Depth half width q

[boogievan]

Oy, Got a call from this weirdo who said be mindful page M-5 of the new compendium of Basic Disc Mastering articles and industry documents has a strange chart on Scully's 1969 Width/GPI chart for unmodulated grooves nearly (not) touching. It's showing the test cut groove widths that correspond to the grooves per inch.
But the numbers for "width" are really those for "depth." So, double each of those numbers if thinking in "widths," I reckon.

(e.g., 400 - .0018" CCIR _maximum_ _depth_, 40 microns, _maximal _width_.0036", 80 microns))


But the numbers with the 90-degree included I've used and seen in the other Scully field bulletins are more like .0025 mil depth with the 400 GPI if even 20% land is available. That's why I've asked for your base depths. 3, or less?

toodle pip,
Tim E.

[Serif]

HI Tim, I have that book here, and I think you're onto something. Those do look like depth numbers, rather than width numbers, and they are certainly used for groove depth calibration with a known GPI.

...
But the numbers with the 90-degree included I've used and seen in the other Scully field bulletins are more like .0025 mil depth with the 400 GPI if even 20% land is available. That's why I've asked for your base depths. 3, or less?

toodle pip,
Tim E.

Other than that one would need to move the decimal place over three positions to the right for your 0025 figure to be still in mils, yes.

I wonder if it is possible that groove "width," which is at all times twice the depth, perhaps used to be used poetically like diameter is used for radius on a disc diameter ruler (which measures each half-inch in "inches" of diameter).


- Serif

[boogievan]

Well, I hadn't thought of it as a deliberate mis-nomer. I reckon that the 1,8 mil figure was meant to be groove depth and perhaps not even for the new micro-groove tip? "001,8 mils" looks a lot more like the defacto depth created by a 110-degree included angle when the GPI, just nearly touching, is 400. So, it is neither the "width," nor really correct for a 1969 Scully, is it? I hope it's good and there's something to learn, here. But this info from page M-5 has got me buggin.'

-Tim E. (host, BuggerConvention'94)

[mossboss]

Got a call from this Weirdo? I thought we are all here posting away
Why a call?
Cheers

[boogievan]

I can't say why he called. Perhaps he wanted a better Turing test. I can say that he were weird. Odd cadence and accent - as if he were acting, poorly. He called to point out the error in the Basic Disc Mastering book and wanted to know if it was merely an error in compending, or an error in the original Scully 1969 information. Or, as Dingbat has suggested, a poetic license sort of use of the word, width. Half-width I granted his depth.


-boogiE Tim

[mossboss]

This forum is blessed with the presence of Serif and boogie-man enhancing the mundane terminology of the cutting world with such wonderful metaphors
Keep at is guys
Cheers

[boogievan]

Reading ahead, page N-1 seems to present more confusion to us. This is the Appendix N in Basic Disc Mastering, which is on the Scully auto variable depth controller from 1960 (DC-601).

"The total width change available from the depth unit is approximately .002"."

Does this mean that the depth can only automatically change by 1 mil in this system?
{Width of groove = twice groove depth. }

Or does it mean that Scully is talking "land" (as in, groove spacing?) when he writes, "width?"

If one starts with no vertical information and proceeds to cut a groove of width .004", which equals a groove depth of 2 mils, why would adding the vertical information make the depth get shallower (e.g., going from .004" to .002", constituting a depth change to 1 mil? (ouch?)) Shouldn't the width get bigger when the vertical signal is added to the lateral feed computing?

Just want to be sure I am knowing what I'm trying to read. Here's hoping width used to mean land.


Fanks,
Tim E.

[boogievan]

Ok, problem solved. Almost any time someone tells you the depth of his cut he is mis-speaking. He is actually telling you the width of the cut at the top of the groove.

Where a manual such as that of the LS-76 states to calibrate the automation by setting the base pitch to 400 LPI and the base depth to 2 mils, they have written down a prescription for horrendous twining. The instructions should actually read:

Set base pitch to 400 LPI and base depth to 1 mil. This is a followable set of instructions. It might also read:
Set base pitch to 400 LPI and base WIDTH to 2 mils. This is the same set of instructions, since the depth of the centerline of the groove is always half the distance measured between the top of the left and top of the right walls of that groove.

Q: What is the spacing of the grooves (centerline to centerline) in mils when at 400 LPI?

A: 1"/400 = 0.0025" = 2.5 mils. Ergo, if the base depth really were 2 mils and the base pitch really were 400 LPI, with no audio sent to the lathe, the groove just cut would be nearly bisected by the following groove as it would occur just beyond the centerline of the previous cut. Clearly, the width is all we can measure and all that we are trying to make be equal to 2 mils for this calibration. Then, at 400 LPI, with a depth of 1 mil, we shall have about 2 mils groove WIDTH, with 85% cut out and 15% still land.


So, Boden is right and Scully are wrong! (in their -76 manual, that is... In the earlier manuals, they use the word, Width.... I move we all switch back to both Width and also GPI (Grooves Per Inch)!)


- boogie (got this info from Laarso's latest post, Width Sympathy (http://www.discolathe.com/viewtopic.php?f=15&t=83))

[Steve E.]

I want to translate this for my own brain.

A modern microgroove stylus is 90 degrees, yes?

If I draw a triangle with a 90 degree wall and two equal sides, the other 2 angles are 45 degrees. We tip this triangle so the 90% angle is pointing to the center of the earth, and we have a cross section of a groove wall.

The hypotenuse of this triangle (longest side) is the groove width at the top of the walls. If we draw a line from the middle of this hypotenuse downwards to the opposite, 90 degree angle, the line is exactly half the length of the hypotenuse.

So, yes, in a 90 degree microgroove stylus, the groove depth is theoretically exactly half of the groove width.
or,

the depth of the centerline of the groove is always half the distance measured between the top of the left and top of the right walls of that groove.

What's the calculation for a 110 degree old-world 78 stylus?

[Steve E.]

EDITED:

By the way.....Somebody recently told me that a record groove is an isosceles triangle, and therefore the depth is the same as the width.

If a groove cutting angle were 60 degrees, this would be an actual isosceles triangle. And actually, you would need a groove with a 53 degree angle at base to get a depth equal to width.

So, no, that is not true.

[Steve E.]

What's the calculation for a 110 degree old-world 78 stylus?

First of all, I am basing this solely on Boogievan's contention that standard ("coarse") cutting styli were 110 degrees. In an earlier thread, I posited that that they might be 100 degrees.

Further caveat: Tipping the stylus from perpendicular to the cutting surface increases the included angle, which decreases the depth in relation to the width. Remember that!!!

I used this Tan calculator:
http://www.rapidtables.com/calc/math/Tan_Calculator.htm

This is what I came up with for the theoretical depth of cut for an old standard groove with a 110 degree included angle.

If the groove width is w, the depth = w * tan35º/2 = w * 0.70020754 * .5 = w * .35010377.

So, to simplify: On a pre-1948 Standard (78-style) groove of 110º, the depth would be .35 of the width. (It's also 7/20, but that's a rather obscure measurement.) It's slightly bigger than 1/3.

In case the pre-1948 standard were 100º, the calculation would be:

If the groove width is w, the depth = w * tan40º/2 = w * 0.83909963 * .5 = w * .419549815

Call it 42/100 = 21/50 or pretty much 2/5.

[Cubed]

I'm no expert so please take this as a grain of salt:

Using general geometry "should" answer at least the width of a groove. But first we must know:

-The depth of the cut from the surface to the bottom of the groove.

-A microgroove is cut at 90 degrees.

-The total count of all 3 angles of a triangle is 180 degrees

Ok, so lets say if you look at the stylus and depth where it's cutting to and see it as a triangle.
if you run a a line dividinig in the middle, this triangle to the tip, it will divide the triange in two with the angles:
90, 45 and 45

So these new triangles have 2 sides that are the same length, the length of the new line and the length from the new line to the outside of the stylus triangle. ok since these lines are the same, we multiply by 2 (because we divided the stylus trangle into 2) and can conclude this:

If the depth is 3 mils (just an example! please don't kill me!), the the width of the cut is 6 mils.


I'm not good explaining things but I can draw it out if no one understands what I'm talking about, in which I blame myself for that.

[Steve E.]

I'm going to post the results in a sticky here (which will be continually updated, unlike this current posting):

https://www.lathetrolls.com/viewtopic.php?f=9&t=4631

To summarize:

Depth of a modern 87º groove: ~1/2 of its width. (=.52689)
Depth of a modern 90º groove: 1/2 of its width. (=.5)
Depth of an old coarse/standard 100º groove would be ~2/5 (or ~3/7) of its width (=.41955)
Depth of an old coarse/standard 110º groove would be ~1/3 of its width (= .350104)


Some extra info, just for interest:

In a groove with a 53.2º angle, the depth would be ~1/1 of the width (=.9985)
In a groove with a 60º angle, the depth would be ~6/7 of the width (=.866025)

The formula for calculating depth:

If the groove width is w, and the angle of the base of the groove is G, the depth d is:

d = w * .5 * TAN( 90 - (G/2) )

or

d = w * [TAN( 90 - (G/2) )]/2

That's using degrees, of course, not radians. In Excel, the TAN function expects radians so you need to convert.

I welcome any corrections. This seems like a pretty crucial batch of info.

[Steve E.]

- - - - just sent me things from the old Read recording book, and it seems to contend that the old "Standard" Capps styli were also 87 to 90 degrees!! They just had a larger radius on the tip at bottom!! So I may have been barking up the wrong tree all this time??? Where were we getting 100 to 110 degrees?

I wonder if this old thread of mine, about a steel cutting needle, was the source of any of the confusion:

https://www.lathetrolls.com/viewtopic.php?f=8&t=3256&p=19585

angle-compare.jpg

side-angle.jpg