Why 27 TPI?

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SteveM
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Why 27 TPI?

Post by SteveM »

I know that microphone stands have the odd pitch of 5/8x27. Never thought much of it and bought the tap so that I could make things to go on microphone stands.

I'm sorting thru my dad's taps and there are quite a few other sizes of taps that are also 27 tpi.

Is there any history to why there would be so many in 27tpi?

There are a lot of taps in different sizes in 32 and 40 tpi, but I know that some of those are model engineering sizes.

Steve
spro
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Re: Why 27 TPI?

Post by spro »

Brass fuel fittings come to mind.
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NP317
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Re: Why 27 TPI?

Post by NP317 »

Old British motorcycles and turn of the 20th Century Locomobile engines seem to have many 24 tpi threads of different diameters.
Our shop gang figured their lathes were set up for 24 tpi (no quick-change gear boxes then) so they just made everything 24 tpi!
That's our story and we're sticking to it.
~RN
SteveM
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Re: Why 27 TPI?

Post by SteveM »

NP317 wrote: Fri Sep 21, 2018 12:29 amOur shop gang figured their lathes were set up for 24 tpi (no quick-change gear boxes then) so they just made everything 24 tpi!
The thing is, with a standard American QC box, you CAN'T do 27tpi. My Atlas can't, nor can a South Bend.

A chart for a South Bend change gear lathe shows that you can do 27.

Steve
earlgo
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Re: Why 27 TPI?

Post by earlgo »

Recall that 1/8 NPT is 27tpi and perhaps the early microphones were mounted on a pedestal that required the wire to pass thru the stand. Maybe this is where the 27tpi is held over from.
Just a theory.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
ChipMaker4130
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Re: Why 27 TPI?

Post by ChipMaker4130 »

SteveM wrote: Fri Sep 21, 2018 8:07 am. . .The thing is, with a standard American QC box, you CAN'T do 27tpi. . .
Just depends on the level of lathe. My mid-70's era Sheldon does 27, among a great many (59) other pitches.
On the other hand, I have to use change gears to cut metric threads.
Mr Ron
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Re: Why 27 TPI?

Post by Mr Ron »

I have a Sheldon 11" S series made around 1944-45. Great lathe and great condition. Didn't look like it got much use. It came off a Liberty ship. I paid $850 for it, fully tooled with chucks, taper, metric, etc. I don't think I would trade it for any other lathe, except maybe for a Monarch. I bought it in San Francisco and it now resides in my shop in Mississippi. There are few things I have bought that I feel were good buys, but the Sheldon is one of them.
Mr.Ron from South Mississippi
SteveM
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Re: Why 27 TPI?

Post by SteveM »

The 11" Sheldon is on the top of my list for lathes, as it has several advantages over a South Bend heavy 10 (more clearance and more travel), but I've only found one and it got away.

Steve
John Evans
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Re: Why 27 TPI?

Post by John Evans »

My 5914 Clausing does 27 along with the rest of the odd pipe threads and with the taper attachment pipe threads no problemo! My 18" Enco also has 27 TPI in the gear box.
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wlw-19958
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Re: Why 27 TPI?

Post by wlw-19958 »

Hi There,

South Bend Lathe did make an optional QC Gearbox (referred
to as the wide range gearbox) that had a 27tpi setting. These
were available for the Heavy Ten and larger lathes.

The SBL Parts Manual does list a replacement stud gear that will
allow the regular gearbox to cut 27tpi. The number of teeth
depends on the size of the lathe and which type of gearbox it has
(single tumble or double tumbler).


Good Luck!
-Blue Chips-
Webb
Conrad_R_Hoffman
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Re: Why 27 TPI?

Post by Conrad_R_Hoffman »

Yet another reason I like my plain change gear Logan, 27 is no problem.
Conrad

1947 Logan 211 Lathe, Grizzly G1006 mill/drill, Clausing DP,
Boyar-Schultz 612H surface grinder, Sunnen hone, import
bandsaw, lots of measurement stuff, cutters, clutter & stuff.


"May the root sum of the squares of the Forces be with you."
SteveM
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Re: Why 27 TPI?

Post by SteveM »

Conrad_R_Hoffman wrote: Wed Sep 26, 2018 3:39 pm Yet another reason I like my plain change gear Logan, 27 is no problem.
Touche!

Steve
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