TAP PROBLEM

[riversidedan]

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

 

[hman]

I may be a bit rusty on this ... but I've always thought that SAE thread pitches were equivalent to NF (national fine) threads. The NF thread pitch for 3/8" is 24 TPI, not 16 TPI (which is NC, equivalent to the old USS standard). But then, you did check both the bolt and the tap against the same gauge, and tapped according to the chart. Guess I'm stumped.

OK, silly question ... might the bolt be left-hand threaded????

 

[Ken226]

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 bolt 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

What kind of tap are you using? As in brand and material?

What type of material are you tapping? With aluminum you can get away with poor taps. With tougher steels, not so much.


If your already using a quality HSS tap, disregard the following info.


When I first started tapping holes, I bought cheap carbon steel tap sets from local auto parts stores.

Not one of them, ever worked very well in steel and in some cases, tougher steels like 4150 or in titanium, the hole would remove the teeth on the tap.

If you haven't already, try a HSS tap. Something like this:

Amazon.com: MaxTool 3/8-16 Spiral Point Taps HSS M2 Thread Taps 16 TPI FULLY GROUND RIGHT HAND; SPF02W01R24 : Industrial & Scientific

Buy MaxTool 3/8-16 Spiral Point Taps HSS M2 Thread Taps 16 TPI FULLY GROUND RIGHT HAND; SPF02W01R24: Spiral Flute Taps - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

 

[projectnut]

Is it possibly a tapered tap rather than a plug tap? If you're using it for a blind hole it may not be cutting full threads at the end. A taper tap usually has between 7 and 10 chamfered threads at the lead end. That could be over 1/2" on a 3/8-16 hole. They're great for starting a threaded hole, but should be followed by a plug tap (3-5 chamfered threads) on through holes, or a bottoming tap (1-2 chamfered threads) on blind holes.

 

[SLK001]

My guess is that you're using the wrong tap to start your hole. A bottoming or plug tap is to finish a hole (blind), while a tapered tap starts a hole (and can be used for the entire hole, if it is a thru hole). Although there are more types of taps, the usual progression of a tapped hole is start with a taper tap, then move to a plug tap, then move to a bottoming tap. In most materials, you can skip the plug tap and go from taper to bottoming tap.

 

[Karl_T]

most cetainly you have either the wrong or a poor quality tap. A picture would tell the story.

 

[RJSakowski]

If the tap won't start in the hole, it is most likely due to either the wring style tap or a bad tap. as stated above. Taps sold at DIY and auto parts stores are usually plug taps. One get-around us to drill the start of the hole just slightly smaller than the thread o.d. for a few threads which will allow the tap to get a bite and draw itself in. This isn't possible in all circumstances though. Another is to tap with a drill press or lathe with the quill providing pressure to push the tap to get it started. These are Band-aids though and the proper procedure is to get the proper tap.

 

[markba633csi]

5/16" is the correct size tap drill for 3/8"-16 threads so the tap must be the culprit. Remember you have to push a bit to get the tap started, chamfering the hole helps. And you have to check carefully that the tap is started straight as you begin cutting- past a certain point you won't be able to correct the tilt if it gets started crooked
Forward, back, forward, back, forward, back is the motion you want to use when hand tapping
-Mark

 

[projectnut]

My apologies for my first post. I misread the original post thinking the bolt wouldn't go in the threaded hole. I see after reading it a second time the tap won't go in the drilled hole. I'm wondering if you have a roll form tap rather than a thread cutting tap. Roll form taps usually have the designation RF on the shank.

A 3/8-16 roll form tap requires an 8.8mm (.346") drilled hole while a 3/8-16 thread cutting tap requires a .312" (5/16") drilled hole. The roll form tap in this case needs a hole .034" larger than one for a thread cutting tap.

 

[Richard King 2]

If I were you I would measure and look at everything again. Measure twice and cut once as the old saying states. How did you drill the hole, on a machine or by hand? If it is cockeyed it may not start. When I was an Apprentice I made a Tapping block . so I could keep the tap straight when I ran it into the hole. Also the members above all make some good points. If your new to the trade, buy only Industrial Grade drills and taps from an Industrial supply. I would stay away from imported ones. If a tap breaks, you will play heck getting it out. Also use tapping fluid. Buy a starting tap, a plug tap and a bottom tap and practice. Another thing you can do is to drill the hole one letter or size bigger if the final tapped hole does not support weight. I do this a lot when I install way wipers If nothing helps please show us some photo's. Good luck

PS: Look at the tap and look for letters RT meaning right hand. If it says LF that's the problem