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Thread: cutting 27 tpi on a hercus
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12th October 2010, 06:50 PM #1Senior Member
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cutting 27 tpi on a hercus
Does anyone know how I can cut a 5/8 27tpi thread using an imperial Hercus. I'm sure I read a document once on how to do this but I can't remember where I saw it and no amount of searching is yielding anything.
The thread is to match the thread on a gooseneck of a microphone boom which I am using to position the light on my mill.Peter
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12th October 2010, 07:21 PM #2GOLD MEMBER
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What Model lathe,I am guessing its an A.
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12th October 2010, 07:26 PM #3Senior Member
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Yep. Model A, 8TPI leadscrew.
Peter
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12th October 2010, 07:47 PM #4GOLD MEMBER
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Do you have Transposing gears for cutting Metric threads.
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12th October 2010, 08:05 PM #5Senior Member
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Yes. I do.
I've got the 100/127, 26, 28, 32, 36, 40, 44 and 48 change gears.Peter
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12th October 2010, 11:51 PM #6Senior Member
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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.
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13th October 2010, 02:12 AM #7GOLD MEMBER
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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.
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13th October 2010, 11:43 AM #8
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 150014cheers
David
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13th October 2010, 12:41 PM #9Mechanical Butcher
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According to South Bend 9A info, to get 27tpi they supplied an optional gear with 42 teeth.
Jordan
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14th October 2010, 12:25 PM #10Senior Member
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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.Peter
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14th October 2010, 12:37 PM #11GOLD MEMBER
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Yes.
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14th October 2010, 08:36 PM #12Mechanical Butcher
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14th October 2010, 08:45 PM #13GOLD MEMBER
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Microphone thread is listed as 27TPI must of been a printing error.
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14th October 2010, 09:03 PM #14Cba
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There is a "thread" on this 27TPI "thread" here:
27 tpi - Practical Machinist - Largest Manufacturing Technology Forum on the Web
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14th October 2010, 10:04 PM #15Mechanical Butcher
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