Cut my first single point thread-- questions now...

[jacampb2]

I finally decided it was teach myself how to single point threads. I ordered a little center/threading gauge and read up on the operation and decided to give it a stab.

The thread I cut is 1/2-13, all went fairly well, but here are the questions:

1) How do you know how deep to cut the threads? I just kept test fitting a nut (which didn't fit, more on that later)

2) Do you feed with the cross slide, or compound? I had the compound set at 29.5°, but the way I see it, if I feed in with the compound, then my cutter will not be in the same position as when originally synced up with the lead screw/thread dial. I fed with the cross slide, maybe that's why it didn't come out perfect.

3)I kept test fitting the nut, I got to a point where it would thread on 2-3 threads and bind. I finally decided I was doing something wrong, and in an experiment, removed the piece from the lathe and threaded on a 1/2-13 die. My thought was that I would see where that cut, and be able to determine if it was a pitch error, or depth or what. I ran the die down the first inch or so, and it cut a little, but not much, the pitch was right. I tried to thread the nut on, and it threaded on all the way, past the part I cut with the die, and down to the single point part. It was actually a bit loose on the lathe cut threads, but it did not bind at all. I'm guessing that the unsupported end of the work was deflecting enough in the first inch that I never got the root of the thread deep enough. Does that make sense?

Here are some pictures, you can see the difference in the die and lathe threads in the second picture.





Thanks,
Jason

[vpt]

Looks to me like you don't have a sharp enough point on your threading bit. The bottom of the threads look a little rounded and shallow but they still come to a point at the top of the thread.

From what I have read on most threading topics is it is a good idea to use a livecenter on the tailstock when threading anything longer than 1.5" or so.

[motorworks]

Hi
This has been ask/answered many times, just do a quick search
Here one answer by Spope14

"here are some formulas (or is this formulii in plural) for all of you machinist guys....I gave these to one of my most respected machinist friends, he had not heard of some of these. before you use ANY of these formuli, it is of the absolute importance to have backlash the right direction before cutting.

If you are cutting a relief groove for threading at the end of the shaft, use the formula .613 / n +.0025 (/ means divided by, n is the number of threads per inch) for the depth of the groove from the thread major diameter. This means if you have a 1/2 - 20 thread, and the major is .495, you would cut the groove depth (one side of the part only) .0332 deep. This works on a .001 on the dial equals .002 on the diameter lathe. Double this on a .001 = .001 on the diameter lathe for the relief groove depth plunge.

For the infeed on the compound rest after touch off, use the formula .708 / n. For a 1/2 - 20 thread, this would mean tht you move the compound rest set at 29/61 degrees (depending on your angle setting on your lathe) .0355 total on the compound rest dial. Be sure to take out backlash before cutting or this is a non factor. Also, take a .001 or .002 clean-up pass (.001 on a .001 = .002 on the diameter lathe, .002 on a .001 = .001 lathe). I can hit threads 99% 0f the time with this including almost split limit on the pitch diameter on a pitch mic ot three wire check.

here is a rule of thumb for you. Back to the 1/2 - 20 thread. .500 is the nominal major diameter. However, with a general use tap, the tap will also cut to .500 diameter. To fit a .500 shaft thread in a .500 hole requires a cheater bar. The rule of thumb for thread OD-s for fractions is the nominal diameter (1/2 for the example) - .005 on the diameter. Or, cut the thread major to .495 first before threading. After thredding, hit the thread with 80 grit sandpaper to remove burrs. Machinery's handbook has a 1/2-20 thread at about .497 to .486 diameter. This works on all threads.

3/8-16 UNC - 2B example

Major diameter turned - .370
Relief groove (should you chose this) = .0408
Compound rest infeed = .044 +.001 clean-up (seperate)

I teach these formulii to each student. My student won the VICA turning contest in NH by 80 points (out of 200 possible) by just knowing these and not having to guess on the threads, and wasting time. he had the only perfect thread out of 24 made."
Spope14

[MotorradMike]

Hi Jason:
I've been learning how to cut threads too.
Here's what I've found.

I finally decided it was teach myself how to single point threads. I ordered a little center/threading gauge and read up on the operation and decided to give it a stab.

The thread I cut is 1/2-13, all went fairly well, but here are the questions:

1) How do you know how deep to cut the threads? I just kept test fitting a nut (which didn't fit, more on that later)
You can look up the minor diameter and cut to that but I just do what you did. Have a nut handy and start trying it when the thread looks like it's getting close.

2) Do you feed with the cross slide, or compound? I had the compound set at 29.5°, but the way I see it, if I feed in with the compound, then my cutter will not be in the same position as when originally synced up with the lead screw/thread dial. I fed with the cross slide, maybe that's why it didn't come out perfect.
Everybody says feed with the compound set as you described. The thread stays in sync, you just end up cutting with only one side of the tool instead of the whole edge which tends to chatter. Some say cut the last cuts with the cross slide but very small cuts.

3)I kept test fitting the nut, I got to a point where it would thread on 2-3 threads and bind. I finally decided I was doing something wrong, and in an experiment, removed the piece from the lathe and threaded on a 1/2-13 die. My thought was that I would see where that cut, and be able to determine if it was a pitch error, or depth or what. I ran the die down the first inch or so, and it cut a little, but not much, the pitch was right. I tried to thread the nut on, and it threaded on all the way, past the part I cut with the die, and down to the single point part. It was actually a bit loose on the lathe cut threads, but it did not bind at all. I'm guessing that the unsupported end of the work was deflecting enough in the first inch that I never got the root of the thread deep enough. Does that make sense?
I get that too and think you are right. The threads are usually looser near the chuck.

Here are some pictures, you can see the difference in the die and lathe threads in the second picture.
I couldn't see your pics. 1/2-13 is a pretty solid workpiece, I did one in brass but haven't tried steel.

Mike






Thanks,
Jason

[Machinist-Guide]

Point Threading

Hi Guy's
Here is a link to some videos on point threading. These videos were made by a retired machine shop teacher. He does a good job on his videos and I think they would be helpful to you guys.

Threading Videos

[Black_Moons]

A big problem with test fiting is burrs, if you get a burr rolled up at the start, a nut may bind on the burr, when infact its clears the threads fine and you just needed to remove the burr.

Deflection is another problem, but it can be minimized by taking several 'spring' passes along the work (Usally just for the final passes), that means taking a few passes and not moving the tool in any, as then the 'sprung' areas at the end get recut as they are pressing against the tool, but the supported areas near the chuck do not because they where cut to the proper depth the first time.

You can find some cheap 'thread pitch' micrometers on ebay ($30~ each, covers 1 inch range per micrometer) that will let you messure the thread height/depth
Or you can grind some cheap metal calipers (the manual spring kind used to transfer messurements to a ruler or a digital caliper) to have a sharp profile to fit into threads.

Test fiting with a nut is quick and easy and works well for loose fit, but nuts vary (You might be using a loose nut, and the rest of your nuts are tighter), burrs are annoying, And messurements never lie (they just get done wrong sometimes!)

PS: your thread finish looks very good for a first time in mild steel, My first few threads finish was so rough it looked like some kinda file, I was impaitent and expected my giant lathe to be able to move lots of metal in each pass.
Yea, it did allright, but the metal that remained did not enjoy the process. :P

[Machinist-Guide]

Point Threading

A couple suggestions.

1-I would use a center on a thread this long.
2-Every 2 or 3 cut's take a free pass till it stops cutting.

What is a free pass?
Take a cut with out feeding in. You will notice when you do this you will remove material even tho you don't feed your cutter in. What is happening here is your part and tool will deflect. A free pass will remove the material left by the deflection.

Watch the videos. You will be glad you did

Donnie

[Machinist-Guide]

Spring Pass Free Pass

PS: your thread finish looks very good for a first time in mild steel, My first few threads finish was so rough it looked like some kinda file, I was impaitent and expected my giant lathe to be able to move lots of metal in each pass.
Yea, it did allright, but the metal that remained did not enjoy the process. :P

I agree with Black Moon. What he is calling a spring pass is what the guys in this region call a free pass.

My first thread also looked like a round file or woodworkers rasp.

I would stick with the nut check at first. When you start getting the hang of it then you can advance to some of the measuring methods.

[dp]

Tubalcain's videos are great. He's very good at describing the craft without a lot of stammering and struggling for words. Well organized and focused. And his video stage craft is great, too. It's clearly planned ahead as though he's working from a storyboard which is exactly what it takes to make a first rate video. His video editing of the breaks, segues, and use of multiple camera angles work well.

And he's prolific - he's done a bunch of them and is giving them away on YouTube. That's pretty nice, too.

Sol at Glacern.com is another who's giving away some valuable educational videos that are very well done.

[Machinist-Guide]

Tubalcain

I like the way he teaches. He don't confuse beginners with a lot of technical terms. He has 3 videos on lathe bit grinding that includes grinding a threading tool. I like the way he uses large wooden blocks as models of lathe tools. I thought that was a great teaching aid.
I came to know Tublecain via email while getting his permission to use his videos on my site. He is a very nice helpful person.

Here is a link to his videos showing how to sharpen lathe bits including threading tools

Lathe Bit Grinding Videos

[John Garner]

Jason --

A sound understanding of screwthread geometry makes it MUCH easier to understand threadcutting.

I wrote the following essay to explain the geometry of the most common screwthreads; it's long, but I think it'll be worth the time for you sit down with a pencil and pad of paper and make sketches as you read through it:

There are three different forms of screwthreads that have been in widespread use in the industrialized world in the last century or so that we should talk about here, and probably several dozen more forms that I'll tactlessly ignore. All three of these threadforms -- the Sellers (aka Franklin Institute, National, American National, and US Standard), the Unified, and the ISO Metric -- are descended from an earlier form, the "60 degree Sharp V".

The 60 degree Sharp V threadform has been obsolete since the 1800s, but it provides a good place to begin our discussion.

Let’s start by imagining a bolt with a 60 degree Sharp V thread. Now imagine that we cut that bolt lengthwise, so that the plane of the cut contains the central axis of the screwthread, and then look closely at the profile of the thread in the plane of the sectioning cut.

If we were having this discussion over a cup of coffee in the breakroom, I'd be drawing sketches to show you what I'm trying to explain. Since we aren't, though, you might want to get out a pencil and paper and try to make your own sketches while I talk.

The Sharp V screwthread profile looks like a row of equilateral triangles, each with one side resting on a straight edge with their points pushed together. All sides of these triangles are the same length, so the “points” of successive triangle away from the straight edge are separated by the length of the triangle side . . . we'll call this distance The Pitch of the screwthread.

There is another row of these little triangles on the opposite side of the sectioned bolt, offset along the length of the bolt by a half Pitch, with their not-on-straightedge points pointing in the opposite direction from the first row's not-on-straightedge points.

Following so far? Ok, now lightly draw two parallel lines, one connecting the away-from-straightedge triangle points on one side of the bolt and the other connecting the away-from-straightedge triangle points on the other side of the bolt. These two line are separated by the Major Diameter of the screwthread.

The on-straightedge sides of the two rows of triangles are separated by the Minor Diameter of the screwthread. The away-from-straightedge points of each row of triangles are off of the straightedge by the Single Depth of Thread, which is The Pitch x Cosine 30 degrees.

The Double Depth of Thread is twice the Single Depth of Thread, 2 x The Pitch x Cosine 30 degrees. The Double Depth of Thread is also the difference between the Major Diameter and Minor Diameter.

If you were going to cut this thread on a lathe using a single-point toolbit with the compound rest slewed to feed along the flank of the screwthread, and assuming that you zero the compound when the sharp point of the toolbit just touches the already-cut-to-Major-Diameter workpiece, you'd have a complete threadform when you'd fed the tool into the workpiece by a distance equal to The Pitch. After all, all three sides of each triangle are the same length.

While the geometry of the 60 degree Sharp V screwthread is nice and simple, it has practical problems. The sharp point on the toolbit breaks or wears very quickly, the sharp ridges at the Major Diameter of bolts and Minor Diameter of nuts get banged up very easily, and the sharp grooves at the Minor Diameter of the bolts are "stress risers" weakening the bolt. By the 1860s the American industrialist and machine tool builder William Sellers proposed a modified version of the earlier threadform, one with 1/8 Pitch flats at both the Major Diameter and Minor Diameter, as a new standard.

This new threadform, the Sellers threadform, was fairly well accepted, but it didn't actually become the official US Standard threadform until well into the first half of the 20th century.

So let's modify those sketches. The general spacing and angles stay the same, but the new profiles have flats instead of sharp points at the Major Diameter and same-size flats instead of sharp grooves at the Minor Diameter. Both flats need to be 1/8 Pitch long, which reduces the length of the angled flanks AS MEASURED ALONG THE AXIS OF THE SCREWTHREAD to (Pitch - 1/8 Pitch at the Minor Diamter - 1/8 Pitch at the Major Diameter) = 6/8 Pitch = 3/4 Pitch.

The length of the flank is reduced by the same ratio, and the other Pitch-dependent calculations are adjusted accordingly.

Flank length (along-flank infeed using slewed-to-feed-along-flank compound rest) for Sellers threadform = 3/4 Pitch.

Single Depth of Sellers Screwthread = 3/4 Pitch x Cosine 30 degrees.

Double Depth of Sellers Screwthread = 2 x 3/4 Pitch x Cosine 30 Degrees.

The Sellers threadform served the US's needs well enough until World War II, when the difference between the United States' and the British Standard threadforms created major logistical headaches . . . British equipment could only be repaired with British-standard hardware while US equipment could only be repaired with US-standard hardware.

Once WWII had been won, the US, Great Britain, and Canada (which, interestingly enough, had fifty years of experience struggling to supply the appropriate British Standard and US Standard hardware when and where needed) put their collective heads together to develop a single standard that all three nations would use. To "share the pain" of forsaking a traditional standard screwthread, a new threadform was developed that both the US and Britain would need to learn to use. This new threadform was called the "Unified" threadform, and it incorporates the easier-to-tool 60-degree angle with flats at the Major and Minor Diameters of the Sellers screwthread, but with different proportions.

A decade later, the fundamental geometry of the Unified threadform was incorporated into what could be considered a metric version of the Unified form intended to replace the various European national standard threadforms. Since the International Standards Organization developed and promoted the new metric standard, it was christened the ISO Metric threadform.

The major difference between the Sellers and Unified threadforms is that the length of the flat at the Minor Diameter of the Sellers threadform was doubled to 1/4 Pitch for the Unified threadform. The flat at the Major Diameter of the Unified and ISO Metric threadforms is the same as the Major Diameter flat of the Sellers threadform, 1/8 Pitch.

So, for both the Unified and ISO Metric threadform the axial length of the flanks is reduced still further to (Pitch - 1/4 Pitch at Minor Diamter - 1/8 Pitch at Major Diameter) = 5/8 Pitch.

Flank length (along-flank infeed using slewed-to-feed-along-flank compound rest) of Unified and ISO Metric threadforms = 5/8 Pitch.

Single Depth of Unified and ISO Metric threadforms = 5/8 Pitch x Cosine 30 degrees.

Double Depth of Unified and ISO Metric threadforms = 2 x 5/8 Pitch x Cosine 30 degrees.

Minor Diameter = Major Diameter - Double Depth of Thread.

Well, that's the basic geometry of these screwthreads.

As you've already pointed out, the dimensions of real hardware are properly perturbed by allowances and tolerances. External screwthreads cannot be larger than the dimensions derived from the basic geometry, and internal screwthreads cannot be smaller than the dimensions derived from the basic geometry if the external and internal screwthreads are to fit together. The along-flank infeed calculated from the basic geometry assumes that the Major Diameter of the to-be-externally-threaded workpiece is right at the basic Major Diameter AND the flat on the toolbit is the proper width (1/4 Pitch for Unified and ISO Metric threadforms, 1/8 Pitch for the Sellers threadform) . . . or that the Minor Diameter bored into the to-be-internally-threaded workpiece is right at the basic Minor Diameter AND the flat on the tip of the toolbit is the proper 1/8 Pitch width (any of the three threadforms we've talked about).

As for the charts . . . even though the Unified threadform replaced the Sellers threadform as the official US standard threadform a half century ago, many of the tables in various "reference works" have been carried forward from edition to edition with values appropriate to the Sellers threadform.

I realize that my posting is long-winded, but I hope it's clear enough for you to follow. If not, post back and I'll try to answer your questions.

[jacampb2]

Hi Guy's
Here is a link to some videos on point threading. These videos were made by a retired machine shop teacher. He does a good job on his videos and I think they would be helpful to you guys.

Threading Videos

I just watched the video. It was excellent. It appears I did everything wrong! I ran the machine at ~400RPM, but my slowest speed is 34RPMs, should I really run it that slow??? I fed with the cross slide instead of the compound, and I was taking .005" per pass, which is probably 2-3xs the DOC he was taking with the compound angle...

Good info, I don't normally go in for training videos, I like written/illustrated material much better, but this was worth watching.

Thanks to all else for the other info as well. I am not sure I completely follow the first mathleate post, but the essay above was interesting.

Thanks again,
Jason

[Carld]

You could start at about 75 rpm but here is something you have to find out. On some lathes when you change speeds the headstock is not kept engaged with the lead screw on others it is.

To find out set the lathe up for a thread, any thread and then a speed, say 75 or so. Now spin the chuck by hand and you will see the lead screw turn as the chuck turns. Now move one lever to change a speed as you spin the chuck by hand and if the lead screw doesn't stop turning when you get between the gears going to the next speed then you can change speeds any time when cutting threads.

If when you get between the gears and the chuck turns without the lead screw turning you can't change speeds during the time your cutting threads. This is good to know because sometimes you may want to increase or decrease the spindle speed when you start cutting a thread. It's best to try this with all the levers that change spindle speeds because one lever may disconnect the spindle and lead screw while none of the others will.

One lathe in a shop I worked had three levers and I could move two of them but if I moved the third lever it disconnected the spindle and lead screw. I had to select a range that I could move up or down in and only move any of the other two levers to change the speed.

Of course this applies to gear head lathes.

[Paul Alciatore]

NUMBER ONE is sharp tools. Dead sharp! Stone it before cutting and examine the edge under 10X or 20X magnification. Stone it until you can't see any visible edge. DEAD SHARP.

There are many ways of threading. Most machining books have at lease one chapter on it, many have two or more. I have a whole book on thread cutting in the lathe and it probably does not exhaust the subject. Feed with compound, feed with cross slide, feed with cross slide and use compound at 90 degrees, YES I SAID 90 degrees or parallel to axis of lathe, and calculate amount of feed on it to move the tool point so that only one flank is being cut like the 29.5 degree technique. Use this cutting fluid, use that cutting fluid, cut dry, etc.

Many, many techniques. All can work and each has advantages and disadvantages. Start with some fairly standard technique and make chips. Change the technique only as needed.

As for the die, the easiest way to get an accurate thread is to cut it single point in the lathe for about 90 to 95% of full depth and then chase it with a die for the full length. The die will finish it off quite well and if it is started single point, the thread will be straight and concentric (a die used alone can cut a drunk thread - sometimes very drunk). But not all threads can be cut this way. I have never seen a 1 1/2 - 8 die. They probably exist, but I have never seen one. Probably cost quite a bit and are not worth it for one or two threads. So many threads are single point all the way.

Two tips if you are finishing to full depth on the lathe. One, cut the OD a little on the small side. This will allow any burrs to "hide" in the bigger flat. By a little small, I mean no more than 1 or 2% or even less. A 1/4" thread could perhaps be turned down to 0.245". You will never notice the difference in any practical application.

Second, use a metal bristle brush to clean up burrs BEFORE measuring or testing with a gauge or nut. For steel, a steel brush would be OK, for softer metals use a brass bursh. Just run it across the newly cut threads while the lathe is still turning. Three or four strokes should surfice: don't over do it. This will clean up both burrs and any chips left in the grooves and make the measuring or gauging more accurate.