Drilling Properly Sized Holes

I have a tapered dreamer for 1/8-27 but have never tried it out I always just drill and tap your probably just going to deep with the tap.
 
If you are going to be tapping lots of NPT holes, get an L1 plug gage.
 
Thanks for all of the responses. Yeah..I may have tapped a little too deep on the prototype and should have used a little smaller drill like the "Q". I did a little playing around with a piece of scrap this afternoon just looking at hole size. Here's what I found on drill bit sizes:

11/32 - decimal .344"
Triumph drill bit diameter .342"
Costco drill bit diameter .3405"

Both drill bits produced produced approximately the correct hole size on exit ~= .344". However, on the topside it was closer to .349". It may be the entry chatter causing this. This may be remedied by lower rpms on entry. I used a set of SPI hole size gauges to do the measurement. This may be also do to flexing of the jobber drill bits during the drilling operation. I think I'll be using screw machine bit in the future for tapping holes.

I'll do a little more playing around with this tomorrow.

-Joe
 
don't be afraid to go smaller on the drill hole when using a taper pipe tap.
when in doubt use a reamer of the correct dimension.
the taps cut is going to make a threaded taper while it's cutting anyway, as long as the fitting starts and threads in a reasonable distance you'll be good!

mike:))
 
The wide end of a pipe tap is much larger than the required thread diameter at the top of the hole. Lacking an L1 gauge, take your pipe tap and thread it by hand into an existing, good pipe fitting. Take a piece of electrical or masking tape and wrap it around the upper exposed part of the tap. That will give you an idea of how deep to tap. Use it for a reference. I have known old timers who used annealed wire wrapped around the tap and twisted tight at the correct spot.

One thing I may have missed, are you tapping by hand or under power? That can affect the fit.
 
Rule of thumb is to run tapered pipe taps in stopping with four threads left showing on the tap. Like was said, test one in the material your tapping or similar, to be sure of the correct/necessary depth.
 
As a part time gun smith. When drilling holes that must be the correct size within 0.003 it's always recommended to drill slightly under size hole and finish with a reamer to the correct size. and stay away from those cheapo bitts, The size variations are ok for most things but not when size is critical. Dave.....
 
National Pipe Taper 3/4" per foot included angle, if the hole is .020+ big at the entrance it will still be smaller then the minor diameter. The only way to assure a NPT thread is accurate is with a plug gauge.
You can not measure them directly with simple tools.

I can not begin to express my loathing for NPT threaded components, this is the year 2016, the thread standards were written in 1862, there have been far superior methods of fluid sealing developed over past last 150 years yet people resist change of any kind.

http://www.threadcheck.com/1-16-27-...ages-gauges/?gclid=CNbKuZOA9soCFQNkhgodsKEKcg
 
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