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

[chip_slinger]

It's a link in the original post. Click on "drill_chart_with_tip_lengths.pdf" in the original post.

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

 

[OldMachinist]

This is the chart I've used for several decades.

 

[chip_slinger]

Very cool! I'm sure there are lots of these type of charts produced over the years.

This is the chart I've used for several decades.

View attachment 88244

 

[Steven Dzupin]

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.

View attachment 88218 View attachment 88219

Many thanks for making this chart available.

Regards,

Steven D.

 

[chip_slinger]

Many thanks for making this chart available.

Regards,

Steven D.

You're welcome, glad it came in handy.

 

[Karl_T]

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

Actually the above calculations ignore the very slight reduction in tip length due to the flat at the drill tip. So, 0.3 is probably a better value for 118 point drills.

 

[Alan H.]

Mike, Thanks for sharing. A time saver that minimizes ciphering. I have some holes to drill today that this will come in handy for.

Printed a copy for my notebook in the shop.