Thread: cutting 27 tpi on a hercus

What Model lathe,I am guessing its an A.

Yep. Model A, 8TPI leadscrew.

Do you have Transposing gears for cutting Metric threads.

Yes. I do.
I've got the 100/127, 26, 28, 32, 36, 40, 44 and 48 change gears.

You should be able to use spindle driver 20 with leadscrew follower 27 and lathe selector gearbox set to 20 tpi to get 27 tpi.
Not sure about the Hercuse gearbox ratios as I have a Sheraton.

The set on the banjo would require an intermediate gear, any size is OK.

Note that Martin Cleeve in his book, Screwcutting in the Lathe (Workshop Practice Series # 3), makes the point that perfection is not an engineering necessity - the best economical workable compromise is all that is required.
26 tpi or 28 tpi for the length required might be OK.

I have 20 and 27 gears.
Send a PM if you would like a loan.

John.

For what you are trying to cut,a Metric .9mm pitch thread should work without much trouble.
There is only about .001" difference in the pitches over an inch.
Stud 36, 127/100, 54, Gear train, C/3 Gearbox.

I acquired a 42 tooth gear for this purpose
20 stud, 80 idler, 42 on gear box, C2 selected gear box (36tpi) worked for me.

Vernon provided a really useful gear calculator
(I get on better with version 1 in this post) and that also says
20 stud, 80 idler, 54 on gearbox with B8 selected (28 tpi)

If you can compound some of your existing gears:
20 stud, 36/32, 56 gearbox - B6 (24 tpi) looks like it works
but setting up the compound is a new challenge as the gear bore is smaller for the stud and gear box gears than for the idler position

This is the gear ratios for the Hercus 9" Model A I calculated for use with version 1 of Vernon's program.
Attachment 150014

According to South Bend 9A info, to get 27tpi they supplied an optional gear with 42 teeth.

Jordan

Originally Posted by nadroj

According to South Bend 9A info, to get 27tpi they supplied an optional gear with 42 teeth.

Jordan

That's what I think I'd read somewhere and was looking for.

Thanks for the help guys.
The thread is not critical as it's only holding a gooseneck and light so I think I'll try the 28tpi first and see how I go. Failing that the 0.9mm metric pith sounds like it's worth a try.

HavinaGo:
I'm still trying to get my head around that gear calculator but I got one from somewhere for the SouthBend. Interestingly based on Pipeclays suggestion of using the metric 0.9mm pitch (27.21429 tpi) I did some playing around and found a 20 stud, 127/100, 26 gear train and C/5 gives me 27.12357 tpi which is even closer.

Pipeclay: Are you still making changegears? It sounds like having a 42 tooth gear might be handy. I'll send you a PM if you are.

Yes.

Originally Posted by wannabe

The thread is not critical as it's only holding a gooseneck and light so I think I'll try the 28tpi first and see how I go.

If it's for a light fitting, British Standard Brass threads were often used in that application. They are 26tpi across the size range. 27tpi is only used for Gas thread.

Jordan

Microphone thread is listed as 27TPI must of been a printing error.

There is a "thread" on this 27TPI "thread" here:

27 tpi - Practical Machinist - Largest Manufacturing Technology Forum on the Web

Originally Posted by pipeclay

Microphone thread is listed as 27TPI must of been a printing error.

You got me there. Well spotted, Pipeclay.
Must be the microphone makers use Gas thread!

Jordan