Tap Drill Recommendation

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Philjoe5

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I've been using a chart published by Little Machine Shop for choosing tap drill sizes. Recently I began using a new 10-32 NF tap with the recommended tap drill in steel for 50% threads, a #18 (0.1695). The tap seems to cut with too little resistance, compared to the older tap I had been using. Screws fit OK but I'm wondering if there is some other factor that might cause this tap to cut so easily. It's a plug tap BTW. Is a 50% thread in steel typical? Thanks for any advice,

Cheers,
Phil
 
Philjoe5, My book recommends 70-75% for mild steel. The chart shows #21 for 10-32. For tuffer materials less % is shown. Monel and high speed steel @ 50% thread. For stainless or tool steel, or some tuff alloys, 60% is OK. For brass, babbit or rubber 80% is good, and for thin sheet metal 100% is OK. Your intuition seems to be right on. Tap drill diameter does matter.

Dave
 
Hole size does matter. Type of tap also matters tremendously.

If you've never used anything but boxed sets from Sears or similar, give yourself a treat and buy a purpose-built tap. You'll be amazed at how much easier they cut threads. I've all but quit using my boxed sets unless I'm in a hurry and can't find a tap. If you standardize hardware sizes in your shop (pick a "small, medium, and large") and buy the taps for just a few sizes, it isn't even that much money.

I bought several types in 1/4-20 (one of my "standard" sizes) to experiment with one time:

P3023853.JPG


From left to right we have a 3 flute 45 degree spiral flute plug tap, a thread forming plug tap, and a thread forming bottoming tap. Thread forming taps are interesting: they rolll or press the threads in without doing much of any cutting. They're wonderful for aluminum, and they'll also work on steel. The spiral flute tap is great because it cuts the chips real long and shoots them out the top. I seldom have to do the old clockwise 1 turn, counterclockwise 1/2 turn to break the chip. Just keep going. Essential for things like tapping heads, but also really nice for hand tapping.

There are more kinds of taps, but even the normal tap designs seem better than the box sets. If I was only going to buy one type, the spiral flute tap was the winner with by far the lowest cutting force. I have $9 for a name brand from Enco.

Cheers,

BW
 
Marv, I downloaded the program. It’s very useful and I like the fact that it recommends different wire drill sizes depending upon the % thread one wants. I’m thinking the 50% thread recommended by Little Machine Shop is conservative, though probably quite satisfactory. So to test out the program I ran a #20 drill for tapping the 10-32 threads in some mild steel. When I ran the tap in, I sure do feel like I’m cutting something now. I'm thinking a slightly higher % thread on something like set screws may prolong thread life, am I correct?

Thanks to everyone for the tips.

Cheers,
Phil
 
A #20 will give you about 70% DOT which is quite reasonable.

For a 10-32, the standard formula yields:

TD = 0.19 - 1/32 = 0.159 (#22 drill)

for 77% DOT. So you should feel some resistance but not as much as with the standard rule of thumb.

It all depends on the application. LMS's 50% seems a mite low to me for steel. I generally use 60% as a good compromise between engagement and ease of tapping.

I don't see any obvious connection between DOT and thread life. Perhaps someone else will have some wisdom on that point.
 
Almost all published tap drill charts such as this one are based on a 75% thread.

In a condition where the fastener will be removed and re-torqued over a long life span a 75%
thread is engineered to have a built in safety factor.

Drill a hole to be tapped 2-56 for a 75% thread and you have an adventure ahead of you.

My personal methods will never be published! :D
I'm no engineer, but I can tell you that when the tap drill size calls for a #50 I use a #49
That makes the hole 4% larger. What effect does that have in the % of thread?
I have no idea...

What I do know is I break fewer taps and I've never had a thread pull or
fail in a model from the practice.

If you are building a clock that is expected to require frequent, regular maintenance, make
those threads 75%. If you're building a model that will run 3 minutes at a time until you
move on to your next project...............................

Rick

 

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