Stripped drawbar on SB - what to do?

I went to use the 3A/3C collet on our 9" South Bend the other day, and was
mystified as to why I was able to keep turning the drawbar handwheel
without the collet tightening. It was pretty easy to figure out: the
threads are stripped out.
So now I'm left wondering how to approach fixing this little problem.
Here are the options I've thought of:
1) Buy another drawbar
2) Buy a commercial tap of the appropriate thread (which I'm assuming is
available at MSC - I haven't looked yet.) Use this tap on a piece of new
stock. Trim the diameter down to a shoulder to allow some relatively thin
tubing to join the new threads to the old bar. Harden? How much? Cut
off the old bar and make a recess for the tubing to fit on and hold them
together. Silver solder or braze.
3) Shorten the existing drawbar (it's about 5/8" too long anyway). Bore
the new end out to so that the commercal tap will fit down far enough.
Tap. Does it need to be annealled first? Hardened afterward? How much
tempering?
4) Keep using the 5C collet chuck and forget about the 3A's and 3C's.
TIA for your comments,
Alden
Reply to
Alden Hackmann
Loading thread data ...
That's the ticket. You'll never regret that you did. Besides, allows you to go up 3/4" stock (almost) through the chuck and spindle as against 1/2". Not to mention square to 3/4", hex to 7/8, inside collets to 6" and pot collets, ditto. Pay the extra bucks and get the adjust-through or "set-through" Bison. Well worth the extra cost.
Boris
Reply to
Boris Beizer
Those draw bars are mild steel, not heat treated.
I would shorten it, if it really is too long.
Then thread the end to take the collets, IIRC the thread is 0.628 - 26 inside of it. The draw bars lead a rough life and get full of chips and junk, the collets are heat treated so the drawbar threads wear, and then finally wear out.
Don't bother with a tap, you'll have a tough time finding one. Just single point the threads, using your machine.
Jim
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Reply to
jim rozen
I'd just cut it a little shorter and put new threads on the drawbar. Then I'd build another just in case. After all, it is a lathe and that is what lathes do!
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works evevery time it is tried!
Reply to
Bob May
"Stripped drawbar" is the same as making a new drawbar, which is what I am trying to do. It was suggested that we simply buy a tap and tap out the drawbar. I just called MSC and they would be happy to get me a .640X26 tap for $104 US . That leaves two options :A- grind up a teensy threading tool and cut the thread on the lathe, or 2- make a one off tap and cut the drawbar with that. (Why are 3Ccollets such an oddball thread anyway?)
option 2 has the appeal of external rather than internal threadcutting, easier by far to my mind, and if I screw up(no pun intended) I can make another one. But what do I make it out of? will a mild steel tap cut a thread in mild steel? or do I have to harden, anneal and grind?
Option A has the advantage of one the cutter is made I can just go ahead and cut the drawbar, but if I mess up the thread, then I have to find another thinwall tube of the right size and start all over again
Comments? which route should I take?
Thanks in advance,
John
Reply to
John Hall
ID thread it, single point. The tool is easy to grind and will be a lot easier than making some kind of tap which will have to be heat treated btw.
If you make a mistake, just silver solder a piece of 3/4 inch CRS stock on the end, turn down the OD, bore it out, and try again.
Jim
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Reply to
jim rozen
That makes sense. Thanks
John
Reply to
John Hall
Because they need to be a fine pitch to keep from cutting too deeply into the collet or the drawbar, and weakening whichever. Look at how deep the threads would be for some standard 5/8" thread (which is at least fairly close to your OD.)
[ ... ]
Well ... I changed spindles on my Clausing 12x24" lathe -- from an original 2-1/4x8 threaded spindle to an L-00 long-taper keyed spindle. This meant that I needed a different closer nosepiece (which I bought -- Royal -- through Scott Logan. But it also meant that I needed a longer drawbar -- by about an inch. What I did was to take some bar stock, turn down a step and thread to fit into the original drawbar (it was a lever-style closing system, and it was not obvious how to separate the mechanism from the tube without damage), and then reverse it (holding it in a 6-jaw chuck to minimize distortion of the thin material), bore it out, and internally single-point thread it to match the collets. This had the advantage that I did not risk damaging the original tube, but kept it intact with the lever-closing mechanism.
Once it was verified that the length was right, I Loctited the extension collar into the original drawbar tube, and have used it happily ever since. (Be sure to test it with *all* of your collets and other attachments. I discovered that it was a bit tight on the Chinese small 3-jaw chuck on a 5C body which I got somewhat later.
Anyway -- this means that you can do the internal threading -- and if something goes wrong, you can cut it shorter, and put in an extension as I have done.
I gather that you can easily remove the handwheel from yours, which will make it easier to set it up to thread internally. I was lucky in that I had to extend, instead of to shorten, mine. :-) (Also lucky that the extension was long enough so I could thread deep enough to accept the worst-case collets. If I had needed to extend it by only 1/2 inch, I would have had problems, and would proably have had to look into how to separate the mechanism from the tube.
Granted -- with 5C collets, you've got more room to do the internal threading, but I could have handled the 3C size as well. Just be sure to cut a generous runout groove to give you room to stop each threading pass.
Good Luck, DoN.
Reply to
DoN. Nichols
I understand that, yet MSC carries 5/8X24 and 5/8X27 taps , so they are presumably more "standard" than .640X26 which just a bit off from them for no benefit that I can see.
John
Reply to
John Hall
And why is a 5C 1.041-24? ^^^
Dan
Reply to
Dan Allen
LOL. The C in 3C that you are talking about was "Cataract" because that was hardinge's brand name at the time - the 26 tpi was standardized well before there *were* any standards. You might really be better off asking, "why do all the taps and dies in the MSC catalog have non-standard thread pitches?"
The 26 tpi threads were there a long, long time ago, and gradually became non-standard over the years.
Jim
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Reply to
jim rozen
Success!
I'm either getting better or luckier! grinding the thread cutting bit out of a piece of 3/16 boring bar was a snap and cutting the 26tpi internal thread was a snap. rather than cut a deep groove to run the tool into, I simply cut a long thread, about an inch and a half (since the draw bar is plenty long ), and pulled the tool out a little earlier each pass. worked like a charm.
John
Reply to
John Hall
Great stuff. Once you do this sort of thing a few times, it becomes a whole bunch easier. All the trepidation goes away.
I should have mentioned, another way to run the thread out in the bore is to set up a dial gage for the carriage travel. Then you rig it so that zero on the gage is where you want to stop the thread, and you simply unlock the half nuts at that point.
Then the threading tool cuts a groove around the bore there, each time.
Jim
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Reply to
jim rozen
AHA!
John
Reply to
John Hall

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