Milling Cutters
Home Up Milling Cutters Boring Head Tricks Indexing Head

 

Homemade Milling Cutters (from old engine bolts & twist drills):

n.b: this page is in the process of being created.  Please be patient, it might take me a while to finish it.

The page will include:

Examples of homemade cutters.
How I make them.
The calculations I use to get the depth & position of cut to generate the cutting faces.

 

Three of these milling cutters were made from high tensile 'big-end' engine bolts.  The fourth (obviously) is a re-ground twist drill.
All have been used & work well with low speeds & light cuts - remember (other than the re-ground drill), they aren't HSS, but carbon steel.
Note the remnants of the bolt's original thread.
The one on the left is used as an end mill (it's really a mini fly-cutter with 2-cutting teeth.
The two in the middle are Woodruff key slot cutters.
The one on the right is a Tee-slot cutter I used to deepen the recess on my lathe cross-slide.
The 6-tooth cutter has a 5-degree cutting angle.  The 4-tooth cutter was supposed to be a six tooth cutter, but in setting up I accidentally added 4 numbers instead of adding 3 & subtracting 1 - whoops.  It works but is better for softer materials due to the acute cutting angle.  The 3-tooth cutter was shaped by hand.

Guided Reamer & Counterbore:

Although not necessarily a cutter for milling with, it's near enough to show how I make them.  This tool will be used for making the 9mm bearing seat inside a housing.

From 1/2" silver steel, the required diameters are machined between centres.  A square is then milled on the end (so I can use a tap wrench with it).
Flipped over in the (piston) Indexing head, I can now start to mill out the spaces between the teeth, set using the formulae below.  In this case, the depth of cut is just down into the guiding tip.
The guide is to centre the cutter in a 7mm hole.  The counterbore is to make the seating for a 9mm ball race.  As I will be using it in aluminium & for only a few holes, I don't think it will need hardening or tempering.
Side on, showing the teeth spaces as milled out.

I will provide a small amount of relief by hand filing the back edges & at the tips of the ends.
End on, shows the partially formed teeth, prior to the final hand shaping & honing.

The final shaping of the non-cutting surfaces (relief & clearances) was done with careful use of both needle files & needle riffler files.
Finished & ready to use.

I will ream out the bearing mountings by putting a centre in the milling quill, apply a small amount of down-force via that, whilst turning the tool with a tap wrench - Until the required depth of counterbore is achieved.

 

 

Fluting a Milling Cutter.

Dimensions for a cutting angle of 5-degrees - This is the general purpose angle I chose for the teeth on my milling cutters. 

D = cutter diameter
x = offset to the side of the centreline
y = the depth of cut - you might need to vary this to suit your need - less depth gives more land, deeper cuts give shorter land.
The 6-toothed cutter - end result after milling

The back of the tooth can then either receive a second cut to remove the corner, or be carefully filed by hand, into a curve.
Depth of cut for Milling a flat on a shaft.
  D = Diameter of shaft
  FW = Width of flat required
Across the Flats = 0.75D to 0.78D

Depth of cut = 0.125D to 0.11D
     Approx = 12% of the Dia.

 Milling a square on a shaft.

 

 Milling flutes in 4-flute taps.

Formula as I worked it out:
Use  ball end of 1/2 tap diameter,
about 1/4 to 1/3 ball diameter deep
A is typically 5-degree
 

Reamer Dimensions.

Dia. Overall
Length		 Flute
Length		 Square
Length		 Size
Square		 Number
Flutes
1⁄8 3 1-1⁄2 5⁄32 0.095 4 to 6
5⁄32 3-1⁄4 1-5⁄8 7⁄32 0.115 4 to 6
3⁄16 3-1⁄2 1-3⁄4 7⁄32 0.14 4 to 6
1⁄4 4 2 1⁄4 0.165 4 to 6
5⁄16 4-1⁄2 2-1⁄4 5⁄16 0.21 4 to 6
3⁄8 5 2-1⁄2 3⁄8 0.255 4 to 6
7⁄16 5-1⁄2 2-3⁄4 7⁄16 0.305 6 to 8
1⁄2 6 3 1⁄2 0.35 6 to 8
9⁄16 6-1⁄2 3-1⁄4 9⁄16 0.4 6 to 8
5⁄8 7 3-1⁄2 5⁄8 0.445 6 to 8
3⁄4 8-3⁄8 4-3⁄16 3⁄4 0.49 6 to 8
7⁄8 9-3⁄4 4-7⁄8 7⁄8 0.61 8 to 10
1 10-7⁄8 5-7⁄16 1 0.705 8 to 10
1-1⁄8 11-5⁄8 5-13⁄16 1 0.845 8 to 10
1-1⁄4 12-1⁄4 6-1⁄8 1 0.935 8 to 12
1-3⁄8 12-5⁄8 6-5⁄16 1 1.03 10 to 12
1-1⁄2 13 6-1⁄2 1-1⁄8 1.125 10 to 14

Tap Dimensions

Dia.

Length

Thread
Length

Square
Length

Shank
Dia

Square
Size

Number
Flutes

1⁄8

1-15/16

5/8

3/16

0.141

0.110

3

5⁄32

2-1/8

3/4

1/4

0.168

0.131

4

3⁄16

2-3/8

7/8

1/4

0.194

0.152

4

1⁄4

2-1⁄2

1

5⁄16

0.255

0.191

4

5⁄16

2-23⁄32

1-1⁄8

3⁄8

0.318

0.238

4

3⁄8

2-15⁄16

1-1⁄4

7⁄16

0.381

0.286

4

7⁄16

3-5⁄32

1-7⁄16

13⁄32

0.323

0.242

4

1⁄2

3-3⁄8

1-21⁄32

7⁄16

0.367

0.275

4

9⁄16

3-19⁄32

1-21⁄32

1⁄2

0.429

0.322

4

5⁄8

3-13⁄16

1-13⁄16

9⁄16

0.48

0.36

4

11⁄16

4-1⁄32

1-13⁄16

5⁄8

0.542

0.406

4

3⁄4

4-1⁄4

2

11⁄16

0.59

0.442

4

7⁄8

4-11⁄16

2-7⁄32

3⁄4

0.697

0.523

4

1

5-1⁄8

2-1⁄2

13⁄16

0.8

0.6

4

1-1⁄8

5-7⁄16

2-9⁄16

7⁄8

0.896

0.672

4

1-1⁄4

5-3⁄4

2-9⁄16

1

1.021

0.766

4

1-3⁄8

6-1⁄16

3

11⁄16

1.108

0.831

4

1-1⁄2

6-3⁄8

3

11⁄8

1.233

0.925

4