Gunco Forums banner

1 - 13 of 13 Posts

·
Registered
Joined
·
2,071 Posts
Discussion Starter · #1 ·
I'm installing some 4-40 bolts through my rails. I've got the tap ready, but am curious to what kind of sizing methods you use when drilling your hole before tapping it. I've got a Drill bit size conversion sheet that also lists the NC and NF tap sizes, but have never tapped holes before. :dunno: :dunno: :dunno:

1) What does NC and NF stand for? On my sheet an "NC" tap of 4-40 is equivalent to a #43 drill bit which is .0890.
2) Do you just use the size 43 bit and drill your hole and then tap, or use a few sizes smaller so that the thread will be thick enough when you tap it?
 

·
Registered
Joined
·
229 Posts
NC means "national coarse"
NF means "national fine"

Both refering to the American National Standard for threads.

Use the recommended drill size or a little bigger - especially with the smaller taps (below 1/8" or so). Small taps are very easy to break. (easier than WinnR breaking end mills). I've broken quite a few tapping electrical panels. Go slow and back off every 1/4 turn to clean the threads. I would recommend using a tapping oil as they work much better than regular machine oil. Make sure you are only turning the tap, and not putting any bending force on it.

Taping is not hard, but as I said, be carefull with the small ones. Taping thin material such as the rails and receiver is a lot easier than thicker material.
 

·
Registered
Joined
·
2,071 Posts
Discussion Starter · #4 · (Edited)
7.62x39 said:
Josh,

The tap drill size for 4-40 is #43, the clearance hole drill is #32 (close fit)

Chart available here. http://www.efunda.com/DesignStandards/screws/tapdrill.cfm

But there is no need to tap the rails, since you will be securing them with a nut

So if I were about to tap a hole because I have some "4-40" bolts and nuts, I would drill a hole with a #43 drill bit, and then tap the hole with the "4-40" tap. Where does the "clearance hole drill" come into play?

So you don't see a necessity in tapping the rails and the receiver or just the railsr? I didn't have any intentions on tapping the rails, but figured tapping the holes on the receiver that go through to the rails might help. How bout this?


edit: I checked out the link. I think I'm a little more confused about which drill sizes to use and when.
edit:spelling
 

·
Registered
Joined
·
2,836 Posts
yosuthnmasa said:
So if I were about to tap a hole because I have some "4-40" bolts and nuts, I would drill a hole with a #43 drill bit, and then tap the hole with the "4-40" tap. Where does the "clearance hole drill" come into play?

So you don't see a necessity in tapping the rails and the receiver or just the railsr? I did have any intentions on tapping the rails, but figured tapping the holes on the receiver that go through to the rails might help. How bout this?


edit: I checked out the link. I think I'm a little more confused about which drill sizes to use and when.

Josh, perhaps we need to start from the top. (Hey guys jump in any time with your thoughts.) I take it you are trying to attach rails to your receiver. Correct? If so I take it you have decided to attach the rails via screws. If I am right so far let me know. Jack
 

·
Registered
Joined
·
2,071 Posts
Discussion Starter · #6 ·
You're right. The next thing is..would it be helpful to thread the hole that goes through the receiver only?
 

·
Registered
Joined
·
2,836 Posts
Josh, forget the clearence stuff use the proper drill for the tapping bit. The receiver is too thin and the rails may not align properly. Use the nuts you purchased to hold the rails. Any other way you will find the threads may bind. Don't forget to use thread locker on your final assembly.
 

·
Registered
Joined
·
2,071 Posts
Discussion Starter · #8 ·
I wasn't planning on threading the hole in the rails though. If its ok to attach them with the 4-40 screws and nuts, then I'll move right ahead.

I am confused about what size drill bit to use for this tap. On the list it says #43. But then 762 mentioned a clearance hole drill of #32. What is this for? Do I use a 43 or 32?
 

·
Registered
Joined
·
2,836 Posts
yosuthnmasa said:
I wasn't planning on threading the hole in the rails though. If its ok to attach them with the 4-40 screws and nuts, then I'll move right ahead.

I am confused about what size drill bit to use for this tap. On the list it says #43. But then 762 mentioned a clearance hole drill of #32. What is this for? Do I use a 43 or 32?
Use the drill for the tap. I checked my chart and the #43 is correct. Jack
 

·
Premium Member
Joined
·
6,900 Posts
Josh,

The tap drill size is #43, meaning that a #43 hole is smaller in diameter than a 4-40 bolt, so you can cut threads and screw the bolt in.

The clearance drill size is #32, meaning a #32 hole is just large enough to slide a 4-40 bolt through it.

Bottom line is that you want to drill your receiver with a #32 in any case, so the bolt can pass through it.
If you want to tap the rails (not necessary, but you could if you wanted to) drill them to #43, then thread them.
If you are not going to tap the rails, drill them to #32 along with the receiver, the bolts will pass through both the receiver and rail, and secured by the nut.

Even if you do tap the rails, I would still recommend using a nut on the inside.
 

·
Registered
Joined
·
2,071 Posts
Discussion Starter · #13 ·
762,

Understood! Thanks for clearing that up for me. #32 is the size for clearance of the bolt with no threads. #43 is the size I would use if were to tap the hole. Thanks alot!
 
1 - 13 of 13 Posts
Top