I just finished putting together the latest installment of my CNC Chef videos for Cutting Tool Engineering Magazine. More on how to get that video later (they’re editing it), but the subject is 7 ways to avoid breaking taps. One of those most important is to Mind the Depth on Blind Holes.

I never thought real hard about exactly how deep the tap can go for a blind hole.  I knew it had to stop short of the bottom of the hole, but it turns out there are detailed calculations you can make to determine exactly how much clearance to leave.

Here’s the slide from my video that gives all the formulae:

One of the key things I discovered is that the clearance you need to leave is quite a bit more than I would have thought.  For a 1/4-20 thread roll form tap, it’s 0.214–almost a quarter of an inch!

That information came to me from the Tapmatic people, who surely do know a lot about tapping.

The calculations aren’t hard to do, but given their necessity whenever a blind hole is to be tapped, I thought it’d be convenient to do the calculations for you in G-Wizard Calculator.

Figuring out stuff like that so you don’t have to is exactly why G-Wizard exists.  So starting with version 4.00 (click this link to download the first release of the New Year!), there’s a new Blind Hole Clearance Calculator located on the Threads Tab in G-Wizard:

Just click the “Blind Clearance” button below the Tapping Drill chart to bring it up.  Once you’ve got it, tell it what kind of tap you’re using and it’ll calculate the rest based on the thread you have currently selected.


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Maximum Tapping Depth on Blind Holes [ Stop Breaking Taps! ]
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