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Why is center gauge marked "Double Depth of Sharp Threads" ?

Joined
Feb 4, 2004
Location
Metuchen, NJ, USA
I'm puzzling over the markings on an old Starret No. 391 Center Gauge. (The 390 is the same tool, without the temper.)

The markings are a table which is labelled "Double Depth of Sharp Threads" The table entries seems to be TPI, running 4 to 25 mostly even but 5, 7, 9, 11, and 25 are included. Each entry has a decimal depth associated with it.

Why DOUBLE the depth? What method of measurement is assisted by this table? Is it the method using the 60 Degree thread micrometer? The three-wire method? What?

John Ruth
 
John Ruth --

The theoretical Minor Diameter is calculated by subtracting the Double Depth of Thread from the theoretical Major Diameter.

For a sixty-degree Sharp V screwthread, the [Single] Depth of Thread is Pitch x Cosine 30 Degree, so the Double Depth is 2 x Pitch x Cosine 30 Degree.

So why's the Double Depth on a Fishtail? Well, the theoretical Minor Diameter is the theoretical minimum Tap Drill Size, resulting in a 100% thread.

Of course, the 60 degree Sharp V screwthread has been 99.99% obsolete for a century.

In the US the 60 degree Sharp V threadform was replaced by the Sellers (aka Franklin Institute, US Stanard, and American National) threadform. The [Single] Depth of Thread of a Sellers thread is calculated as 3/4 x Pitch x Cosine 30 Degree; the Double Depth is 1.5 x Pitch x Cosine 30 Degree.

Then, fifty years ago, the Unified threadform replaced the Sellers. The [Single] Depth of Thread of a Unified thread is calculated as 5/8 x Pitch x Cosine 30 Degree; the Double Depth of a Unified thread is 1 1/4 x Pitch x Cosine 30 Degree.

The ISO Metric threadform is geometrically identical to the Unified, so the same calculations work.

Reaching down to the bottom of the barrel, though, there is a very straightforward way of calculating Tap Drill Size that, for most purposes, works well for the Sellers, Unified, and ISO Metric threadforms: Tap Drill Size = Major Diameter - Pitch.

John
 
I had the same question, so I contacted Starrett and this is their reply.

EXPLANATION OF
"DOUBLE DEPTH OF THREADS"
(as found on # C391, old # 40 etc.)
As you no doubt understand, the Center gage is primarily used to align a tool bit
(threading type) with a work piece in a lathe when you wish to cut threads.

The gage is set against the side of the work piece and the threading tool bit is aligned with one of the 60 degree notches in the side of the gage, to ensure that the tool bit is square with the piece to thread.

The compound slide is typically set to 29 ½ or 30 degrees to allow for side clearance.

Once this is done. the tool bit is advanced to just barely touch the work. The cross slide and compound dials are now set to “0” and you are basically ready to start cutting threads after setting the lathe for the right thread pitch.

Think of the following formula, and you will see where the "double depth" comes in.

If you take the cosine of 30 degrees, divided by the threads per inch, multiplied by .75 (using a nominal 75% engagement) you will have the basic depth to cut one side. Double that number, and you have the number stamped on the gage.

For example. if you were cutting a ¼-20 thread, the cosine of 30 degrees is .8660254, divided by 20. times .75 equals .03247, double that and you have .06495, the double depth figure stamped on the gage for a 20 T.P.I.
You will be advancing the cross slide 003247' to cut the depth of thread.
Although your cross slide may be graduated to increments of .001 of an inch, it actually only advances half that as a cut on one side which actually cuts the whole diameter. As you undoubtedly know.

Now on the older gages, there are different numbers than you would see on the newer gages. These are figures for double depth on “Sharp threads” which are very, very seldom seen any more. If a chart of these is required, it can be requested from Starrett Tech in Athol.
 
I wondered about that, but it didn't keep me up at night.

I wonder at what is written by them, as I was taught, and taught in turn, that the cross slide was for retracting the tool clear, and returning to zero to restart the cutting, and the COMPOUND was where you fed in to increase the depth of cut. Y'know. The whole reason for offsetting the angle. Because, well, NOT a straight in feed.

I cannot recall ever finding a compound that directly read off half the actual in feed.
 








 
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