How to measure the primary relief angle of drill bits

[TDegenhart]

The has been many threads on the sharpening of drill bits but I have yet to hear about measuring the results.

http://www.practicalmachinist.com/vb/general/drill-sharpening-233564/

This source lists relief angles

http://www.neme-s.org/2005 May Meeting/drills.pdf

So now that the tip is nice and sharp, how do I know that it has the correct relief angle? I have tried using an optical comparator and hand tools without success. Ideas?

Perhaps this should go into the metrology section?

Tom

 

[Bruce Nelson]

To get an idea whether or not your relief angle is correct, look at the drill point from the end of the drill and examine the chisel point on the end of the drill bit. It should lean to the right about 15 to 20 degrees. If it is straight up and down, you don't have any relief angle, and the drill cuts very poorly, or not at all. If it leans to the left, you have a negative relief angle, which is no good. If it leans to the right more that 15 to 20 degrees, you will have too much relief angle, and the cutting edge will be weak and will chip out.

If you would like the drill bit to cut with less pressure, you should thin the web. This narrows the chisel point, which reduces the pressure to make the drill cut. If you are enlarging a previously drilled hole, or are drilling brass, you should grind a flat on the cutting edges, thereby eliminating the back rake angle which causes the drill bit to dig in and grab.

After grinding a drill several times, the drill gets shorter, and the web gets thicker and the chisel point gets broader. In this case, it is recommended to thin the web, as a broad chisel point makes a poor cutting edge.

Lord Byron

 

[dkmc]

So now that the tip is nice and sharp, how do I know that it has the correct relief angle? I have tried using an optical comparator and hand tools without success. Ideas?

Is it possible you are over thinking this?
If it cuts, does it really matter -what- the angle is?
If angles are your hobby,I guess I understand....sorta.
Life is short.....poke the goddam hole and move on....to a Reamer or
Boring Bar perhaps?

 

[TDegenhart]

My question is based on several aspects. One, as an engineer, I like to be able to measure things for feedback. Kinda like not having a micrometer to check a setup. Stick the thumb in the air, wave it back and forth as you squint, and say "That's about right". Second, and to me more important, is to decipher claims about what drill grinders can and can't do. My cheeep $7.95 (when it bought it) drill sharpening jig produces a quick drill point that for the most part works very well. I would like to know what it's set at. I have other fixtures that require 20 minutes to setup and I can't get as good a grind. I would like to know why. To the issue of just poke a hole and go on, my experience is that only a rigid boring bar can correct a poorly drilled hole. Drills are one of the most commonly used cutting tools. I would certainly not setup a T&C grinder for a mill cutter without some means of checking my work.
Tom

 

[CarbideBob]

Toolmakers microscope or surface illum on comparator.

or get one of these, the most common gauge for this stuff.
http://www.pg1000.com/PG1000/
Vee block fixture holding the tool on the gauge pivots to let you line up lips then rotate 90 to see the side.

Cutting tips must be aligned with optical axis or you will get differing measurements.
Angle is checked on the margin.

Bob

 

[dkmc]

My question is based on several aspects. One, as an engineer, I like to be able to measure things for feedback. Kinda like not having a micrometer to check a setup. Stick the thumb in the air, wave it back and forth as you squint, and say "That's about right". Second, and to me more important, is to decipher claims about what drill grinders can and can't do. My cheeep $7.95 (when it bought it) drill sharpening jig produces a quick drill point that for the most part works very well. I would like to know what it's set at. I have other fixtures that require 20 minutes to setup and I can't get as good a grind. I would like to know why. To the issue of just poke a hole and go on, my experience is that only a rigid boring bar can correct a poorly drilled hole. Drills are one of the most commonly used cutting tools. I would certainly not setup a T&C grinder for a mill cutter without some means of checking my work.

After 20+ years, I can freehand a drill point that will drill an accurate hole, with point centered on drill body within .005-.007 (measured with dial calipers), margin thinned down to almost nothing so it does not rub, but holds up, no real idea what all the angles are, but does not ever seem to matter. Always seemed 'about right' to me....
If it's 'reeaal fussy'.....I use a brand new drill.....and move on

 

[Bruce Nelson]

As I said in my post #2, the amount of clearance angle that you have below the cutting edge of a drill is directly related to the amount that the chisel point leans to the right, when viewed on the end of the drill point. The amount that I said in my post is a good indication of a properly ground general purpose drill point. If you are interested in a direct measurement of the amount of clearance angle that you have, I suppose that you could use the protractor head of a combination square to make that measurement.

When I taught my students to grind drills when I was a instructor in a Technical College, I taught them to grind drills by hand on a properly dressed pedestal grinder, using a drill point gage. The only critical measurements are: 1. Both lips must be ground at the same angle. (usually 118 degrees included) 2. Both lips must be ground to the same length. 3. The chisel point must lean 15 to 20 degrees to the right when viewed from the end. If you meet these requirements, you will get a drill that cuts an accurate size hole.

If you grind the lips to unequal angles or lengths, this will resuilt in a oversize hole. If you have a decent drill grinder, and know how to use it, of course this is a good thing. We used a Sterling drill grinder.

If this explanation of how to determine a how to obtain a satisfactory ground drill does not meet your requirements, you will have to look elsewhere. You may be trying to over analyze the problem to your engineers mind set.

Lord Byron

 

[antique iron]

Here's a little giz-whiz that Oliver came up with to check such matters.

Drill Point Checking Gages

 

[cypherp]

The above fixture shows "difference in the height of the cutting lips", not used to actually measure relief as I understand it.



Two methods detailed in the preview of this article:

SpringerLink - Measurement Techniques, Volume 12, Number 2

Refers to conical relief, I guess.

 

[John Garner]

Bruce --

The chisel edge leaning 15 to 20 degrees to the right sounds very low to me. Virtually all of the good factory-ground new drills and machine-ground resharpened drills I've encountered have had the chisel edge leaning a whole lot closer to 45 degrees . . . using the official jargon, they've had Chisel Edge Angles very nearly 135 degrees.

Refer to Fundamentals of Machine Tools for illustrations.

It's worth noting that the attachment's Figures 4-11 and 4-16 differ in their illustrations of Chisel Edge Angle, one being 90 degrees greater than the other.

John

 

[henrys57wagon]

Very simply I put two nuts together(120*) and as long as the back relief is lower than the point that is close enough for the 35+ years as a machinist. As far as the second grinding machine mentioned goes, a lot of things are on the market that simply do not function like they should and they are designed by engineers.

 

[ttok]

Guys -

How about using one of these:

http://www.titantoolsupply.com/Optical_Drill_Geometery_Anaylzer

(For some reason the above link does not work directly. It does reach the Titan Tools home page. There, you click on "microscopes" and then on "other microscopes" - there you will see the drill geometry analyzer - note the mis-spelling of "geometery")

A.T.

ps. I changed the link to "Anaylzer" and it works now! Someone is lisdexic!

 

[FlatBeltBob]

cheap drill releif gage

Take a clean sheet of paper .
mark off a line 6" long
Mark off a vertical line 1 1/4" at one end
connect the start point of the line to the top of the 1 1/4 line to make a long triangle .
Cut this out
wrap the paper triangle around the drill , till the ends of the base line overlap each other .
The angle formed by the hypotenuse of the paper triangle should match the relief angle at the circumference of the drill .
In reality , the clearence at the tip should be slightly more .
( at least this is how they checked hand ground drill back in shop class 40+ yrs ago )

Thanks for the photo of the Oliver drill indicator . I have tried to remember how we used to check them long ago , in my toolroom days .
FBBob

 

[Screwmachine]

Toolmakers microscope or surface illum on comparator.
Bob

Optical comparators and toolmakers microscopes can be had for so cheap nowadays every shop that needs to do oddball measuring can have one. I had a comparator since forever but finally picked up a toolmaker's scope a few years ago, just because it was so cheap; had no idea what I'd do with it really and now I wouldn't live without it. Very very handy for checking angles and dimensions on stuff that would be tricky or impossible with regular measuring tools.

 

[Bruce Nelson]

Bruce --

The chisel edge leaning 15 to 20 degrees to the right sounds very low to me. Virtually all of the good factory-ground new drills and machine-ground resharpened drills I've encountered have had the chisel edge leaning a whole lot closer to 45 degrees . . . using the official jargon, they've had Chisel Edge Angles very nearly 135 degrees.

Refer to Fundamentals of Machine Tools for illustrations.

It's worth noting that the attachment's Figures 4-11 and 4-16 differ in their illustrations of Chisel Edge Angle, one being 90 degrees greater than the other.

John

John:

I agree. The point that I was trying to make is that the amount that the chisel edge leans to the right from vertical directly relates to the amount of primary clearance behind the lip of the drill. 45 degrees will make a satisfactory job of cutting. If you go less than that the cutting edge on the lip of the drill will be stronger. 90 gegrees means there is no clearance angle. Were the illustrations from Moltrecht?

Lord Byron