# Drilling Properly Sized Holes



## catoctin (Jan 3, 2015)

I have drilled holes for years on a drill press and never thought too much about hole size until recently.  I was drilling and tapping holes for 1/8"-27 NPT ports on a project.  The fit was a bit loose and decided my cheapo drill bits were the problem.  Here's my setup:

PM 935 TV Mill
Glacern R8 drill chuck
Keo 90 degree spot drill
1/8"-27 NPT tap
Tap guide
11/32" jobber drill bit - 2000 RPM
1/4" thick 6061 aluminum
WD40 cutting oil for drilling and tapping
Costco 115 piece drill bit set (10 years old) 118 degree

I decided my cheap drill bits were the problem and picked up a set of Triumph 118 degree bright bits from Enco on sale.  Things got much worse.  The 1/8"-27 fitting were pretty loose in the newly tapped holes even more so than the tapped holes made with the Costco bits.  The Triumph bits chatter and screech a lot even when withdrawing the bit to clear chips.  I clear chips maybe every 1/32" of an inch while drilling.

I know drill bits are typically under size.  The Costco bits were more undersized than the Triumphs.  Does anyone have any ideas for things for me to check on?  I was really surprised by all of the noise the Triumphs made even when the holes were spotted prior to drilling with the xy gibs locked during the entire process.  The end goal is to get a properly tight thread.

Thanks,
-Joe


----------



## Dave Smith (Jan 3, 2015)

Joe---I think if you are tapping with an 1/8-27tpi tap then your drill bit should be a couple sizes smaller than 1/8"----doesn't make sense drilling your hole larger than your tap ---Dave-------------*note edit I see it's a pipe tap so just use a size or two smaller bit than the 11/32 drill and don't run the tap in quite so far


----------



## chips&more (Jan 3, 2015)

Don’t think drill bits are normally undersize. The tap drill should be “R” or 0.339” not the 0.343” drill bit. 2K rpm is pretty darn fast, I would slow it way down, what’s the hurry and it’s only aluminum, have fun and be safe. It’s going so fast, the bit is probably wobbling around and making a bigger hole in that 1/4” plate aluminum. You are tapping a tapered thread, right? Maybe don’t tap so much/deep. The more you tap the bigger the tapped hole gets…Good Luck, Dave.


----------



## Dave Smith (Jan 3, 2015)

Joe---you didn't say what type of fittings you were tapping the holes for---some new fittings are just smaller and don't have the full upper side taper to snug tight--so don't run the tap in very far and you should be good----also make sure that the fittings are tapered and not just straight threads---this could be your problem if they aren't tapered---Dave


----------



## catoctin (Jan 3, 2015)

The pipe thread specs are a bit misleading in terms of size.  The drill size used is correct for 1/8"-27 NPT according to the published NPT thread drill specs.



Dave Smith said:


> Joe---I think if you are tapping with an 1/8-27tpi tap then your drill bit should be a couple sizes smaller than 1/8"----doesn't make sense drilling your hole larger than your tap ---Dave-------------*note edit I see it's a pipe tap so just use a size or two smaller bit than the 11/32 drill and don't run the tap in quite so far


----------



## John Hasler (Jan 3, 2015)

catoctin said:


> The pipe thread specs are a bit misleading in terms of size.  The drill size used is correct for 1/8"-27 NPT according to the published NPT thread drill specs.



There are also several different types of pipe threads, not all tapered.

http://www.reed-machinery.com/pdf/external-pipe-threads-tech-data-master.pdf


----------



## wnec65 (Jan 3, 2015)

When I want to drill a hole to size, I always centerdrill then drill a hole that's undersize then redrill to size.  On a large hole I might step it up 2 or 3 times.


----------



## Eddyde (Jan 3, 2015)

I always used a 21/64" or letter Q drill for 1/8" NPT taps in soft metals. Try those sizes you might have better results.


----------



## Karl_T (Jan 3, 2015)

too large a drill for tapping, Use a Q size drill
http://www.engineershandbook.com/Tables/npttap.htm

like others have said, drill undersize first, then finish with a Q drill


----------



## darkzero (Jan 3, 2015)

I just tapped this size a week a go. Some charts list Q & some charts list R for the drill bit size for 1/8"-27 NPT. Some charts list 11/32" cause that's just the closest fraction drill bit that will work. Letter drill bits aren't as common & some people haven't even heard of them.

For aluminum I use Q. It's not so much the starting hole size that is probably causing your issue, 11/32" would work just fine too. Since 1/8" NPT is a taper thread, you'd shouldn't be getting a loose fit if tapped properly. Sounds like you are tapping too deep.

With a taper tap I never just run the tap in like with a straight thread tap. I always test fit with a fitting I trust, then tap deeper if needed. I run the tap about halfway, then check. Some taper pipe taps taps have a reference mark on them. I like to end up with about 5-6 threads showing on the male fitting.

The ones I did last week were for these nitrous fittings, they don't have a long thread. I tapped the holes so they left about 2-1/2 threads showing which equaled out to 5 threads showing on the test fitting I like to use.


----------



## Ebel440 (Jan 3, 2015)

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.


----------



## NightWing (Jan 3, 2015)

If you are going to be tapping lots of NPT holes, get an L1 plug gage.


----------



## catoctin (Jan 3, 2015)

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


----------



## Ulma Doctor (Jan 3, 2015)

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)


----------



## NightWing (Jan 4, 2015)

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.


----------



## Fabrickator (Jan 4, 2015)

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.


----------



## Daddy Dave (Feb 13, 2016)

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.....


----------



## Wreck™Wreck (Feb 13, 2016)

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


----------

