Tap Drill Charts are Wrong?

[basalt]

Hi Guys,
I noticed that all the Tap Drill Charts I've seen are giving the tap drill sizes as 75% of full thread depth but in the old American National which works out to 90% Unified National.
For example a .250-28-UNF tap drill size is .213" on the chart which is 80% of full thread depth American National = 96% Unified National. To get 75% Unified National the tap drill size would have to be increased to .221"
As a result I have calculated the tap drill size for a unified thread is D-.812P to get a 75% full thread, whereas the old american national was D-P.

Has anyone else noticed this? Do you think it matters?
Thank you for your input.
Colin

[fizzissist]

Tap drill charts are for when you're fixing your lawnmower or BBQ.

For real machining you get out your Machinery's Handbook.

Though somewhat facetious, charts are starting points, and each job has unique requirements. Most of the time a 65% thread is just fine. Sometimes if you have a nasty material you may have to drill a bigger hole that results in a 65%, simply because of tapping issues. Much more than 80-85% thread isn't buying you much anyway, since you're well into diminishing returns on strength.

[basalt]

Thank you for your response,
My main point was that I assumed I would be getting about 75% of full thread depth when using the charts but in reality it is closer to 90% (UN). I do agree with you that if I want to know for sure I should consult the Handbook and use the UN formula %=[(D-TDS) x TPI] / .01082
Colin

[fizzissist]

The problem with charts..... Who made it, why, what's it for, who double-checked it, etc, etc.....

I had a chart that actually showed drill sizes for 2 DIFFERENT percentages...

For the sake of my beloved taps (and to avoid the dreaded burn-it-mill-it-chip-it-etch-it out and still end up with a scrap part) I always consider the application (and of course the after plating spec if any) and use the biggest drill I can legally get away with, so my beloved tap can lounge on the way down.

Even more critical, by an order of magnitude it would seem, when drilling for forming taps.