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1. ## Help reading Thread and drill charts

Hi, I need help with how to read and interpet the thread chart.
I came across a Chart that gave the dimension for the threads but I'm not understanding what the numbers mean. For Example:

Screw Size

Diameter = 1.1875"
TPI = 18
Minor Diameter = 1.1193
Drill Size is 1.1250

Does that mean that the Diameter with thread is 1.1875" and the Minor Diameter is without threads?
How about the Drill Size? Does that mean its 1.1250" without the thread?

That Chart can be found here: http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

What I'm trying to do is to add threads to my model.

Thanks.

2. Originally Posted by cotang
..Screw Size

Diameter = 1.1875"
TPI = 18
Minor Diameter = 1.1193
Drill Size is 1.1250

Does that mean that the Diameter with thread is 1.1875" and the Minor Diameter is without threads?
How about the Drill Size? Does that mean its 1.1250" without the thread?

Thanks.

Long answer, not that you need for a drawing, but at least you will know why the hole is bigger than the minor diameter.

The major diameter of a thread is the OD to to top of the crest of the thread; presumably you are dealing with standard unified thread from with a 60 degree thread angle. The flanks of the thread do not come to a sharp point they are blended into a top radius, the crest of the thread; similarly the bottom does not go to a sharp vee it also has the radius, the root of the thread. The size of these radii depend on the pitch and you can probably find it in Machinery's Handbook; for 18 tpi it is only going to be a few thousands of an inch.

The minor diameter is what you get when you subtract two times the height of the thread; that is the vertical distance from the crest to the root.

The tapdrill size is bigger that the root diameter so yes some of the thread is missing when you bore a hole this size and use a tap for the thread. You could bore a hole the exact size and use a full profile internal threading tool and then you would have a full thread to the minor diameter of 1.1193".

An 'oversize' hole is generally used for tapping because the driving torque needed for the tap gets very high if the tap is cutting from the true root diameter. In your example the tap would probably be strong enough to do it but with smaller size threads the tap will likely break. So the hole is made bigger than the minor diameter to reduce the load on the tap.

This does weaken the thread but only very slightly; a thread tapped into a hole that results in only 75% of the full thread height being present will have more than 90% of the strength of a full height thread and a 65% thread resulting from an even larger hole still has over 75% of full height strength. But the torque needed to drive the tap through the 75% hole is possibly less than half that need to tap a full height thread and the 65% may be down a half again. It is largely a matter of playing off the reduction in tapping torque with the reduction in thread strength, and to compensate for reduced thread height and strength it is always possible to increase the numbers of thread in engagement.

3. 1.1193" is the minor diameter, meaning the inside of the thread grooves. 1.1875" is the major diameter, which is the peaks of the thread -- the max diameter. 1.1250" is drill size that you should use in Aluminum, Brass, Plastics to yield a 75&#37; thread after tapping. Notice that the drill size recommendation is different for tapping steel, btw.

The numbers on the right are the drill sizes you would use for the screw to slide right through without any female threads. They provide info for close fit and free fit.

See this also... http://en.wikipedia.org/wiki/ISO_metric_screw_thread

Cheers,
-Neil.

4. Originally Posted by Geof

Long answer, not that you need for a drawing, but at least you will know why the hole is bigger than the minor diameter.

The major diameter of a thread is the OD to to top of the crest of the thread; presumably you are dealing with standard unified thread from with a 60 degree thread angle. The flanks of the thread do not come to a sharp point they are blended into a top radius, the crest of the thread; similarly the bottom does not go to a sharp vee it also has the radius, the root of the thread. The size of these radii depend on the pitch and you can probably find it in Machinery's Handbook; for 18 tpi it is only going to be a few thousands of an inch.

The minor diameter is what you get when you subtract two times the height of the thread; that is the vertical distance from the crest to the root.

The tapdrill size is bigger that the root diameter so yes some of the thread is missing when you bore a hole this size and use a tap for the thread. You could bore a hole the exact size and use a full profile internal threading tool and then you would have a full thread to the minor diameter of 1.1193".

An 'oversize' hole is generally used for tapping because the driving torque needed for the tap gets very high if the tap is cutting from the true root diameter. In your example the tap would probably be strong enough to do it but with smaller size threads the tap will likely break. So the hole is made bigger than the minor diameter to reduce the load on the tap.

This does weaken the thread but only very slightly; a thread tapped into a hole that results in only 75&#37; of the full thread height being present will have more than 90% of the strength of a full height thread and a 65% thread resulting from an even larger hole still has over 75% of full height strength. But the torque needed to drive the tap through the 75% hole is possibly less than half that need to tap a full height thread and the 65% may be down a half again. It is largely a matter of playing off the reduction in tapping torque with the reduction in thread strength, and to compensate for reduced thread height and strength it is always possible to increase the numbers of thread in engagement.
Ditto on Geof post.

cotang I would not use that chart you are referring to this is the first I have ever seen this in the Private Industry to include Aero Space and to have different sizes for steel .... it would never happen they apparently are using this chart for other purpose then main stream application.

http://catalog.starrett.com/catalog/...30801&Mode=REF
http://catalog.starrett.com/catalog/.../2900/2881.pdf

5. Pilot diameter = nominal screw OD - 1/n where n = threads/inch
eg. for 1/4-20 UNC , .250-1/20=.200 inch drill or#7

cheers

6. Originally Posted by cam1
Pilot diameter = nominal screw OD - 1/n where n = threads/inch
eg. for 1/4-20 UNC , .250-1/20=.200 inch drill or#7

cheers
Good reference, brings me back to when I was in college, this will get you there close until such time you need a class 2 or 3 threads.
But everyday use this is a good one to remember.