DTI travel.

Good afternoon,

I've come across a couple of DTI's with 1.6mm of stylus travel as apposed to the more common 0.8mm.

It would be useful to have that extra travel, but is it likely to compromise accuracy?

Sounds like a lever indicator rather than plunger type, so it is not recommended for accurate measurements anyway. The accuracy of readings depends on a number of things like angle of probe to work, length of probe (some DTIs come with several different length probes) and so on. Generally they are used for comparative rather than absolute measurements. So as long as the thing works smoothly over its range, I wouldn't worry about accuracy per se.

Dial test indicators are just that, indicators. They are not intended for measurement. In practical terms the cosine errors will be slightly larger with the larger travel. But not significant, provided the stylus is more or less at right angles to the direction of travel.

Andrew

Hi Bosun

I use a 0-30mm dial indicator as a check for X axis positioning on a cnc lathe and find it is in unison with the cnc tavel of the cross slide so would say it could be used to measure but that is not normally used for that, just to provide an indication of the program X datum.

Emgee

Thanks all, I hadn't considered the issue of cosine error.

Verdict lever type indicators have a shaped tip which reduces the cosine error over a greater range that other types can manage. I only use these for zeroing shaft runout and would only rely on fine accuracy over a short distance, say within 0.005" with the lever at right angles to the movement. The original length of lever also matters.

The verdict DTI probe uses an involute form, like our workshop gears (but not clocks), so that a given linear movement of the work gives a proportional angular movement to the probe, which is displayed on the dial. The body should be at an angle to the workpiece so that the probe is tangential to the work, and no, I don't know that the pressure angle should be.

For general info.

I’ve just copied this link from one of my old posts:

**LINK**

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=3161963A&KC=A&FT=D&ND=3&date=19641222&DB=EPODOC&locale=en_EP

Recommended reading for details of the Verdict ‘pear shaped’ tip.

MichaelG.

Thank you, I sort of assumed that was the reason for the shaped tip on some DTI's, although I guess it shouldn't be relied upon for definitive measurement.

Considering accuracy, it's worth viewing one of Joe Piecziyski's videos (Joe Pie) on youtube, regarding the errors in using plunger and lever-type dial indicators: This refers to angle of contact with the work rather than preciion of internal mechanism, which I take to be the OP's concern. As an inspector, I used long travel plunger type dial clocks for checking tooth size on gear shaper cutters. The lever-types were used for comparing close to the zero reading.

Errors in usage of dial indicators

Thanks, Michael, for locating the patent, and its reference to the British patent. With your careful attention to detail, I am surprised that you didn't pick me up on the angle of the body, when I should have said the angle of the probe.

British patent - they didn't waste any time in applying for the US patent!

Bill

.

No need for me to pick you up on anything, Bill

I provided the document reference … You read it, and interpreted it correctly

Job done

MichaelG.

And, reading the patent, the author refers to a cycloid. The Wikipedia article on cycloidal gears says (paraphrasing) that it is formed by a point on a circle rolling on another circle (base circle). If the rolling circle is infinitelty large, it is a straight line, and the curve is an involute.

Given that involute gears were common by the time of these patents, and the diagram showing the generation of the end of the probe (fig. 4) shows a circle rotating without slip on a straight edge, I'm surprised that it wasn't described as an involute.

But I'm talking to a small audience, perhaps only myself.

But thanks, MG, it was an interesting rabbit hole to explore. Sorry, Bo'sun for hijacking your question.

Bill

Try this hypothesis, Bill

As you have noted, the Involute is a special-case of the Cycloid … therefore, for mitigation of the risk of ‘patent avoidance’ it is perhaps better to use the all-encompassing term.

Is there a Patent Lawyer in the house ??

MichaelG.

I'm hungry for more on how Inspectors work please Bill. I know a bit about practical home measurement and fitting, and have an interest in Metrology, but there's much I don't know about the how, what, why, and when of inspection. I'm sure there's more to it than haphazardly poking stuff with a GO/NO-GO gauge. Might make a good MEW article - Neil's looking for new material.

For example, within tolerances, even a simple hole has to be made in the right place, with the correct diameter, and depth, straight, circular from end-to-end, and with a given surface finish. In my workshop, drilling is often imperfect, with holes off-centre, not perfectly round, veering off axis, tapered, depth off, and scratched. Mostly doesn't matter because what I do rarely needs better than about 0.05mm.

How would an Inspector check a Duffer built part, say 7 off 5mm holes drilled 10mm deep on an accurately centred 100mm diameter PCD, all dimensions within ±0.005mm?

Dave

Dave

Sounds like you are flirting with GD&T.

Geometrical Dimensioning and Tolerancing.

The modern way of going about things that seeks to unambiguously and accurately define all such infelicities working from sound theoretical principles. I guess it's fundamentally aimed at sorting the confluence between CAD, CNC and machine inaccuracies. Workable in computer space where all is defined I suppose.

Presumably the way that attempting to follow GD&T causes normal workshop folks brains to leak out of their ears in short order is just one of those unintended side effects.

The book I looked at left normal English usage behind about midway through chapter 1 and by the end of chapter 2 was accelerating rapidly over the comprehension event horizon at a rate likely to crack the light speed barrier before the last page.

The quick introduction guide sounds so simple and logical then ....

Clive

Dave, you have a PM.

Bill