Offen imperial bore mic gauge.

[Graeme SeedParticipant @graemeseed34272]

Hi All, here’s an Offen, UK made, extremely well made bore gauge. Complete set, including a pin to engage/disengage the friction thimble. Measuring between 0.485/0.965, 0.080 between changes of anvils. There is a line above the datum zero which corresponds to 9/10ths, as a vernier would do. Photos show different settings but I can’t see the correlation between datum and vernier to read 1/10th. Each thou is divided for 2/10ths. I can see 1/2 a small division but that’s a guess, where is a true reading? Thanks, Graeme.

 

[JasonBModerator @jasonb]

There was a thread a few months ago about reading the different types of 1/10th scales.

[Graeme SeedParticipant @graemeseed34272]

Hi Jason, I can’t believe in search, anything about Offer bore gauge, your assistance please.

what’s confusing is that anything on the datum line added to 10ths above the line always equals 9. The photos show this.

[Graeme SeedParticipant @graemeseed34272]

Hi Jason, I can’t find anything in search, anything about Offen bore gauge, your assistance please.

what’s confusing is that anything on the datum line added to 10ths above the line always equals 9. The photos show this.

[JasonB]

On JasonB Said:

There was a thread a few months ago about reading the different types of 1/10th scales.

On Graeme Seed Said:

Hi Jason, I can’t find anything in search, anything about Offen bore gauge, your assistance please.

what’s confusing is that anything on the datum line added to 10ths above the line always equals 9. The photos show this.

I think this was the thread on tenths, but Graeme was party to it

Somethings missing or is it in plain sight?

There was a previous one about Offen, but again Graeme will be aware of it.

Offen screw type telescopic gauge.

Bill

[Graeme SeedParticipant @graemeseed34272]

Thanks for finding the threads Bill. Yes, I started these threads, one about the shardlow mic but the Offen one didn’t continue to tenths. Michael G sent the copywrite docs, but these don’t explain tenths either.  One of my photos with datum at 5 is slightly out of parallax. Strangely, having both cataracts done, I can see that the tool must be rotated to see the exact position of the top line even though they’re only 3/16″ apart. The upper line is there for some reason and on datum zero, it reads 9. Maybe the tool only reads to 2 tenths, but why the line? Going to 9 strongly suggests a vernier scale but there’s no scale at the line!! Move the thimble anywhere and nothing correlates to a vernier number. I’m mystified but enjoying trying to fathom this out. Graeme.

[Nigel Graham 2Participant @nigelgraham2]

My thought is a vernier effect but to “half-tenths”, i.e. 0.0005″, only.

You could experiment if you have another bore-measuring instrument by comparing results of measuring various accurately-made items like bearings.

[JohnFParticipant @johnf59703]

Unless I am not understanding what Graeme is saying I see no problem in reading the bore gauge ? The vernier is there only to split the 0.0002″ marks to 0.0001″ and to me the readings in the photo’s are: –

1 = 0.005″  [less a tad ! probably a photo parallax error ?]

3 = 0.0004″

4 = 0.0007″

5 = 0.0000″

The reading on the barrel would of course be added to those on the thimble — photo 5 shows zero

[alecsParticipant @alecs]

It is your one-tenth line. The reading on the main fiducial line is to the nearest 2/10ths, as each thimble graduation  is 2/10ths.

But if the thimble graduation goes past the main fiducial line far enough for that second short fiducial mark to line up with a thimble line, you add one tenth to the main reading, ie half a thimble graduation.

Hard to explain but much simpler to use in practice than the traditional 10 on 9 set of lines. It’s basically half of a 10 on 9 vernier scale with all the lines except the fifth erased.

EDIT. John F beat me to it, simultaneous post.

[Graeme SeedParticipant @graemeseed34272]

Hi All, many thanks for all of your contributions. Alecs and John F are almost spot on. You’re so right about the vernier line. Here’s my EURIKA moment and a few photos. Alecs mentions ADDING where my assumption is SUBTRACTION.
Here I go —— set any thou number to the datum, the vernier will always read 9 [subtracted 1 ]
Set the mic anywhere to the datum. In this case, between 14 and 15. Look at the numbered thou line IMMEDIATELY ABOVE THE VERNIER LINE, 13 , in this case. Count down the divisions from 13 > 14 to the vernier line = 4 = 8 tenths.   Now the vernier width size of 9/10 of a thou comes into action. SUBTRACT 1 from 8 tenths = 7 tenths. Read the datum = 7 TENTHS . This  method works every time.

Parallax view is most important,  the thou readings are obvious, the tenths are found by going to the immediate  numbered thou above the vernier line, counting divisions down to the line [ including  1/2 divisions ] and subtracting 1 to achieve the correct datum size.
Another example included. Graeme.

1 set to any size.
2 count back from 13 to 14 and minus 1
3 read result = 7
excuse  edit on photo, I don’t know how to write on a photo, please ignore.

[alecsParticipant @alecs]

Seems complicated and potentially prone to operator error. But who knows? Would love to see the original documentation. Can’t find anything remotely similar on Google.

[JohnFParticipant @johnf59703]

Graeme, with respect you are vastly overcomplicating taking a reading and going “backside forrard” !

The way to take a reading is : –

1  Read the barrel in this case its 0.0000″  [it could be 0.025 or 0.050 or 0.075 etc where you would add this to the thimble reading instead of zero]

2  Read the thimble number before the barrel fiducial line 0.014

3  Read the whole tenths on the thimble 0.0006  then because the line does not coincide with the barrel fiducial —

4  Read the vernier –the number it lines up with on the thimble is of no consequence it just confirms that it is one tenth extra thus the reading is 0.0147″

Only simple mental arithmetic needed to make the reading and no different to using a standard micrometer with one thou readings on the thimble and one tenth vernier around the barrel.

Hope this helps

John

[Graeme SeedParticipant @graemeseed34272]

Hi John, thanks for your input. A bit strong ” vastly overcomplicating”. As you say, its very simple arithmetic, as yours or my idea both work. I seem to be using the vernier line and it looks like you don”t seem to be, is that an ” assumption” that the vernier reads +1 or -1? Its got to be one or the other?
However, I’m grateful for your fathoming, but have to disagree that this is not designed as a standard 1/10th micrometer. Give this tool to someone not familiar with it and I’m sure that they would be scratching their head, as I was. Moving onto an appropriate, famous quote from Kenneth Wolstenholme, “They think its all over——it is now”  Graeme.

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