Part 13 Spiral / Twist Drill Grinding
UNIVERSAL TOOL AND CUTTER GRINDER
From the U2 Operating Handbook.
The Twist Drill Grinding Attachment is for grinding Twist Drills 1/8” to 11/16”. The Lip Angle is always 116° and the Back Angle is adjustable.
From the Author:
The Spiral / Twist Drill / Reamer Grinding Attachment is for designed for grinding Twist Drills or Reamers 1/8” to 11/16” on their cutting tip. The Lip Angle and the Back Angle are adjustable. The Lip Angle may be adjusted for a Positive or Negative Angle.
In my research on sharpening drills, the earliest Home Shop Grinding Fixture I could find was made by the Yankee Tool Company.
The following tool is from a 1898 tool catalog. I could not find the price but I estimate it was under $100.00.
Founded 1878- The North Brothers Manufacturing Company (North Bros. Mfg. Co. or more simply North Bros.), was an American manufacturer based in Philadelphia, Pennsylvania that specialized in the making of hand tools, small appliances and some specialized power tools. They were family owned and operated for over 60 years before being acquired by the Stanley Works in 1946. They are probably most well known for their line of tools, particularly the "Yankee" brand of ratcheting screwdrivers.
You can see that basically they all work the same:
- Clamp the part into the fixture
- Swing the tip across the grinder
- Flip the bit over and repeat
HOW DOES OUR FIXTURE WORK?
The Drill Grinding Fixture is mounted into the R-8 Tool Head with a ½” Collet.
Locking the Index Head and Main Axle makes mounting the fixture easier,
unlock / loosen them when you are ready to grind.
The drill/reamer is clamped in the Swivel Arm using the clamp and Drill Flute
Indexing Tip to position the cutting edge. The fixture is pivoted by hand so the
point of the drill is swept across the grinder stone.
Without changing any settings, the drill is turned over and replaced to index
the other flute that is then sharpened. Swing the Index Head away from the
machine to allow room to install and set up the Drill Grinding Tool.
Install The Fixture into the R-8 Collet Tool using a 1/2" R-9 Collet and the Calibration Aperture.
Setting the proper angle for 118°
Point Drill by using the Cross Slide Drum:
With the Drill Grinding Tool installed in the R-8 Tool Holder,
the position of the Swing Arm by default is 45°.
Common Drills use 118° degrees for their point. So let’s use this as a
reference although any tip angle can be ground with this tool.
The Cross Slide Drum should be set at zero or 90° to the Mail Axle to begin
the adjustment. From that 90° position Rotate the CSD +14° or 14° CCW.
(Truing / setting the degree rings for the grinder will be addressed in a later article)
This explains the conclusion:
118°÷ 2 = 59°
59°+ 45°= 14°
Setting the Relief Angle:
It seems that we machinist’s cannot agree on the terms used
for this area of the common twist drill. The terms that I have
found for the relief area behind the cutting edge are:
Chisel Angle, Chisel Edge Angle, Cutting Edge Relief, Flank, Flank Face,
Flank Face Angle, Heel, Land, Lip Clearance, Lip Clearance Angle, Lip Relief Angle
Major Flank Face, Rake Angle.
For our purpose we will go with the term from the tools
Inventor, Michael Decker: RELIEF ANGLE.
Relief Angle- The material directly behind the cutting edge of a tool. This angle
may be positive or negative depending on the material that is to be machined.
To understand more about proper relief I began by measuring a Chicago-Latrobe ½”,
never used, made in the USA Twist Drill. The Relief from the cutting edge to the back
edge was .038”. The area length measured at the drill circumference was .384”.
That is approximately .100” drop per inch or 6°.
(The Drill Doctor 750x grinds a relief of about 5°).
Adjusting the Grinder Settings:
The Index Quadrant Ring should be set at Zero. This ring is adjustable
so make sure the setting is parallel and true to the Main Axle.
(Truing / setting the degree rings for the grinder will be addressed in a later article).
The Cross Slide Drum can be set at zero or -90° to the Mail Axle
to begin the adjustment. This ring is adjustable so make sure the setting
is -90° true to the Mail Axle.
Our example is for a 118° Point. The setting for the CSD is 14° CCW from
the zero point. From that 90° position Rotate the CSD +14° or 14° CCW.
The actual reading on the Quadrant Ring will vary according to how you
have set the zero and/or if you have made the second Fiducial.
I am working very hard on this section, please bear with me. More information coming
Terminology of parts Disclaimer:
Many of the identification of parts have been taken from the Deckel Catalog. To insure you, the reader, will always know what part I am referring to I have applied the identification of parts from other machines, catalogs and some I created for a clear description of the parts function (eg. Top Slide Tool Holder).