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Joined
Mar 28, 2004
Location
Cedar Rapids, IA, USA
I have several thread gauges and they all have a 34 TPI leaf. I'm a real observant fellow and after 6 or 7 years of owning this lathe I finally noticed it does not have a 34 TPI selection on the quick change. I have 32 TPI and 36 TPI but no 34 TPI. Of course if I do want 34 TPI that also happens to be metric 0.75 which I can get though a gear change.
Now the question is why does my lathe skip 34 TPI? Is that a normal situation on other lathes? Mine is an ENCO 12x36 gear head. So this is more or less a survey question to see if other lathes have one or more selections missing from the quick change gear box. FYI, on mine that is the only one missing from the "standard" thread series.

Richard
 
My English Colchester Chipmaster also does not have 34 tpi (or 17 or 68). That's no biggie. What is sometimes a pain is that it doesn't have 27 tpi (for small pipe threads & straight lamp fitting threads) or 11.5 tpi (also a pipe pitch). I can get pretty close to 27 tpi with one of the uncharted metric pitches, but 11.5 is not possible without a change gear, like a 69-tooth. By the way, this lathe only approximates metric pitches by using a 55:65 compound gear.
 
Richard,

That's the way the cookie crumbles. On a quick change one can't have EVERY gear combination - just not practical. Matter of fact, there's probably a number of thread pitches that you'll never use. One other thing to notice: Look down the rows of pitches on the quick change chart - they double each time.
 
Hello Mike, Yes, I agree putting in all the pitches would be a near impossible. So what do I do with 152 TPI? I've got a whole row of selections I wouldn't know what to do with. Threads so fine it's hard to tell from a roughing cut.
I think this is a situation where the designers never used a lathe in real life. Same situation as for not having common pipe threads or the lamp socket threads. At least this lathe has all that.

Richard
 
"By the way, this lathe only approximates metric pitches by using a 55:65 compound gear."

Wow, that seems pretty far from the actual ratio, 100/127=.7874 rounded to 4 places.

55/65=.8462

That's almost a 1/16" difference length in an inch of threads, unless I'm missing something (not unusual)

51/65=.7846
55/70=.7857, even closer.

There may be an even closer "more common" ratio, but these are using at least one of the gears that apparently the manufacturer found expedient to make anyway.

smt



[This message has been edited by stephen thomas (edited 07-18-2004).]
 
Richard,

On most lathes the gear box also serves to input motion to the carriage. The pitch selection that you don't use may serve a carriage feed rate that might be useful. Remember, each gear in the quick change gear box provides a number of feed rates or thread pitches, of which only some of them may be useful or practical. Look down the row on the 152 one and see if there are some you might use - especially the feed rates.
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[This message has been edited by Mike Burdick (edited 07-18-2004).]
 
The number plate on a quick change lathe's gear box is the numerical array of the permutations and combinations of rates of advance provided by two or three gear transmissions running in compound with one aother.

Some of them happen to be useful thread cutting pitches. Most of the finer ones are only practical as feed rates.

The 9 inch South Bend quick change box has a fine pitch of 224 TPI. Nobody ever cuts that fine of a thread on that kind of lathe. That setting gives .0015 in. per rev. and that is a very useful feed rate on the small diameter delicate work that lathe is commonly used for.

Most lathes don't have the power or the mechanical strength to pull the 2 pitch or the 4 pitch setting, those gears are there so that the machine can cut the 8 pitch treads.

You get the 2 and 4 for free, yeah right, like you are going to use them!
 
I used to run a doggy old lathe for a guy who made material hoes fittings for sandblaster equipment. 11 1/2 threads for 2 NPT did not appear on the QC box. Tradition was to cut 'em 11 TPI and make them a little large so they'd barely engage. The material was soft and easily deformed when the hose fitting was screwed into the tank boss.

34 TPI is an offball thread pitch that does not appear in the NIST handbook for standard and preferred pitches. There might be a use for it in a differential screw instrument movement (34/32 for example) where very small increments of movement are desired from a full turn of the screw.
 
Yes, the variable lead threads are truly a bit of a special case.

We have a couple of turning jobs that use multiple start variable lead threads as a type of ballscrew.

Many CNCs will do variable lead, in fact I was only able to find one that did not do it, but that's a topic I wont dive into yet, still too close to going off on a wild tyrade .
 
Stephen,
Regarding the 55/65 compound gear approximation for metric threading on my Colchester lathe:
1.5 * 55/65 = 1.26923
whereas the ideal combination is
127/100 = 1.27000

So the error is
100% * (1.26923-1.27000)/1.27000 = -0.0606%

That translates to an error of 0.0006" per inch of threads. That's fine for most lathe thread work. I'm sure most fansteners have much worse lead errors allowed.
 
Of course the fine threads are also used for feed. Forgot that point-- well made. The 34 TPI is interesting from the point that thread gauges have that leaf even as Forest says it is not a standard. Anyway it just happens to be the same as a metric 0.75 which is used a lot in optical parts. Since that's what I do that's the reason I keep bumping into this type thread. The good news is today I timed how long it takes me to change gears on the lathe to make it metric. I spent a whole 8 minutes on the job! That's start to finish including putting stuff away. So it's not that bad a deal. BTW, the lathe does cut good metric threads. They fit perfectly even over a 1/2" of threads which is way too much but I did it for a check.

I've also tackled the 11 1/2 TPI and other pitches not available on the QC. Long ago I determined that there are metric pitches that are either right or so close they pass inspection. I made a chart. The best way is to test a US gauge against a metric and find the right pitch that way. It's quick and sure.

Richard
 








 
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