View Full Version : Drill chart for length of point chart

Ridgerunner

01-25-2019, 03:54 PM

Does anyone know if there are drill charts for length of point for standard length jobber letter drills? In the Machinery Handbook they have a chart for fractional drills but I can't find anything for letter drills. The "point" is if you touch off the point of a new 118° drill to zero, what is the depth of a new drill where the flutes would touch? Or, if you will, to be able to calculate how deep to drill for thread depth hole clearance. Also, for a 135° drill. I know resharpening can change things so new drills. Trig or a formula can work but it would be nice to have a chart.

Illinoyance

01-25-2019, 04:10 PM

I have Machinists Calculator on my iPhone. It does the calculation. Don't know if it is available for other operating systems.

mklotz

01-25-2019, 04:51 PM

It's a trivial calculation...

A = drill tip included angle (typically 118 deg)

D = drill diameter

then X, the "length of the drill tip before the flutes touch" is...

X = (D/2) / tan (A/2)

Lew Hartswick

01-25-2019, 04:54 PM

Well the "point height" is just :

H = Tan 31 deg x the radius of the bit "ignoring the tinny flat on the point."

if your mathematically challenged the radius is half the diameter.

...lew...

I see Marv is faster than me. :-)

lynnl

01-25-2019, 05:51 PM

Well the "point height" is just :

H = Tan 31 deg x the radius of the bit "ignoring the tinny flat on the point."

i :-)

Yeah, this formula is what first occurred to me too, and then I remembered that little bothersome chisel tip. Is there some standard for the size (width) of that? If so, its height would be calculated in the same way, and then subtracted from the first calculated value.

Paul Alciatore

01-25-2019, 05:58 PM

As I am sure you are aware Marv, that is the worst case formula. It does not take the flat, chisel tip into account. So the actual distance from first contact to where the full diameter is being cut is actually a bit shorter.

It would be difficult to give a general formula because the width of that chisel tip varies with the thickness of the drill's web at that point AND with the actual angle of that chisel tip with respect to the web thickness. If you measure the web thickness, you can subtract it from the diameter in Marv's formula and get a better estimate of the desired distance.

A = drill tip included angle (typically 118 deg)

D = drill diameter

W = web thickness at the tip

then X, the "length of the drill tip before the flutes touch" is...

X = ((D-W)/2) / tan (A/2)

The web thickness can vary from one drill bit to another. And the only real way to calculate the actual distance would be to measure the actual width of that chisel tip. The best thing I can think of for this would be an optical comparator.

If you want to insure that the hole is drilled with a full diameter for a given depth, then you can use Marv's formula. Your actual hole will be just a bit deeper, but in most cases that does not matter.

It's a trivial calculation...

A = drill tip included angle (typically 118 deg)

D = drill diameter

then X, the "length of the drill tip before the flutes touch" is...

X = (D/2) / tan (A/2)

mklotz

01-25-2019, 06:37 PM

Well the "point height" is just :

H = Tan 31 deg x the radius of the bit "ignoring the tinny flat on the point."

if your mathematically challenged the radius is half the diameter.

...lew...

I see Marv is faster than me. :-)

Ignoring the chisel tip probably has less of an effect than using this formula with a drill with a point angle other than 118 deg.

Let's face it...if you want the hole to be cylindrical to a precise depth, you should be boring it; drills are bulk removal tools.

Paul Alciatore

01-26-2019, 04:04 AM

When I want to insure that I have a certain depth at the full diameter I ...

Blue the part.

Use the depth stop on the drill press.

Carefully increase the depth by small increments as I start the hole, looking for a slight edge to appear at the OD.

When I see that small edge, I set my zero and drill from there.

So far, that has been quite accurate enough for any work that I have been doing.

If I really needed a precise depth, I would drill it in the mill. Use a center cutting end mill and set zero when it starts to scratch. And go from there. That works +/- a thousandth or two.

Ridgerunner

01-26-2019, 07:26 AM

Thanks for taking the time to respond. I will just make a chart.

When I want to insure that I have a certain depth at the full diameter I ...

Blue the part.

Use the depth stop on the drill press.

Carefully increase the depth by small increments as I start the hole, looking for a slight edge to appear at the OD.

When I see that small edge, I set my zero and drill from there.

So far, that has been quite accurate enough for any work that I have been doing.

If I really needed a precise depth, I would drill it in the mill. Use a center cutting end mill and set zero when it starts to scratch. And go from there. That works +/- a thousandth or two.

If you want to know the depth of just the point: Borrowing a page from Paul's post I would incrementally drill into the work and measure with a pointed instrument like a pointed depth mic or indicator with a point attachment from the surface to the bottom of the cavity.

Mcgyver

01-26-2019, 10:07 AM

A chart, trig calcs, indicators? You guys are exacting types :). I feed in until the Mark I eyeball sees the corner of the lip meet the work, note the reading on the scale or make a pencil part on the quill and feed away until its to a the depth I want. Offhand, I can't remember a drilled hole that needed much more than that.

Cuttings

01-26-2019, 11:46 AM

Machineriy's Handbook Pocket Companion table 5, page 115 - Length of point on Twist drills and centering tools.

It gives length of point when the included angle is 90° and 118° for both number drills and fractional drills up to 3/4".

I am with Paul..

Why dick around with a formula when you can just watch the drill and set depth..

And for 95 percent of the holes I drilled it did not matter.

mklotz

01-26-2019, 12:53 PM

On page 945 of the 24th edition of MH, there's a table of drill point lengths for 90 and 118 deg included angle points. I compared the values for several drills with my formula. Column A is the drill size, B is the table length for 118 deg angle, and C is the calculated value from my formula.

A B C

#60 0.012 0.0120

1/4 0.075 0.0751

#30 0.039 0.0386

3/4 0.225 0.2253

Not only do these values match very well. The table values for a 90 deg point are exactly half of the drill diameter which is exactly what the formula predicts when tan (45) =1.

So, it appears this table was generated with the formula.

Ridgerunner

01-26-2019, 01:55 PM

Machineriy's Handbook Pocket Companion table 5, page 115 - Length of point on Twist drills and centering tools.

It gives length of point when the included angle is 90° and 118° for both number drills and fractional drills up to 3/4".

Thanks for that. It is on a different page in my 2000 book but what I was looking for was a chart for length of point for letter drills. This is just for convenience to save some steps so no big deal. Now, when using a letter drill I have to look at a chart for the decimal measurement, then use some math to calculate the length of point. I was hoping somewhere there might be a chart for the letter drills.

LKeithR

01-26-2019, 02:49 PM

A chart, trig calcs, indicators? You guys are exacting types :). I feed in until the Mark I eyeball sees the corner of the lip meet the work, note the reading on the scale or make a pencil part on the quill and feed away until its to a the depth I want. Offhand, I can't remember a drilled hole that needed much more than that.

Exactly. Any hole that requires finishing to a precise depth is going to be finished with and end mill or boring

bar. Touch off on the end face and move in accordingly. A drill is just a starter. In 40+ years of machining

I've never measured the length of the tip of a drill. I do it exactly as Mcgyver does...

danlb

01-26-2019, 04:31 PM

Exactly. Any hole that requires finishing to a precise depth is going to be finished with and end mill or boring

bar. Touch off on the end face and move in accordingly. A drill is just a starter. In 40+ years of machining

I've never measured the length of the tip of a drill. I do it exactly as Mcgyver does...

A case where it's handy to have the formula is when you are designing a part where you want to drill a hole deep as possible without going through the side of the part. Many will just take the thickness and subtract some fraction of an inch based on guesswork or experience. Others will wait till they make the part using Mcgyver's method and then mark the drawing "as made". I prefer to figure it out first and make the parts to spec.

Dan

Mcgyver

01-26-2019, 04:56 PM

A case where it's handy to have the formula is when you are designing a part where you want to drill a hole deep as possible without going through the side of the part. Many will just take the thickness and subtract some fraction of an inch based on guesswork or experience. Others will wait till they make the part using Mcgyver's method and then mark the drawing "as made". I prefer to figure it out first and make the parts to spec.

Dan

Not sure I follow, but do you think any experienced guy is suggesting an approach where you change the drawing to suit the result you got?

old mart

01-26-2019, 05:09 PM

With a common drill point of 118 degrees, the tip extends 15% of the diameter, simple and easy to remember without any chart.

Ridgerunner

01-26-2019, 05:15 PM

With a common drill point of 118 degrees, the tip extends 15% of the diameter, simple and easy to remember without any chart.

How much is 15% of "S" ;)

old mart

01-26-2019, 05:27 PM

Don't you have anything like this where you come from? https://www.ebay.co.uk/itm/272728872114 It still has antiquated sizes in it.

danlb

01-26-2019, 05:29 PM

Not sure I follow, but do you think any experienced guy is suggesting an approach where you change the drawing to suit the result you got?

That's an interesting question. If it was meant tongue in cheek, the proper answer is "No, of course they would not change the drawing. The next guy should be able to figure it out like they did." If it was serious, In the home shop you'd better mark your "as built" changes so that you can recreate it if you blow one of the final steps. In a professional environment, I've had IBM provide "as built" docs for prototypes to document the changes that they found they needed to make to the plans in order to get everything installed. Those were fed back to the engineers so that the next installation would match what we built in my lab.

So yes, that is one way of doing it. As I said, that's not my preference. The engineer should look up the size of the point in those pesky tables so the machinist does not have to do the math in their head while machining.

Dan

Mcgyver

01-26-2019, 06:48 PM

I don't think you got what was being suggested, unlikely that we're on the same page.

To answer Dans post, previous page...

Ok .5 thick part I usually try to drill .45 or .46.

Deeper the hole the quicker you can tap.

Unless it's only 1/4 of thread or something shorter than thickness.. bottom line I always check with a fastener..

sandiapaul

01-27-2019, 08:37 AM

Very simple formula, for a 118 degree drill: drill diam x .3

mklotz

01-27-2019, 10:04 AM

Very simple formula, for a 118 degree drill: drill diam x .3

Yes

From the formula...

K = 0.5 / tan(59) = 0.30043

Then...

Tip length = K * drill diameter

Lee Cordochorea

01-27-2019, 01:45 PM

A - 0.070"

b - 0.071"

c - 0.073"

d - 0.074"

e - 0.075"

f - 0.077"

g - 0.078

h - 0.080"

i - 0.081"

j - 0.083"

k - 0.084"

l - 0.087"

m - 0.089"

n - 0.091"

o - 0.095"

p - 0.097"

q - 0.100"

r - 0.102"

s - 0.104"

t - 0.107"

u - 0.110"

v - 0.113"

w - 0.116"

x - 0.119"

y - 0.121"

z - 0.124"

Ridgerunner

01-27-2019, 03:33 PM

Thank you very much for doing that.

Paul Alciatore

01-27-2019, 05:23 PM

Fifteen percent of "S":

"-"

How much is 15% of "S" ;)