threading

[Loose nut]

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.

 

[itowbig]

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.

 

[itowbig]

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

 

[chuck foster]

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

 

[Bogstandard]

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

 

[mklotz]

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.

 

[itowbig]

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

 

[Bogstandard]

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

 

[SmoggyTurnip]

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.

 

[mklotz]

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.

 

[jwsvandr]

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

 

[rake60]

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

 

[Mcgyver]

.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.

 

[Bluechip]

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.

 

[itowbig]

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 ???

 

[moconnor]

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

 

[ksouers]

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

 

[itowbig]

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

 

[moconnor]

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.

 

[Holescreek]

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