Threading Left and Right Hand

Wheels17

Wheels17

Active User
H-M Supporter - Silver Member
I fabricated a firewood carrier for my brother's tractor that uses 5/8" bolts. As a joke, and to improve my novice skills and understanding, I'm making a couple of left and right hand double start 5/8" bolts and nuts to "improve" the carrier.

As I went to cut the left hand thread, which has to start within 1/4" of the 3-jaw chuck for a complicated set of reasons, I started thinking about the compound angle. For right hand threads, threading toward the chuck, I swing the crank end of the compound away from the chuck 29 1/2 degrees from perpendicular, so I'm advancing the compound into the thread in the feed direction. For internal threads, the compound is swung 90 degrees counterclockwise from that position, and again advanced in the feed direction. Conventional wisdom as I understand it.

For the left hand thread, I could not swing the compound to advance in the feed direction due to chuck clearance, so I left it in the right hand external thread position and fed in as usual. It seemed to work fine, and the threads look good. After juggling three wires and a micrometer, they also seem consistent in three places over the length of the thread. (While writing this, I realized I might have had the clearance by swinging the compound to the internal threading position, with the work over the compound.)

Thinking about the geometry of the cutting, in the normal case, the cutting force is taken in the direction of the compound screw, and in my left hand case, it is at right angles to the compound screw, against the gib side of the dovetail. So I suppose there's a chance that the compound could float in the screw direction due to backlash, but I didn't see any evidence of that occurring. I had the gib fairly snug for the process.

I'd be interested in any others thoughts/experiences with this issue. It sure is a lot less nerve-wracking threading away from the chuck for the left hand threads. (My lathe has a threaded spindle, so reverse isn't an option.)



Something that I've only ever seen on one site really helped my understanding of what marks to use on the threading dial. On my South Bend, at least, the marks are not arbitrary, they are inches. Move from 1 to 2, the saddle has moved 1 inch if the lead screw is not turning. This really helped me figure out what marks to use for what thread, by figuring out when the exact repeats of each thread would occur, particularly when I was making double start threads
 
Hi...

I never quite understood all the fuss about 29-1/2 degrees for thread cutting -but then again, I'm not a professional machinist. I'm interested in hearing other people's opinions on this as well. Anyhow, I always leave the compound at 0 degrees, use a carefully ground bit with a 60 degree point and call it a day. The only time the threads come out damaged is when I screw-up and engage at the wrong number on the chaser or, disengage too late or soon. Wherever possible, I usually follow-up with a die and the threads look beautiful. -And sometimes I cheat too... When I'm threading the end of an expensive piece of stock, I often cut the threads only 2/3 deep for the length of the desired thread and cut about full depth for the leading 1/4" or so. Then I follow-up with a die. Starting the threads on the lathe just ensures they go on straight. Even though the incidence of totally screwing up is pretty darn low, when I'm working on an expensive shaft, I take the safe route and minimize lathe time.

Anyhow, did you make a custom nut for the double-threaded shaft? I've never done a double thread and would like to try that or possibly a triple-thread.

Ray
 
You don't have to use the 291/2 degree setting to cut threads. Many guys do as Ray suggested and have the compound set to zero. I set my compound to 291/2 out of habit for threading jobs, but did test out the zero setting and feed in with the crosslide. It does work fine, but I did notice the threads were rougher than usual. It was nothing that a few spring passes couldn't take care of with generous amount of cutting oil.

Cutting threads up close to a shoulder, or chuck can be a little tense. Having a screw on chuck shouldn't stop you from running in reverse, providing your taking very light cuts. My previous machine had a screw on chuck, and I did many theads in reverse without mishap. Though when threading and running in reverse, you better get the half nut closed dead on your mark, or risk having the tool bit dig in, and you know what may happen next.

I usually thread around 220- 250 rpm anyway
So as long as the DOC is light, you can do it safely. 291/2 using the compound, or set at zero and feed in with the crosslide, both ways work fine.
 
I'm no machinist either, I use the 29.5° method cause that's how I was taught & is what I'm used to. I tried the 0° method a couple of times & on aluminum I could not really tell a difference so I just keep doing it the way I do. I thread Ti 6-4 quite often, more than I do aluminum, & with Ti I do notice a difference. There seems to be a lot more strain on the tool when I tried feeding straight in & I have to take smaller cuts which eats up time. They say the 29.5° method is a lot easier on the cutter & improves cutter life but I have been using the same insert & the same first edge for two years now & it's still sharp (but I'm just a hobby machinist). It's the strain put on the tool when threading SS & Ti are the reasons why I don't feed straight in.

I was also taught that it doesn't have to be 29.5°, you could feed in at less of angle but the idea of the modified flank method is to avoid cutting more on both sides.


Wheels17 said:
For right hand threads, threading toward the chuck, I swing the crank end of the compound away from the chuck 29 1/2 degrees from perpendicular, so I'm advancing the compound into the thread in the feed direction. For internal threads, the compound is swung 90 degrees counterclockwise from that position, and again advanced in the feed direction. Conventional wisdom as I understand it.
Click to expand...

When I cut internal threads, my compound is set exactly the same way as if I was cutting external threads. But my internal threader is mounted upside down & cuts on the backside of the bore. It's much easier this way for me & I like that I can see at least something when I'm cutting internal threads. :))
 
@RayC Haven't made the nuts yet. The first one is just set up in the 3-jaw, drilled for boring, but not yet bored. Had part of the daughter's boyfriend's garage roof blow off in a big windstorm, that kept me busy. The double thread wasn't too bad to deal with. It's a 5/8-11, which means the actual thread is a 5/8-5 1/2, done twice to the 5/8-11 depth. I started by putting a sharpie in one of the toolholders for the QCTP and drawing threads on a piece of stock. When you mess up, dykem cleaner takes the bad threads right off. One starts on an even number on the dial, one on the odd.

@8ntsane Hadn't thought about threading on the back of the bore. I like that. You're brave!! I use back gear, slowest speed, about 58 rpm based on the charts and that makes me nervous. I suppose for small threads it's not too bad to go faster. I haven't made anything under 1/2-16. My internal tool is home-made out of O-1 rod
 
Wheels,

The whole 29-1/2º degree thing is total nonsense and all it accomplishes is to cause a lot of concern and confusion. For whatever thread you cut, just set the compound on 30º and feed the tool in with the compound until the thread is nearly complete, then finish it up by feeding the tool in with the cross slide. Your dial will be reading direct diameter this way and it'll be easy to know exactly how much material needs to be removed for the finish cut. Doing it that way also turns your threading tool into a form tool and produces a thread profile equal to your tool, negating any misalignment of the compound. For higher pitched threads, it's not necessary to use the compound at all, just cut the entire thread with the cross slide. Just remember that when doing it that way you are doubling the amount of tool in contact with the work so reduce your depth of cut accordingly.

Tom
 
I believe that most of the thinking on the compound method is based on old techniques such as are described in the older textbooks like How to Run a Lathe, etc., when HSS was the common threading tool material. The normally positive rake ground on such a tool would yield better results if fed in at the 29.5 or 30° compound angle simply because the trailing edge of the tool would not have the optimum rake angle to cut the right hand flank. With the advent of carbide, many threading tools are neutral in rake, and present virtually the same cutting angle on both flanks. The only difference is the angle developed by the helix, or lead angle of the thread. Generally, that is a minor difference and is of no consequence. In other words, straight in works fine in most cases.

If a person were to grind a HSS threading tool in the same neutral rake as a carbide tool, straight in would also be acceptable, with the operational differences being the same as carbide vs. HSS in most other applications. Given the same DoC, plunging straight in would conceivable present a greater load on the machine, and hence a lighter machine would not perform as well as a heavier, more rigid machine. So it would seem prudent to take lighter cuts if cutting with this method.
 
I am a machinist by trade. I've made several double lead nuts and shafts over the years.Mostly valve stems and nuts, and they are usually left hand acmes. So, if your doing a 5/8 11 double lead, you will actually be cutting two 5 1/2 tpi threads. Im curious as to how you are cutting your second lead? Some ppl set the compound at 180 and dial the pitch over for the second thread. Feeding in on the cross slide only. The old adage is,even thread-any line, odd thread-any numbered line, 1/2 threads- even numbered lines, and 1/4 threads-same line every time. The simple way to do this thread and split them evenly would be to catch an even numbered line then an odd numbered line. Feeding in after cutting both leads. Does this make sense? I would do the bolts first and measure the pitch diameter over wires. On the nuts i would undercut a small counterbore the double depth of the thread, when you lightly touch the counterbore you should be close to your finish depth. You can face the counterbore off at the end. The compound angle dosent really matter. just set it where you can and finish your thread with light cuts on the cross slide. I always take a couple of spring passes at the end. I thread alot but ive never tried to explain it. Hope i havent confused you. Good luck!
 
It's kind of funny when you do something for years without thinking much about it, then try to explain it without making sound difficult, isn't it? The way you describe is the way I've cut multi-start threads all along. Just an offset in -Z- equal to half the pitch for a double start, for example. Just have to watch out for the end of the thread if it's blind and you're working off an indicator. The zero isn't where it was on the first thread.
 
Yes it's funny trying to explain something that's second nature to you. One thing I've noticed on this sight, is that ppl are really concerned about the angle the compound is on. I really never give it a lot of thought. All the lathes in my shop are usually. Close to 30 degrees. With fine pitches, say 16 tpi I just feed in on cross slide. . The most convenient thing about 29 1/2 is the formula for depth. .750/tpi=amount to feed in on compound. When I'm close I feed I'm on cross slide anyways, I like to clean up the back side of my threads. I can usually tell just by appearance . When my threads are close. If you haven't threaded much, one thing to watch for is you'll raise up A burr and the OD of part will increase just debur well and mic OD before you fit to a nut
 
You must log in or register to reply here.
Back
Top