Thread: Metrology for Amateurs

Originally Posted by Stustoys

You arent going to make me check all my dial gages are you? hehe

Yes...

Originally Posted by Stustoys

But what do you set the limits with? you either need a dti that is "correct" or you'd have to set the limits with gage blocks(which could be ticky)

True, but once you have set those limits, and if it's a go/no_go type of parameter, all you would actually need is something like red and green led's the actual position of the dial is not important as log as it's within the limits.


For the type of things that Phil is talking about I can see that actual numbers measuring things like run-out are required for specifications.

Regards
Ray

PS... Chris, Smoothness, I'd find hard to measure, but some cheap DTI's are jerky in operation, that is they seem to take a bit before they start moving and then jump around.. hard to describe, I did repair some dial gauges a while back and got some whizz bang oil for the job... I forget the type, I could go and find the bottle if you are interested, I remember Stuart got some of the same stuff about the same time...

Originally Posted by RayG

got some whizz bang oil for the job... I forget the type, I could go and find the bottle if you are interested.

Yes please

it occurs i can check my dti's with my gauge blocks. Speaking of those Ray i had forgotten you had the bigger ones. Has Josh zapped em yet?

Originally Posted by Ueee

Yes please

it occurs i can check my dti's with my gauge blocks. Speaking of those Ray i had forgotten you had the bigger ones. Has Josh zapped em yet?

Hi Ewan,

This is that oil,
Precision Lube THE Ultimate Synthetic Clock Watch OIL | eBay

I'll check with Josh, about the gage blocks.

Regards
Ray

Originally Posted by RayG

Hi Ewan,

This is that oil,
Precision Lube THE Ultimate Synthetic Clock Watch OIL | eBay

I'll check with Josh, about the gage blocks.

Regards
Ray

Thanks Ray, no rush on the blocks, the thought just occurred to me whilst contemplating the DTI checking.

Does anyone else want some oil? at $11 for post but only 99c for each extra item it may be getting a few in one lot.

Originally Posted by cba_melbourne

- The force needed for actuation matters a big lot too, the greater this force the greater the deflection in the overall mount setup. A cheap DTI may require a stronger mount setup for this reason.

I thought about checking this but my scales only go down to 2 grams so I figured it wouldn't tell me anything useful? maybe I need one of those cheap scales off ebay?(or maybe Josh should have talked min into buying those 0.00something1 gram scales)

Originally Posted by cba_melbourne

- what you call "smoothness of operation" is not that easy to quantify.

I noticed something like this in my best DTI, be hard to explain. I'll try coming up with a more useful explanation or some figures tomorrow. Given that my best DTI has been sitting on a shelf for 15 years....

Originally Posted by RayG

Yes...

I'll get back to you in a month or so then . How about I pick out say 5 and test them?

Originally Posted by RayG

True, but once you have set those limits, and if it's a go/no_go type of parameter, all you would actually need is something like red and green led's the actual position of the dial is not important as log as it's within the limits.

If you were setting an "unknown" DTI you would need an undersize part and two sets of slips(or two parts, one bottom one top).
With a "known" dti you can use any measured part and set the DTI to that.
Would depend how many parts you were making.

Stuart

Originally Posted by Ueee

Does anyone else want some oil? at $11 for post but only 99c for each extra item it may be getting a few in one lot.

I'm in, thanks.

Originally Posted by cba_melbourne

- Accuracy matters on a DTI too, when measuring runout to a limit. For example, a service manual may say something like "replace item when runout exceeds 0.03mm".
Chris

Originally Posted by Machtool

I hear it time and time again, especially on P.M , always from American's, that a Dial test indicator, doesn't need to be accurate over its travel, as its only a comparative instrument. I.e you are only interested in zero - zero. Whilst that's true for dialling in a 4 jaw, or indicating a bore.

So I'm in the camp that a D.T.I is actually a measuring device. It has that nasty cosine thingy. But if you are aware of that and keep the stylus mostly flat / horizontal, (Unless you have one of those 12 degree thingy's). You can almost take its readings to the bank.

Regards Phil.

At the risk of being pedantic (and perhaps boring too), I'm not sure that a DTI or a DI can strictly be 'accurate'. A long time ago I was taught that measuring equipment had 3 important properties

• Accuracy - how close it was to the true value
• Repeatability - how consistent it was in giving that value
• Precision - how well it showed what the true value was

A vernier or micrometer gives an absolute value so we can talk about how accurate a micrometer or vernier is. However, a DTI gives a value relative to another. It needs to be repeatable compared to an absolute standard. That is, if I measure a slip stack 23.45 high it should always indicate the same value, and if I measure a stack 23.75 high, it should always show a 0.3mm difference. The DTI will not tell me that the stack is 23.45mm high though*, which is what we should be thinking of when speaking of accuracy. The example given of a shaft run out of 0.03mm is again a relative measurement. Provided the DTI is repeatable I should be able to use the same indicator I used to compare the slip stacks and get the same result - but I can't tell you what the part diameter is because I can't give an absolute (true) value so therefore it is not strictly accurate in the sense defined above.

Michael
*Alright - a long travel version might, but only by referencing off the surface the stack is on.

Originally Posted by RayG

Hi Ewan,

This is that oil,
Precision Lube THE Ultimate Synthetic Clock Watch OIL | eBay

I'll check with Josh, about the gage blocks.

Regards
Ray

Do you know the difference between the different types? The watch oils are currently sold out. I wonder if the oil is different or just the tip...

Hi Michael,

If you were ever going to be pedantic I think this would be the thread for it anyway.

Originally Posted by Michael G

The DTI will not tell me that the stack is 23.45mm high though*.

Thats because its outside its range........ a 150mm vernier wont tell you your stack is 160mm high either.

Originally Posted by Michael G

but only by referencing off the surface the stack is on.

But by that definition my master height gage cant be used for measuring in absolute terms either? or my normal height gages for that matter.

Originally Posted by Michael G

but I can't tell you what the part diameter is because I can't give an absolute (true) value

Yes you could. If the DTI is set from check a 50mm dia part, if the run out was 0 but the DTI was reading 0.1mm you would know the actual diameter was 50.2mm. If the run out went from +0.2 to -0.1 the actual diameter would be 50.01mm

Stuart

G'Day Gents,
The question I asked earlier about Mitutoyo standards was how much do they vary from the stamped value ?,
Regards,
Martin

Originally Posted by Ueee

Do you know the difference between the different types? The watch oils are currently sold out. I wonder if the oil is different or just the tip...

Oils aint oils, Sol.

Had to throw that in. Actually I have not got a clue but I would like to put my name down for a bottle.

Dean

Originally Posted by Michael G

A vernier or micrometer gives an absolute value so we can talk about how accurate a micrometer or vernier is. However, a DTI gives a value relative to another.

At the risk of being obtuse... A micrometer reads relative to its anvil, a height gauge relative to the base plane, an indicator relative to whatever you zeroed the dial to. What's the difference?

Hi Martin,
You mean the 1" standard that comes with a 1"mic?

Stuart

Originally Posted by Ueee

Do you know the difference between the different types? The watch oils are currently sold out. I wonder if the oil is different or just the tip...

Somethink to think about:

watches are mostly worn on a warm body, and tick all their life. DTI's may be stored in an unheated cold workshop, and are used only a few short minutes a day, sometimes not at all for weeks.

I would think one of the thinner synthetic watch oils would be best suited for a DTI. It is amazing how many different watch oils are made by one single maker like the Swiss Moebius alone. They even make an arctic instrument oil (Moebius 9040, 2mL cost about $30..... that would make it $ 15,000 for a Liter, hmmmm, that is still slightly cheaper than gold ....). Chris