Threads off axis

I have had a problem on a couple of my recent projects. Both involved 1/2"-13 inside threads. Both were cut with a tap on a lathe.
My procedure was to drill the hole on the lathe, and then hold the tap in the tailstock chuck to start threading until the tail stock chuck slipped, then lossen and slide back the tailstock, put a tap wrench on the tap and finish by had. Two out of three times the thread ended up significantly off axis.
I had assumed that I would start the thread on axis with the tailstock chuck and it would stay on axis when I finished by hand.
What is a better procedure? I have cut a few external threads, but am leary to cut internals. I currently have no tools to cut internals threads but I suppose I need some if that is the only way to do this.
CarlBoyd
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Several good ideas in the other posts. You probably know that there are 3 kinds of basic taps: Taper Plug Bottoming.
Taper taps have as many as 6 to 10 tapered threads, Plug taps, 3 to 5 or so and, Bottoming taps, about 1 1/2. Depending on which tap you are using, your tailstock chuck is going to slip sooner, the taper tap being deeper in the hole and therefore straighter before the friction goes 'way up. If you are buying your taps at the hardware store, you are probably getting "plug". I like using the dead center in the tailstock to align with the hole in the end of the tap or in the tap wrench. But, I also have a little device that is a spring loaded "center" for the tailstock or mill spindle that does the same thing while freeing you from having to crank to keep the tap aligned. Also, could it be that your tap(s) are worn out? Causes same problem. Be sure to countersink the hole, too. Try a new GOOD one anyway. I always keep one brand new tap of the most used sizes so I have a way to check if I suspect that the tap I'm using might be dull. (A REAL machinist would probably throw the things away on a regular basis).
Pete Stanaitis -------------------------
Carl Boyd wrote:

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Hi, Carl
Does your tap have a female center at the back? I think you'd be better served, after starting the tap, to put a hard center in the tailstock and use it to align the tap. Turn the tap with a wrench while keeping tailstock pressure on the center. You can start the thread this way also, and save your chuck jaws the beating they take when the tap slips. Put a nice countersink in the hole before starting the tap. I was taught this way when going through the USAF machinist course in (gad!) '69.
You are backing off the tap every half-turn or so, to break the chips, are you not? That greatly eases the job.
If you supply yourself, or make, internal threading tools, you'll enjoy the results. If you doubt your grinding, you can start by screwcutting with a pretty-good tool, and finish with the tap. That'll provide better alignment, too. Have fun, I sorta envy where you are on the learning curve. I had fun learning all this stuff.
Mark

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Mark
Thanks for the advice.
My taps do not have a female center in the back.
I did not countersink, the one that came out straight had a fairly long 1/2 inch counter bore that I am sure helped to keep the tap straight. I'll remember the counter sink idea.
Yes I am backing the, but it seems to cut fine for about 2 turns and then get tight. Thats when I back it off.
I have ground my own external threading tools, the nuts fit the threads fine, I have no idea if they meet spec or not. Grinding for an inside thread seems harder, maybe, I'll give it a try.
CarlBoyd
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    A 1/2-13 without a center hole? That sounds like cheap taps. Smaller taps 10-32 and smaller) may have a male center instead of a female center. I don't think that I have ever seen a 1/2-13 with that, however.
    The spring-loaded center gadget which was mentioned before has a replaceable follower -- one for the female center taps, and one for the male center taps.

    Yes -- the counterbore will certainly help to guide the tap in straight. Starting with a taper (starting) tap will help too.
    Tapping in the lathe tailstock chuck could be helped by using gun taps (spiral point taps), which chase the chips ahead of the tap instead of collecting them in the flutes where they can jam if not cleared by frequent backing. But you can't go all the way to the bottom of a blind hole with those, because the buildup of chips gets too big. Use the gun tap to start the threads, and switch to a plug tap to continue down to the bottom -- or finish up with a bottoming tap if you need maximum thread length.
    I forget whether you mentioned the workpiece material. A thread forming (rolling) tap instead of a thread cutting tap can deal with such holes without generating chips if your workpiece is not too hard -- but it needs a different staring hold diameter. The standard will jam it and break it off in the holes.
    Note that there are releasing tap holders made (designed to mount in a turret on the lathe, but you can adapt them to the tailstock or the carriage). These feed in with the tap until you reach a stop (part of the turret -- if you are doing carriage work, you'll have to set up a bed stop). Once you reach the stop, the tap self-feeds for about another turn after which it disengages a dog clutch in the tapping head, so the tap is free to spin. Then you reverse the spindle and pull back on the tap holder to back the tap out of the hole. This (of course) needs a gun tap or a thread-forming tap, since it offers no provisions for backing out every half turn or so.

    Hmm -- you should back more frequently than that -- before it gets tight -- unless you have a gun tap or a thread forming tap.

    A 1-2" internal thread is a rather tricky one to start with (too small). Learn using something larger first, like a 1" ID. Among other things, the direction to crank the cross-feed to clear for backing out is different from external threading, and it is easy to make a mistake with old habits. And a larger hole lets you see things like how close to the bottom of the hole you are getting. For small holes, run your tool in without cutting (spindle not turning) until it hits the bottom of the hole. Then back it out a little and put some bright-colored tape around the shank lined up with the end of the workpiece to tell you when to disengage the half nuts. Then, start the spindle, crank the tool out (into the workpiece walls) to cut a groove to full thread diameter, and move it slowly by hand towards the bottom of the hole to produce a runout groove at the bottom of the threads, so you don't have to be totally precise as to when you disengage the half-nuts.
    And one particularly important factor in the single-point internal threading on your lathe is what is the minimum spindle speed. Especially when first cutting internal threads, you want it slow enough so that your reflexes are a lot faster than what you need. My 12x24" Clausing will go all the way down to 35 RPM at the slowest belt speed plus back gear. This is probably where you want to be for your first internal threads. Later you can speed up to whatever you are comfortable with.
    Some of the small import lathes have a minimum speed which is frightening when considering internal tapping. :-)
    [ ... ]

    Note that the relief angles will be reversed for internal threading.
    Good Luck,         DoN.
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I am using a Craftsman Tap, I've gotten a lot from the set, although I am sure they are not industrial quality, I don't consider them cheap (i.e. Harbor Freight is cheap). A good set bought new would cost me more than my lathe. Budget has been tight but may loosen up some next year. I picked up 3 Union Butterfield, 3 tap sets last month at reasonable prices. I'll continue to keep my eyes open.

The workpiece material has varied from CRS to an old lawn mower crankshaft.

2 turns is pretty much where it starts to get tight. When just starting I do tend to back off quicker, but when the tap is well started I turn until I feel the resistance increase. A 4 flute 1/2-13 tap has a fair amount of room for chips before they start to bind up. With some taps and some materials I back up every 1/4 turn. Why is 1/2 turn the magic number?

My 16" LeBlond goes pretty slow. I cant remember of the top of my head but I believe it is under 20 RPM

My external tools I ground with symetrical relief angles and pretty much just eye balled them, but I have not done any very coarse theads.

Thanks, I need it.
CarlBoyd
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    Well, yes -- if bought new and all at once. I have a lot of taps which I have bought a few at a time. I tend to prefer gun taps for general use, even when hand tapping.
    But I also got a set of TRW taps -- 0-80 through something like 1"-8 -- all three taps and a corresponding die. Metal boxed, with a molded foam liner which accepted all of the taps and dies and kept them in place. Large -- something like 3' wide by 2' deep IIRC. A ridged rubber liner under the lid which you can use to set taps on and not damage them. The whole thing went for a little over $100.00 -- but it had no photo, and I was just taking the description as true at that price. When I got it, it looked as though none of the taps or dies had ever been used. :-)
    You can also get good taps with matching drills in a special index. I've picked up one of those for common metric sizes, and the taps are TiN coated gun taps so I am quite pleased with them.

    Good luck there. And watch eBay too -- though this was close to ten years ago,a nd things have changed over the years.

    O.K. I would probably go for HRS for roll tapping, unless I were to consult with the tap vendor to pick the right version for the material. 360 brass and aluminums would be my material of choice for roll taps without some research.

    Because beyond that is where chips in some materials curl into a complete circle and can sometimes pretty much lock the tap in place. Backing off to cut the chips shorter increases your chances to keep the taps from breaking off in the wokrpiece. But I don't back at all with gun taps, since they are designed to put the chips out of the danger zone.

    Great! That would be a good choice to start with. Plenty of time to train your reactions.

    O.K. The last personally ground threading tools which I have made were for cutting an ACME thread on a test stub, and then an internal ACME thread in bronze on a replacement nut for a friend's log splitter. There, I wanted the minimum clearance for strength, so I had to calculate the helix angle and grind specific clearance angles on each side -- and I did manage to make my first try for the internal thread with the reverse of the needed angles -- fine for making a left-hand thread, but not for what I needed to make. :-)
BTW    For tapping the end of the shaft -- perhaps you should turn a     guide bushing to slip over the OD of the shaft and provide a     hardened hole to hold the tap centered while you're starting.
    Good Luck,         DoN.
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On Mon, 30 Jun 2008 20:49:33 -0400, "Carl Boyd"

Greetings Carl, I (almost) never have trouble with tapping in the lathe. When tapping the tap is gripped hard enough in the drill chuck so that it doesn't slip until the tap starts to bottom. Of course, large taps or really tough materials may make the tap slip in the chuck. Tap sets often have taps made for hand tapping. These taps will have 4 flutes instead of two or three. And the chips do pack into the flutes. Stringy materials like mild steel can pack up real tight because the chip tends to be continuous. This why the tap is backed off every so often. I generally use spiral point taps or roll taps. The spiral point taps push the chips ahead of the tap and so don't tend to fill the flutes. If you examine the end of a spiral point tap you can see how it will push the chips forward. Also, the tap thread is actually ground with a decreasing pitch diameter at the end of the tap. This makes them cut easier. Hand taps just have the first threads ground away at an angle. Spiral point taps do pack chips into the bottom of the hole though. So blind holes can be a problem. I try to use roll taps whenever I can because they don't make chips and they make better threads. Roll taps require a larger hole because the metal is only displaced during tapping. The holes also need to be closer to the exact size because of the metal being displaced. A slightly oversize hole will result in the minor diameter being too large and a slightly smaller hole will require much more torque and may break the tap. Roll (AKA form) taps work best in ductile materials. This means that wrought aluminum, copper and most of its alloys, mild steel, most stainless steels and zinc are good candidates for roll tapping. Some cast aluminums and some cast irons do not tap well with roll taps. The metal fractures from the displacement and this weakens the threads. The weak thread may not be obvious until it fails under pressure. Cheers, Eric
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<snip>

<snip> I have learned that Craftsman tap and die sets are what's call rethreading sets. The are high carbon steel, not HSS, and it makes a big difference as they last much longer and cut soft-semihard steel, unlike the high carbons which dull very quickly, and is most likely the source of your problem. I have used the same technique of chucking the tap in the tail stock. It generally works great. If the dies are all hex in the set, this is a indication that they are high carbon. HSS will be clearly marked on them if they are.
SteveK
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How about using a Steady Rest to keep the tap on center away from the tail stock. Fixed or 'traveling' rest.
The tap would have to be round or ground round if not.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
Carl Boyd wrote:

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"Martin H. Eastburn" wrote:

^^^^^^^^^^^^^^^^^ Maybe cobble some way to hold a nut to serve as a steady rest. If your steady rest has three arms, maybe they would hold a hex nut on axis.
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On Sat, 28 Jun 2008 22:10:49 -0400, "Carl Boyd"

==========Sometimes a taper tap in a straight hole without a guide can be a problem. It may be helpful to use a taper reamer and then thread. The thread quality will also be higher, and a taper reamed hole is generally needed for dryseal quality threads.
for examples see http://www.hermanscentral.com/product/qual-tech-18-taper-pipe-reamer-spiral-flute-2936.cfm?sid=google http://www.use-enco.com/CGI/INPDFF?PMPAGEg&PARTPG=INLMK32 most mill supplies should stock.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
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There used to be a tool called an "Extension Tap Holder", I believe. I had one in ancient history, but it got lost in moving. It looked like an ordinary Tee-handled tap holder, with a shaft extending coaxially from the tool, (opposite the tap), and it had a bearing: ie, the tap-holder swiveled on the extension shaft. You would put the shaft in a chuck on the tailstock, run the tailstock in until the tap was just taking a bite in the hole in the end of the workpiece, and turn the tap by hand, keeping pressure lightly with the tailstock.
I think that would get your job done right.
I have used the dead-center point in the back of the standard tee-handle tap-holder method, with decent results.
However, these showed up in a Google search.
http://www.patentstorm.us/patents/4818158.html
http://www.vintageprojects.com/machine-shop/tap-holder-attachment.html
And this might just be overkill: http://nbutterfield.com/d.aspx
But this one looks simple and could do the job. I think I'll make one for myself, anyway. http://homemetalshopclub.org/projects/lathetap/lathetap.html
And this guy has taken this idea to a higher level of the craft. http://members.shaw.ca/aldobler / (Scroll down to the last article)
About half-way down this page, "Tailstock Tap Holder" http://www.stellar-international.com/lathe.html#Revolving%20Tailstock%20Drive
Maybe something there will work for you.
Flash

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On Sun, 29 Jun 2008 00:22:17 -0400, "Flash"
<snip>

<snip> ====for current production see ===http://www.use-enco.com/CGI/INPDFF?PMPAGE0&PARTPG=INLMK3&PMITEM18-0150 most mill supplies should have something similar in stock.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
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Carl, Your procedure is fine. I have used the same one hundreds of times and never had a problem. I believe you are starting straight, but your thread is probably not deep enough to prevent applied side trust to tilt the tap. It is ultimately the applied side thrust that is causing the problem. Once the thread is adequately started (4-5 threads), removing the work to a vise and using a T wrench will go a long way to control side thrust. Pay careful atention to the side yhrust and you will be fine. Steve

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Drills aren't necessarily going to bore straight, best to use an internal boring bar after starting the hole with a smaller drill. Tap shanks are hard, the chuck jaws aren't going to hold the shank and spinning the tap in the jaws isn't going to improve them. I use a T- handle tap wrench with an accurately centered center hole for doing this sort of thing in the lathe, if the tap is a larger size, it will have center holes and I use a dogbone. In either case, I use a dead center in the tailstock to align the tap. I lock the headstock while doing this and it's all manual, not under power. I go about 1/4 to half a turn before backing out and breaking the chip off. Getting in a hurry leads to busted taps and more work.
If you need to use these screw holes for location, don't do it! As I've said before, threaded fasteners are to be used in tension and only for fastening parts together. Design in a feature, ledge, collar, whatever, to locate your two parts or drill, ream and insert dowel pins to do the location. Then it doesn't matter so much how much your tapped holes are off. If you just gotta have threaded holes accurately located, then you've got to bore and single-point thread in the lathe. The only way you get good at it is to do it! Chuck up some scrap and practice! If you've got a threaded lathe spindle, make an external copy of the nose for practice, then make an internal mate, these come in handy. You can go down in size as you get better at it. If you don't have a threading gauge, get one, they're cheap. About the only way you can grind and set up tool bits.
The South Bend Lathe book shows a set of threading stops that used to be available. I made a set of these and the time spent making and learning to use them was well worth it, particularly for internal threading. I also made a set of boaring bars and bits, for the smaller sizes I ground 1/4" Tantung bits, these last a long time without sharpening and puts off the time when you've got to stop and sharpen in the middle of a hole. Learning to pick up threads again in the middle of a job is another skill, internals are more fun than externals.
Stan
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Others have given good advice, so I'll only add a few hints.
The tailstock on my lathe droops from student abuse. When it's properly centered front-rear it turns a parallel cylinder and the droop isn't evident. Check yours. I think the tendency was to manufacture them on the low side of tolerance so the buyer could shim it higher if he noticed.
I use a very short Collis center-drill holder to start a hole because a chuck is off too far. A chucked drill bit jumps upwards noticeably when it starts into the hole, although they usually drill true within 0.005" or better. They sell tap holders which are like square Morse collets.
The female center hole in a chuck arbor will guide a pointed-end tap fairly well because it doesn't extend out as far as a chuck. A female cup center works better if you can find one. Sometimes closing the chuck jaws enough to bear on the tap's conical end guides it straighter than chucking it. I grind shallow flats in the shank end of the threads for a wrench.
Make the part long and bore a straight guide section that the tap barely bites into, or make a slip-on guide. I make my custom tooling for standard drill rod sizes rather than specific for a job. 0.500" and 1.000" have worked well so far.
Tap from the wrong side with a *sharp* spiral-point tap and good tapping fluid. The exit should be straighter and tighter than the entrance. If the tap starts way off you can bore the hole out a little.
A long nut / extension / pulley tap shows any angular error in the chuck and then flexes back parallel to minimize it, assuming the hole is straight. Spiral point nut taps are the most generally useful if you have to buy those expensive things new.
Use a hand-held angle grinder to rough out an internal threading bit quickly. They cut fast and cool and are a lot safer than trying this on a chop saw. Make the shank recess at least as long as the width of your bench grinding wheel. If you cut even a rough partial thread with it the tap will cut much easier. Internal threading goes easier if you swing the compound back behind the ways so it feeds directly into the cut. If at all possible cut a good-sized groove to end the thread in. A good, easy practice part is threaded bushings to hold screws straight and centered on the threads. Mine are the length of a 5C collet's gripping surface and recessed so a 1/2" long screw will protrude a little.
What you did works fine on my mill but not on my lathe.
Jim Wilkins
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Thanks for all the good advice. I went out to the shop and looked at what I had. The tap I was using was a plug tap, It does not have a center hole, but I found I have a couple of old Tap wrenches in the back of the box that have center holes in the back.
I have been on the lookout for quality taps and have picked up a few UB 3 tap sets, but not in this size yet.
Thanks again.
CarlBoyd
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...

I have quite a few of those and except for one Starrett they hold taps crooked.
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I bought three brand new Starrett tap wrenhces about 10 years ago. NONE of them would hold a tap straight. They appeared to have switched from machined to investment cast jaws. I sent them all back & they replaced the jaws with ones that were a bit better, but not exactly perfect. I was more than a bit disappointed, especially given the prices.
Doug White
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