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hwingo
11-12-2007, 05:36 AM
Hi guys,
I am inquiring about dual lead threads . Years ago I had a South Bend lathe that was capable of "splitting threads". Some shotgun chokes use duel lead threads. A well seasoned machinist called this "Dual Lead" therefore I am calling it such.

As an example, the SB lathe might be set to turn 12 tpi. The chasing dial on my lathe had (to the best of my memory) even and odd numbers on the dial with un-numbered lines between each. To cut even number thread one had to make each cut on an even number (odd tpi on odd numbers). The first run is made starting on, say #2. Watching the dial (and here's where things get foggy), the next cut is begun on #4 or perhaps a non-numbered line just past the #4. The point is, by doing this, you are still cutting 12 tpi but the lead on each cut is 180 degrees out from the first cut and when finished, it would appear that you had threads on the work that was 24 tpi when in fact you had 12 tpi ..... it was two sets of threads the starting point being 180 degrees out for each thread.

Sure enough, I could easily do this on the SB. I now have a new lathe (as in brand new) but it's not a SB (it's an import).

Should all lathes be able to make a "Dual Lead Thread" or was this merely a unique feature on the model of SB I once owned?. Keeping in mind that one starting lead should be 180 degrees from the other, if this can be done on all models, how would it be done on a lathe having no numbers on the chasing dial?

Harold

DickDastardly40
11-12-2007, 05:53 AM
Harold,

There are a couple of other ways of doing this, the easiest is to cut the first start with the compound set to 90 deg to the crossslide and the backlash taken up, plunge cutting the thread.

Once you've done that, move the compound half the pitch for twin start or a third of the pitch for 3 start etc and cut the next start. With this method you engage the half nuts at the same place or you can leave the half nuts engaged as you would cutting a metric pitch on an imperial leadscrew and run the saddle back to the start point after withdrawing the tool.

Others will chime in with other ways I'm sure.

Al

John Stevenson
11-12-2007, 06:17 AM
Harold,
If you don't want to follow Monsewer Dastardly's good idea of running at 90 degrees and want to infeed at 1/2 the thread angle as many have been taught, then you can cut one thread using the thread dial as per normal then disengage the gear train and carefully turn the chuck 1/2 a turn and re-engage the gears.

You second pass, still using the same settings as before will now be between the original threads.

.

hwingo
11-12-2007, 06:28 AM
Great Heavens Above Al!

It's not even 0500 and you're up answering questions.:eek: What's a youngster like you doing up at this hour? This is when all us old folks are suppose to be up and about. I'm on patrol walking perimeter defense keeping a vigilance on my new lathe.:p Can't have the enemy sneak through my lines of defense and peculate my "precious darling".;)

oops! Another trooper is on patrol standing guard over this thread. Just got an incoming alert. Before I dive into you answer, let me see what the next answer is and get back to you.

I can't believe you guys are up so early. It's still Zero Dark-Thirty at my AO.

Harold

DickDastardly40
11-12-2007, 06:33 AM
Me again,

Just to clarify most multi-start threads I've cut have been square form therefore plunge cutting was easiest.

When cutting the nut for vee form I don't infeed at 29 1/2 deg so the 90 deg method works too. Always move the compound in the direction that you've taken up the backlash in.

Al

PS: 1130 AM here in Blighty, sat shining my @rse at work.

oldtiffie
11-12-2007, 07:35 AM
Harold,
If you don't want to follow Monsewer Dastardly's good idea of running at 90 degrees and want to infeed at 1/2 the thread angle as many have been taught, then you can cut one thread using the thread dial as per normal then disengage the gear train and carefully turn the chuck 1/2 a turn and re-engage the gears.

You second pass, still using the same settings as before will now be between the original threads.

.

Agree John.

That is the most efficient and fool-proof way in the general case.

But a couple of "traps" to be wary of.

I presume that the gears to be dis-engaged will be the one mounted on the drive-train end of the spindle and the next. That being so, the method you suggest will work fine providing that gear has an even (and not an odd) number of teeth.

There is a "trap" with lathes with a geared head like mine. There is a 4:1 reduction within the gear-box between the lathe spindle and the first drive gear in the external drive train. Rotating the lathe spindle 1/2 turn will result in the first drive-train gear only moving (1/2 x 1/4 = )1/8 turn so that gear needs to be a multiple of 2, 4 or 8 (I think).

Fortunately I have a 6-lobe "Dog" clutch on my lead-screw so I can disengage and engage the lead-screw in 6 different but equal divisions without disengaging my gear-train. Which in my set-up is very handy.

Al's (DickDastardly40) method has a lot of things going for it as he can leave his gears alone and use the cross-slide in conjunction with the compound/top slide to get a similar effect as the "offset the compound by half the thread included angle" method and still us the same settings to cut his multiple-start (or any) thread/s.

Because of my lathe set-up (my lathe is "metric" with a 3mm lead-screw) I agree with and support the logic with Al's method as I am able to avoid the multiple disengage-engage of the gear train. This is particularly the case if I am cutting "inch" threads no matter how many "starts" there are.

So - I favour and use the method as described by Al.

Others preferences and circumstances may well differ from mine - that's fine - but whatever works works.

joegib
11-12-2007, 09:11 AM
Sorry, misspost.

Paul Alciatore
11-12-2007, 10:14 AM
I believe some of the methods above may depend on the lathe or on the exact TPI of the thread. They may work in some cases and not others. Setting the compound to 90 deg and using it to offset the thread is good in all cases but could be less accurate for small threads as the offset could be just a few thousanths.

Another method is to mount the work in a manner that allows it to be rotated 180 degrees in the chuck after cutting the first start. Of course, it must be perfectly aligned horizontally for both starts. This would work for any number of starts by rotating 120, 90, etc. degrees.

As for your SB trick using the divisions on the threading dial, that only works for some TPIs, even on the SB. But it would work on any lathe that has the multiple divisions on the thread dial. The trick here is that the TPI would normally REQUIRE starting on an exact number or on an even number or even on a number as opposed to an inbetween (un-numbered) division. And the rules would be different with different threads. For instance, I have an 8 TPI lead screw on my SB. If I am cutting a 8 TPI thread, I can engage on any division and it will still cut the original path. I could never cut a multiple start thread of 8 TPI with this lead screw. On the other hand, a 13 TPI thread would require that I start on the same number each time. So if I started on even numbers for the first start then starting on odd numbers for the second start would product a two start thread. And I could make a four start, 13 TPI thread by using the half divisions: even, then odd, then on a half division following an even number, and finally on a half division following an odd number. But I couldn't make a three start thread this way. There are several different combinations for different TPIs. I could NOT cut multiple start metric threads with this method using my 8 TPI lead screw and a transposing gear.

hwingo
11-12-2007, 11:30 AM
OK Guys,

I am sooooo confused. You guys are far above my head. :eek:

What information about my lathe should I provide so you guys can help me through this? I have a manual for the lathe (though it's not the best manual in the world) and maybe it would provide me information that I can convey to you so you can get me on the right path.

I will be cutting in TPI most all the time and rarely in metric (though I will have a need for metric from time to time). However, dual lead threads will be cut in "inches" using the V design.

Harold

tdmidget
11-12-2007, 07:11 PM
I challenge you to one. Suggest spelling books at 20 paces.

J. Randall
11-12-2007, 07:31 PM
Harold, another method is to thread between centers using a driving dog. For a two start leave the four jaw chuck on and index the dog off of opposite chuck jaws, or a three start with a three jaw chuck. You could also use a face plate or dog plate and lay your indexing out on them by drilling holes or using clamps in existing holes.
James

hwingo
11-12-2007, 07:59 PM
I challenge you to one. Suggest spelling books at 20 paces.

Yes, I made a mistake .... several times. I have made the necessary corrections to put your mind at ease. Now, would you like to make a significant "on topic" contribution so I may learn more?:)

Harold

hwingo
11-12-2007, 08:18 PM
Harold, another method is to thread between centers using a driving dog. For a two start leave the four jaw chuck on and index the dog off of opposite chuck jaws, or a three start with a three jaw chuck. You could also use a face plate or dog plate and lay your indexing out on them by drilling holes or using clamps in existing holes.
James

Hi James,

Thanks for the reply. I got confused beginning with the first reply. I can see a great importance in doing it the way Al has suggested. If doing it as Al has suggested, this means that dual lead threads can be done on most any lathe using Al's method. That which confuses me is how Al's method starts a second thread 180 degrees from the first pass.

Also, I made another mistake (which is not at all uncommon). I had stated that my chasing dial had no numbers and this was incorrect. I have numbers 1, 3, 5, and 7 on the dial with only one line between each number. So help me out on this ............. To cut "even number threads" must I start on a number ....... just any number? If so, does this mean that to cut odd numbers I must start on a non-numbered line ...... any line? Finally, what's the significance of odd numbers on the chase dial? Does each number have a special significance?

Thanks,

Harold

lane
11-12-2007, 08:31 PM
Their are a number of ways depending on the lathe because some are different. But my method is this .
1.even threads any line any time . 2,4.8,12,16,

2. odd threads same number or line every time.9.13, so on.
South Bend made a lathe with a 6 pitch screw which was really wearied.

Paul Alciatore
11-13-2007, 12:57 AM
Some help on how to think about threading:

Perhaps the best way to think about it is that a threading dial is actually a linear measure. It shows the distance that the thread has traversed as the lead screw rotates. Thus, using my SB for an example, the lead screw is 8 TPI and the SB threading dial uses a 32 tooth gear or 8 X 4 teeth. When the lead screw has rotated 8 times, the carriage and thus the thread has traversed one inch. And that's 1/4 of the 32 teeth so one complete turn of the dial represents 4 inches of travel. The numbers, 1 to 4, on the dial represent inches. The un-numbered divisions between the numbers represent half inch increments.

With the above in mind, what you need to ask yourself is how far must the thread you are cutting traverse to come back in sync with the lead screw TPI. There is an opportunity for this every rotation of the lead screw but not all threads will match at every rotation. Obviously, an 8 TPI thread on the work will match on every rotation. As will any whole multiple of 8 TPI (16 TPI, 24 TPI, 32 TPI, etc).

Even numbered threads like 10 or 12 TPI will synchronize after a half inch and can be started on any mark on the dial since they represent half inches. Odd numbered threads like 7 or 11 TPI will synchronize after one inch so they can be started on any numbered mark. Threads with a 1/2 fraction will not synchronize until a full two inches have been traversed and can only be started on even or odd numbers to match the first start. Finally, threads with a 1/4 or 3/4 fraction must be started on the same number as the first start.

Since the threading dial only represents four inches, it's ability to synchronize threads ends with fourth fraction threads. Also, note that it can not synchronize a thread with any other fraction like thirds or fifths: only halves and fourths.

That's how a thread dial works. Metric dials are more complicated and often (always) have multiple gears for different pitches. This is because the fractions are more complicated, like fifths and tenths. With different gears on the dial, it can represent different distances and handle different fractions in the equation.

When you use a dial for multiple starts, you are actually starting out of sync with the first start. If you want to cut a two start thread, the TPI must have the inbetween divisions on the dial that would not work for a one start thread. Thus, the multiples of 8 TPI would not allow this as any mark on the dial would sync them. The other even TPI threads would allow it, but you would have to stare half way between the marks (that's half way between a numbered mark and an un-numbered one). And you could only make multiple starts that are a multiple of 2 (2, 4, 8, etc).

The important thing to think about is the synchronizing distance.

All of the above numbers are for an 8 TPI lead screw. Other lead screw pitches will change things, but the rules are the same. The things that you must know about a lathe to figure this out are the pitch of the lead screw, the number of teeth on the thread dial gear, the number of divisions (numbered and un-numbered on the thread dial, and the pitch of the thread being cut. That's all.

DickDastardly40
11-13-2007, 05:17 AM
That which confuses me is how Al's method starts a second thread 180 degrees from the first pass.

Harold,

I don't want to muddy the waters any more than they may already be, I hope this further expanation helps.

A notional two-start thread has say a pitch of 2TPI or 0.5" between crests. This will be one very slow spiral. You cut the first start to say half the normal depth. There will be a big land between each groove of the spiral. By moving the compound half the pitch, in this case 0.25", you are moving the point of the tool into the middle of the big land.

When you begin cutting the second start it will begin 180 deg opposite the first start by default. For a 3 start if you'd moved the compound 0.166" it would cut 120 deg from the first and the next start would be 0.166 further on. You'd only cut to 1/3 normal depth to maintain a decent form.

Clear as mud? I hope I've helped.

Al

oldtiffie
11-13-2007, 05:34 AM
Hi James,

Thanks for the reply. I got confused beginning with the first reply. I can see a great importance in doing it the way Al has suggested. If doing it as Al has suggested, this means that dual lead threads can be done on most any lathe using Al's method. That which confuses me is how Al's method starts a second thread 180 degrees from the first pass.

Also, I made another mistake (which is not at all uncommon). I had stated that my chasing dial had no numbers and this was incorrect. I have numbers 1, 3, 5, and 7 on the dial with only one line between each number. So help me out on this ............. To cut "even number threads" must I start on a number ....... just any number? If so, does this mean that to cut odd numbers I must start on a non-numbered line ...... any line? Finally, what's the significance of odd numbers on the chase dial? Does each number have a special significance?

Thanks,

Harold

Hi Harold.



QUOTE=hwingo
Thanks for the reply. I got confused beginning with the first reply. I can see a great importance in doing it the way Al has suggested. If doing it as Al has suggested, this means that dual lead threads can be done on most any lathe using Al's method. That which confuses me is how Al's method starts a second thread 180 degrees from the first pass.

Perhaps I can clear this up for you as regards Al's method.

Let's say you are cutting a 2-start thread with a lead of 1/2". This means that each of the 2 threads that you cut will advance 1/2" for each turn of the chuck or work-piece. As there are two inter-spaced threads, the distance from a point on one to the same point next thread will be 1/2" /2 = 1/4" pitch.

So if you cut the first thread - all is well.

Now the second thread must fit in mid-way between the groove formed by the first thread - that is to say 1/2 the lead = 1/4" which is the distance a thread will advance in 1/2 turn (180 degree) of the chuck or work-piece.

To use Al's method, cut the first thread/groove, leave the lead-screw half-nuts engaged and move the top-slide left or right by 1/4" (0.250") and your threading tool will be positioned mid-way between the first thread groove/s and you are ready to cut the second thread - just as you did for cutting the first one.

I suggest you try it out on a piece of scrap as it will "work" and when it does it will all become evident to you.

First of all - all threads are spirals.

A normal thread is a spiral and is in fact a single-start thread.

Multiple-start threads are everywhere. A twist drill is a good example of a 2-start (dual?) spiral. A four tooth end mill is an example of a 4-start spiral.

I hope this helps.

hwingo
11-13-2007, 12:32 PM
Hi guys,

You’ve given me a wealth of information to digest all of which has been valuable. I suspect you guys are instructors at a school.

Let’s fractionally distill Al’s approach allowing me to paraphrase (in elementary fashion) what I’ve gleaned from this information and focus on the actual technique.

1. The compound is to be set at 90 deg to the cross feed (i.e., parallel to the work). The tool is perpendicular to the work (unlike the method of rotating the compound 29.5 degrees with the tool set perpendicular to the work piece).

2. TPI selection is made and the first “scribing” cut is made and successive cuts there after until the first thread is cut to ½ the depth. Cross-feed is used to control depth. For example, if cutting a double lead at 12 TPI, and using the cross feed, you would cut to ½ the depth since the second thread (@ 12 TPI) will “point up” the first thread at half the height.

3. The next task is to determine the distance between two “peaks” of what would be a fully cut thread. Once that is determined, the compound is advanced ½ that distance before starting the second cut. Naturally, successive cuts are made using the cross-feed to control depth until all threads “appear to point”.

4. I may use any number on the chase dial but I *must* use the same number with all cuts until the work is completed
.
Does all of the above sound about right?

One other question I have is regarding the term “plunge cut”. In my way of thinking, to “plunge” is to take something to depth in one great stroke, e.g., to “plunge” a knife into someone’s heart suggest this is not done in small increments. When you use the term “plunge cut”, are you suggesting that a thread is cut in one pass rather than incrementally?

Harold

tattoomike68
11-13-2007, 12:52 PM
By plunge cut I think they mean use the cross slide and not the compound.

I dont use the compound to thread till the pitch is 5 tpi or less myself.

DickDastardly40
11-13-2007, 01:50 PM
When you use the term “plunge cut”, are you suggesting that a thread is cut in one pass rather than incrementally?

Harold

No by plunge I meant straight in as opposed to the angular 29 1/2 deg method but always cut incrementally. Generally I reduce each cut as the depth increases because the pressure on both sides of the tool increases too.

You have the method correct otherwise.

Hope we've helped.

kap pullen
11-13-2007, 03:36 PM
Just for another method, here is the Monarch Lathe thread indexing attachment.

http://img.photobucket.com/albums/v206/kappullen/DSCN0989-1.jpg

With this tool you can cut a kazillion different multilead thread options.

The 3, 6, and 8 stamped on the ball... oops on the ring, are common indexing options.

The sliding key in the center of the photo engages a gear and has at least 72 teeth to index from.

The chuck indexes in relation to the lathe spindle.

This tool can be made using locating pins, or even graduations to do a fair indexing job.

Kap

GadgetBuilder
11-13-2007, 03:53 PM
Martin Cleeve's "Screwcutting in the Lathe" has 6 different ways to handle multi-start threads - this book is one of my favorites, well worth having if you do much threading. Cleeve apparently made his living with a 7" Myford and much of his work involved threading.

One of his techniques for dual start threads automatically cuts both threads alternately so you make two passes between advances of the tool. He sets up carriage stops on left and right to facilitate this and avoid errors. The exact setup depends somewhat on your leadscrew and threading dial so you'd need to read and follow his writeup.

This method allows using Cleeve's standard technique of advancing the compound half the amount the cross slide is advanced, approximating the 30 degree thread flank angle. I use this method now and prefer it to setting the compound to 29 degrees because it allows reading thread depth directly from the compound dial. Plus, once you arrive at depth you can widen the cut section by adjusting the compound and not the cross slide - this means you can leave the threading tool point sharp enough for fine threads and simulate a wider point as needed for coarser threads. No need for multiple threading tools or resharpening for different pitches.

I used plunge cutting for a while, taking 7 thou on the first pass and decreasing on subsequent passes. This cuts on both flanks so the tool works much harder as depth increases. The tip sometimes broke off the threading tool (depending on the material) plus chatter was often a problem and the threads were often rough. Internal threading was especially prone to rough threads. These problems essentially disappear when advancing down the thread flank using Cleeve's technique.

Also recommended is Cleeve's jig for sharpening threading bits, see:
http://www.gadgetbuilder.com/ThreadingTools.html

John

hwingo
11-14-2007, 09:40 AM
Hi Guys,

This has been a great help and the different approaches to an end has been most interesting and informative. Your directives have served as the impetus for me to get out in the shop and give this a go.

I will likely have additional questions on this matter so I hope I have not worn you down still finding time to address additional issues regarding this shop challenge.

I *really* do appreciate your willingness to offer direction and clarification. :)

Harold

hwingo
11-14-2007, 12:24 PM
Martin Cleeve's "Screwcutting in the Lathe" has 6 different ways to handle multi-start threads - this book is one of my favorites, well worth having if you do much threading. Cleeve apparently made his living with a 7" Myford and much of his work involved threading.


Also recommended is Cleeve's jig for sharpening threading bits, see:
http://www.gadgetbuilder.com/ThreadingTools.html

John

John,
As a "heads up", I have sent an email via your link so I hope it makes the trip and you receive the email.

Harold

tdmidget
11-14-2007, 09:08 PM
Sorry to ruffle your feathers about the spelling. Next time my choice of weapons will be"sense of humor" assuring me of victory. Al's method is what I have been taught , tho never used. All of the methods suggested will work. For those terminally enamored with the 29 1/2 degree compuond, it will still work. The compound movement is a trig exercise probably beyond my limited abilities but can be done. The Martin Cleeve method is very interesting and seems the simplest method for multi-start threads. Got to try it.

hwingo
11-19-2007, 01:56 PM
Sorry to ruffle your feathers about the spelling. Next time my choice of weapons will be"sense of humor" assuring me of victory. Al's method is what I have been taught , tho never used. All of the methods suggested will work. For those terminally enamored with the 29 1/2 degree compuond, it will still work. The compound movement is a trig exercise probably beyond my limited abilities but can be done. The Martin Cleeve method is very interesting and seems the simplest method for multi-start threads. Got to try it.


No problem. :)

Harold

recoilless
11-20-2007, 01:50 PM
Hwingo:An addendum to dickdastardly40's method, in your case a double start 24 tpi. First, set the QC to 24 tpi, engage and take your first cut, couple thou, so you can see the tool mark, but just lightly. Then switch to 12 tpi, run out to full depth. Now, when setting compound over, watch you dial vs. the first mark you made at 24 tpi. Theoretically should be exact, but... use a magnifying glass if your eyes need help.
Clear? All these methods work, just be careful and take heed of abovementioned pitfalls. Good luck, remember, threading is FUN!!