Friday, December 4, 2015

Wrong Size Drill

I have been working on a new model of ScopeRoller; actualy two different new models.  I was struggling to tap 1/2:-13 threads in an aluminum block, and I figured out why.  This size tap uses a 27/64" drill bit (.422")  When I first started tapping this size, not having a .422" drill bit, I used a .408" bit.  That works fine when tapping acetal; but not for aluminum.  As a temporary measure, I used a drill bit that would give me a loose fit.  What a difference!  I am replacing the tap and buying a .422" bit.  The tap is still fine, but sharp taps are a joy to use.


Will said...

Are you using a lubricant when tapping? Most drill sizes recommended for taps are calculated to give around 60-70% thread engagement, as more contact surface doesn't give a stronger joint. This gives you a window to work with, if you don't have the preferred drill bit available. However, aluminum can be a problem material in this regard, especially if done dry, as it can end up with surface displacement in the thread, due to it being a bit sticky or gummy. 6061 alloy in particular is noted for this.

Clayton Cramer said...

When tapping metals, I ALWAYS use oil. With acetal, no need.