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Vertex RT Vernier Anomaly

electrosteam

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#1
The vernier scale on my Vertex 6" RT appears incorrect.

The RT is 90:1 = 4 degrees per rotation.
The main scale is graduated 0-30-1-30-2-30-3-30-0, and the attached photo shows the dial 0 to 30 minutes, with the secondary marks at 10 minute intervals and the individual marks at 2 minute intervals.

The vernier is marked 60-0-60, which I take to be +/- 60 seconds from 0, or 2 minutes long, the same as the main scale individual marks.
So, the vernier should span from one individual mark to another, but it appears too long, or too short.
The photo does not lie, the vernier 60 mark is aligned with 0 on the left, and the 60 mark on the right floating between primary marks, with a span of about 11 1/3 primary marks.

It seems to me that the vernier should span a number of marks divisible by 6 (or 3), with the obvious candidate being 12 primary marks.
Then any vernier mark aligning with a primary mark would denote a multiple of 10 seconds, with interpolation to 5 seconds possible.

Which is crazy, me or the vernier ?

This subject comes up because I am using the RT to mill out a 6-way splined bush with some critical fit requirements.
If the vernier is wrong, what about the rest of it ?

How can I check that a 60 degrees step is always 60 ?

RT Vertex Vernier 1 comp.JPG

John.
 

f350ca

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#2
It does appear to be out a little. The second 60 should as I see it line up with the 11 mark (22 min). Where its sitting now your at 10 min on the main dial and 60 sec or 1 minute on the verier for a reading of 11 minutes.
On the vernier the right alignment looks to be out roughly .1 of a division 2 sec, but thats on twice the scale so really the reading might be out 1 second.
Been a long day hope someone else pipes in.
Greg
 

Bob Korves

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#3
Yours reads 11 divisions on the main scale versus 3 divisions on the seconds scale, and that is correct for 20 second increments. Edit: remember that the dial is 4 degrees per revolution...

For a reality check, you can use a pair of dividers set to the radius of the circle to manually and geometrically divide the circle you are dividing. The dividers will divide the circle into six equal segments, each spanning 60 degrees.
 
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electrosteam

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#4
Thanks Greg and Bob for such prompt responses.

Just spent some time getting my head around the workings of verniers.
You are both correct, the vernier should align at the the 11th mark.
Then, 6 steps of the vernier (= 2 minutes) should get you to the 12th ( 2 minutes greater than the 11th mark).

Makes sense once the brain is engaged !

But, the vernier is indeed wrong, enough to confuse you at first, but I can allow (sort of) for the error.

Bob,
The spline I am milling has slots 1/4" +/- 0.001" on a 0.9" nominal bore.
This equivalent to +/- 1 part in 2827 = +/- 0.127 degrees = 7 minutes and 38 seconds.
But the cutter has to have some tolerance, say half the allowance.
So the RT has to set to about 3 minutes, which appears achievable on the scale, if the scale repeats correctly all the way round.

John
 

Bob Korves

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#5
I see, John. If you have or have access to a dividing head, it would be a better choice for that job. The RT should be able to get it done for you.