I'm looking at some insert threading tooling to improve my threading on carbon steel, SS and Al. I use the flank method to feed into the cut. Generally cutting 8-24 tpi 60 degree threads, sometimes finer. I'm using self ground HSS and get acceptable results but I want better than that. I want to make this as good as I and my machine can do.
I'd like higher precision of the profile and very important to me is the finish of the cut surface. I'm generally threading to a shoulder and need to stop close but I can run the spindle backwards and use a LH tool on the back side to thread away from the shoulder. I'm thinking that might help the finish since I could run the spindle faster and not fear a crash. I might be wrong as I've never done that.
I'm seeing some inserts that have various chipbreakers and positive rake angles and wonder what effect they'll have on the finish. I want the thread finish clean, shiny and smooth like the outside turning finish I'm getting using coated carbide positive rake inserts.
Any comments will be appreciated.
Hi Henry,
I don't think insert tooling will give you a better finish than your HSS, unless your HSS is dull or the geometry is wrong.
If I may ask, why are you so concerned about finish of the threads? Don't get me wrong, I like a nice finish whenever possible, but for threads destined for a fastening application, you'll never see the surface once assembled. If it's something that comes apart regularly, the finish is gonna get perturbed anyway. If it's a showpiece sort of thing like a fishing reel, OK, I get that .
Anyhoo, one thing to try is when you get close to the final pass, instead of advancing the compound, advance the crossfeed ." at a time. This ensures that you get a light cut on both flanks (if your cutter is well-shaped). Otherwise, the cutter can drag on the trailing flanks instead of cutting.
Regards.
Finegrain
I think the shiny finish is a function of the cutting speed. So it may be a question of your personal capability to chase threads at higher speeds like a cnc would manage. This is likely not possible on small diameter work, and the heat concentration on the tip of the insert at high speed is high enough that it would likely spoil a keen tip on any HSS tooling.
When grinding threading tools manually, I suppose it would be good to try to obtain a very smooth finish on the rake face of the thread tool. This will promote better chip flow and reduce the incidence of built up edge, as will using a good thread cutting oil.
Finish usually degrades with ultra light finish cuts, but you may get better accuracy from one end to the other, using light cuts. The choice is up to you.
I get the best finish by taking the last couple of cuts straight in using the cross-slide, instead of on an angle using the compound slide. This is OK for the tool for light cuts and a spring pass. It makes the thread flanks look the same and makes nailing a pitch diameter mike reading easier (the PD change is 1:1 with the lathe collar reading going straight in). Use good lube of course, and a very nicely honed tool. For light cuts at manual lathe speeds, carbide inserts don't offer much advantage, except time savings for making the tool. Faster surface speed is better, but unless you are cutting camera lens threads or such, you can't turn fast enough during threading on most manual lathes to get into the speed regime where high speed helps the finish. Chipbreakers don't really help surface finish; they break chips. Unless you are highly sophisticated in grinding your tool profile, the top of the tool needs to be flat and on center, with no rake.
I put a long-travel indicator on the on the bed and use it to tell me when to back the tool out and drop the half-nuts. I find it easier to go fast without a relief groove if I watch the hand on an indicator (along with a Sharpie mark on the plunger) instead of watching the thread or, gawd ferbid, numbers flying by on a DRO. My lathe has a clutch, and I use that in combination with the indicator when threading to a shoulder.
I can do left-hand threads just as easily. Just flip the tool and run the spindle backwards, and still thread towards the headstock.
If you want really purdy threads, mill them or grind them! The Myford and Schaublin folks have adapted milling spindles to the cross-slides of their manual lathes and have made gorgeous threads that way. The shop at school used to have a Rube Goldberg Pratt & Whitney benchtop thread miller that was the cat's pajamas. Nowadays, CNC thread milling is common.
Good tips so far.
I've used black thread oil but now nearly always LPS Gold #1 - its great stuff and not stinky or too smoky.
It might be that I'm wanting finishes like what I see made on high speed CNC equipment. Someone mentioned high end fishing reels which might be a good example of what I'd like to turn out. The camera thread comment is on point too. I want that. And, I'm seeing what I believe to be manual machine work that's better than mine.
I know a fastener that is doing its job generally hides the thread so ultimate finish doesn't matter. I guess this is also about me being completely proud of what I show people. And for myself just to know that I took time and care to produce excellence or as near to it as I can.
If I'm wanting something I can't get, that's fine, just so I know I'm doing my best.
It might be that I'm wanting finishes like what I see made on high speed CNC equipment.
Yeah, that's gonna be tough to replicate with a manual lathe. CNC can generate single-point threads at hundreds of SFPM and healthy feeds+DOC, at which point the metal is flowing, and you get that gorgeous shine. Unless you have a Hendey or some other manual lathe with high-speed threading mechanism, it's gonna be sporting trying to back the compound out at the right spot fast enough to not run into the shoulder. You can precut an escape groove, but that detracts from the aesthetics IMO. As for starting on the left, you still have to get into the cut quickly, if you're going for high SFPM and fluid flow.
My best threads cut with my manual lathe have a very clean-cut finish, but that's typically a matte finish that happens at 10's of SFPM and light cuts, not the shiny "blurred" finish you get at 100's of SFPM and healthy cuts.
Regards.
Finegrain
Cutting to a shoulder will place limits on your machining speed. Carbide will not produce a better finish a low speeds.
As 67cuda suggested a full form insert might help some. Correct lube and light finish cut will help also. I seem to remember some company making HSS replaceable inserts as well. The price was reasonable and they were machine ground.
Working with what you have now I would make a few small changes and check the results.
1. Use a diamond wheel and magnifying glass to finish grind your bits.
2. Get a few high end lube samples and do some controlled testing.
3. Get your RPMs up as high as your reflexes will stand.
4. Use as much positive rake as you can get away with and still maintain good enough geometry. (I like a nice chip curl)
5. Try different grades of material. 12L cuts better than . Again, get some drops and do some testing.
6. If you want to be consistent you will need plenty of notes so you can repeat the best results.
You may not get CNC speed and finish but you will improve your results.
Walter A.
Many good comments on this "thread". I get great finishes using HSS on , 12l14 using HSS with very good tool life on a manual machine. For steels I do use black cutting oils as noted, and I really hone my HSS bits quite well on the edges. Positive rake of about 8 degrees with a slight "hollow grind" on all edges and the rake to aid the honing - including th rake, which I have seen far too often ignored in the honing process (most of you may think this silly that I mention this, but it is something I really have noticed over the years). Probably a .002 to .003 nose radius.
One very important trick is centering as mentioned. I "touch off" the tool to set my zeros on the OD and if a very slight "hair width" chip does NOT appear on touch off, then I know that somewhere in the process I will encounter a slightly high condition on the front flank (I use the 29 degree infeed method from the compound, or 60 and 1/2 degrees off of parallel to the part C/L) or wear and pressure on the tip will eventually become a problem as thread tools are extremely sensitive to centering.
When I am at the high limit end of the pitch diameter using a pitch mic, I do- as noted above, make one or two cuts using the cross slide of about .001 DOC, checking with the pitch mic each time. This adds the finish to both sides of the thread and works well. Maybe even a "dwell" cut at the finish depth - kind of like spark out on a surface grinder. When one thinks about it, feeding a thread tool direct from the crossfeed is like using a form tool, where a bit of dwell will help clean up a cut. Being you can't directly dwell a thread tool, the last "non depth" pass acts as this dwell.
One idea for all, determining deoth of threading when using the compound rest. A math formula I found years ago that gets me to the high limit or even close to the Basic (split limit) of the pitch diameter is taking .708 and dividing it by the number of threads per inch you are cutting (or .708 x pitch of thread). For a 1/2-20 thread, this come out to feeding the compound rest in . or 35 and 1/2 thousandths. Works 99% of the time, the source was Machine Tool Technology by Kibbe et.al. Then the .001 DOC finish passes - but checking always with the pitch mic (or three wires if so inclined)
Never forget a lube when cutting, even when threadding brass (I have used canola oil for brass with great results, but get this off your machine and part pronto, it gets sticky fast).
I have watched a few videos and believe I understand the basics of threading on a lathe.
My Grizzly mini-lathe has a threading dial and change gears so I'm pretty sure I'm ready to try to actually cut some threads on it. I understand the difference between watching a video and actually making chips and I fully expect to deal with a learning curve.
My question, however, is about my Seneca Star 30 lathe. It has a threading chart on an embossed plate and what appears to be a full set of change gears and I get how all that works. What has me scratching my head is the fact that there is no threading dial on the apron. This leaves me with three questions. I suspect I know the answers as unhappy as that makes me:
1. Is there a way to cut threads without a threading dial? (I suspect not.)
2. Is it likely that the lathe came with a threading dial and it has been removed at some point? (I suspect so.)
3. If I'm correct about Number 2, what are the chances that I can find a threading dial that will work on my lathe? (Slim and none would be my suspicion.)
1.) Please post some picts, that'll give those of us who don't know a Seneca Star 30 from our Logan 210 a chance to help.
2.) I have spotted folks doing up the body, shaft, and the dial face via 3D Printing.
3.) Then all we need to do is figure out the Spur gear.
4.) Perhaps someone here has one that we can reference.
5.) and we need to find out what they look like. Shouldn't be too hard to come up with a replacement for you.
(famous last words.)
6.) May need to start another thread with a title something like "Threading Dial for Seneca Star 30". . .
1. Is there a way to cut threads without a threading dial?
10. Position your tool for the start of the first cut.
20. With the lathe running (slowly), engage the half nut.
30. when you've threaded far enough, turn off the lathe
40. Back the tool out.
50. Reverse the lathe, let the tool run back beyond the start.
60.Turn off the lathe
65, engage forward.
70. Run the tool in just a bit more.
80. goto 20
Repeat until you've reached depth of cut.
I haven't written code for 18 years. it feels good.
10. Position your tool for the start of the first cut.
20. With the lathe running (slowly), engage the half nut.
30. when you've threaded far enough, turn off the lathe
40. Back the tool out.
50. Reverse the lathe, let the tool run back beyond the start.
60.Turn off the lathe
65, engage forward.
70. Run the tool in just a bit more.
80. goto 20
Repeat until you've reached depth of cut.
I haven't written code for 18 years. it feels good.
The implication of line 80 is that the half nut has been disengaged. Once the threading has been started , the half nut should not be disengaged until the thread is complete. Basically, the procedure is that followed for metric threading on an Imperial lathe or vice versa.
Thanks for replies!
I'd seen that restoration but had forgotten that he built a thread dial for it. At the moment (and for the foreseeable future) 3D printing and gear hobbing is beyond my skill set and equipment availability.
I will get pictures of the overall lathe, apron and lead screw later this morning.
How does one tell if the lead screw is Imperial or metric? I
assume it's Imperial, but you know how that works. I don't have an Acme pitch gauge if such a thing even exists.
You can tell from the number of threads per inch. An Imperial lead screw will have a nice round number of threads per inch; 8, 12, or 16. If it's metricthere will be a number of threads plus a fraction in an inch. Not having used a metric lathe, I don't know what the common pitches for their lead screws are but most likely some multiple of .5mm.