Info on threading

[UGLY OLDS]

Hi kids .. I have another of those "machinist" type questions ....  
 
 I was making up some linkage rods that required 7/16-20 threads ...  
I had some .500 bar stock handy ..  
Looked at my handy tap & drill size chart.. I showed that I needed a diameter of .3906  to produce 7/16-20 threads .    
 
I turned the bar stock down to .391 & threaded it by hand with a die ...  
 Complete failure .. The diameter was too small , to the point where a 7/16-20 nut would almost slide on the threads ..  
 
Being the creative feller that I am , I found a piece of "Factory Threaded" linkage  that had 7/16-20 thread & measured the O.D. right above the threaded area .. It measured .430......  
 
I turned another piece of .500 bar stock to .430 & again cut threads using the die ...  
 VOLIA !!!  ..Well almost .. The threaded area came out a little oversize on the diameter but running the die up & down the threaded area a few times made the fit acceptable ....  
 
I always use "Tap-Ease" as a lubricant  when cutting threads ...  
 
My Question ....  
 
Where did I screw up on the diameter     I know of no different thread charts for inside vs. outside threads ..    
 
What am I missing here ??  
 
  Thanks in advance .....  
 
Bob ......  
 

[50 F1]

A tap / drill chart is for internal threads.

If you drilled a .3906 hole and then tapped it with a 7/16 -20 tap the OD of the tap is .437 which will make a good thread

On external threads the OD of your part to be threaded  has to be the size of the thread---- 7/16.  Like the .430 dia. you used on the good part.

Mike

[phat rat]

A tap / drill chart is for internal threads.

If you drilled a .3906 hole and then tapped it with a 7/16 -20 tap the OD of the tap is .437 which will make a good thread

On external threads the OD of your part to be threaded  has to be the size of the thread---- 7/16.  Like the .430 dia. you used on the good part.

Mike

Bingo! Boy that must have been really easy cutting threads on the .391 stock

[Bruce Dorsi]

sorry!  - I couldn't find a delete button.

The question has already been answered.

[unklian]

7/16 - 20, the Major Diameter is 7/16";
or a few thousands smaller.

As posted, a Tap/Drill chart, gives the Minor Diameter,
or a few thousands larger.

[Harry]

http://www.marylandmetrics.com/tech/cvtchtfdm.htm

Print one of these...

[UGLY OLDS]

THANKS GUYS ....    

 By Jiminy ...I think I've got it ..  

Harry ... Thanks for the chart info .. It is printed & hangin' on the wall  
   

Bob.....

[chimp koose]

Tap Drill Size is easily calculated. Major Dia. - pitch=tds ,to put it in other terms     1/2 20NF bolt would calc. a tds like this; 1/2 - 1/20( 20 threads per inch means one thread is 1/20" ) Convert to decimal looks like .500-.050 = .450 as a tap drill size. If you dont have a drill that size the next one larger is likely the best fit.Tap drills are calculated to give 75% thread contact,little benefit to drill a smaller hole as fastener strength is not improved and risk of tap breakage is increased.At about 65% thread contact fastener strength will decrease.Metric threads do not need a tap drill chart as calculating is too easy to bother. Metric threads are listed by major dia, and pitch  so simply subtracting pitch from major dia gives the tap drill size.

[chimp koose]

If you are trying to run a threading die over a shaft, try to ensure it is .002 to .005 " under the nominal thread size.A chamfer on the end of the shaft also helps to start the die easily and straight.

[UGLY OLDS]

MORE good info ....  

 Thanks Guys .... );b(


Bob.....