Shred's comment reminded me of what I had written about the numbers on the fishtail gauge. It's worth repeating here despite its length. If you don't have the patience to read the whole thing, at least read the first part where George Thomas' technique for thread cutting using those numbers is described. It's a "quick and dirty" technique that you may find useful and it explains why those numbers are on the gauge.
Some time ago I wrote a treatise on the fishtail gauge (see below the =====
line) where I puzzled about the utility of the "double depth of sharp thread"
numbers on every one of these gauges I've seen. I concluded that, since the
tool is advanced using the (angled) compound, these numbers weren't terribly
useful and wondered why they were there. The numbers do not take into account
the effect of the angled compound so they seemed useless.
Since then I've come across some new information that may provide a solution to
my puzzlement. I'd like to pass that along since it will be of use to those of
us who screwcut threads.
In his book, "The Model Engineer's Workshop Manual", the author, George H.
Thomas, describes a simple and intriguing method for cutting a thread.
1. Using the cross-slide, bring the tool up to touch the work and zero the
cross-slide dial. Also zero the compound dial.
2. Move the carriage to the right to clear the end of the workpiece and feed
the cross-slide in by "the thread depth". Depending on how your cross-slide is
calibrated (diameter reduction or actual movement), this is either the number
on the fishtail gauge or half the number on the gauge. Rezero the cross-slide
dial.
3. Back off the *compound* sufficiently such that, the carriage can be moved to
the left with the tool opposite the work again.
4. Feed the *compound* forward until the tool tip just touches the work piece.
From this point you can begin cutting the thread. Keep feeding in on the
compound until the dial reads zero again, at which point you'll be at proper
thread depth. The cross-slide can still be used to withdraw the tool at the
end of the cut. It is reset to its zero for the next cut.
There are several advantages to this technique. Note that at no point do we
need to concern ourselves with the angle of the compound. Whether you set the
compound to 29, 29.5 or 30 degrees doesn't matter. This technique
automatically compensates for the angle used. Secondly, there is no finish
value to remember for the compound - you go to zero and that's it. And
finally, all the information you need to do this is right there in your hand
on the fishtail gauge that you used to set the tool perpendicular to the work
piece.
Now you (and I) know why those numbers are on the gauge!
==============================================================================
Re our recent May meeting discussion of threading, and specifically, the depth
of thread...
Like most folks who thread on the lathe, I have a fishtail gauge (also called a
center gauge) which I use to ensure that the thread cutting tool is
perpendicular to the work.
On said gauge (and most others that I've seen) is a set of numbers labeled
"double depth of sharp thread". Specifically, the numbers on mine are those
given in columns A (tpi) and B (double depth of sharp thread) in the chart
below. I've always guessed that these numbers were somehow useful in deciding
how much to feed in when cutting a thread but I never took the time to sort out
how to use them. (For me, it's always been easier to draw a picture of the
thread and derive the depth I need using mathematics.)
A couple of questions at the meeting made me decide to puzzle out, once and for
all, what those numbers really are and how to make use of them.
Mathematically, the height of a thread, measured perpendicular to the thread
axis from sharp root to sharp crest is given by the following equation.
h = .5*pitch/tan(30)
where:
h = height of thread
pitch = 1/tpi
The little program I wrote prints out two times 'h' in column C in the table
below. As you can see, 2*h agrees perfectly with the numbers printed on the
fishtail gauge.
So the numbers on the gauge are indeed as described - the "double depth of
sharp thread".
So now the question becomes, "Why are those numbers on the fishtail gauge?"
Those numbers aren't particularly useful when cutting the thread. Most of the
time the question is, "How much do I need to feed in the compound when it is
set to angle 'ca' (compound angle)?" Mathematically, the answer to that
question is:
cin = h/cos(ca)
where:
cin = compound infeed
I've printed out cin for ca=30 deg in column D. For this case, we have:
cin = .5*pitch/[tan(30)*cos(30)] = .5*pitch/sin(30) = .5*pitch/.5 = pitch
and you'll note that the numbers in column D are exactly equal to the pitch of
the thread with tpi as given in column A.
So, the bottom line here is that I still don't know why those numbers are
there. Perhaps an old school machinist can explain how to use them but I
don't see any immediate value to them. (I can't imagine a machinist
multiplying the number in column B by .5/cos(ca) to get the compound feed
depth he needs.) If you didn't angle the compound at all when cutting threads
(i.e., feed straight in with the cross feed) and your crossfeed was calibrated
in diameter reduction (a .001 feed reduces diameter by .001), then the numbers
in column B would be your infeed to cut that thread. But what competent
machinist wouldn't angle the compound?
I don't know the answer but I do know this...I'm going to continue to ignore
the numbers on my fishtail gauge and base my calculations on what I understand.
A B C D
4 0.433 0.433 0.250
5 0.346 0.346 0.200
6 0.289 0.289 0.167
7 0.247 0.247 0.143
8 0.217 0.217 0.125
9 0.192 0.192 0.111
10 0.173 0.173 0.100
11 0.157 0.157 0.091
12 0.144 0.144 0.083
14 0.124 0.124 0.071
16 0.108 0.108 0.062
18 0.096 0.096 0.056
20 0.087 0.087 0.050
22 0.079 0.079 0.045
24 0.072 0.072 0.042
26 0.067 0.067 0.038
