Now we are getting into the nitty-gritty of this sharpening stuff. You will see in the diagram below that Drill No.1 has the cutting edges, A & B, equal in length and that the corners are the same height. The included angle of the edges is 128 degrees. A drill sharpend like this is on its way to cutting true size. I still haven't shown the clearance under the cutting edge at this point. The shape of the point is the first part to copy as you begin grinding.

Drill No.2 has the edges at the same height but the edge A (in red) is shorter than edge B. The drill will still pivot on the point but because edge B is longer than A, the drill will cut oversize.

Drill No.3
The point of the drill is in the center but the edge A is not shaped to the same angle as edge B and the corner is lower. The steeper angle of the edge will actually push the drill off center and an oversize hole will result once more.

You may be tempted to think that the size difference in the oversized hole wont really matter but it is very important to have the drill cut to true size. If you were drilling a hole to tap a thread eg. a 1/4-28 UNF, the oversized hole would not give you enough thread depth and the threads would strip. A drill point that was off center by only .015" would cut a hole that would be .032" bigger or thereabouts.

To regrind and sharpen a drill, one requirement is absolutely essential. You MUST have a grinding wheel that runs as true as possible and has the face perfectly straight. There must not be any grooves or ridges in the face of the wheel. The are two or three ways to achieve a true wheel. The best way is to cut the wheel with a diamond. Grab your wife's diamond ring and... You can buy an industrial diamond that is mounted in a 1/2" dia. steel bar about 5 inches long. This will cut the wheel to a true circle but needs practice and a steady hand to get the face flat without any grooves.
There is a tool called a "Star Wheel Dresser" that really eats the wheels and can produce a flat face. It can be difficult to get the wheel nicely round 'cos these things tend to bounce about when the wheel is out of round to begin with.
Perhaps the best way to start is to use a "Dressing Stick". This is a square stick of very hard stuff (Carborundum usually) and can true the wheel and make a flat face. It might be difficult to find one in most of the common tool shops.
I use a stick made by Norton Abrasives. It is call a Norbide Stick and lasts a lifetime. The one I have was bought about 35/40 years ago and it cost me an arm and a leg. I dont know the cost of one today. If you can find one, buy it!!
The next section will be grinding the cutting edges, or as some like to call it, the Black Art! Actually, the secret is knowing how the geometry of the drill point works and shaping it to the required form.

Kody

The drawing below shows a drill on the left with no clearance. Whilst the drill on the right shows what the required clearance will look like. You will notice that the angle of clearance starts off at 5 degrees and increases to about 17 degs. This is how I grind my own drills. You are not restricted to conform to my own style of grinding. The important part of sharpening at this point of the job is to get the primary edge clearance at about 5 degrees. This clearance is also known as "Lip Clearance" and is very important that it be correct. There are two basic secrets to proper drill sharpening. One is having the knowledge and understanding of the required geometry and the second is to get lots of practice. I find that the best size/s to practice on are diameters from 1/4" to 5/16" (6.5mm to 8mm). These are quick to grind and big enough to see what you are doing. Once you have mastered these sizes, you will be able to tackle 1/2" then 5/8" and even 3/4" and bigger. For sharpening the very small Number series or drills less than 1/16", you cant do better than to use a diamond hone and shape them by hand. Using a powerful magnifying lens, it is easy to sharpen drills of 1/32" dia. One of the most challenging jobs I ever had was to sharpen a 1/32" drill so that it cut consistent holes at 0.0325" in dia.

Kody

Spot on there Joe. Like I said, who needs to go to TAFE when we have you online?

When we moved from the fitting shop to the machine shop, our first machining excercises were accurate drilling preceeded by drill sharpening. That was preceeded by grinding wheel dressing with a Star Wheel Dresser. Forget about diamond sticks and carborundum sticks.

A gauge tool for drill sharpening we were taught to make and use, consisted of an old file with the teeth ground off on one side and the tang ground to a point and then bent in a U shape. It is only useful for drills having a centre hole in the end of the shank (ie morse taper drills) or for smaller drills held in a Jacobs chuck with a morse taper and centre hole.

Where the file teeth were ground off was chalked. After grinding, the drill butt end is placed on the point of the tang and the outside points of the drill's cutting edges used to scribe lines through the chalk. Assuming one has the chisel point right by eye, if the drill scribes lines (ie two), the cutting edge angles and therefore lengths were unequal. Back to the grind stone. Very accurate.

Old files were aplenty. A flat piece of mild steel suitably shaped and bent is just as good but does tend to get confusing grooves cut into it. The file and drill materials being of similar hardness, the drill point tends not to dig into the file but does leave a shiny mark through the chalk.

Cheers Cisco.

Well done Paul. Keeping drills for ten years and just sharpening them on the grinder with a bucket of water is certainly an achievement. I guess the cruicial point is being able to teach a newbie how to do it and this is why I have written the post. Can you sharpen a drill to drill a hole in brass shim? If you need the info then keep reading as I will post the info as soon as I can clear the sinus's and eyes of the rotten flue.
cheers,
Kody

little grind,cool,little grind,cool..................rolling the drill on the face of the grinding wheel just gently,

[b]Kody said...

All high quality drills are made from a steel called High Speed Steel (HSS). The golden rule when buying any form of tool is :- "You get what you pays for".
Kody

Would that these rules still hold true, but in today's world there are so many shades of things. Once infallible brand names sold off to big conglomerates and quality downgraded for the alleged benefit to consumers of cheaper prices.

Why did Rolls Royce go broke? Because when you bought a Roller you never needed to buy another car.

I was using a drill recently that was marked as HSS and I was wondering why I was having so much trouble with it. Then the penny dropped. I smacked myself on the forehead and said "You idiot!! HSS doesn't stand for High Speed Steel, it stands for Hardly Suitable Steel. It's like things marked 'Made in USA' doesn't mean it comes from America, because there is a town in Japan called USA."

I am not kidding! Adios Amigos, Cisco.