Tapping blind holes in hot rolled steel

OP here:

I was shocked to see the 0.020in difference in suggested tap drill size chart from Little Machine Shop for brass vs steel.
Going 0.020in over the recommended tap drill size, however, gives me the heebie jeebies.

Those are just guidelines, why not just try the next largest drill? Something like G or H.
Using a new quality drill so it cuts clean and to size, might help prevent the heebies.:)
Looking forward to hearing how it goes for you.
 
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Going 0.020in over the recommended tap drill size, however, gives me the heebie jeebies.

Understood.
Note too that those recommendations from LMS are also for 75% thread for soft materials and 50% thread for hard materials.
It really depends on the application what the required thread depth is.

Since you need to re-setup anyway, it would be a great time to run some practice with different over-size holes to see how they tap, how they look, and finally how much you can tighten them before they give out.

-brino
 
I have always used an "F" drill for 5/16 NC, used with good quality taps, I have never had problems with any material, most of the time, not needing to back up the taps to break up chips, this being with ordinary hand taps.
 
Just put the LMS chart in my cell phone. Really good discussion.
 
Update:

I have new taps on order from McMaster but was quite curious about the tight tap issue. The first tap I tried was indeed a carbon steel tap but I found a high-speed steel US made tap hiding in my shop and thought I would just try it in the original "F" size hole.

Lo and behold it tapped just fine! It was a bottoming tap and I had to back out all the way and blow out the chips a couple times, but it tapped with much less force on the tap wrench. I tapped all five holes without mishap, with approx 50% over the minimum 1.5 times the thread diameter.

I suspect the hardware store tap was tight because the minor diameter was simply ground too big. I wish I had a thread mic to confirm. But I learned something new about taps today!
 
There was a brief discussion on taps a while ago. Most carry a GH designation the G means they are ground, while the H designates the diameter. I believe the H numbers go as high as 12, with each number meaning the tap is .0005" larger than the previous one in the series.

An H1 designation means the tap will form threads to the nominal diameter of the fastener plus .0005". The most common H number used in non plated materials is H3 meaning the tapped threads are .0015" larger than the nominal diameter of the fastener. That means a 1/4-20 H3 tap will cut threads to a diameter of .2515" while a 1/4-20 H12 will cut threads .256".
 
OP here.

I'll have to look on the hardware store Irwin tap I bought that is too tight for the "F" size hole and see if there is a designation.
 
There won't be . These are not machine taps .
 
My experience has shown that with taps more than any other tool, quality is paramount, carbon steel just doesn't cut it, and anything less than fully ground thread form from a recognised quality tool maker is generally not worth the hassle.
 
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