Can I modify a large bottoming tap so that I can start a tap with it?

[WAE]

Hi everyone!

I am converting my HF mini mill to CNC with plans from fignoggle.com. One of the parts, the z-axis nut flange, calls for a 15/16 tap through 1/2" thick aluminum. I bought two taps from e-bay (came as a set of two) but after I got them, I realized that they are bottoming taps (no taper on the end). I tried tapping with it but it won't start. Can I grind a small taper on the end and use it? I don't want to buy a tap that I will never use again. What do you think?

Also, the end of this tap has a 1/2" square, which obviously won't fit in my tap wrench or my mill. How do I hold this thing?

 

[Tin Falcon]

if you have a tool post grinder you can probably do it . IMHO hand grinding will not cut it because the tap will wander . If yuo do grind it hone the flutes to take away the burs. I did not want to buy a tap either so I single point threaded on my lathe. No tap needed.
as far as holding it put a center point in your mill then turn the tap with a regular wrench.
Tin

 

[gbritnell]

You can make a starting tap out of it. Depending on how good your eyesight is here's what I do. I first grind a small taper on the tap, back about 4 threads. Try to get each flute even looking. I then put some marker ink on the taper and then go back and grind somewhat of a relief up to to about .01 of the starting edge. I use the ink so that I can see where I'm grinding. Ideally you would want the relief to go right to the edge but if you're close it will still work. If you don't have a tap wrench big enough make up a piece of brass stock with a 60 degree point on it. Chuck this up and put it in the end of the tap. Now turn the tap with a small wrench on the flats. Maybe you have a small spring center, if you do use that.
gbritnell