Drilling Properly Sized Holes

[catoctin]

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]

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]

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]

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]

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.

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]

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]

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]

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]

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]

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.