Drill Tip Lengths - A Drill Chart to Share and Some Math

[chip_slinger]

I often find myself having to calculate drill tip lengths to make jobs with blind holes easier. Actually, after having to do it twice I whipped up an Excel sheet and made a chart that calculated the tip lengths for all number, letter and fractions drills up to one inch for both 118 and 135 degree varieties. I'm an engineer - I've been taught to do work upfront so I can be lazy later. I thought I would share it with all of you so you can be lazy too, with the added bonus of not having to do the work. The chart is attached and is in .pdf format. It is sized for legal paper, 8.5 x 14. If you have any inclination to use it, I suggest printing it out in color and laminating it. I keep a few spread out in the shop and one at my desk, and find them equally handy as just a drill chart.

Here is the chart -> View attachment drill_chart_with_tip_lengths.pdf

This is a preview of what the chart looks like:



The math is simple trigonometry, and I thought the best way to show it was visually:



Define any drill by its diameter (D) and the included tip angle (A). The tip length (L) is given by a formula which can be written several ways. The most direct is using simple division, and is written as L = (D/2) / tan(A/2). If we were to write that out, it would be "the tip length is equal to half the drill diameter divided by the tangent of half of the tip angle". The formula can also be written using multiplication instead of division, and is shown in the diagram. It is L = (D/2) x tan(C), where C = 90 - (A/2).

For example, a #37, 118 degree drill. It has a diameter of 0.104 (D=0.104) and an angle of 118 degrees (A=118). So, the tip length is L = (0.104/2) / tan(118/2) = 0.052 / tan(59) = 0.052 / 1.6643 = 0.031.

Another example, a 13/32", 135 degree drill. It has a diameter of 0.406 (D=0.406) and an angle of 135 degrees (A=135). I'll do this one using the other form of the formula. So, tip length is L = (0.406/2) x tan(90 - (135/2)) = .203 x tan(90 - 67.5) = .203 x tan(22.5) = .203 x .4142 = 0.084.

I hope others can find this useful. Enjoy.

 

[AaronD]

Added to my chart folder.

Thanks for taking the time to make and share!

 

[Holescreek]

Back in the old days we just multiplied the drill diameter by .3. Seems to be pretty close compared to a couple of numbers I tried. We only had 118 degree drills though.

 

[Mark_f]

Thank you, this will be very useful. I was always guessing how much to add to a hole depth.

Mark Frazier

I often find myself having to calculate drill tip lengths to make jobs with blind holes easier. Actually, after having to do it twice I whipped up an Excel sheet and made a chart that calculated the tip lengths for all number, letter and fractions drills up to one inch for both 118 and 135 degree varieties. I'm an engineer - I've been taught to do work upfront so I can be lazy later. I thought I would share it with all of you so you can be lazy too, with the added bonus of not having to do the work. The chart is attached and is in .pdf format. It is sized for legal paper, 8.5 x 14. If you have any inclination to use it, I suggest printing it out in color and laminating it. I keep a few spread out in the shop and one at my desk, and find them equally handy as just a drill chart.

Here is the chart -> View attachment 88217

This is a preview of what the chart looks like:

View attachment 88219

The math is simple trigonometry, and I thought the best way to show it was visually:

View attachment 88218

Define any drill by its diameter (D) and the included tip angle (A). The tip length (L) is given by a formula which can be written several ways. The most direct is using simple division, and is written as L = (D/2) / tan(A/2). If we were to write that out, it would be "the tip length is equal to half the drill diameter divided by the tangent of half of the tip angle". The formula can also be written using multiplication instead of division, and is shown in the diagram. It is L = (D/2) x tan(C), where C = 90 - (A/2).

For example, a #37, 118 degree drill. It has a diameter of 0.104 (D=0.104) and an angle of 118 degrees (A=118). So, the tip length is L = (0.104/2) / tan(118/2) = 0.052 / tan(59) = 0.052 / 1.6643 = 0.031.

Another example, a 13/32", 135 degree drill. It has a diameter of 0.406 (D=0.406) and an angle of 135 degrees (A=135). I'll do this one using the other form of the formula. So, tip length is L = (0.406/2) x tan(90 - (135/2)) = .203 x tan(90 - 67.5) = .203 x tan(22.5) = .203 x .4142 = 0.084.

I hope others can find this useful. Enjoy.

 

[royesses]

Thank you. That sure can be a time saver.

 

[atunguyd]

Not to take away from your chart but what I do is the following :

Start the hole and as soon as I see the edges of the drill start to disappear into the piece I zero my tailstock dro (if you don't have one of these check my other post on how to use your Y Axis dro for the tail stock.)
Now just use you tail stock dro to drill to depth.

Sent from my GT-I9505 using Tapatalk

 

[Sandia]

Thanks, this will be a big time saver. Never seen a chart like this before.

 

[6530]

Great idea! For some reason I can't see the chart itself but can see the preview...

 

[John Hasler]

Back in the old days we just multiplied the drill diameter by .3. Seems to be pretty close compared to a couple of numbers I tried. We only had 118 degree drills though.

For 118 degree drills the exact value is .30043. .3 is close enough for me.

 

[chip_slinger]

Thanks for all of the positive feedback.