DTI finger

[mzt]

I was told yesterday the finger can be swung on a 180deg. arc to have it positioned in the most convenient way for the job at hand.
Some can take measurements in both directions, too. (Not sure about mine, yet)
Now, I'll be seeing the hatred thing under an entirely new perspective.
Think of it, and take a good laugh.

Marcello

[Bluechip]

You're not alone. I didn't realise you could wave the damn things around until Mr. Stilldrillin showed me ...

    

BC

[Jonfb64]

Hi Guys,

Could anyone post pic's to demonstrate as don't understand.

Jon

[mzt]

Or worse...

Marcello

[andyf]

Marcello, here's a good laugh    but it's at myself. It was three months before I realised the finger on my first DTI could be pushed to a different angle. It needs a bit of force, which feels odd when the mechanism is so delicate.

Andy

[mzt]

It was three months before I realised the

six years.   

But I do see fun into the above icons

It needs a bit of force, which feels odd when the mechanism is so delicate

in fact:

....
- "just swing the needle"
--- "really?"
- "Yep!"
--- "Come to the shop! We'll check"

....

--- "You see: NO WAY You can move the finger"
- "gimmie" ... "You see: it MOVES".
--- I see,




Marcello

[AdeV]

I discovered this trick fairly early on in my machining life, when I accidentally power-fed the mill table the wrong way -  AARGH  DISAST.... wait..... OOooh, that's neat 

[Jonfb64]

Just tried it, was only a tad scary it being such a delicate piece of gear.

Once you get past the sweaty moment and realise its mounted on some kind of friction pad its a doddle.

Marcello thanks for without you I would never have known
 

[75Plus]

While the finger is movable I have been told by more than one master machinist or tool and die maker that the greatest accuracy is obtained with the finger in line with the DTI body. Not sure why but I don't argue with experience.

Joe

[hopefuldave]

Quoting Joe:
greatest accuracy is obtained with the finger in line with the DTI body
End quote.

I was always told that the reading matched the calibrations when the finger was at 10 degrees to the surface / displacement was 80 degrees to the finger?

Although they seem delicate and fragile (and are if you drop 'em on the concrete floor!), the slipping joint makes 'em pretty robust when in use - the actual measuring mechanism hits a hard stop at each end of the +/- 15 thou" (or whatever). I've not tried bolting one to the lathe carriage and running it into Something Solid though! I use a sprung-arm holder, so it can get out of the way when I make (one of my frequent) mistakes

Dave H. (the other one)

[rowbare]

While the finger is movable I have been told by more than one master machinist or tool and die maker that the greatest accuracy is obtained with the finger in line with the DTI body. Not sure why but I don't argue with experience.

Joe

The error you get when not in line is called cosine error. CNC Cookbook happens to have just posted an article about it: http://blog.cnccookbook.com/2012/06/19/cosine-error-indicator-measurements/

bob

[kwackers]

"greatest accuracy is obtained with the finger in line with the DTI body"

I don't think that's right.

The cosine error is when the arm of the indicator isn't parallel to the surface (hence the blob on the end) as opposed to simply being in line with the indicator.

If your indicator is calibrated to be correct at say 10 degrees then that means the angle of the arm to the work should be 10 degrees, the actual angle relative to the body is irrelevant.

It's pretty much irrelevant, does anyone actually measure stuff with one? I think the only thing to know is that the nearer the arm is to being parallel to the work the more sensitive it is.

[Lew_Merrick_PE]

kwackers is right.

Depending on the class & grade of your DTI, the mechanical drive is a set of flexure linkages that mechanically multiply the angular rotation of the indicator point's driven pivot.  The point itself is a friction fit to the driven pivot -- which is what allows you to manually change the angle with respect to the DTI's centerline.  The pivots themselves (at least in good quality DTI's) are jewel mounted pivots with spring-loaded connections to the other pivot levers in the multiplication chain.  The indicator point's shaft should line up within 10° of the surface you are indicating.

I made my first DTI as part of my apprenticeship.  As I was 13 years old when I made it, it is nowhere near as pretty as I had hoped as I had to hand saw and file all the parts except for the jewel locations (done using disk locators on a drill press).  However, between 1968 and 1988 it managed to pass the NBS/ANSI calibration as being accurate to within .0001 inch.  (I have not done a job requiring my indicators to have formal calibration since 1988.)  This unit is in storage and I more normally use commercial units in my shop as it has been more than a decade since I have had enough space to set-up my full tool set (which is why I have no pictures to share).

The thing is that the accuracy of the indicator depends more on the symmetry of the pivot-link connecting pinch springs than anything else -- though the position and alignment of the jeweled pivots comes a close second.  If you stop and think about it, a traditional .0001 inch resolution DTI measures .0040 total in 270° of rotation.  That's 40 divisions per 270° or 6.75°/.0001 inch of deflection.  If you do the math on the ratios involved, you will start thinking that .0001 inch accurate DTI's are cheap!

[Stilldrillin]

You're not alone. I didn't realise you could wave the damn things around until Mr. Stilldrillin showed me ...

    

BC

Ah, yes Dave! 

That's  given me a smile. 

If you could have seen your face........... 

Defor.

[DaveH]

I made my first DTI as part of my apprenticeship.  As I was 13 years old when I made it,

Bluudy heck,......... Most Countries had abolished child labour in the 1800's , unless of course you are very, very old 

 
DaveH

[Lew_Merrick_PE]

I made my first DTI as part of my apprenticeship.  As I was 13 years old when I made it,

Bluudy heck,......... Most Countries had abolished child labour in the 1800's , unless of course you are very, very old

The you would not want to hear how I started my first apprenticeship in 1963 when I was ten?

Apprenticeships are practical, hands-on education.  I was not indentured for either of them.  Once I got beyond the oil the machines & sweep the floor stage, it would have taken armed men and dogs to keep me away -- which is the ideal way to learn!