# threading



## Loose nut (Mar 26, 2008)

If you go to this web site you can download videos on lathe work, including threading. The are from the '40's but are still relevant for manual machining.

http://www.shinyobjectdesign.com/staff/paul/sbvideo/

Place your mouse on each picture, right click at hit "save target as", sometimes they won't download if you don't do it in this way.


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## itowbig (Jun 14, 2008)

ok i cut my first threads and they are not what they are suposed to be. 
i have the lathe master lathe and im trying to cut 20 threads per inch. acording to the book im 2 use top gear 70 2 middle gears 60/80 bottom gear 50 this does not work or im doing something very wrong. can you guys shed some light please.
ill keep pluggin away till i chew up my metal.


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## itowbig (Jun 14, 2008)

never mind i got it figgered out. i had the gears out of place. the 80 gear was turning the 50 gear and it is supposed to be the 60 gear turning the 50 gear. now i got to get the cutter to make cleaner cuts. i had to make my own cutter. the one i had is for bigger threads. anywho thanks i got it


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## chuck foster (Jun 14, 2008)

using change gears was and still is not an easy job (for me anyway) i always got one of the gears wrong !

i had an import lathe and the change gears were plastic and to boot they ALL had the number of thee stamped on them WRONG.
example: the 20 tooth gear was marked 19 and the 30 tooth gear was marked 31, there was allot of nasty language in my shop till i figured out the number of teeth was marked wrong :wall:

so i'm glad you figured it out, it gives you a good feeling knowing that now you can cut any thread you want 8)

have fun and keep your stick on the ice ;D
  chuck


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## Bogstandard (Jun 14, 2008)

If you do a web search for 'change gear setups' for blah, blah lathe, you can sometimes come up with results that show not just the manufacturers thread ranges, but others that have already been calculated out for you. Saves having to do it all yourself.
I have reference charts for my old lathe that allow me to cut a lot of weird and wonderful threads, just using the standard set of change gears.
The information is out there, it is just a matter of finding it.

John


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## mklotz (Jun 14, 2008)

Or, shameless plug here, you can use my CHANGE program which, among other things will compute change gear setups for approximating oddball pitches or pitches not included in your lathe's documentation.


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## itowbig (Jun 14, 2008)

marv  you got quite a bit of info there dont ya. you realize now ive got to read all this stuff and check it out.
thank you


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## Bogstandard (Jun 14, 2008)

Oooh! Marv,

A little bit blatent there. 

I had totally forgotten about your little prog, sorry.

Just a little note about generated 'near enough' threading charts. They can really get you out of the smelly stuff at times. If you come across a weird thread you have to match, say 17 TPI, a generated 17.00237 TPI can really work for you. As long as the mating screwed faces are not too long, you can easily get away with a near enough machined thread. Just machine the matching thread until you get a nice smooth fit.
When somone comes to you and says 'it will take them a month to get the special part in', you can gain real good brownie points by getting the machine going again until the new part arrives.

John


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## SmoggyTurnip (Jun 23, 2008)

What I have been doing lately when setting up for thread cutting (because I have ruined many parts by putting the wrong gears in). If i want to cut say 20 tpi I will put a 20 tpi bolt in the chuck and run the setup without putting the bit in the threads. Just get the bit close to the threads and see how it tracks and if all is OK then remove the bolt and replace it with the part to be threaded. It has saved my part more than once.


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## mklotz (Jun 23, 2008)

SmoggyTurnip  said:
			
		

> What I have been doing lately when setting up for thread cutting (because I have ruined many parts by putting the wrong gears in). If i want to cut say 20 tpi I will put a 20 tpi bolt in the chuck and run the setup without putting the bit in the threads. Just get the bit close to the threads and see how it tracks and if all is OK then remove the bolt and replace it with the part to be threaded. It has saved my part more than once.



Doing that might mean disturbing a setup in order to insert the test bolt. Us old guys just make the first threading pass a scratch (maybe 0.002" DOC) and measure the pitch with the handy little scale that's there for just that purpose on your fishtail gage. If you've done something wrong, the scratch will disappear in the cut when you've got the correct gears mounted.

I've got a quick change so I don't have your problem (of mounting the wrong change gears). However, if I did, I would draw a little picture in my shop notebook showing the correct setup for the pitches that I use. It'll take less time than remaking just one part.


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## jwsvandr (Sep 25, 2008)

I finished my die holder for the lathe yesterday and am happy to report it works. I ran into a problem. I wanted to thread 1/4 in threads on the end of a shaft. I made made a guess and turned the end to .240 and threaded it. It worked great but the threads were quite tight. Not a great problem except that I wanted them a little looser ( I wanted to use a nut that was hand tightened but with these threads I need to use a wrench). Is there a formula or rule of thumb that you use to get different fits. I can experiment but would like to tap (pun intended) the wisdom of this group.


by the way I had to tap a 4mm thread to make the die holder. My hats off to you who tap this small and smaller on a regular basis. It is scary


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## rake60 (Sep 25, 2008)

Threading has no perfect answer.

The form of the threading tool, flex of the tool post and machineability 
of the material being threaded, throw too many variables into the mix.

A "fish tail" center gauge will have the double depths printed on it.
In a perfect world touching off the turned OD and advancing the threading
tool in to a total of 1/2 that double depth cuts a perfect fitting thread.
Don't count it it! Making a viable fit usually takes going .004 to .008" deeper
on a side than the published double depth.

You can thread to a size piece. If that is a nut, you thread to .010" away from 
the published double depth and try the nut for fit. It won't go but it's a safe 
place to begin checking. From there I go .002" on a side deeper checking the 
fit after each cut. When it will go but it's tight, I take that same cut over again
That spring cut will usually bring it to a Class 3 thread. One that can be easily
assembled by hand.

If you don't want to thread to a size piece, you can mic the threads between cuts
using thread mics or a regular mic with the assistance of thread wires or thread triangles.
They will give you a perfect answer as to how much deeper you need to cut the thread.

Rick


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## Mcgyver (Sep 25, 2008)

.240 seems too small to me, a couple of thou under nominal should be fine.....BUT...this isn't that is going to determine thread fit. that is determined by how deep the V is which as Rick mentioned can be measured a couple of different ways. 

if you look at the dies, they should be split, some of them have a screw. 90 degrees in each direction the split you'll likely find dimples in the die. you should have set screws in the die holder that engages these dimples; you literally control the fit of the thread by putting more or less pressure on these.

there are various classes of fit to threads you can read up on, but from using a commercial die and commercial tap an interference fit is not good. as you might imagine, for each fit there are different tolerances and clearances; fine tuning to get where you want is what those set screws are for

btw, measuring the thread can be useful when you are diagnosing problems or when single pointing, but more practically, you just make the male to the female. If you have a male thread that is too tight, chase with the die tightened up a little.


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## Bluechip (Sep 25, 2008)

Hi Folks

Not exactly 'On Thread', but before I forget .. it may help someone ..

Some EU manufactured dies now seem to have an OD of 25mm

I got some 3, 4, 5 mm two years ago, could not get any sense out of them, was going to send them back, until I saw it does say 25mm OD on the packaging.

Marked KKK on die, Made in EU on wrapper.

What a bugger eh ? :

Just made another die holder .. dies are OK, no problem there.

Dave.


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## itowbig (Jan 27, 2009)

help what am i doing wrong. i did one in steal & it dont look good & i did one in brass same settings and it looks great.
im making a rotory table and need this part. but its not threading right.
am i going to fast or what. i dont know the speed but i am using 8x14 lathe
help ???


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## moconnor (Jan 27, 2009)

Hello Sid:

It appears that you lost your synchronization on your steel part and wiped out your threads. That can happen if you miss the number on your thread dial or if the part moves in the chuck. Since you don't have a shoulder on the shaft, it could have moved in if you went too far on one of your passes. The work piece could also have rotated in the chuck jaws if you ran into a full depth cut at the end of your thread by pulling out too late. Since you made a good thread in the brass part, your set-up must be OK. Do double check that your tool bit hasn't moved and is still square to your lathe's axis before your next try.

Regards,
Mike


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## ksouers (Jan 27, 2009)

Sid,
You might also want to make sure all your gibs are tight, tight, tight. Not so tight that you can't move anything but you want to make sure there isn't any movement in the saddle while it's threading.

Threads on my C2 look like that if the saddle gibs are even a little bit loose.


Kevin


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## itowbig (Jan 27, 2009)

moconnor i would agree except that i use reverse and dont disengage nothing.
this is the third one ive done. now in brass it comes out sweet and clean but steal no go.

ksouers everything is pretty tight but the tool post will flex a bit if i take a heavy cut. ive tried taking very light cuts and still no go in steal.
ill recheck everything. and try again

thank you both


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## moconnor (Jan 27, 2009)

Hello again:

I just had another thought. If your part can accept it, why don't you try machining a thread run-out groove at the end of the required thread on your shaft. It can be machined with a parting tool or a grooving tool and in general needs to be 1-2 thread pitches wide and machined to the thread's minor diameter. If you are concerned about creating a stress riser at the end of the thread, use a full radius on your grooving tool. The thread run-out groove will give you a wide target to disengage your half-nut and retract your cross slide. It is certainly easier that trying to stop at the same point each pass and will prevent your tool from crashing into the shoulder and taking a full-depth cut.

*For example*: *3/8"-16 NC 3A*

*[P= 1/N]* where: *P= Pitch * and *N= No. of Threads Per Inch*

P= 1/16 = .0625 So, for this thread a 3/32" - 1/8" wide thread run-out groove would be OK.



*[External Minor Diameter (d) = Major Diameter (D) - 2(DOT) ] where: DOT= Depth of Thread (Single)*

*[DOT= .64952/N] where: N= No. of Threads Per Inch*
Ex. 3/8"-16 NC 3A  DOT=.64952/16 = .0406"

External Minor Diameter (d) = D- 2(DOT)
                 d= .375-2(.0406) = .375- (.0812) = .2938"

Give the thread run out groove a try. It is definitely easier to use one if you can when you are learning. Hope this helps.

Regards,
Mike 

P.S. The formula for DOT is also known as a Single Thread Depth. DDOT is the 'Double Depth of Thread' and a useful piece of information is that the DDOT or what we subtract from the thread Major Diameter to find our Minor Diameter is printed or engraved on your Center Gauge next to the Number of Threads Per Inch. These formulas are for American National 60 Degree Thread Form which are still commonly used in tool rooms and maintenance and repair shops. The Center Gauge is the tool that is commonly used for checking the 60 degree included angle of the tool and setting the tool perpendicular to the lathe's axis. 

For our example above 3/8"-16 NC 3A, if you look next to the '16' on the center gauge you will find the DDOT or in this case (.081). Just subtract that from 3/8" (.375) or the Nominal Major Diameter and you have the Minor Diameter.


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## Holescreek (Jan 27, 2009)

Brass and steel cut completely different with regards to tool rake angle. Typically brass wants less rake angle and steel wants more. I have more questions than answers at this point but I would start with the tool bit top relief angle assuming you have the rest of your settings dialed in correctly. 

As far as the tool bit for steel goes: 60 degree included angle, slight radius on the end (no thread has a sharp V) and the top relief angle should be around 12-15 degrees. Stone all of the sides to remove burrs. Your tool bit appears to be mostly flat on top. Great for brass, bad for steel. 

Also, is your compound set to 29 degrees? and are you infeeding with the compound and not the cross slide? -Mike


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## Holescreek (Jan 27, 2009)

Oops, missed something in the photo. The bit mst also be perpendicular to the work and the tip on center with the work.


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## ksouers (Jan 27, 2009)

How heavy of a cut are you taking? With steel I only take about 2 thou on a pass then a spring cut or two about every fourth pass. Takes me a while, but it works for me.


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## steamer (Jan 27, 2009)

When you finish the pass, back the tool out before backing it up

The backlash in your leadscrew is probably to blame....they all have it....not just your lathe.

Dave


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## moconnor (Jan 27, 2009)

Hello Sid:

How many passes are you taking and at what depths?

You should take no more than a .005" depth of cut (DOC) maximum for your first pass or two depending upon the total thread depth. Then reduce the DOC as you progress, finishing with .001" and taking several 'free passes' or no infeed along the way to remove the spring in your set-up. The average thread will require anywhere from 15 to 25 passes to complete depending on the material and thread depth, etc. 

For example: 3/8"-16 NC 3A

DOT = .0406" (This is the single depth of thread.)

[DOC= .75/N] where: DOC= Depth Of Cut with Compound Rest at 30 Degrees, N= No. of TPI or

[DOC= .7463/N] where: DOC with Compound Rest at 29.5 Degrees from perpendicular to lathe axis.

Just using DOC 30 Degrees= .75/N = .75/16 =.0469" call it .047" total compound rest infeed. Break this down into 15 to 20 passes. You reduce the DOC based upon the thickness of the chip as you progress.

As a general rule, I will subtract .005"-.008" from the calculated DOC applied with the compound rest and remove this amount before measuring the pitch diameter with wires. (In the 3/8"-16 NC 3A example, I would remove .042" with the compound rest.) The thread is then finished by feeding in the remainder with the cross slide straight in .001" at a pass. This will then clean up both flanks of the thread. By applying the finish cuts with the cross slide, you now have a direct relationship with the cross slide micrometer collar and the thread's pitch diameter measured with the wires. If you are using a gauge or a mating part, I use the same procedure taking .001"/ pass with the cross slide until the desired fit is achieved. In both cases it is vital to take several free passes with no infeed to compensate for spring and deflection in the part/ work/ set-up. It is important to note that your infeed with the cross slide will be more than .005" that you stopped short on the compound because you are now feeding with the cross slide at 90 degrees and it depends how your dial read (diameter or radial reduction) as to what that total will be.

This is very confusing for a beginner because there is a lot going on and many variables. The best advice I can give is to understand the process thoroughly, have a plan and practice, practice, practice. There is a great deal of hand-eye coordination that needs to be developed for successful single point threading on the lathe and like any worthwhile skill, takes practice to develop.

Regards,
Mike


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## rake60 (Jan 27, 2009)

Are you using any kind of a cutting fluid Sid?
A little cutting oil brushed on the part between cuts makes a 
hell of a lot of smoke, but it also does a lot for keeping the chip
moving. The steel will cut cleaner and tool will not load up as
much. In short chip materials like brass and cast iron you won't
have those issues.

Rick


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## itowbig (Jan 27, 2009)

hello again i dunno about all that math jumbo but its my machine for sure.
i just came back from garage and did a couple test. one i stop using reverse 2 i put a dial indictor on 3 use the dial that is on the lathe  now i turned it again and got some pretty good threads. (BUT) i need to do it two more times to make sure then ill be back and post pics of what i did if it indeed works out the way the first one did.
im taking a number of passes all light. i cant do the runout grove the part need to stay the same size all the way. the bearings im useing are flat and the nut that i still got to make will tighten the bearings. there is something between the gears on the lathe and the gears in the lathe bed thingy thats not right ill have to look futher. but it seems as though when i reverse it thows it out of synk like a tooth jump on the gears from what i saw. i dunno ill keep trying.


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## Hal (Jan 27, 2009)

Sid

What type of steel are you trying to thread?  Some of the cold roll doesn't thread well, it rips and tears , try a piece of stress proof.

Are you using the compound to feed in at 90*, if so set your compound to a a true 29* (some compound are marked different)
By doing this you are only cutting with one edge of your cutter and putting less load on your lathe.

Hal


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## itowbig (Jan 27, 2009)

ok i got it figured out. this worked for me. i said the heck with reverse and used my dial thingy & the dial thingy on the lathe. fed the cross slide doohickey straight in. i have the other slide set at 29degree mark but have not had luck using it.
(but ill keep trying) any who the threads are not real perfect but darn near close.
now ive got to make the real part and a nut (oh wait im already made) :big: hope i can get this done so i can show it off ;D


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## ksouers (Jan 27, 2009)

Sid,
That looks like a big improvement! Congratulations!

Keep working on using the top slide. It really is easier to do it that way.


Kevin


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## rake60 (Jan 27, 2009)

Looking better Sid!

Single point threading is a craft in it's self.
I do believe you've got it!.

Well done! 

Rick


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## Holescreek (Jan 27, 2009)

> i said the heck with reverse and used my dial thingy & the dial thingy on the lathe. fed the cross slide doohickey straight in. i have the other slide set at 29degree mark but have not had luck using it.





> Are you using the compound to feed in at 90*, if so set your compound to a a true 29* (some compound are marked different)
> *By doing this you are only cutting with one edge of your cutter and putting less load on your lathe.*



Sid, From your last photo we can see that you have the compound set at 29 degrees from horizontal instead of 29 degrees from vertical. Rotate your compound to 90 degrees (perpendicular to the shaft) then 29 degrees from there. Infeeding must be done with the compound handle, not the crossfeed handle as stated above. Infeeding with the crossfeed causes the bit to cut on both sides of the angle. This is a bad thing. -Mike


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## itowbig (Jan 28, 2009)

Holescreek  said:
			
		

> Sid, From your last photo we can see that you have the compound set at 29 degrees from horizontal instead of 29 degrees from vertical. Rotate your compound to 90 degrees (perpendicular to the shaft) then 29 degrees from there. Infeeding must be done with the compound handle, not the crossfeed handle as stated above. Infeeding with the crossfeed causes the bit to cut on both sides of the angle. This is a bad thing. -Mike



could you please show me what you mean i was under the influence that was how it was done in my pic. i would love to know how its done as threading is a real test for me. 
thank you


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## lugnut (Jan 28, 2009)

The way my machine is set up I have to use 61° mark instead of 29°mark, I think it's in how the compound degree is placed??? Just a thought, I know that I read this somewhere when I was first trying to thread and it worked. I also us a hand crank to turn the lathe while threading. I looked back in the information I had saved for threading and found this little discription for threading. Hope it helps:
Insure the cutting tool is sharp and a true 60*. Set the compound slide over 29° or 29.5°  (on my lathe I use the 61° mark) from being perpendicular to the lathe axis. To the right for RH threads, to the left for LH threads. Now, using a thread gauge make sure the tool point is perpendicular to the lathe axis. Feed for the thread you are cutting is done with the compound slide NOT the cross slide. Start the cut some distance from the work to remove slop in the halfnut and lead screw. The cut should be done in several passes (5 or more) with two zero feed passes to remove any spring in the work. Each cutting pass should be at least .003". Use the travelling steady behind the piece to avoid deflection when required. 
Stop the machine at each pass and retract the cross slide enough so the tool clears the work when you reverse the lathe WHILE THE HALFNUTS ARE STILL ENGAGED. Return the cutting tool to the same distance from the work you began the first cut with, stop the lathe and feed the cross slide back to the same point it was before retraction. In-feed the compound for the next pass and start the lathe. Repeat until thread has achieved the final depth. Double check with a thread mic or use the 3 wire method to check the thread diameter. 
Use a cutting fluid on the threads while cutting - this will give the best possible finish on the threads. Nice bright & SHINY - I like Shiny! 
The reason for the set-over of the compound is that it cuts a flat chip. It also allows the one cutting edge to act as a wiper giving a smoother finish than the other side. If you feed it in perpendicular to the axis the chip formed is a "V" - a great way to bugger everything up. 
Was that you John?


Mel


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## Majorstrain (Jan 28, 2009)

Well that's another thing I've learned, thanks Lugnut. :bow:

Now if someone could post a pic of the setup from overhead the cross slide, I can make certain that my brain cells are linking up in the right order. 
(re. right offset for right hand thread etc.)

Thanks
Phil


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## Maryak (Jan 28, 2009)

Two ways to skin the threading cat.






Hope this helps. ???

Best Regards
Bob


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## Majorstrain (Jan 28, 2009)

Maryak  said:
			
		

> Hope this helps. ???



Yep, the old brain cells are in the right line. 

I assume it's 27° or 27.5° for a 55° thread, and 29° or 29.5° for a 60° thread.

I'm gonna have to make some chips to try this, that is when the temp in the shed drops below 40°C 

Thanks,
Phil


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## crankshafter (Jan 28, 2009)

Hi Sid.
Are you using the tailstock center : just a 2cent.

I'm leaving for some days.Grand Canary(Spain)
CS


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## itowbig (Jan 28, 2009)

CS no i did not use the tailstock i know i should but this piece is 1 inch in size so i thought it was ok as long as i did not take deep cuts.

Bob yes that really clears it up for me Thank you. i can now see my setup was all wrong. Thank you all for your kind help. :bow: :bow: :bow: now ill go apply what i have learned. :bow:


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## Holescreek (Jan 28, 2009)

Maryak, I have only used the compound set paralell (in your right view) for cutting acme threads. wouldn't using that method for a V thread require infeeding the compound (back to cutting on both sides) with the weakest part (tip) of the bit? We can get away with it on an acme because the tip is blunt.

1. My process is to set the compound similar to Maryak's left view but at 29.5 degrees. 
2. Set the tool bit on center and perpendicular to the work. 
3. Touch the tip to the work and set my cross slide dial to zero.
4. Make one pass with the tip just touching.
5. Back the cross slide out at the end of the cut.
6. Return the tool to the right end of the shaft with the X axis handwheel.
7. Return the cross slide to the zero position.
8. Feed in 3-5 thousandths with the compound.
9. Make a pass cutting at compound depth.
10. Repeat steps 5 through 9 until the pitch diameter is reached.

This has never failed me and is how I have taught others in the past. The cross slide is a zero reference only and is only used to infeed on acme threads. FWIW. -Mike


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## itowbig (Jan 28, 2009)

;D i got it yeeeeee haaaaaaa
BEFORE...




 AFTER...




 thanks maryak (Bob) this is what i needed . now i have even better looking threads
 :bow: :bow: :bow: :bow:


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## rleete (Jan 28, 2009)

itowbig  said:
			
		

> ;D i got it yeeeeee haaaaaaa



That's a great feeling, isn't it? A while from now you'll look back and wonder why it wasn't obvious from the beginning. I know I felt the same when I did my first thread (after messing up the first couple). The next two went surprisingly easy. Congratulations on learning a new skill.


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## shred (Jan 28, 2009)

If you've not seen it, Marv has a great writeup on threading and using the double-depth-of-thread numbers on the fishtail. I've tried it a couple times and prefer it.


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## Maryak (Jan 28, 2009)

That's Great Sid, :bow:

Glad you found the way you like to make them. 

Best Regards
Bob


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## rake60 (Jan 28, 2009)

Congrats Sid!
I do believe you've got it! Thm: 

Rick


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## mklotz (Jan 29, 2009)

Shred's comment reminded me of what I had written about the numbers on the fishtail gauge. It's worth repeating here despite its length. If you don't have the patience to read the whole thing, at least read the first part where George Thomas' technique for thread cutting using those numbers is described. It's a "quick and dirty" technique that you may find useful and it explains why those numbers are on the gauge.




Some time ago I wrote a treatise on the fishtail gauge (see below the =====
line) where I puzzled about the utility of the "double depth of sharp thread"
numbers on every one of these gauges I've seen. I concluded that, since the
tool is advanced using the (angled) compound, these numbers weren't terribly
useful and wondered why they were there. The numbers do not take into account
the effect of the angled compound so they seemed useless.

Since then I've come across some new information that may provide a solution to
my puzzlement. I'd like to pass that along since it will be of use to those of
us who screwcut threads.

In his book, "The Model Engineer's Workshop Manual", the author, George H.
Thomas, describes a simple and intriguing method for cutting a thread. 

1. Using the cross-slide, bring the tool up to touch the work and zero the
cross-slide dial. Also zero the compound dial.

2. Move the carriage to the right to clear the end of the workpiece and feed
the cross-slide in by "the thread depth". Depending on how your cross-slide is
calibrated (diameter reduction or actual movement), this is either the number
on the fishtail gauge or half the number on the gauge. Rezero the cross-slide
dial.

3. Back off the *compound* sufficiently such that, the carriage can be moved to
the left with the tool opposite the work again.

4. Feed the *compound* forward until the tool tip just touches the work piece.

From this point you can begin cutting the thread. Keep feeding in on the
compound until the dial reads zero again, at which point you'll be at proper
thread depth. The cross-slide can still be used to withdraw the tool at the
end of the cut. It is reset to its zero for the next cut.

There are several advantages to this technique. Note that at no point do we
need to concern ourselves with the angle of the compound. Whether you set the
compound to 29, 29.5 or 30 degrees doesn't matter. This technique
automatically compensates for the angle used. Secondly, there is no finish
value to remember for the compound - you go to zero and that's it. And
finally, all the information you need to do this is right there in your hand
on the fishtail gauge that you used to set the tool perpendicular to the work
piece.

Now you (and I) know why those numbers are on the gauge!

==============================================================================

Re our recent May meeting discussion of threading, and specifically, the depth
of thread...

Like most folks who thread on the lathe, I have a fishtail gauge (also called a
center gauge) which I use to ensure that the thread cutting tool is
perpendicular to the work.

On said gauge (and most others that I've seen) is a set of numbers labeled
"double depth of sharp thread". Specifically, the numbers on mine are those
given in columns A (tpi) and B (double depth of sharp thread) in the chart
below. I've always guessed that these numbers were somehow useful in deciding
how much to feed in when cutting a thread but I never took the time to sort out
how to use them. (For me, it's always been easier to draw a picture of the
thread and derive the depth I need using mathematics.)

A couple of questions at the meeting made me decide to puzzle out, once and for
all, what those numbers really are and how to make use of them.

Mathematically, the height of a thread, measured perpendicular to the thread
axis from sharp root to sharp crest is given by the following equation.

h = .5*pitch/tan(30)

where:

h = height of thread
pitch = 1/tpi

The little program I wrote prints out two times 'h' in column C in the table
below. As you can see, 2*h agrees perfectly with the numbers printed on the
fishtail gauge.

So the numbers on the gauge are indeed as described - the "double depth of
sharp thread".

So now the question becomes, "Why are those numbers on the fishtail gauge?"
Those numbers aren't particularly useful when cutting the thread. Most of the
time the question is, "How much do I need to feed in the compound when it is
set to angle 'ca' (compound angle)?" Mathematically, the answer to that
question is:

cin = h/cos(ca)

where:

cin = compound infeed

I've printed out cin for ca=30 deg in column D. For this case, we have:

cin = .5*pitch/[tan(30)*cos(30)] = .5*pitch/sin(30) = .5*pitch/.5 = pitch

and you'll note that the numbers in column D are exactly equal to the pitch of
the thread with tpi as given in column A.

So, the bottom line here is that I still don't know why those numbers are
there. Perhaps an old school machinist can explain how to use them but I
don't see any immediate value to them. (I can't imagine a machinist
multiplying the number in column B by .5/cos(ca) to get the compound feed
depth he needs.) If you didn't angle the compound at all when cutting threads
(i.e., feed straight in with the cross feed) and your crossfeed was calibrated
in diameter reduction (a .001 feed reduces diameter by .001), then the numbers
in column B would be your infeed to cut that thread. But what competent
machinist wouldn't angle the compound?

I don't know the answer but I do know this...I'm going to continue to ignore
the numbers on my fishtail gauge and base my calculations on what I understand.


 A   B    C    D

 4  0.433  0.433  0.250
 5  0.346  0.346  0.200
 6  0.289  0.289  0.167
 7  0.247  0.247  0.143
 8  0.217  0.217  0.125
 9  0.192  0.192  0.111
10  0.173  0.173  0.100
11  0.157  0.157  0.091
12  0.144  0.144  0.083
14  0.124  0.124  0.071
16  0.108  0.108  0.062
18  0.096  0.096  0.056
20  0.087  0.087  0.050
22  0.079  0.079  0.045
24  0.072  0.072  0.042
26  0.067  0.067  0.038


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