How to Cut 27 TPI Threads On Lathe ?

[Degull]

Hi everyone, I am a newbie. I have a Standard Modern 1120 lathe with quick change feed box. 27 TPI is not listed on the feed box chart, how would you cut 27 TPI?

 

[Dan from Oakland]

You probably have 54 TPI. Check your end gearing from the spindle to the input of your quick change box and see if you can swap things around to use the 54TPI selection and rearrange the end gearing to double the ratio- example- if you had a 30 tooth gear driving a 60 tooth, swap their position to double the ratio.

 

[Limy Sami]

Is this manual any help? http://www.standardmodernlathes.com/docs/manuals/standard-modern-1120-1334-manual.pdf

or this one http://www.standardmodernlathes.com/docs/manuals/standard-modern-utilathe-11-13-inch-manual.pdf

 

[randyc]

..... a 30 tooth gear driving a 60 tooth, swap their position to double the ratio.

Wouldn't that actually quadruple the ratio ? 30:60 = 1/2 and 60:30 = 2, 2 div by .5 = 4

 

[Forrest Addy]

27 pitch? Unless you have a suitable change gear for the index you're pretty much up a creek. 27 pitch is used almost exclusively for small tapered pipe threads, some mike stands, and the straight brass pipe thread used in lighting fixtures. If the engagement is short and the thread is a set used like in a light fixture where mechanical strength is of little consequence you can use 28 threads per inch and bore or thread for clearance. It won;t fit right but it will do the job.

Pipe threads have to seal but since 1/8 27 threads engage but 3/16 to 1/4" and the material is ductile, you can cut the one thread 28 pitch and count on one time assembly, pipe dope and local deformation of the weaker material to make the seal.

These are hold-your-nose-and-get-it-done expedients no careful craftsman likes to perpetrate but if it gets the job done...Back in my South Bend days, I've resorted to them a few dozen times on grease fittings, gland flush connections, and the like with no call-backs. Needless to say my present lathe cuts 27 pitch.

 

[L Vanice]

Hi everyone, I am a newbie. I have a Standard Modern 1120 lathe with quick change feed box. 27 TPI is not listed on the feed box chart, how would you cut 27 TPI?

I suggest a tap and die. The 1/16 and 1/8 pipe are easily found. Other sizes are more or less special and I usually look first at Victor.
High Speed Steel Special Pitch Taps - many hard to find sizes and threads
High Speed Steel Special Pitch Round Dies

Larry

 

[Dan from Oakland]

Randyc- you are correct- BUT it would work if you had a 108 TPI position on the QC box!!! good catch- thanks for pointing out my dumb error. Suppose "engage brain before you let out the clutch" applies here.

To Forrest- a mans got to do what a mans got to do! and the pitch is fine enough that most would not notice anyway!

Dan

 

[EPAIII]

How would I cut 27 TPI? I have a 8 TPI lead screw and I do not have one of those tremendously limiting quick change gear boxes so I can cut many odd threads easily. In this case I would use a 16 tooth gear on the stud and a 54 tooth gear on the lead screw. I would probably need an idler gear between them. That does it.

For your lathe it would depend on the details. First I would look at the QC gear box and see what threads it does have. 54 TPI, which is two times 27 TPI, would be a good start. You would only need to speed up the lead screw by 2X. For that you would look at the gearing between the spindle and the QC gear box and see what is there. You would need to change one of the gears for one that has either twice as many or half as many teeth, depending on weather it is on the driver side or the driven side of the chain.

Cutting threads is all about prime numbers. If you don't have 54 TPI, then look for a thread with as many of the prime numbers in 27 as possible. 27 = 3 x 3 x 3 so you are looking for threes. Threads like 9, 18, 36, 45, 63, 72, 81 TPI would all have two threes. 54 TPI has three threes and 81 has four. Here are some fractions you could use with various threads:

9 TPI x 3 = 27 TPI
18 TPI x 3/2 = 27 TPI
36 TPI x 3/4 = 27 TPI
45 TPI x 3/5 = 27 TPI
54 TPI x 1/2 = 27 TPI
63 TPI x 3/7 = 27 TPI
72 TPI x 3/8 = 27 TPI
81 TPI x 1/3 = 27 TPI

The fractions in each line would indicate the CHANGE needed in the gear train leading to the QC gear box.

Another way to go would be for whole number fractions of 27 TPI: 13.5 and 6.75. These are not very likely on your QC gear box.

If you could post a photo of the threads available on the QC gear box and a CLEAR one of the FULL gear train between the spindle and that gear box, then we could probably figure out what gears you could change. Count and post the number of teeth on those gears. Also give us a list of any additional gears that you have with the lathe.

All that being said, 27 is one of those odd values that is often problematic for some lathes. This is because standard, English threads do not include 27 TPI or multiples of it. Early lathes usually had gears in sets that were multiples of 4 or 5 teeth: 16, 20, 24, 28, etc. or 15, 20, 25, 30, 35, etc. In either of these sequences you do not get three threes until you reach 4 x 27 = 108 teeth or 5 x 27 = 135 teeth. Normally the supplied gears sets did not go that high so the lathes could not easily cut 27 TPI. So the sequence of English threads was based on the gearing available on the available lathes of the day and did not include 27 TPI. Why 27 TPI was later selected for pipe threads, I do not know. Perhaps some manufacturer wanted a proprietary thread that others could not easily duplicate. If so, they succeeded.

 

[SeaMoss]

it would work if you had a 108 TPI position on the QC box!!!

Gotcha

driver/driven x driver/driven ahem

 

[Degull]

Here are some pics of my feed box chart and gears

 

[EPAIII]

You have 9, 18, and 36 TPI. That may be a start. Do you have any extra gears that match the ones between the spindle and the QC gear box? And what are the tooth counts on the gears in the photo?

 

[iwananew10K]

damn hard to count from here but if those are dual 48 tooth gears then it would seem you could sneak a 64 tooth gear into the lower position and use the 36tpi setting IF your banjo will let you do it...looks like it swings off the upper 48 tooth sliding gear??

My head hurts now so I might be all screwed up.

 

[SeaMoss]

damn hard to count from here but if those are dual 48 tooth gears then it would seem you could sneak a 64 tooth gear into the lower position and use the 36tpi setting IF your banjo will let you do it...looks like it swings off the upper 48 tooth sliding gear??

Barber-Colman 10,000 Four-Gear Ratios... if ya got the big bucks, Pfauter has a version that must be ten million four-gear ratios, it's about six inches thick.

My head hurts now so I might be all screwed up.

Set up a Cleveland some time

 

[billmac]

If you have a set of loose gears and a good Norton gearbox, then you probably just need to do the sums to select the correct combination of loose gears to be placed on the banjo, in conjunction with the gearbox.

My Smart & Brown 1024 is like this - the manual provides the simple formula, and you solve it for the available Integer gear tooth counts (about 26 in the standard set) and the available tpi in the 40 speed Norton gearbox (mostly integer apart from a few fractional tpi). Because you are not solving for Real numbers, you mostly need to use a suck it and see approach - try some munbers and see what errors you get - keep trying until you get a good combination, which may well use compound gearing. I wrote a little program to help me with a recent job which required internal screwcutting for a Thury -5 thread. No - I had never heard of this before and the pitch was 1.69 mm. I got a reasonable solution and cut the thread (Imperial lathe).

 

[HuFlungDung]

27 pitch? Unless you have a suitable change gear for the index you're pretty much up a creek. 27 pitch is used almost exclusively for small tapered pipe threads, some mike stands, and the straight brass pipe thread used in lighting fixtures. If the engagement is short and the thread is a set used like in a light fixture where mechanical strength is of little consequence you can use 28 threads per inch and bore or thread for clearance. It won;t fit right but it will do the job.

Pipe threads have to seal but since 1/8 27 threads engage but 3/16 to 1/4" and the material is ductile, you can cut the one thread 28 pitch and count on one time assembly, pipe dope and local deformation of the weaker material to make the seal.

These are hold-your-nose-and-get-it-done expedients no careful craftsman likes to perpetrate but if it gets the job done...Back in my South Bend days, I've resorted to them a few dozen times on grease fittings, gland flush connections, and the like with no call-backs. Needless to say my present lathe cuts 27 pitch.

My Summit lathes actually came with an extra gear or two to cut 27 tpi. But be damned if I'm gonna disturb all those end gears just to cut a $1 thread I cut 28 almost to finish size, and then run a die onto it.

 

[iwananew10K]

assuming I can count to 48(questionable)
A 46 tooth gear on the gearbox input will get you DARN close form the 26tpi position,..27.12 tpi and would certainly fit.

 

[Cole2534]

27 pitch lead is 0.0370", while 28 is 0.0357. Just back your compound out .0013" gradually through the thread and you're golden!

 

[SteveBausch]

The chart indicates 9 TPI, just a matter of slowing the leadscrew to 1/3 of the rotation (for 9 TPI).

Don’t change anything, yet. You need to confirm the quadrant is correct; cut some threads, a light scoring is adequate. Does the result match the expectation?

If yes, then open those storage areas and see if any extra quadrant gears are stowed.

If no gears are found, and the gear quadrant has adequate adjustments, and you can justify the cost, contact Standard Modern and see what is offered, if they agree it’s possible, etc.

 

[JohnEvans]

This got me curious ,so went and looked at my 4 lathes. 2 will cut 27 per inch in the QC box. 18"ENCO and 5914 Clausing with taper attachment.Heavy 10 and 12" eastern import won't.

 

[awake]

Barber-Colman 10,000 Four-Gear Ratios... if ya got the big bucks, Pfauter has a version that must be ten million four-gear ratios, it's about six inches thick.

My Smart & Brown 1024 is like this - the manual provides the simple formula, and you solve it for the available Integer gear tooth counts (about 26 in the standard set) and the available tpi in the 40 speed Norton gearbox (mostly integer apart from a few fractional tpi). Because you are not solving for Real numbers, you mostly need to use a suck it and see approach - try some munbers and see what errors you get - keep trying until you get a good combination, which may well use compound gearing. I wrote a little program to help me with a recent job which required internal screwcutting for a Thury -5 thread. No - I had never heard of this before and the pitch was 1.69 mm. I got a reasonable solution and cut the thread (Imperial lathe).

Some years ago I wrote a program that lets you input the gears available (one time - it saves this info for future uses), select what pitch you want (including tpi, mm, dp, and module), and it will spit out the best combinations with the least possible error. If anybody is interested, I'm happy to make it available as freeware ...