Understanding drill and tap sizes???

Looking some charts that identify what size drills to use for particular size taps and have a question. A portion of the chart looks like this:

Drill ...... Tap
7...... 1/4-20
13/64......
6......
5......
4......
3...... 1/4-28

So does this mean that the 1/4-20 tap is used for drill sizes 7 through 4, and starting with 3 you use the 1/4-28, and so on?? Thanks-

 

I enlarged the page out of The Machinist Handbook, can't
remember how it is organized right now.

I have such a mess of drill bits, I use the decimal and use
a caliper to find a drill bit that's close.
TZ

 

drill chart drill and tap 2 another drill and tap 3

 

Dan,

What your chart is telling you is to use a #7 drill for a 1/4-20 tapped hole. #6, 5, & 4 aren't used for any tapped holes. #3 is used for a 1/4-28 tapped hole. In a pinch you can use one size smaller than recommended, but that's extra wear on your tap.

Ed

 

when you see a chart it will give you different size twist drills to use for a specific tap. It all depends on the engagement you want on the threads, 70% engangement is about right. Sometimes you don't have one the appropriate size so you look on the chart to find one thats close.

 

Ed-
That makes perfect sense now, thanks for pointing that out. A little closer studying of the chart probably would've made that obvious to me!

 

There are different systems for organizing drill sizes.
They include fractions,letter,numbers,millimeters.

That chart is for different percentages of full thread.
The standard charts will give you approx. 70% thread form.
Using a slightly larger drill will give you a lower percentage.
This is usefull on hard,tough,difficult to tap materials,
to reduce the chance of breaking taps without compromizing strength.

#7 drill is .201 dia. and will produce 75% thread
13/64" drill is .203" dia. and will produce 72%.
#6 drill is .207 dia. and will yeild approx. 71%.
etc.

 

You may want to buy the right drill size for each tap and drill some holes in a piece of wood or something and match them up with the taps . That way you won't be spending so much time looking for the right ones.You do know the different taps and when you use what, right?
Taper tap has about 8 threads that taper to nothing,used to start the tap process,(also called starter tap)
Plug tap has about 4 threads tapered.
Bottoming tap will have about 1 or 2 threads tapered. use for bottom tapping, very hard to get started straight without useing the taper first.
All taps should be backed up after every 3/4 turn or so .You will feel it cut the metal threads off when you do this. The most common reasons for breaking a tap is misalignment and a dull tap. I sharpen my taps ,take a small stone grinder or even a cut off wheel and run it the lenght of the tap on the LEADING edge,(where it does all the cutting.)
I also buy only taper taps and as they wear I cut the ends off ( with a cutoff wheel ,slowly so as not to screw up the heat treat)to make bottoming taps. The most wear area is removed and you can get double life out of them.

 

Just buy Spiral Point taps,
and some good Tapping Compound.