Quick change gear box - 27 tpi

I'm looking to buy a South Bend. I haven't found one yet, but I have noticed that a number of lathes with the quick change tool box don't have a setting for 27 tpi. As I do a lot of work with lighting, and the standard lighting conponent thread is 27 tpi I'm going to be needing this gear ratio.

Would it be a realitivly easy thing to rejiger one of these gear boxes to get me to 27 tpi, or should I hold out for one that gets there stock?

B

As far as I know, the only SB quick-change lathe that came equipped to cut 27 tpi is the 10L ("Heavy Ten") lathe with 1" collet capacity and "wide range" gear box. All other quick-change models require an additional gear, which varies by model. My advice is to look for a 10L that already has 27 tpi capability. Very good machines.

Paula

"Would it be a realitivly easy thing to rejiger one of these gear boxes to get me to 27 tpi, or should I hold out for one that gets there stock?"

This is a standard application for change gears, even if you have a quick-change box.

For my Logan, 27 tpi is achieved by using a 16 T stud gear, a 54 T screw gear, and a 72 T idler gear.

In actual practice, the idler gear can be any teeth count which mechanically fits.

P.S. some "toolmakers" (AKA, "toolroom") lathes, a Monarch 10EE being a very good example, have a lot more pitches than "engine" lathes.

P.P.S. Boston Bear Works makes 16 DP 14-1/2 PA change gears in every teeth count from 20 to 128, 127 included, but these have a 3/4" bore and must be bushed to 5/8" or whatever your lathe requires. For my Monarch 10EEs, 3/4" is just dandy as that is the dia. of the so-called "end" gear (really, it is the first gear in the gear train), and no bushing is required.

P.P.P.S. Download a copy of Boston Gear Works change gear catalog Here . The Change Gears download link is the eighth one.

if you post this question on the yahoo southbend list and mention that Logan does it with the gears mentioned, chances are that you may get a reply from scott logan who reviews that board fairly often, and he could tell you how to do it on your SB. Logan is still actively in business and they may also be able to sell you the gear to get it done if you would rather go that route than BG. he is a very congenial and very helpful person. thanks to peterH for mentioning the logan solution.

otherwize, there are other change gear solutions that could also work. you might want to see what combinations your gearbox would do ( like a 54 tpi or a 13.5) that would come out to a relatively easy math job to use a different stud or other gear. its seldom that elementary lathes with QC gearboxes can ONLY do what is listed on the box, usually there is a workaround for things like this even if its using another sized stud gear. be glad we can use these methods, the more complicated enclosed machines permit somewhat fewer mods like this.

OTOH, IIRC the 1/8 american pipe die is 27 tpi, you could possibly ( hand)grind one of these to the proper output size ( remove a small part of the teeth in the tightest taper area) so that you can make running threaded sections that maintain wall thickness and correct thread profile. not the most elegant solution but it works and certainly is an easy setup if you are making stem sections and parts. the same concept goes for using the pipe die - cut down the tapered section teeth to leave the part that threads to the proper size and thread profile for internal threading on finials, nuts, etc. and bingo-- you have a 27 tpi straight die.

Threading is all about ratios ... the ratio of the spindle to the (usually) 8 tpi leadscrew.

The change gears and the quick-change box simplify this to some degree.

Since 16 is not a gear which is easy to obtain, 32 would work, but the screw gear would then have to be 108.

Simply doubling the teeth count of both gears maintains the required ratio for 27 tpi.

With the quick-change in the middle of all this, the 108 requirement could be met by other means.

Most quick-change boxes are set up for English units, and metric units presents a greater challenge, as the fine pitches, the so-callled model makers' pitches, including a few of the conventional pitches, are required to be a multiple of 0.05mm, whereas the coarse pitches, the so-called conventional pitches, which include a few of the model makers' pitches, are required to be a multiple of 0.25mm.

Logan's reverse and change gear web page is Here.

Logan's 16 T change gear is its P/N LP-1036.

Check with Scott Logan as to the DP, PA, bore and width. I'd guess it would be 16 DP, 14-1/2 PA, 5/8" bore and 7/16" width.

Incredibly, although Logan Actuator (the company's present name) makes no new machines, almost every service part is still available, and some complete accessories are still being made to this day.

In one of my 10L parts manuals there is a page entitled "How to cut 27 threads per inch" (or something to this effect). As I recall it just requires an auxiliary pinion which may still be available from LeBlond.

I'm away from my shop on travel, but can post a scan when I get back (if nobody beats me to it).

Hi There,

I have a sheet from SBL (Form 968, Section 3, date: 4-5-65) that covers cutting 27 tpi. It states:

It is possible to cut 27 threads per inch on any South Bend Lathe by procuring an additional gear. Our present 9" and Light Ten Models B & C Lathes and 10"-1" Collet Lathes with wide range gear box are already equipped to cut 27 threads. For other South Bend Lathes see list below:

Click to expand...

And in the chart, it lists for 9" and 10K Model A, Double Tumbler.....Part No: PT32K42NK1. In addition, it has the proper set-up for this gear in the gear train. For the 9",10K and Heavy Ten (without the wide range gear box) the stud gear is the 20 tooth gear and the gearbox input gear on the gearbox would be the above mentioned 42 tooth gear. The gear box tumblers are set for C-2 (left tumbler set on "C" and right tumbler set in the second position).

Personally, I would have thought of using a 54 tooth gear instead of the standard 56 tooth gear on the input to the gear box and then set the tumblers to 28 tpi.

As a point of referrence, some 10R's (small spindle Heavy Tens) came with the wide range gear boxes. I have two of them at present (built in 1953 as I remember).

Good Luck!
-Blue Chips-
Webb