TAP PROBLEM

riversidedan said:
so we gotta 3/8 bolt with SAE threads , the chart sez to drill the part with 5/16 drill then tap with a 3/8 16 tap.........
the pitch gauge reads 16 and lines up with the tap and bolt but when I go to tap doesnt wanna go in........cant figure it out :rolleyes:
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If your drilling is particularly accurate you may try larger than 5/16"(77 % thread), letter O is .3160 at 72 % thread. I do this in hard to tap materials
all the time, going as low as 50% thread engagement. And this is using low run out drills in a mill with high quality taps. The first 3 threads hold over 90% of the load . According to Regal Cutting Tools, increasing the thread engagement from 60% to 72% in 1020 steel reguires twice the torque but increases strength by only 5%.
 
am wanting to tap into some simple 6061 ...here you see the 3/8 bolt thread gauge and 3/8 16 tap

BTW Ive had good luck with previous tapping adventurers this one thru me a curve

I also put the tap in the drill chuck on the lathe turning the chuck by hand but the tap woudlnt start cutting.... while looking at the tap , the 1st 3 rows are flat and not sharp from the top on down so Im thinking the tap is garbage and the wrong shape

in the 2nd pic the 1st 3 cutting edges are flat not sharp like the rest are on down, so again am thinking thats the culprit
 

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The tap you show is a plug tap. To start a thread with that tap will require a considerable amount of force. Get a taper, or starter tap.
 
thats what I was kinda thinking but didnt have a taper one in the lot...........am also assuming the 5/16 drill should be the right one for that size tap. correct me if Im wrong
 
Looks like I misunderstood your post when I first answered. 5/16" is indeed the correct drill size for tapping 3/8-16, especially for aluminum. Sometimes you want to go a skosh larger if tapping something tougher, like steel. Guess I'll second the opinion of other posters and suggest a good quality (ie, HSS) taper tap to start with. You can also try slightly beveling the outer edge of the hole with a countersink.

I myself like using name brand "spiral point" taps for all my tapping. These are generally only available in "plug" style, but have worked well for me in steel, aluminum, plastic, etc.
 
I had a similar experience. Despite all factors correct the tap required so much force it would have broken if I continued. Went in the drawer and got a high quality HSS tap and it went right in.
 
Buy a new tap. That tap looks like it's dull and as SLK said, that's a plug tap. Practice some power tapping on some scrap.
 
what is your guys thoughts about Irwin taps?
 
That doesn't look like a plug tap to me. Rather it appears to be a bottoming tap. A plug tap has 5 to 7 chamfered threads on the leading end. A taper or starter tap has 7 to 10 tapered threads on the leading end, and a bottoming tap has 1 to 2 chamfered threads on the leading end.

This tap appears to have only a single chamfered thread at the leading end, and is consistent with a bottoming tap. If you're tapping 6061 aluminum a plug tap should be easy to start and is all that's needed if you're doing a through hole. If you're doing a blind hole the plug tap should be followed by the bottoming tap to get threads to the bottom of the hole.

I don't know who's rating Irwin taps at at 4 1/2 out of 5 stars. They are almost always carbon steel and made in China. They might be good for a couple holes in aluminum, but that's about it. In the professional world they are rated as one step below junk. Go with HSS taps from name brand manufacturers like Morse, Brubaker, Triumph, Precision, Regal, YG 1, Viking, Hertel, Dormer, Sandvik, Union Butterfield, Kennametal, Guhring, or any of a hundred other name brands offered by industrial suppliers.
 
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