Lapping my granite surface plate part 2

# Thread: Lapping my granite surface plate part 2

1. Plastic
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## Lapping my granite surface plate part 2

I have just finished the video on lapping my granite surface plate. I found it to be very much easier than I had expected. I bought a granit surface plate, (A Rahn 3'x3') to check it for true I built a Repeato metre. This is all much easier to do than one would imagine!

2. Titanium
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what is the hysteresis of your indicator and the repeatometer? what flatness can you get with repeat measurements? is there an accepted formula?

3. Plastic
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dian,

I'm sorry, but I have no idea! I cannot answer any of your questions, at the end of the video, I go over the 91cm square surface plate with my Repetometer, over that surface, I have less than a micron of movement!

I'm sorry, but I suffer from "Math" anxiety! Anything mathematical gives me an instant short circuit between the ears!

Regards, Matthew.

4. Titanium
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i have the following suggestion. take something flat and put a shim under it on one end. preferably a 5µ shim. approach the middle (or a mark) from both sides with you indicator or repeatometer. im sure you will see a difference in measurement. this gives you an idea of the "stiction" of the instrument. i have several micron and sub micron indicators, one of them of similar construction as yours. i took it apart, cleaned and oiled it and it still has a range of 3-4µ where it indicates nothing. also i quit holding precision idicators by the stem, as this can make them stick.

5. Plastic
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dian,

thank you for your comment. When I first tried my "Repet-o-meter",I found exactly what you describe. The clamping screw it a knurled nob, and I only tighten it enough to ensure that it stays in place. i check for free movement in the plunger! I just tried it again and found a 3µ piece of dirt once removed, business as usual. Reading the same as in the video!

Thanks again, Matthew

6. Titanium
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you might want to get an inditator with a smaller range, like 50µ. a mikrokator for example. the gearing on those like you show is so steep that they dont really work.

7. Plastic
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dian,

Those are 1ù increments. It works!

Regards, Matthew

8. Titanium
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Originally Posted by dian
... this gives you an idea of the "stiction" of the instrument. i have several micron and sub micron indicators, one of them of similar construction as yours. i took it apart, cleaned and oiled it and it still has a range of 3-4µ where it indicates nothing. also i quit holding precision idicators by the stem, as this can make them stick.
You're talking about dead zone, not hysteresis. Dead zone can be caused by a number of things, but one that stands out is that you oiled something. Indicators do not like oil! A tiny drop on bearing points applied with a needle is probably ok, or even advisable, but all linear components especially should be clean, but not oiled. Cleaning should be done with a fast evaporating, no residue solvent.

If that is all in order, and you still have measurable dead zone, then something is sticking, or loose. Wear in the plunger slide bearings allowing lateral movement of the plunger can translate to dead zone, but more likely is one of the pinions in the gear train being a bit sticky, possibly from oil, or from misalignment. If not that, then the spring that preloads the gear train may be weak or uncoiled.

Clamping indicators on the stem is not usually an issue, unless the clamp is not bored well.
Last edited by gregormarwick; 05-14-2020 at 09:37 AM.

9. Plastic
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Thank you.

Regards, Matthew.

10. Diamond
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Going back to the lapping, rather than the measuring:- Did you clean the coarser diamond grit off the lapp before applying the finer grit? and if so, how?

Regards
Mark

11. Plastic
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Mark,

Well, I didn't worry too much, as I'd got to a point where I felt that the charge had worn off, I could feel it wasn't cutting much and I would have had to charge with diamond flour anyway. I answered this question on youtube, David B suggested using a fine silicon carbide stone, another way is to use a block of Aluminium which being softer should "pull the diamond from the cast iron.

Regards, Matthew

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gregor, do you know these indicators? they have a range of maybe 1 mm with 1/1000 mm graduations. if you touch the tip the needle disappears, because it spins so fast. very difficult to use unlike a millimess, hikator or mikrokator.

matthew, do you have/are familiar with the federal-spec, where the repeatometer is described? from memory, they stipulate a 10 microinch instrument.

13. Plastic
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Originally Posted by dian
gregor, do you know these indicators? they have a range of maybe 1 mm with 1/1000 mm graduations. if you touch the tip the needle disappears, because it spins so fast. very difficult to use unlike a millimess, hikator or mikrokator. My indicator does not spin so fast it disappears! Matthew.

matthew, do you have/are familiar with the federal-spec, where the repeatometer is described? from memory, they stipulate a 10 microinch instrument.
dian,

Like you, I live in a Metric country, I use microns as a consequence. I don't have to abide by federal-spec.
You may not realise it, but because when I made my "Repeat-O-meter", I didn't have the dimensions, I made it longer than standard. So, my "Repeat-O-meter" is more precise than a metric "Repeat-O-meter"!

Regards, Matthew

14. Titanium
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Originally Posted by dian
gregor, do you know these indicators? they have a range of maybe 1 mm with 1/1000 mm graduations. if you touch the tip the needle disappears, because it spins so fast. very difficult to use unlike a millimess, hikator or mikrokator.
We are talking about a basic plunger type dial indicator that is very high resolution and short travel? I assumed that because that is what Matthew appears to be using in his video.

If so, yes I have a few such indicators and have worked on them to maintain their function, hence my previous reply.

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