SB 6K collet threads

[piper184]

Hi everyone,

I'm getting ready to attempt making a hand wheel style collet drawbar for the 6K collets. I have some with really nice threads on them and have the dimensions listed in the SB literature.

I know that they are 26 TPI and the major diameter is listed as .767". Also from looking closely it appears the thread angle is 55 degrees, not 60.

Does anyone know if SB used any kind of thread "standard" for the collets or was it a proprietary SB thread?

It appears that British Standard Brass (SBS) is really close for 3/4" with the major diameter is .750". Thread depth of SBS seems to be uniform at .0246" at any diameter.

I don't have any thread measuring wires so my plan is to make some test threads in some cheap pipe. I've ground an internal thread tool to 55 degrees and am ready to start testing but I don't have a starting ID for the bore. I'm thinking Major diameter minus 2X thread depth (.767-.0492 = .7178).

Am I missing anything here?

TIA,
Stuart

 

[Illinoyance]

Hardinge shows the thread as .762-26. Since South Bend was a US company, I would be very surprised if the thread was other than 60*.

 

[piper184]

That's the part I can't figure out. Looking at the South Bend catalog they specifically list the Thread Diameter at .767" and 26 TPI, which makes it not anything standard. They show the thread length as 21/32".

They also used the 26 TPI on the number 3 collets for the 9" lathe but had a thread diameter of .642" and thread length of 3/4".

Since the collets are not a standard thread and the catalog doesn't list thread angle or minor diameter, I started looking for the minor diameter to bore for internal threads.

I can't find any thread charts that show a 26 TPI for anything close to 3/4" diameter except for the British Standard Brass. Here is an excerpt from Britishfasteners.com:

BRASS THREADS (British Standard Brass) Because brass tubing has a uniform wall thickness, irrespective of the tube diameter, any thread cut on, (or in) it, would have to have the same depth, so 26 tpi is standard on all diameters. An example would be the thread cut on the brass trumpet horns of the 30s cars, although many were die-cast rather than brass. If it were 1" O/D, the thread would be 1" Brass, 26tpi.

BSB has a 26 TPI pitch through the whole range and is often confused with BS Cycle thread the difference being that brass thread has an angle of 55 degrees whereas Cycle is 60 degrees. Brass threads are usually confined to fittings and adjusters on cables. Brass thread taps and dies are sold and advertised as 26 tpi and are identified by the letters BSB.

Seeing this made me curious and looking really close shows that my 60 degree threading tools did not fit the collet threads properly. I re-ground one of my tools to 55 degrees and now it fits the collet threads perfectly.

The thread chart from the website above lists all diameters as having a thread depth of .0246" and lists tapping drill sizes for what one might consider "normal" sizes. 3/4" is the closest but at .750" major diameter a 45/64" (.7031) bore would be too small for the .767" listed by SB.

Starting with a bore of .07178" (or as close as I could get) I was able to put threads into some soft iron schedule 40 pipe. I was able to get them close enough that the collets will thread into the pipe. I had to keep going past the initial bore size and the surface finish was terrible so the engagement was not as smooth as I would like.

The plan is to keep practicing on water pipe until I can make nice threads then give it a try in better quality tube that will be used for the actual hand closer. This way I can keep trying larger and larger starting bore sizes until I find the one that works best. I was hoping to find the specs somewhere to flatten the learning curve a little.

 

[piper184]

This project got postponed for several weeks while other items replaced it in the priority list. Blizzard, tractor repairs, shop furnace died, etc...
In the end a new ceiling mount furnace means the shop now has floor space available for a mill. (Yay!)

Several more practice runs with 1/2" black pipe and internal threading was getting consistent results. Time for the real thing. Borrowing from several other's designs I managed to make something that works for me. The collar that contacts the spindle has a small counter bore so that it slides over the end of the spindle and does not bear on the taper of the spindle. It is a close fit but not tight. The outside is just small enough to not rub on the gear cover. There is a 7/8" needle thrust bearing between the handle and the collar. In hindsight, I could have made the collar so the bearing was recessed and out of sight. Oh well.

One of the examples I used for inspiration said that the threaded end of the tube had a larger outside diameter than the rest of the tube, just like the SB original. I couldn't see that feature in any literature but thought it a good idea for strength under the threads. As it turns out, it does serve as a retainer for the collar as it is a sliding fit at the handle end but it captured by the larger diameter and can't come off without removing the handle.

The handle was turned from a chunk of 1" thick 3" bar that was in the scrap bin. Nothing special or specific about it. Bored it out to be a press fit on the tube and used some 648 retaining compound. I don't see it coming apart without some heat and a press. I was worried that I might not be able to turn the small 3" handle hard enough to lock the collets but it works just fine so far.

A small note about the collet threads. The SB catalog lists the major diameter at 0.767. My small collection of used collets has South Bend collets marked either with South Bend or <-R-S-B-< and two collets by Hardinge. Some are well worn and had various amounts of rust on them especially in the threads. Some even have had encounters with grinders (Ugh!). After cleanup all the SB ones mic'd out at pretty close to 0.762 on the outside of the threads. Looking closely SB left a short shoulder at the collet end of the threads that is full major diameter. I cut out the first couple of threads in the drawbar to accommodate it although it might not matter as the collets probably never get threaded in that far anyway.

The two Hardinge collets were the in the nicest condition and showed the least wear/use of all. However they were the farthest "out of spec" with one being 0.764 and the other at 0.753 major diameters. I didn't want to make the drawbar so loose that the larger one would fit so I used a needle file and carefully made the collet fit the drawbar. It works now but I will have to be carefull with the smaller one and not over torque it and strip the drawbar. Also the Hardinge ones have a recess cut at the end of the threads instead of the shoulder that SB used.