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