Sharpening Drills

Improperly Sharpened Drills (unequal edge length, unequal point angles, inadequate relief angles) will either rub, drift and/or drill oversize.

The picture above is an illustration of a standard Twist Drill.

Regardless if you sharpen it holding it by hand or with a drill grinding machine, the results should be the same.

Grinding the relief behind the cutting edge:

Referring to the above picture, the drill is swung around the A-axis of an imaginary cone while resting in a support which hold the drill at one-half the point of angle B with respect to the face of the grinding wheel.

Recommended angles for Twist Drill sharpening:

Point Angle:

118 degrees for low and medium carbon steels.

118 degrees to 135 degrees for high carbon steels.

90 degrees to 140 degrees for Aluminum.

Helix Angle:

24 degrees to 32 degrees

Lip Relief Angle:

10 degrees to 15 degrees for low and medium carbon steels.
7 degrees to 12 degrees for high carbon steels.

Note: the lower values of these angle ranges are for large diameter drills and the higher values are for smaller diameter drills. For drills less than .250" diameter the lip relief angles are increased beyond the listed max....up to 24 degrees.

Drill Point Thinning or "Splitting the Drill Point"

The Chisel edge is the least efficient Operating surface on a trist drill point because it does not cut. The chisel edge squeezes or extrudes the work piece material.

Point thinning is desireable on large diameter drills and also on drills that have become shorter because of grinding due to usage. The thickness of the web increases toward the shaft of the twist drill, which increases the chisel edge. The above picture illustrates what a thinned drill point looks like.

Twist drill grinding by hand takes patience and practice. There are other types of drills such as Spade Drills that have different geometry. This post focuses on Twist Drills.

Speeds and Feeds

Tool life is influenced most by Cutting Speed, then by the Feed Rate and least by the Depth of Cut. The proper selection these are Important for time and cost.

(The proper coolant is also a factor. Some materials will work harden with the wrong coolant. This post does not cover this....you'll need to further research how to select the best lubricant/coolant.)

The phrase speeds and feeds refers to two separate velocities in machine tool practice, cutting speed and feed rate. They are often considered as a pair because of their combined effect on the cutting process. Cutting speed may be defined as the rate (or speed) that the material moves past the cutting edge of the tool. It is expressed in units of distance along the workpiece surface per time (typically surface feet per minute [sfm]).
SFM varies depending on the material grade of the cutting tool, the type of cutting tool, the type of material being removed and hardness of material being removed. The Machinery Handbook has time proven charts of recommended Cutting speeds (SFM) to use. Also, manufacturers of cutting tools and steel manufacturers have charts as well.

To determine the Spindle Speed in Revolutions per Minute (RPM): use the following formula:
(12 multiplied to the SFM) divided by (3.1414 multiplied to the Diameter)
note: in the above formula, Diameter refers to the Diameter of the mill cutter/drill or the Diameter of the work piece being turned in the Lathe.


Feed rate is the velocity at which the cutter is fed, that is, advanced against the workpiece. It is expressed in units of distance per revolution for turning and boring (typically inches per revolution [ipr] or millimeters per revolution). It can be expressed thus for milling also, but it is often expressed in units of distance per time for milling (typically inches per minute [ipm] or millimeters per minute). Cutting speed and feed rate together determine the material removal rate, which is the volume of workpiece material (metal, wood, plastic, etc.) that can be removed per time unit.
Reference the Machinery Hand Book or other charts for recommended amounts of material removal per cutting edge and "Depth of Cut"

Formula for IPM: IPM=(RPM) x (Number of cutting Edges) x (Amount of material removed per cutting Edge)

Formula for IPR: IPR=(IPM) divided by (RPM)

Formula for Tap Feed: (IPM)=(RPM) divided by (Thread Pitch)

Once you've determined the Speed and Feed, you may have to adjust it to compensate for variables such as cutter geometry, variations in the material hardness/toughness, the rigidity of the machine tool, workpiece set-up and the Horsepower of the machine. Every CNC machine center that I've ever used has Feed and Speed overrides.

Yes, there are speed and feed calculators that you can download from the internet, and master cam calculates speeds and feeds when it post a program. I feel that you should at least know what you are doing when you use feed and speed overrides on a CNC machine tool. Also, you may not always have your PC available to use the downloadable calculators. Computers and machines don't compensate for the variables discussed in the previous paragraph.

Every Machinist should have reference books and charts in their work area with formulas, cutting speeds, chip removal rates and depth of cut.

Removing a Broken Tap

Removing a broken tap can be challenging. In a perfect world, taps would never break, however, this is not a perfect world. The method of removal depends on how the tap broke and the size of the tap, and the size and shape of the work piece that it broke off in.

If a tap breaks off laterally and part of the tap is out of the work piece far enough...grab it with vise grips or pilers and twist it out.


If a tap breaks off laterally and you can't use vise grips or pliers, then use a Tap Extractor to twist it out.



















I would not recommend welding a screw to the tap, the heat will make the tap even more brittle and weld splatter can become an issue.


Easy-Outs DO NOT work for taps, they only work for screws and bolts.


If the tap shatters inside the work piece (which is most often the case), The method used would greatly depend in the size and shape of the work piece.


EDM Electrode is the best method if possible. You can use a Portable EDM or a EDM Electrode machine.


There are times when you don't have the EDM equipment availability. I would not recommend Drilling it out because the drill point will deflect to the softer metal of the work piece.



I would recommend using a centercutting carbide endmill with a smaller diameter than the minor diameter of the tap, securing the workpiece in a milling machine. When using this method, use a very high spindle rpm and a slow peck feed and compressed air to remove debris. If you are using a Bridgeport mill, then don't use the quill to feed it, use the table and slowly peck feed up. When using a CNC mill, then manually peck feed it in using the "Handle" function. If the Workpiece is too large for the nearest available mill or EDM, then use a hand Dremeling Tool and a carbide bit to grind it out. Regardless if you EDM, Mill or Dremel the broken tap out, you will have to use tweezers or a hard sharp pointed object to dig out the small teeth of the tap after the core is removed.


Remember to always use Safety Glasses or Face Shield.