Drill Bit Size?...

[BCB]

I am planning on putting threads into a piece of brass…

The tap size is: 5/16” x 36 tpi

What size drill bit should I use to drill the hole to be threaded?...

Thanks…BCB

[6bg6ga]

You can google a drill tap chart

[6bg6ga]

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

[BCB]

Thanks...

I'll take a look at it...

The charts I looked at showed 32 tpi, but not 36 tpi. Maybe that doesn't matter?

BCB

[6bg6ga]

I didn't give the the answer straight out simply because I wanted you to read the publication I posted. Everything is there complete with percentages of thread and it explains everything completely. It will take about 5 min for you to read it and grasp what it is saying but on the positive side you won't forget what you read and you will probably print out the article complete with the drill tap chart for future reference.

[BCB]

Well, I guess if I use the 90% rule for fine threads, that should give me that answer...

Yet other charts list different size bits for taps with 20 tpi,24 tpi or 28 tpi. No matter what diameter and tpi. So it does seem there are precise sizes of bits for different tpi with the same diameter of screw...

But, I will try the 90% size calculation and hope for the best--I get one try at what I am doing. Or, I will be purchasing another piece of brass...

Thanks...BCB

[M-Tecs]

Post 14 has the info you want. https://thefiringline.com/forums/showthread.php?t=458796

More here and a sources for cheap taps http://www.savageshooters.com/showthread.php?7894-Hornady-COAL-cases-drill-bit-and-tap&

[WJP]

You can drill it .282 - .289. So start with a 9/32 and see where you end up. 9/32 may be a hair under but depending on the bit and machine it may be right on. Or you could use letter L if it comes up undersized. The L will put you on the max side though.

[BCB]

I used a 9/32" and the tap was tight and quite difficult to turn...

M-tecs posted a couple of links and one of them recommended a 19/64". I think that one might work better as it would make the tap less difficult to turn...

The bottom link he posted is exactly what I am doing. It is difficult to hold the brass to drill it and to then tap it...

I placed it between two wooden blocks that I had drilled a hole a tad less than the diameter of a 30-40 Krag--problem: a tapered case. It did hold fairly snug, but it would sometimes spin when drilling the primer pocket. I had to start with a small bit and work my way up to 9/32". The tapping was the problem, but I did get it done...

If a guy had a way to hold the brass solid so it didn't turn, this procedure would not be so demanding...

Thanks...BCB

[BCB]

I read the entire thread on the Savage Shooters website...

One guy says you can use a hammer type boolit puller to hold the brass...

I don't know if that would hold it tight enough or not, but it might be worth a try...

BCB

[M-Tecs]

This may help
https://www.youtube.com/watch?v=pAngKHIZgyA

[country gent]

Drill the case body oversized and use the neck for alighnment. coat case with wax 2 coats and JB weld to "Bed" the split blocks to the case taper, Then open up the case neck a little. This makes a split block that grips 100% on the case body and depending on vise will grip very tightly. Another thing to keep in mind is relief angles are different on drills between steel and brass. A drill sharpened for steel will want to grab and screw itself in. Brass uses shallower relief angles to stop this.

[country gent]

I make my split blocks from hard maple. 3/4 thick. I drill 2 -3 dowel holes in the blocks to maintain alighnment. I then set up and lay out the case hole drill thru with drill size of the case neck and then with a drill .100 bigger than case body / Head. a couple ,030 shims on each side then insert case into block and center it up. Fill with JB weld and let cure. push dowels out and case out cut JB with hacksaw to separate the blocks. I use 32 tpi as its a slightly thinner cut and also dosnt chip the epoxy as bad.

[BCB]

Good info here…

I am certainly going to try the JB Weld method…

Concerning drill bits—do they make bits especially for brass? Could those bits be used to drill aluminum since it is much softer than steel?...

Sounds like a good project to put some time in during Winter…

Thanks all…BCB

[Jeff Michel]

Read Post # 11. You definitely do not want to use the same drill bit for brass as you would for steel. This goes double on thin stuff.

[M-Tecs]

Concerning drill bits—do they make bits especially for brass? Could those bits be used to drill aluminum since it is much softer than steel?...


Thanks all…BCB

For brass they make a slow spiral to prevent the bit pulling in too fast https://www.mscdirect.com/webapp/wcs/stores/servlet/ThematicPageCmd?page=brass-bit-drill.html

The ones of aluminum will have fast spiral for better chip removal.

[country gent]

At work we simply reground them with a little less relief. and on bigger ones flattened the cut edge a little also thinning the web