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What Diameter Stock To Start With Given Thread Choice

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Buggy Chief

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#1
Was just playing with my new lathe and practicing threading on some .250 steel stock. I was running 20 TPI setting. I micd the stock and it read .250. I threaded great and it worked well. Problem is I micd after threading and the .250 stock now read .273.

So the question is..What is the rule of thumb of stock to start with given you know the final diameter? TIA...
 

4gsr

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#2
.250 diameter. What you have caused is a burr on top of your thread you cut when using a die nut or single pointing a thread. Go over it with a flat file and remove the burr and recheck. Should now read .250". May also need to use a triangle file to break the edges of the freshly cut thread, too. A good cutting oil will help, too. Ken
 

Wreck™Wreck

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How did it get bigger? Is that what you are asking, if so it is the burr.

As far as starting diameter begin with the what the thread specifications will tell you which will be at maximum the Major Diameter of the thread for certain classes of thread fit, mostly somewhat smaller however.
 

Bob Korves

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Make sure the tool is sharp and has the correct geometry. That is a pretty big bur for a 1/4-20 thread. It could also be the material you were cutting, perhaps some gummy stuff.
 

Buggy Chief

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Yup. I thought it was a big bur as well. brand new insert at 30 Degree. My first few attempts and am learning a bunch. Just practicing before I start one of my projects that calls for brass:)
 

Wreck™Wreck

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Yup. I thought it was a big bur as well. brand new insert at 30 Degree. My first few attempts and am learning a bunch. Just practicing before I start one of my projects that calls for brass:)
You are in luck, it is difficult to kick up a large burr in brass, be aware the the chips can be annoying and more so with bronze.
Good Luck
 

Tozguy

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#9
Since common 60 deg. external threads have a flat or rounded top you do not have to cut until the tops are sharp.
Burrs will not be so high if you leave a small flat on top. Thread charts will give you the standard tolerances for diameters of various threads. For example, a 1/4-20 class 1A external thread can be as small as .237'' in o.d.
 
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gonzo

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#10
You can find the recommended initial diameters in the machinest handbook if you happen to have one.
The initial diameter is usually a small amount less than the nominal bolt diameter.
If you are not really picky you can just keep cutting until the nut fits.
This works astonishing well if you are patient and make the last cuts very small.
However, I am certain that a REAL machinest would have heart failure upon reading this.
 

Wreck™Wreck

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However, I am certain that a REAL machinest would have heart failure upon reading this.
Not at all, if making a threaded part that someone else will measure then by all means make it within the stated class fit or particular given dimensions.

If making something for personal use then do it however you want, one may also make non standard threaded parts that will confuse the hell out of someone in the future, 1/4-19, 5/16-15, 3/8-17 and so on.

I am guilty of this little joke.
 

boostin53

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#12
If making something for personal use then do it however you want, one may also make non standard threaded parts that will confuse the hell out of someone in the future, 1/4-19, 5/16-15, 3/8-17 and so on.

I am guilty of this little joke.
I'm guilty of doing the same thing! I find the looks on people's faces to be funny when they can't find a nut or a bolt that fits my weird choice of thread for a given diameter. I only do it for laughs then eventually tell them.
 

Wreck™Wreck

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#13
You can find the recommended initial diameters in the machinest handbook if you happen to have one.
This data may be found all over the Web

The Industry nomenclature is:

Major Diameter, this is the nominal bolt size and will include a minimum and maximum.

Minor Diameter, the diameter at the root of the thread, minimum and maximum.

Pitch Diameter, this is the important one, it is roughly halfway between the Major and Minor diameters, also with a minimum and maximum dimension.

Lead, the distance between thread centers, not to be confused with pitch, minimum and maximum values.

Crest and Root, these will either be flat or radiused depending on the thread profile.

Thread Class, the higher the class the closer the dimensions, the lowest thread class is used for common household fasteners, the tightest is used for holding the wings on the airplane.

As seen here, an A thread class designation is external, B is internal.
http://www.engineersedge.com/screw_threads_chart.htm
 
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