Differences in drill angle depths (118, 135, 140, 145)

[ohland]

As I was tripping out with some good 8 O'clock Bean coffee this morning, I stumbled to http://www.1728.org/trig.htm in order to see what difference in tip depth results from changing the drill angles from 118 to 145 degrees. In this case, the test subject is a G2 7-30 barrel, .812 in diameter. So how far in do I have to drill, versus how much wall do I have?

.812 - .285 = .527 / 2 = .2635 so I have .260 ish before I have a ported 7-30 barrel...

Now with the power of math, and Al Gore's Internet, I solved for the height of the right angle. Side ac is .1425.

Drill b Tip
118 31.0 .0856
135 22.5 .0590
140 20.0 .0518
145 17.5 .0449

Assuming the sight threads are 6-48, each thread is .0208, three threads depth for steel is .0624 for threads only. Supposing the tap is a perfectly flat bottoming tap, able to thread all the way down to the end of the cylindrical portion of the drilled hole (it's coffee, not booze!), just add .0624 to whatever tip depth of the four listed sizes...

So easy, a field grade could do it, right?

v/r,
Servant to the plumbous calf
:coffeecom

[Uncle Grinch]

Uuuh, OK. I'l glad someone passed their math classes.

I thought this thread was about the differences in the various drill bit tip angles, 118, vs. 135, etc.

[ohland]

I thought this thread was about the differences in the various drill bit tip angles, 118, vs. 135, etc.

The title clearly states: Differences in drill angle depths. It says nothing about the differences in tip geometries.

All it is about is the length of the tip of the drill back to the cylindrical part of the drill hole. I prefer 135 drills because they walk less (and screw machine length...). Might look at the spotting drills, they go 140+ degrees. At some point, a mill would seem reasonable, but center cutting would be necessary. Plus, a truly right angle at the bottom of the hole would really make a flat bottoming tap a necessity.

[country gent]

You left one out. In industry its not unussual to Flat grind a drill bit to make a flat bottomed hole. Its easy to do and will cut very well when done corectly though the hole has to be "started with a regular drill point.

[ohland]

:coffeecom
You left one out. In industry its not unusual to Flat grind a drill bit to make a flat bottomed hole. Its easy to do and will cut very well when done correctly though the hole has to be "started with a regular drill point.

Odd, then why not cut out the middle man and use a center cutting end mill? I don't feel comfortable with a square corner on the cylindrical part of the drilled hole, but they _do_ make end drills with slightly radiused corners...

[ohland]

Thought that I bought one of these before...

CORNER RADIUS ENDMILLS

McMaster-Carr has 'em, but not the right size...

[JSnover]

:coffeecom

Odd, then why not cut out the middle man and use a center cutting end mill? I don't feel comfortable with a square corner on the cylindrical part of the drilled hole, but they _do_ make end drills with slightly radiused corners...

If I had to guess… Center drilling makes for a more accurate hole and reduces the load on the finish drill.

[Doc Highwall]

I would start with a short center drill, then a standard 118 deg or 135 deg drill and finish with a flat bottom drill.

[W.R.Buchanan]

I would just drill it with a normal split point stub drill by touching the point onto the work and setting the depth based on that. The amount of threads for a #6-8 tap over the distance of the height of the point of the drill over the margin is not worth the effort. especially if you have .260 to work with. I'd stop at about .230 deep and then tap it as deep as I could get it.

Here's the deal on thread depth/strength. Once your depth is past the diameter of the thread everything else is extra and not necessary. Most people say 1.5 times dia for good measure. On a #6 that is <.210 deep for 1.5x and .140 for 1X.

You can always grind the point off a tap and make a bottoming tap out of it. In fact broken taps work really well for this purpose. You will need to start the threads with a regular tap so that the new custom made bottom tap has something to guide it.

Do be careful as breaking a tap is a big problem. IN fact it is best to start the tap and do as much as possible n the same set up you drilled the hole with. That way the tap is assured of going in perfectly strait.

The vast majority of tap failures are directly attributable to two things.

Not having the tap go in the hole strait which is the worst one, (This is why it is best to use some mechanical means of controlling the entry like a tapping jig or by using a mill or drill press)

and not having lubrication on the tap. NEVER run a tap dry.

Goons account for the other 50% [smilie=b:

Randy

[ohland]

Here's the deal on thread depth/strength. Once your depth is past the diameter of the thread everything else is extra and not necessary. Most people say 1.5 times dia for good measure. On a #6 that is <.210 deep for 1.5x and .140 for 1X.

The vast majority of tap failures are directly attributable to two things.
Not having the tap go in the hole strait which is the worst one, (This is why it is best to use some mechanical means of controlling the entry like a tapping jig or by using a mill or drill press)
and not having lubrication on the tap. NEVER run a tap dry. Goons account for the other 50% [smilie=b: Randy

I use a spring loaded center point to tap in my mill. Even when I had the DRO working on my mill, I always drilled then tapped before moving the table. That .140 sounds quite reasonable, as I am quite sure Remington did not drill n tap the front sight on my 788 with a .210 deep hole. I have an urge, whether intelligent or not, to mount a sight block on my G2 barrel.

:veryconfu

[Big Rack]

I'm pretty sure that info is buried somewhere in the Machinists Handbook. I actually had to use it once I believe the the print actually called for the drilled depth of a hole pretty closely.
Something I did was buy a Casio programmable graphing calculator and made it into a program also made little programs for surface speed, right angle triangles ,tap drill diameters and other stuff. They have calculators to do this but the Casio was $30 cheaper and being a ham I could also program some stuff for radio and electronics in it. They must be used in Australia a lot there's a lot of programming info for them on the net for them and they are cheaper than a TI.

[KCSO]

I finish mine with a flat bottom home and tap right to the bottom for max threads, a trick I was shown years ago by a fussy old fellow.

[country gent]

In the trade its normally center drill for precise location drill ( drill anf then flat bottom hole or completly finish hole) chamfer then tap alot put the mill in neutral the tap in the drill chuck and turn by hand or with a rod in one of the key holes ( this keeps the tap square and true to hole and preents side flexing of the tap when turning. Makes taps last much longer. A good cutting fluid is a big plus. Another trick is to pack the hole with beeswax or grease type cutting fluid, this will push chips up and out keeping them from building up in the bottom of the hole. A thick cutting compound filling the hole does this thru hydrolics and the volumne of the tap is much greater than the chips its producing so it does work very well.

[JSnover]

Country Gent is spot on. Everything runs straight and true, taps don't break. The chamfer helps the tap start and eliminates the burr. On the larger sizes, especially if it's a through hole, you can bump the motor in low gear: just nudge it along and hit reverse often enough to break the chip. If you do it carefully, the machine does all of the work.