??correct 6/48 drill bit size?

[nekshot]

I ordered 2 drill bits for tapping 6/48 and I cannot get the taps started. I have been blaming it on my hands shaking from the meds but I got frustrated and took a drill bit out of my cheapo grizzly set #31 and drilled the hole and presto the tap worked. So I measured the grizzly bit and it is .119 and the smaller #32 is.111. So I measured the 2 bits I got from midway specifically for the 6/48 and the one measured .115 and the other .109. Could someone please till me the correct size foe a 6/48 tap? My only referance says use #31 bit. This has really been a frustration to me. I tried a number of projects with these midway bits and had to move to 8/40 bits from midway and they worked perfect. Thank God it finally dawned on me to measure the bits.

[Artful]

http://en.wikipedia.org/wiki/List_of_drill_and_tap_sizes

Tap drill for a 6-48 screw is a #31 (.120") drill.
If a very small hole is drilled, the chance of breaking a tap in increased but gives more thread contact so is stronger.

[SOFMatchstaff]

Tap drill is #31 and clearance is #25. I usually start with a taper and then go to Plug or bottom as needed.
plus what Artful said

[nekshot]

Thanks Artful, then my cheapo grizzly set is fairly accurate in this application. I purposely bought the midway drill bits thinking they would be the best of quality. Once again I should have went to Brownells , o well live and learn!

[oldred]

Thanks Artful, then my cheapo grizzly set is fairly accurate in this application. I purposely bought the midway drill bits thinking they would be the best of quality. Once again I should have went to Brownells , o well live and learn!


Or you might try Enco for true high quality Triumph, Hertel or Precision (Brand) drill bits. They often have free shipping and very fast service, beats the dickens out of Midway's off-brands (that is assuming they were indeed no-name bits from Midway??) Sometimes the really smaller size bits need to be purchased in minimums of ten but something as large as a no#31 could be bought individually, heck even ten of those tiny bits don't cost much! If you are buying machining equipment, except for the specialized gunsmithing items, you might save a bunch of money and get better tools from Enco than any of the shooting suppliers.

www.use-enco.com

[W.R.Buchanan]

There should be some kind of Industrial Supply or tool supply house near you. I would recommend buying "Stub Length Drill bits" since they are shorter and stiffer than "Jobber Length Drills."

Good brands of drills are Cleveland, Precision Twist Drill (PTD) Guehring. Good brands of taps are Cleveland, OSG. Buy only hi-speed steel taps or HS taps coated with TiN or other friction reducing coatings.

I personally use Spiral Point taps mostly and if I need to bottom a hole then follow with a bottom tap or a cut off spiral tap. If I have a bunch of blind holes to tap I use Spiral Flute taps which wind the chip back out of the hole. On soft materials I use Roll Taps almost exclusively since they produce no chips to get in the way and cause problems.

Here's the deal guys. This is one place where you don't want to skimp! Breaking a drill is one thing, breaking a tap is quite another, and breaking a tap on a gun is a Friggin' nightmare!

Buy the best only cry once.

Randy

[MtGun44]

Unless there is some burning reason to stick with the (IMO) silly
6-48 tap, I'd go to 8-40 right off the bat. I don't have access to
my reference book right now and can't find online dimension on them
right now. Let's compare the UNF series, which will be similar in
proportions. The minor diameter of a 6-40 is 0.1065, so the area
at the thread root is 0.0089 square inches. An 8-36 has a minor
diameter of 0.1291 (both for 2A threads - external, like a screw).
This gives the 8-36 an area of 0.1309 square inches. If we assume
150,000 psi yield strength (grade 8 bolts - high strength, even tho
Gr 8 doesn't actually apply this small) you have a max load of
1963 lbs on the 8-32 and 1335 lbs on the 6-40 screw. Many
screws are not this good an alloy, so they don't reach this strength.

The bottom line is that if you use 8-40 you have a screw that is almost
50% stronger. Broken 6-48 scope mount screws is probably one of
the most common problems with scopes. Start with a stronger screw.

If this is for a aperture sight, the loads are far lower than a scope and the
screws are pretty appropriate. I just do not understand why scope mounts
are still attached with these silly little 6-48s when there is PLENTY of room
for 8-40s on any rifle receiver and the strength increase is substantial.

Bill

[W.R.Buchanan]

I agree with the larger screws #6's are about .136 on the od and #8's about .160.

Here's a little tidbit for everyone who doesn't already know.

All black finished socket head style screws (except stainless) are Grade 9. There are no lesser graded screws of this type unless they are imported form China, and even then they didn't do it deliberately.

6-48's are more or less a "traditional" size used only in guns as far as I know. Actually 6-48 NEF (National Extra Fine) I think the original reason for these screws is someone wanted an uncommon size that only a few could get access to. (gunsmiths) I don't see any good reason for them or for 8-40's which are also NEF, either. 8-36's would have worked just fine.

I am starting to see more and more common sized screws on AR's as more and more Machinists get into the gun making business.

Randy

[KCSO]

More threads per inch than 6-32 gives more hold. Same as 8-40 vrs standard 8-32. I have used both 6-48 and 8-40 and hae never had eiter come loose so I usually stick with the now standard 6-48 because that's what all my jigs and guides are set for. Biggest thing with small taps is to get them straight, you really need a good guide when you do the tapping.

[MtGun44]

Fine threads have a slighthly greater cross sectional area, not a big deal on
a 5/16 or larger bolt, but significant on these tiny screws. The actual part cut
out for the thread is shallower, leaving more "shank" in the middle.

Bill