C'est le problème!

[graham-xrf]

Aww.. darn it! (+ Some other robust expressions of an Anglo-Saxon nature!)
Duh - I specially chose those precision ground pins to touch at near half the height of the dovetail.

 

[rabler]

Can you use a gauge block to space it out?
Do you need a precise width or are you looking at relative wear?

(It looks to me like the problem is that the shoulder on the micrometer doesn't let you get a good reading. If I'm missing the issue, feel free to laugh hardily at my expense)

 

[graham-xrf]

@rabler : Indeed - spot on! No way to get the anvils down onto the pins. Relative measure is wanted.
You can see one of the attempted gauge blocks standing loose behind the micrometer.
The measure across pins is around 40.5mm, or about 1,6 inches.
That's too much for a 25mm micrometer, but would be OK for a 25mm-50mm micrometer, if it would get in there.

The problem is the minimum distance to span from the left side pin, to the end of the obstruction, is about 61mm (or 2.4 inches). That much is beyond a 1" to 2" span. This is the sort of measurement where one wants to see consistency to within at least a couple of tenths.
I don't have a "tenths" micrometer that will do 2" to 3".

If I use a really big gauge block, I do have a 75mm-100mm micrometer.
It's even a reasonably good one, reading 0.01mm, which is about 4 tenths resolution, so still not quite up to this job. Keith Rucker's increment variations were marked in tenths. For me, even the "small" side of the micrometer can't sit right on the pin.
I will find a way. It's just that sometimes, one wishes things would work out a little better..

 

[WobblyHand]

@rabler : Indeed - spot on! No way to get the anvils down onto the pins. Relative measure is wanted.
You can see one of the attempted gauge blocks standing loose behind the micrometer.
The measure across pins is around 40.5mm, or about 1,6 inches.
That's too much for a 25mm micrometer, but would be OK for a 25mm-50mm micrometer.

The problem is the minimum distance to span from the left side pin, to the end of the obstruction, is about 61mm (or 2.4 inches).
That much is beyond a 1" to 2" span. This is the sort of measurement where one wants to see consistency to within at least a couple of tenths.

I don't have a "tenths" micrometer that will do 2" to 3".

If I use a really big gauge block, I do have a 75mm-100mm micrometer.
It's even a reasonably good one, reading 0.01mm, which is about 4 tenths resolution, so still not quite up to this job. Keith Rucker's increment variations were marked in tenths. For me, even the "small" side of the micrometer can't sit right on the pin.
I will find a way. It's just that sometimes, one wishes things would work out a little better..

View attachment 395576

Do you have an extra dowel pin to use as a spacer? In the first picture, there seems to be a rectangular block. Could you use the block? It seems close to the right width.

As for not having a 2" - 3" micrometer (50-75 mm) that's a solvable problem. You can get one that reads to 0.0001" for not that much. I bought a set of micrometers a while back, 0-1, 1-2, 2-3, 3-4, reading to tenths. Not as nice as a Digimatic, but still, quite useful.

 

[graham-xrf]

Do you have an extra dowel pin to use as a spacer? In the first picture, there seems to be a rectangular block. Could you use the block? It seems close to the right width.

As for not having a 2" - 3" micrometer (50-75 mm) that's a solvable problem. You can get one that reads to 0.0001" for not that much. I bought a set of micrometers a while back, 0-1, 1-2, 2-3, 3-4, reading to tenths. Not as nice as a Digimatic, but still, quite useful.

I did mention that there is 2.4 inches from left side pin to edge of obstruction.
Sadly, regardless of whatever blocks one can put in there, the end of that micrometer body will never be able to fit over past the end of the shelf. A bigger micrometer, along with bigger gauge block(s) maybe..
There is that "gap" in my collection of tenths-capable micrometers.

 

[T Bredehoft]

Share with us your need for accuracy...

Do we really need a tenths mike? Seems to me a normal eye can distinguish at least half thou, maybe even 1/4 thou, or if it's close to one line, half the thickness of the line. And the purpose of the measurement? To compare both ends and the middle of the dovetail? or to make a mating part? I see the need if the first, the second gets a gib, any way. Do you have a bind in the middle of travel? this could be from wear, but it would be loose in the middle, where the most wear is, not binding.

I don't mean to be mean/nasty, but sometimes tenths are over valued here.

 

[woodchucker]

DISC MIC..
Digital Caliper
Grind down a mic ... painful but doable.

 

[rabler]

@rabler : Indeed - spot on! No way to get the anvils down onto the pins. Relative measure is wanted.
You can see one of the attempted gauge blocks standing loose behind the micrometer.
The measure across pins is around 40.5mm, or about 1,6 inches.
That's too much for a 25mm micrometer, but would be OK for a 25mm-50mm micrometer, if it would get in there.

The problem is the minimum distance to span from the left side pin, to the end of the obstruction, is about 61mm (or 2.4 inches). That much is beyond a 1" to 2" span. This is the sort of measurement where one wants to see consistency to within at least a couple of tenths.
I don't have a "tenths" micrometer that will do 2" to 3".

If I use a really big gauge block, I do have a 75mm-100mm micrometer.
It's even a reasonably good one, reading 0.01mm, which is about 4 tenths resolution, so still not quite up to this job. Keith Rucker's increment variations were marked in tenths. For me, even the "small" side of the micrometer can't sit right on the pin.
I will find a way. It's just that sometimes, one wishes things would work out a little better..

Sounds like an opportunity to buy a new tool. In this case an appropriate micrometer.

Every job is just an excuse to wind up with a new tool.

 

[rabler]

Otherwise, since you're really only looking for relative measure, what about using a dial indicator and some sort of fixture to span it.

 

[graham-xrf]

Share with us your need for accuracy...

Do we really need a tenths mike? Seems to me a normal eye can distinguish at least half thou, maybe even 1/4 thou, or if it's close to one line, half the thickness of the line. And the purpose of the measurement? To compare both ends and the middle of the dovetail? or to make a mating part? I see the need if the first, the second gets a gib, any way. Do you have a bind in the middle of travel? this could be from wear, but it would be loose in the middle, where the most wear is, not binding.

I don't mean to be mean/nasty, but sometimes tenths are over valued here.

Leaving behind my previous world of microns and nanometres, it is in this forum that I start learning about what is usual when making stuff with lathes and mills. I expect to make most stuff to within about 12 or 20 microns. That would be around 0.0005" to 0.0008". Most of us are happy to have stuff within about a thousandth inch, except for certain critical things, like spindle run-out, or oil film clearance, and gauge blocks and micrometer's truthfulness.

I had been getting my norms from this forum, and from Keith Rucker's, video about scraping in a dovetail, along with what is out there by several others, like Stefan, and Robin. It's not that a dovetail needs to be straight and accurate in every respect to within two tenths. Its that when the condition of the thing is totally unknown, and you need to measure it for wear and anything else it may have suffered, you need better than a thousandth resolution in the measuring kit.

Keith was recording tenths. A scraping pass will remove one or two tenths. If I discover it's all within (say) a half a thousandth, or about a hundredth of a millimetre, that would be fine, but the situation is one cannot get any micrometer in there, regardless the accuracy.