Twist Drill Bit

Terminology for multi-facet grinds

General Guidelines for 4-Facet Grind
Drill Bit SIze					Point Angle	Clearance Angle		Notes & Recommended Tormek Jig
Inch Decimal	Inch Fraction	Metric (mm)	Wire Number	Letter		Mild Metal, Wood	Hard Steel
< 0.125	< 1/8	< 3.0	> 31		118°	14°	11°	Can't use the Tormek DBS-22 Drill Bit Sharpening Attachment with these bits -- they are too small. (See also Note 1, below)
0.125 - 0.250	1/8 - 1/4	3.0 - 6.0	31 - 1	A - E	118°	14°	11°	Bits as small as 1/8 inch can be held in the Tormek DBS-22, but they can flex easily. A pin vice can assist with reducing this flex.	Tormek DBS-22
0.250 - 0.500	1/4 - 1/2	6.5 - 12.0		F - Z	118°	11°	9°
0.500 - 0.875	1/2 - 7/8	12.5 - 22.0			118°	9°	7°
> 0.875	> 7/8	> 22.0						Can't use the Tormek DBS-22 Drill Bit Sharpening Attachment with these bits -- they are too large

Notes:

1. One sharpener has used a precision pen vice (like the Starrett 166C ) with the Tormek DBS-22 to hold bits as small as wire number 61 (= 0.0390" ≈ ⁵/₁₂₈" ≈ 1 mm).

Notes & Comments

Information regarding Grindstones

If you pursue sharpening small bits (< ¹/₈ in / < 3.0 mm),

• A pin vice like the Starrett 166C is recommended. Having the pin vice's handle being larger than the knurled area allows the Tormek DBS-22 to hold the pin vice without it shifting. (Yes, that is something I learned the hard way.)
• You will need a good magnifying glass and good light. These are difficult to see with the naked eye (especially in poor light).
• Care must be taken when grinding the secondary facet on bits this small. It is very easy to over grind this. Check after each pass on the grindstone.
• Consider honing the primary grind with a high grit stone (e.g., Tormek SJ-250 Japanese Waterstone). There is no need to hone the secondary facet as it is merely to allow space for chip evacuation.

Tertiary Facets

Addition of a tertiary facet can be useful for helping to remove waste as the hole is drilled. There is no guidance on the angle for these; only that they need to be greater than the secondary facets. If an angle is needed:

• Secondary Facet = 20°
• Tertiary Facet = 45°

The Tormek DBS-22 Drill Bit Sharpening Attachment is not designed for a 6-facet grind, so values are not shown for it. A modification to the DBS-22 was made by Janne for 6-facet grinding, and more information can be found on the Tormek Forum.

Secondary Point Angles (SPA)

SPAs

For tougher materials, an SPA is useful for reducing the stress on the outside cutting lip corners. (This is well outlined in Joseph Mazoff's document {see below}.) However, the Tormek DBS-22 Drill Bit Sharpening Attachment is not designed for a grinding SPAs, so it is not outlined here.

Experience in using larger drill bits shows that SPAs help to reduce wander when drilling deep holes. On GadgetBuilder's web site (referenced below), it was noted:

These extra facets cause the drill to have a stronger self centering action and the sharp outer point on the cutting lip is eliminated so the drill remains sharp longer. The chip is weakened by the difference in cutting action between the primary and SPA facets so long spirals are less common because the chips are prone to break. (...) When through drilling the SPA reduces or eliminates the exit burr on most materials.

Again, on GadgetBuilder's web site, it was noted:

Secondary Point Angles (SPAs) can be added to extend drill life, improve hole finish and minimize the exit burr on through-drilled holes.

Notes:

• As symmetry of the grind would dramatically affect the use of the drill bit, be very careful to make them consistent.
• SPAs are not terribly useful on smaller drill bits.
• Addition of SPAs to a drill bit does not make this a 6-point grind; only adding tertiary facets does that. This may seem like a bit of a nomenclature thing, but it is important to be clear.

More Information

Books & Papers

• Drill Bit Geometry, by Joseph Mazoff. This is a truly great reference about drill bit geometry and is cited often.


• Drill Bit Sizes (Metric, Fractional, Letter, & Number)


• The Drill Press: Its Use and Application from Walker-Turner.


• TM 9-867 Maintenance and Care of Hand Tools from the U.S. War Department.


• Metal Cutting Tool Handbook


• Tap & Clearance Drill Sizes (Metric & Fractional)


• Tormek's "User Manual for the DBS-22 Drill Bit Jig" is quite useful.


• The Use and Care of Twist Drills, from the Cleveland Twist Drill Company. This small booklet (58 pages) is a great read, especially the part starting on pg. 54: "Common Drill Troubles and Causes".

Videos & Presentations

• Sharpen drill bits with Tormek Drill Bit Sharpening Attachment (YouTube video)


• Tormek Live Sharpening Class - Part 6 - Drill bit sharpening (YouTube video). It's time for drill bit sharpening! Learn how to sharpen your drill bits with maximum precision, to the greatest sharpness. We will show you how to create a 4-facet point which gives the ideal cutting performance.


• Tormek DBS-22 (YouTube video)


• Tormek Drill Bit Sharpening Attachment (YouTube video)

Web Sites

• Jigs, Fixtures, & Modifications for the Tormek


• Some DBS-22 Drill Bit Sharpener Modifications are outlined.


• Mica Schmid posted a nice USB spacer jig which makes it easer and faster to set the USB relative to the grindstone, and it also helps ensure the stone is consistently flat relative to the USB.


• GadgetBuilder.com - has a page for building drill sharpening jigs.


• Viking Drill and Tool - Drill Bit Anatomy

Tormek is a copyrighted logo of Tormek AB. Its presentation on this site is used to help the user quickly understand when specific Tormek tools, jigs, or setting are being used. For specific information regarding Tormek AB, or its products, please refer to the www.Tormek.com. Some information presented is from Tormek's handbook, Water Cooled Sharpening of Edge Tools.