DTI vs DI

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GlennW
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Re: DTI vs DI

Post by GlennW »

SteveHGraham wrote: Fri Mar 30, 2018 2:50 pm My coax indicator came with long tips that multiply the measurements by some wild numbers, much worse than big cosines. The indicator has to be much less accurate with long tips, but I've never thought it until today, because all I was looking for was a steady reading.
I tried to explain that zero movement didn't mean zero runout depending on probe length years ago.

I could indicate over a bore using a coax with a long probe and get zero movement, and then follow it with a DTI and have as much as .003 TIR.
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Harold_V
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Re: DTI vs DI

Post by Harold_V »

Heh! Coax. I rank them right up there with thread pitch mics.

Please help me understand why anyone needs either one of those items? Not being a tool junkie, I don't own either of them, nor do I care to.

H
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tornitore45
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Re: DTI vs DI

Post by tornitore45 »

It is possible that my spacial perception, usually excellent, fail totally when I am facing a DTI. I never know which way the dial indicated near or far. Add to that that when a DTI is on the spindle and is rotated all around the mirror image, the autoreversing action and all that I am totally lost to decide which way to nudge the part, ever more so if the swing is over 1/2 turn. I have to tickle the arm to see which way the needle move.
As much as is a pain to raise the head to install the COAX, that is the time when it comes out of the box. Granted it may have poor sensitivity with long probes but once the part is close to location I can deal with the DTI if it only moves a few divisions.
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BadDog
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Re: DTI vs DI

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As I said repeatedly, cosine error only matters if you care about getting a quantified value.

As for the limited range, the lower the resolution, the longer the range (typically). And I generally just center up by eye close enough to get withing DTI resolution. And before making contact, I get it near (within fine adjust range) and spin it to eyeball the gap, tweaking as needed. By the time the probe lands (generally only one or two tweaks if any), it's within it's range. All total, with 2 chuck keys, I can generally get a part centered in a four jaw chuck without a couple of minutes at most (depending on accuracy needs, or problematic parts) from the time I first snug up the part until I'm ready to turn.

But as noted by many, it's really just a matter of preference. For me, DI for general use is just a non-starter, partly due to convenience of DTI, partly due to my desire to leave them for when it does matter. Each has its purpose, and a DI for general self-referencing relative work does not makes sense to me.
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GlennW
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Re: DTI vs DI

Post by GlennW »

tornitore45 wrote: Fri Mar 30, 2018 4:13 pmIt is possible that my spacial perception, usually excellent, fail totally when I am facing a DTI. I never know which way the dial indicated near or far.
There a number of DTI's (B&S BesTest for one) that have Constant Clockwise Rotation of the needle regardless of which direction the probe is moved. Much easier to use if you don't use a DTI a lot!

I have a couple that are not CCR, and occasionally I have to use one and it does make me scratch my head at first until I realize what is happening...
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John Hasler
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Re: DTI vs DI

Post by John Hasler »

I have a coax indicator that I find useful on the mill (It helps that I got it free at a time when my only other indicator was a Gem DTI). It does require that one pay attention to geometry.
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BadDog
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Re: DTI vs DI

Post by BadDog »

GlennW wrote: Fri Mar 30, 2018 4:59 pm There a number of DTI's (B&S BesTest for one) that have Constant Clockwise Rotation of the needle regardless of which direction the probe is moved. Much easier to use if you don't use a DTI a lot!

I have a couple that are not CCR, and occasionally I have to use one and it does make me scratch my head at first until I realize what is happening...
That's the truth. I happened across an Interapid for much cheaper than I had ever seen one, and with the large face, so I snapped it up. Then I find out later it's completely opposite every other DTI I own (mostly B&S BesTest, and a nice Jeweled Swiss Tessa of the same design). I would reach for that one before I would a Starrett Last Word, but not much before. I keep it around thinking maybe one day I'll make it into some sort of fixed tooling or something where the reverse operation doesn't confound me, but so far it just sets at the back of my indicator drawer. Thankfully I've been able to sell all the Last Words I've come into possession of, so at the moment, the Interapid is the bottom of my list.
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SteveHGraham
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Re: DTI vs DI

Post by SteveHGraham »

Harold_V wrote: Fri Mar 30, 2018 3:50 pm Heh! Coax. I rank them right up there with thread pitch mics.

Please help me understand why anyone needs either one of those items? Not being a tool junkie, I don't own either of them, nor do I care to.

H
Wow. Here I was, about to brag about the great deal I got on thread pitch mikes.

How can you not love coax indicators? They're so easy to use.
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BadDog
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Re: DTI vs DI

Post by BadDog »

DTI on an intercol (or clone). Probably my second most used indicator mount after the Noga magnetic.

Something about those coax indicators, but in all my sniffing around I've never found one that was less than what I thought was outrageously expensive for my purposes. You can get good brand name (classic) B&S indicators that look like new for less than $30, sometimes quite a bit less, and $40 to $50 is pretty much the going rate (or at least was when I was looking?). But those coax indicators, seems they are always $100+ (brand names $400 or more). It's been a long time since I cared to look, but that's what my questionable memory says anyway. I'm too cheap to put that into something I would use once in a long while, and where a DTI does it better for a little inconvenience in craning around the back to see the face.

But a little trick when you know that the indicated part is actually highly round, you don't need to see the back side. One you have it left and right, you know what the front (on a spud, back on a bore, unless you reverse the read) will read when concentric. I usually finish up with a single sanity check on the back to make sure it matches, but don't recall ever having it not read what I thought. As such, I almost never need to look at the back side...
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Harold_V
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Re: DTI vs DI

Post by Harold_V »

SteveHGraham wrote: Fri Mar 30, 2018 7:56 pm
Harold_V wrote: Fri Mar 30, 2018 3:50 pm Heh! Coax. I rank them right up there with thread pitch mics.

Please help me understand why anyone needs either one of those items? Not being a tool junkie, I don't own either of them, nor do I care to.

H
How can you not love coax indicators? They're so easy to use.
Pretty much for the same reason I don't like the Starrett Last Word. I prefer an instrument that doesn't lie to me.
I'm not suggesting that a (good) coax doesn't work, but in order to do so, it must have linkage that is prone to error. I don't have any issues with reading a DTI spun in a spindle, as that's all I've ever done, and, to me, it's routine.

Once you understand how easily one of the Last Word indicators can be wrong, it tends to get relegated to non-critical work. The guys with whom I used to work pretty much all used them for removing taper on grinders, where they seem to do an acceptable job.

Funny---me complaining about Starrett---when I have a tool box full of their instruments. The only one I don't like is the Last Word.

H
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Re: DTI vs DI

Post by John Hasler »

> Once you understand how easily one of the Last Word indicators can be wrong...

Could you explain? Until recently a Gem DTI (a Last Word clone) was the only indicator I had and it has never lead me astray since I figured out how to use it (which didn't take long).
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SteveHGraham
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Re: DTI vs DI

Post by SteveHGraham »

Harold_V wrote: Sat Mar 31, 2018 3:21 amI don't have any issues with reading a DTI spun in a spindle, as that's all I've ever done, and, to me, it's routine.
Well, you have admitted to dancing on the mill table, so you are probably more spry than I am.

I will give you the Last Word.
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