HELP: Single Point Thread Cutting

I hope somebody has the patience to read through this :-) I am trying to include all the details because I don't know what I am doing wrong.

I recently tried to cut a 3/8-40 thread on a piece of 1018 HRS. First pass was smooth. With later passes the surface seemed to get rough,
then before reaching final depth small sections of the thread tore out... so I tried again:
I ground a new 60 degree HSS threading tool with 12 deg. relief on both sides and 10 degrees side and back rake, and stoned the top and sides
smooth. It is mounted directly in a turret type tool post, (i.e. the tool bit is horizontal, not sloped upward as in a rocker type tool holder).
I turned a 0.500 diameter section on a short bar. It is held in a three jaw chuck, (no tailstock support).
I set up the threading tool on center height and perpendicular to the surface.
I set up the QC gearbox for 40 TPI, spindle for slowest speed (about 40 RPM), compound slide at 30 deg.
First pass was just barely grazing the surface to ensure that there were no surprises (i.e. mistakes in my setup), zeroed the collars.
I started by feeding in the compound 0.002", reducing progressively to 0.0005" for the last pass.
I measured the OD occasionally. It increased by 0.001" after about three passes, i.e. it appears that a "burr" is being raised - is this
normal? I didn't expect it. I touched up the OD with a file before proceeding.
I seem to be reaching the final thread form before I have cut as much as I thought I needed from the Machinery's handbook, (0.01534" deep, 0.0177" on the compound at 30 degrees).
Finally on one pass I got a rather rough appearance to the thread at the right hand end, although the rest of the thread still appears to be
smooth.
In case it's relevant I am doing this on a 10" Atlas lathe.
1) What can I do to reduce/eliminate the "burring"
2) Any ideas why I ge the rough thread, especially at the right end?
Rob
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1018 HRS, that is your problem! Greg
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I agree. 1018 hrs is terrible to thread. But there are a few things to try. You state that you turn a section to .500 dia. I assume you meant to say that you turned a section to .375 for a length of .500. If so, try going a little smaller. The book says you can go to a minimum of .3691. Unless this is a very important part, I would go to about .365 dia.. This will give you a flat on top of your thread which should help with the burr. Also use a small triangular file to just kiss the burr. With the lathe running, hold the file so that it is about 45 degrees to one side of the thread and let it ride down the thread. Then hold it on the other side and let it ride to the left again. Do this several times. Then, make one more pass with the threading tool set at the final dimension that you used before you used the file. This should clean up any burr you created with the file. You did not mention cutting oil. I like the old fashion sulfur oil. Good luck

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Also make sure the tool height is on center line.

to
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Checked and double checked, it's as close as I can get it, and I'm pretty picky about that.
Rob
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Sorry finished part will be 3/8-40, I went to a test piece after the trouble started.

I'll keep this in mind. I am trying to get as close to the "nominal" thread form and size as possible.

I'll try that.
Thanks Chief,
Rob
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Can you expand upon that? What do you suggest?
Rob
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The lower carbon steels all cut and thread poorly. Higher carbon, to a point, will cut cleaner, too high and it gets hard on tooling. Most CRS cuts better than HRS. Best yet is leaded steel. The stuff cuts and threads like butter, compared to plain 1018 HRS. Then again, choice of material depends on the application. Greg
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Thanks Greg. I wasn't aware that the higher carbon steels would cut cleaner.
Rob
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I had a similar problem turning threads on similar material- I gave up on trying to get a smooth finish on the lathe, and switched to "burnishing" on a wire wheel. Probably rank heresy but it did clean up the burr pretty well. I was doing 1/4-20, that 40tpi might not burnish so well...
Gregm
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writes:

Thanks Greg,
This one needs to be close fitting and smooth running so that wouldn't do here. Besides, this is a point of honor now :-) I want to figure out what I am doing wrong and make a good thread.
Rob
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    Hmm ... the needs specified above, plus the pitch makes it sound as though you are making a micrometer thimble of some sort or other. If you don't need the wear features of a hardenable steel (where you ideally cut the threads oversized, harden, and then grind the threads), and if you don't need to weld it, I would strongly suggest that you contact one of the online metal vendors and get some 12L14. Get more than you need -- you'll find other projects for it, and you'll love working with it.
    Good Luck,         DoN.
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    1018 may not be the best choice -- and depending on where you got it, it could be *very* bad. Home Depot "steel" is rather poor for machining in any case. Very gummy, which could contribute to your problems. And if it is of poor consistency, tearouts are more likely. (See also the cutting oil comments later.)

    I don't see you mentioning *any* lubricant while cutting the threads. I use lubricant even when threading 12L14 (about the nicest threading steel that I have worked). I usually use one of three lubricants, depending on the degree of difficult expected:
1)    The high-sulfur pipe threading oil sold by almost everyone,     including Home Depot, packaged by Rigid (among others).
2)    Molly-Dee (light oil with molybdenum disulfide).
3)    Sul-Flo -- *very* high sulfur cutting oil -- best applied with     an acid brush. It has so much flowers of sulfur in it that it     looks more like mustard. It stinks when cutting, but you     produce beautiful threads.
    Note that I usually use carbide insert lay-down threading tools, typically TiN coated for the larger sizes.

    O.K. Though ideally, there should be more relief on the left-hand side than on the right-hand side to accommodate the helix angle of the thread.

    O.K.
    0.500" for a 3/8-40 thread? Or is this just a test pass with a larger diameter before going to the final diameter?

    Good.
    Hmm ... there are debates about the angle, but the ideal according to some is 29-1/2 degrees -- so it does a light cut on the right-hand flank with each pass, along with the heavy cut on the left-hand flank.
    O.K. Did you turn a runout groove at final thread depth, so you don't have to wind the cross-slide out very quickly (and at precisely the same point each time)?

    O.K.
    Hmm ... for 16 TPI, I start out with 0.010" per pass, and reduce to perhaps 0.002" for the last pass or two. I also make two or three repeats at the final depth.

    I touch up the OD with a file after *finishing* the thread. No point until then.

    Hmm ... my threads program suggest the following for 40 TPI:
=====================================================================izalco:dnichols 23:27 > threads -i 40
For a thread of: 40.0000 TPI the pitch is: 0.0250"/thread
| Thread Style Format | Sharp-V | Trunc | Formed ==================================================================== Single depth: | 0.0217" | 0.0189" | 0.0135" Double depth: | 0.0433" | 0.0379" | 0.0271" 29.5 deg. angle feed: | 0.0249" | 0.0218" | 0.0155"
Note: Sharp-V is top and bottom sharp. (Not often the best choice.) "Trunc" is top truncated only. "Formed" is both top and bottom truncated or rounded.
Select whichever suits your needs and available tooling. ===================================================================== The pitch calculation is for convenience in using a small CNC lathe which I have. "Single depth" vs "double depth" is because some lathes have handwheels calculated in diameter removed, instead of radius. My program calculates for 29.5 degrees, not 30 degrees, as that is the angle which I use. You seem to be getting shallower cuts than my program suggests for a pointed tool and truncated crests.
    However, one other thing comes to mind -- how tight are the gibs on your lathe? If they are too lose, the compound or the carriage will rock to one side under cutting loads, thus possibly tearing out the thread, even in the presence of adequate lubricant.
    Lock the carriage, and press sideways on the toolpost. See how much it rocks. If it is any preceptable amount, you need to tighten the gibs to prevent this. While it is possible to set the point of the tool over the center of the ways, to minimize the sideways thrust, as you crank the compound, you will exit that sweet spot.
    The forces can also cause the tip of the tool to dip, placing it below the centerline of the lathe.

    O.K. How long is "short" compared to the diameter of the workpiece? If there is more than 4 times the diameter sticking out of the chuck, you will have deflection -- to an amount dependent on the cutting forces involved.
    I would support the end with a live center, if it allowed me to get in close enough to do the threading -- otherwise, with a hardened half-center and proper lubrication. (The half-center is ground on one side, so the tool can get closer to the center of the end of the workpiece.
    An alternative is a follower rest. It provides brass wear points behind and above the workpiece, and travels with the carriage, so it supports directly behind the cut. The positions of the wear points counters the two most likely directions of deflection. It is absolutely necessary when threading a long shaft, as even a tailstock center won't support it in the middle.
    Next -- what shape are the jaws in? A common wear pattern on chuck jaws is to wear at the tips (where short workpieces are clamped) more than at the back. So -- when you clamp something using the full length of the jaw faces, that workpiece will be tightly clamped only at the back, and can deflect somewhat at the front. Check the workpiece for deflection with force to see whether this is happening. A solution, if it is *just* the faces of the jaws, is to set them up under load, and run a toolpost grinder (or even a toolpost mounted Dremel with a grindstone) in a little at a time, until you get the full length of the jaw faces cleaned up.
    However, if the jaws have been tightened too much at the tip, too often, you will have wear in the ways in the chuck, and in the jaws. If this is the case, it *might* be possible to make new jaws with tighter way grooves, but it would be quite difficult -- especially for someone who is at your current stage of machining indicated by your question. So -- a replacement (new) chuck would probably be the best bet. Bison seems to make high quality chucks for reasonable prices. (They are from Poland.) I have a nice 6-1/4" one on my 12x24" Clausing. I suggest getting one with two-piece jaws, so you can replace the top jaws if they ever become worn, or so you can fit "soft" jaws and bore to hold a special project. I don't need that too often, but it is really nice to have when I do need it.

    Another set of thoughts?
1)    What kind of spindle bearings? Earlier ones have bronze     bearings which are adjusted by removing shims and tightening,     and then reaming to the proper size for the spindle.
    Newer ones have ball (or roller) bearings, and until they die,     all that is needed is to keep them well lubricated.
2)    What is the condition of the bed near the chuck? A worn bed     will allow the whole carriage to rock as it approaches the chuck     (usually the most worn area), and this could be the cause of     your problem -- or at least one cause. In particular, with the     square bed which the Atlas line used, you could also have wear     in the width of the bed, so the carriage can move forward and     back with force near the chuck (the most worn area).
    Do you get taper when turning close to the chuck?
    Larger at the free end is likely deflection of the workpiece     (either bending or worn/sprung chuck jaws).
    Larger closer to the chuck is more likely to be bed wear.

    File when you are done. This is normal -- especially with soft gummy steels like you are using. Get some 12L14 to see what heaven is like. Even with 12L14, I file after cutting to clean up the crests.
    Use a proper threading lubricant. (*Not* motor oil -- one of the ones which I mentioned above will do -- though Sul-Flo is quite difficult to get in small quantities. A club member who *really* wanted some bought a minimum quantity, and then sold it by the gallon to other club members to share the costs. :-)

    At the very end suggests deflection of the workpiece, though it could be play in the gibs of the cross-slide or the compound. Or a combination of them all.
    So -- you now have a checklist of possible causes. Someone else may well think of something which I forgot to mention, so look at whatever other followups appear, too.
    Good Luck,         DoN.
P.S.    Now that I have spent this time typing, I suspect that at least     *one* other followup will have shown up. There were none when I     started typing.
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I was going to put a couple cents in, but just follow what Don said. He covered my 2 cents and the rest of the buck.
Good info, Don.
michael
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Other replies had much good info, but:

You might want to try threading at the highest speed you dare run at. Since this is a very fine thread, you can probably do the threading at several hundred RPM, at least.

Definitely, the burr is normal.

Tool deflection or workpiece deflection is very common in threading operations. So, I'd expect it to take more infeed to get down to final thread diameter. How do you measure it, with wires or triangles? Without these special tools, measiring the true thread diameter is almost impossible.

Right! Does it make a crunching or snapping sound when threading at that end? If so, you may need either a tailstock center or a follow rest to support the workpiece and keep it from climbing up over the threading tool. Long, thin workpieces will try to do this, and you can actually see the part jumping up and down as the threading tool takes bites out of it. Even without the tearing, you'll never get a proper thread when the work is flexing like that.
Jon
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I am turning to a groove at a shoulder. I am afraid that the tool will hit the shoulder if it is too fast. I will experiment with higher speeds though.

I think that the tailstock support is the first thing I am going to try.
Thanks for the suggestions Jon.
Rob
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Rob McDonald wrote:

Does your chuck thread onto the spindle? If it does, is there a way to prevent it coming loose or could you make such? My chuck is held to the spindle with three bolts so I needen't fear it coming off when cutting in reverse. Thus I place my threading tool _behind_ the work and cut with the machine running "backwards" to the usual. The _huge_ advantage of this is that there is zero chance of running into a shoulder. I simply run off the end, retract the cross slide from its pre-set zero, run back beyond the shoulder, hand move to the start, reset the cross slide, advance the compound and take the next cut. I know it _sounds_ complicated but by the time you've done it twice it is faster than running at such slow speeds.
Ted
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    Agreed -- as long as he has an adequate runout groove, so his stopping point is not too critical. The only time that I thread without a runout groove is on the CNC machine, where the tool automatically retracts at the same point each pass. (I've never tried the trick of threading to a hole, turning the spindle by hand the last turn or so. That used to be seen on threads on old machine tools.)
    [ ... ]

    Agreed -- though if he has something portable enough to which he is fitting it, that is quite possibly good enough. You can get thread micrometers (either with permanent anvils for a single short range of pitches, or with replaceable anvils, to cover a much wider range. The same set of anvils works with both inch and metric (both 60 degree threads), though you need special ones for Whitworth threads (55 degrees). As long as you stick with the same brand, you can use the anvils from one micrometer with the other sizes. I've got 0-1" 1-2" and 2-3" Tesa (Brown & Sharp) thread micrometers sharing one set of anvils. (All obtained in different auctions from eBay.) But they are expensive for only a single thread.
    The over-wires is the most accurate, as I can see the thread mic's blade anvil deforming the thread if you don't use care. But thread wires are awkward to handle -- though I have seen fixtures for them to hold them to the micrometer anvil and spindle.
    The only case when you can get the pitch diameter correct by measuring the diameter over the crests is when adjusting a Geometric style die head, as the thread form is complete in those.
    [ ... ]

    Yep! He said "short" but he didn't (yet) say *how* short relative to the diameter. The fact that he is still having problems when going up from 3/8" to 1/2" suggests that it is either too long, or that the chuck jaws need truing.
    Enjoy,         DoN.
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snipped-for-privacy@d-and-d.com (DoN. Nichols)

Don Agreed -- as long as he has an adequate runout groove, so his

Some of the old, old threads did stop on a radial hole. Done by hand, not under power, by pulling on the flat belts, I presume.
For today's lathe I think the higher speed may produce a better finish--I think of screw cutting in the back gears (the slowest way) more for larger threads.
If you cut to a groove, then grind your tool blank so as to use only the left half of the end of the blank, instead of grinding the vee centered on the full blank. This will allow the thread you form to get further up to the shoulder of the work.
If you reduce the right end of the work to the minor diameter of the thread also, this will give you a telltale as to when your toolpoint has reached that diameter. Frank Morrison
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snipped-for-privacy@d-and-d.com (DoN. Nichols) wrote in wrote:

I got it from a local steel supplier (Elma Steel) it could have come from anywhere.

I used a cutting lubricant I had on and, but I doubt it's as good as any of the ones you mentioned. It is time to invest in some good cutting and threading lubricants.

Good point, I hadn't thought of allowing for the helix angle.

Test piece after the initial failure.

OK
Yes.
Knowing that I had trouble already, and that my Atlas lathe is not very rigid I wanted to minimize deflection/vibration.

This value: Formed Single depth: 0.0135" corresponds to the value for an internal thread in the handbook. I am using the value specifed for an external thread.

The gibs are tight, so I don't think this is the problem (but I'm going to double check them anyway).

The turret tool holder is quite rigid, and the tool is clamped very short to minimize this.

Nowhere near 4X. Test piece is 1" diameter protruding 1-1/2" from the chuck, the last 1" is turned down to 1/2" diameter with a groove at the shoulder - this simulates the configuration I would be machining on the real part.

I'll try that. The fact that the problem is worst at the end of a cantilevered workpiece makes me think "deflection."

That is on my wish list :-)

This is a definite possibility. The jaws are in lousy shape and I have been considering grinding them as you suggest.

I'll look into this, I don't remember them being loose this way but it is an old chuck. If so would definitely replace rather than repair.

Thanks for the suggestion.

It is a babbit bearing headstock. I must admit I have had concerns here, since it is an older lathe which is known to have spent part of it's life in a low production environment. I lack the skills to confidently determine it's condition.

I will look into this.

Yes, this is what I get, possibly related to the battered chuck.

That was my first choice, but the local supplier doesn't stock it. I may pick up an aassortment next time I'm in the city (I'm a 2 hour drive north of Toronto).

I suspect you are right, possibly several things, none of which are bad enough alone to be problem.

Thank you very much for taking the time DoN. As you say, I now have a list of possible causes to investigate. I have been "improving" this lathe one step at a time as I find problems and learn how to deal with them. This will be the next step.
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