as I was suggesting I guess it is a balance between less adjustment force and more flexibility as the arms get thinner.

As for feet, I think it depends on what the device is running on. You don't want something that will ding up the surface so brass might be the go for a CI plate, perhaps CI or steel for granite plates.

Michael

Who's going to be making one of these (or a variant) and needs some spring steel shim?

A few more questions rather than great ideas.

The fine adjustment(other then on the indicator) is that really needed? shouldnt this be set once and forgotten.
To answer my own question "no some of us might what to use the indicator for something else between tests".

Why the angled cut at the hinge?

Re the weight. I wonder if its so heavy just so that moving it around with the handles lifting are less likely to effect it? If one was worried about that you could make handles with a pivot so they wouldn't let you lift it. (though that might be a little over the top)

Why spring steel? Just coz its handy and thin?

Stuart

Originally Posted by Stustoys

The fine adjustment(other then on the indicator) is that really needed? shouldnt this be set once and forgotten.
To answer my own question "no some of us might what to use the indicator for something else between tests".

Yes. Typically when being used you are meant to zero the indicator on the centre of the plate and then compare the rest to it. Zero adjustment may be handy for that too. Remember this is being used with an indicator that has sub micron divisions and the limits depending on the plate grade are between 1 and 5 microns. Some indicator movement is to be expected either due to wear or the initial grind/ lap.

Originally Posted by Stustoys

Why the angled cut at the hinge?

No real reason. It just is and has been done that way. If I had to invent a reason I would suggest it is that way to prevent things being able to easily drop into the gap

Originally Posted by Stustoys

Re the weight. I wonder if its so heavy just so that moving it around with the handles lifting are less likely to effect it? If one was worried about that you could make handles with a pivot so they wouldn't let you lift it. (though that might be a little over the top)

Weight is not the same as rigidity. Personally I think the Rahn is as heavy as it is because it was never refined much as part of the original design process. Some weight is desirable though so it sits well on the plate. Because of the gauge force and the cantilever arrangement my estimate is that 3kg is about the minimum that is practical

Originally Posted by Stustoys

Why spring steel? Just coz its handy and thin?

More or less. A solid hinge is preferred to prevent movement between the main body and the hinged section in contact with the plate (could generate errors). After that it becomes a matter of finding something with dimensional stability that does not work harden and has a good fatigue life.

Michael