Those numbers on a fishtail gauge

[mklotz]

On my fishtail (center?) gauge are printed some numbers labeled "double depth
of sharp thread" for various thread pitches.

I did some math and verified that these numbers are indeed twice the thread
depth, measured perpendicular to the thread axis, for the indicated pitches.

My question is...

What use are these numbers? Since most machinists angle the compound when
cutting threads a number related to feeding straight in seems less useful than
say the compound infeed for some standard compound angle (e.g. 29 deg).

I've never seen a fishtail gauge without these numbers so I have to presume
that they're there for a good reason and 'real' machinists have some use for
them. Perhaps some 'real' machinist can explain?

[lynnl]

Well Marv, I'm no "real machinist", and won't presume to have an authoritative answer. But I've always assumed they were included on the guage simply to save me from having to do the calculation or consult a table.
But in practice I've usually already done the calculation before I ever get around to picking up that gauge, or even thought about those numbers being there.

ps ...as I think more about it, I guess I have found the info handy, to use as a doublecheck against my calculations.

[J Tiers]

The double depth is what you would read on the crosslide dial at proper depth........ if you plunge cut as many apparently still do and more probably used to do.

But as for use, now that sharp threads are a dim memory as far as standards are concerned, the number isn't likely that much use.

[Forrest Addy]

I'm a real machinist but I never use the "double depth to sharp point" for the reason there's no such animal as a sharp point thread and few machinists chase threads by plunging straight in. The Unified Thread and the ISO thread we all use since WW II both have a rounded root not interfering with the creat of the mating thread as the standard thread root form.

I became a student of Unified Screw Threads in early 1973 when a local crisis developed in the shop where I worked. I was appointed to teach a series of classes on the topic. Since then always truncated the tool point for external threads to p/5 (and for internal p/10) as a personally selected hedge to comply with H28 intent to avoid root and crest interferance.

I was taught as an apprentice to "cut it til it looks like a thread", then try the gage, thread mike, wires, what ever.

I did make up a little chart, back in the old days, that showed the compound infeed at 30 degrees for p/5 (external) P/10 (internal) and the tool tip width for external and internal threads for common TPI 32 to 8. The old guys always sharpened their threading tools to a point and stoned it "just a little" across the tip so it would hold up. That "little bit" was seldom enough as the optical comparator showed time and again.

I taught my apprentices and later my students to follow the standards using my self-selected tool tip rules. They made good threads in compliance with the standards.

[mklotz]

Ok, that's four (three plus mine) votes for the fact that those numbers are
pretty much worthless in the world of modern threadcutting.

We've had angled compounds and truncated root/tip threads since at least WWII,
maybe earlier. So why do the makers of fishtail gauges keep putting those
numbers on there? Are they some sort of secret sign of the Ancient Order of
Freemachinists?

[John Stevenson]

It's because the Chinese just copy everything without query

And Forrest, if you had a problem with Unified threads back in '73 you should have got them to revert back to the Whitworth thread.
Funny how the first Standard employed turned out to be the one with the strongest thread all those years ago and still is.
Marvellous bloke that Mr Whitworth.

.

[JCHannum]

Just to add to the fun, I have a Starrett fishtail that gives double depth of American National thread and a Lufkin that has double depth of Sharp Thread. The numbers are different of course.

Next question is why are they called Center Gages?

[John Foster]

Jim, because they can be used to check the 60° point on a lathe center. John

[IOWOLF]

Because you can put it between work and thread tool to see if you are on center,like a 6" scale.

Getting my flame retardant clothes on now.

[japcas]

I use the numbers on the center finder quite frequently. When I'm running a cnc lathe and programming the part where a thread is at, I can simply look at the center finder, find the double depth of thread that I am programming for, divide it by 2 and then I'll add .010 to .020 to it for the chamfer at the start of the thread. Maybe not the best or only use for it, but it's a quick reference.