Saturday, May 11, 2019

Are Spiral Flute Taps Intrinsically More Brittle Than Straight Flute Taps?

I have broken two Greenlee 8-32 spiral flute taps this morning tapping 6061 aluminum with plenty of lubricant.  The spiral tap flutes look like the have a thinner cross section than straight flute taps.  I wonder if this is why the tap sets you find at your DIY stores are almost always straight flute. 

I have long used 8-32 socket head screws to hold the acetal block into which the casters screw inside the aluminum sleeve.  (In this case, I am using aluminum blocks for reason below.*)  I am thinking of switching to 1/4"-20 hex head bolts instead.  The taps are stronger, and the per screw price is not that much higher.  It also reduces the number of different parts I need to keep in stock. 

*I was making a set of ScopeRollers for the table on which my chop saw sits.  I am casterating the table.) They are simpler because they do not need a 60 degree angle.  I am thinking of going back to acetal for these blocks even though acetal is more expensive than aluminum.  Manufacturing is much quicker because there is no need to tap the acetal.  The 8-32 screws self-tap into the acetal.  On the other hand, once I have at least some 1/4"-20 threads in place, hex head bolts are self-tapping when using a socket wrench.


  1. You can modify a screw to be self-tapping. Using a Dremel with a small cutting wheel, cut a groove through the threads, down to the core. Wider is better. A three-sided file works, but is too slow for production. You want the cut to have some width to give the material room to peel off the bore.

    The sharper the edges of the cut, the better. You want the screw to cut new thread, not rip.

    Like a tap, multiple slots are usually better, but may not be necessary.

  2. Good to know. I've used drywall screws in wood to form the hole into which I put an ordinary wood screw, but I wonder how much tearing of the wood that does. I've stopped using drywall screws to fasten wood together because they seem to work out over time where ordinary wood screws don't (so far).