KEVOS® MAKEOS · Manufacturing
Milling Cutter Clearance Grinding Set-up (Clearance Needed, Wheel and Tooth-Rest Offsets, Indicator Drops)
When I sharpen a milling cutter on a cutter grinder, what clearance angle does the cut need, how far must I offset the wheel or the tooth rest from the cutter centre to grind that angle with a disc or cup wheel, and what indicator drop should I see when I check it?
Set-up for grinding the clearance (relief) angle on the teeth of plain milling cutters, face mills and form-relieved cutters by the handbook’s cutter grinding rules: the clearance angle the cut needs from the feed per revolution, land width, depth of cut, cutter diameter and the Brinell hardness of the work, the offset of the disc wheel centre above the cutter centre (wheel diameter × ½ × sin clearance), below the cutter centre (cutter diameter × ½ × sin clearance), the tooth rest drop with wheel and cutter centres in line (wheel diameter × cutter diameter × sin ½ clearance ÷ (wheel + cutter diameter)) and with a cup wheel, and the indicator drops for checking the relief on side and end teeth and the rake on the tooth face over a measuring distance.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Grinding set-up | choice | How the clearance is ground: with the periphery of a disc wheel offset above or below the cutter centre, with wheel and cutter centres in line and the tooth rest lowered, or with the face of a cup wheel. |
| Cutter diameter | mm | Outside diameter of the milling cutter being sharpened. |
| Grinding wheel diameter | mm | Diameter of the disc grinding wheel (not used for a cup wheel). |
| Clearance angle to grind | ° | Primary clearance angle you intend to grind on the peripheral teeth. |
| Clearance the cut needs | ||
| Feed per revolution of the cutter | mm/rev | Feed per revolution of the cutter in the cut (feed per tooth × number of teeth). |
| Width of the land | mm | Width of the land behind the cutting edge; the handbook gives about 0.4, 0.8 and 1.6 mm for small, medium and large cutters. |
| Depth of cut | mm | Depth of cut the cutter takes. |
| Brinell hardness of the work | HB | Brinell hardness number of the work material. |
| Indicator check | ||
| Measuring distance moved under the indicator | mm | Distance the cutter or indicator is moved in a straight line for the indicator-drop check of side and end teeth or the tooth face. |
| Rake angle of the tooth face | ° | Rake angle of the tooth face to be checked with the indicator. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Grinding set-up
- What it is
- How the clearance angle is ground on the cutter teeth on the cutter grinder.
- Units and format
- Choose one of 4 options from the list: Disc wheel, centre above the cutter centre; Disc wheel, centre below the cutter centre; Disc wheel, centres in line (tooth rest lowered); Cup wheel (tooth rest below the cutter centre).
- Example
- The form opens with “Disc wheel, centre above the cutter centre” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Chooses which offset is reported as the set-up.
- Get it right
- Disc wheels are usually set above or below centre; a cup wheel grinds a flat land with the tooth rest below the cutter centre.
What this means: Cutter diameter
- What it is
- Outside diameter of the milling cutter being sharpened.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the offsets below centre and with a cup wheel, and the clearance needed.
- Get it right
- Measure over the teeth after the last sharpening, not the catalogue size.
What this means: Grinding wheel diameter
- What it is
- Diameter of the disc grinding wheel used on the cutter grinder.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the offset with the wheel above centre and in line.
- Get it right
- Measure the wheel after dressing; it shrinks with use.
What this means: Clearance angle to grind
- What it is
- Primary clearance angle you intend to grind behind the cutting edge.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0.000001° to 45°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Every offset grows with the sine of this angle.
- Get it right
- The handbook gives about 3 to 5° for steels, 4 to 7° for cast iron and bronze, 10 to 12° for soft non-ferrous metals.
Clearance the cut needs
What this means: Feed per revolution of the cutter
- What it is
- Feed per revolution of the cutter in the cut, feed per tooth times teeth.
- Units and format
- Enter a number in mm/rev: millimetres per revolution. The form accepts 0.000001 mm/rev to 100 mm/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 mm/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger feed needs more clearance behind the edge.
- Get it right
- Take it from the job’s feed rate divided by the spindle speed.
What this means: Width of the land
- What it is
- Width of the land behind the cutting edge of each tooth.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A wider land needs more clearance.
- Get it right
- About 0.4, 0.8 and 1.6 mm for small, medium and large cutters; it widens with each resharpening.
What this means: Depth of cut
- What it is
- Depth of cut the cutter takes in the job.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A deeper cut needs a little more clearance.
- Get it right
- Use the deepest cut the cutter will take.
What this means: Brinell hardness of the work
- What it is
- Brinell hardness number of the work material being milled.
- Units and format
- Enter a number in HB: HB. The form accepts 10 HB to 800 HB. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 HB as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Harder work needs less clearance in the handbook formula.
- Get it right
- Take it from the material certificate or a hardness test.
Indicator check
What this means: Measuring distance moved under the indicator
- What it is
- Distance the cutter or indicator is moved in a straight line for the indicator-drop check.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The drop grows in direct proportion.
- Get it right
- The handbook tables use 0.005 to 0.062 in for relief and 0.031 to 0.125 in for rake.
What this means: Rake angle of the tooth face
- What it is
- Rake angle of the tooth face that you want to check with the indicator.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 45°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the indicator drop for the rake check.
- Get it right
- Positive rake of 10 to 15° is common on high-speed steel cutters for steel.
Method and formulas
- Clearance needed C = (45860 ÷ (D B)) (1.5 L + (F ÷ (π D)) √(d (D − d))) degrees, with F, L, d and D in inches (mm ÷ 25.4) and B the Brinell number.
- Disc wheel above centre: offset of the wheel centre = wheel diameter × ½ × sin clearance.
- Disc wheel below centre: offset of the wheel centre = cutter diameter × ½ × sin clearance.
- Centres in line: tooth rest below the centre line = wheel diameter × cutter diameter × sin(½ clearance) ÷ (wheel diameter + cutter diameter).
- Cup wheel: tooth rest below the cutter centre = cutter diameter × ½ × sin clearance.
- Indicator drop over a straight measuring distance = distance × tan(angle), which reproduces the handbook’s Tables 3 and 4 for side and end relief and face rake.
- Example: a 100 mm cutter cutting 200 HB work at 1 mm/rev and 3 mm deep needs about 2.8761° clearance; to grind 5° with a 150 mm wheel set the wheel centre 6.5367 mm above the cutter centre.
Limitations
- The clearance formula is the handbook’s general rule for plain milling cutters, face mills and form-relieved cutters; it uses inch units, so millimetre inputs are converted.
- A disc wheel leaves the land slightly hollow, which reduces the effective clearance more with small wheels; the offsets set the angle at the cutting edge.
- The indicator-drop rule is for straight-line checks of side teeth, end teeth and rake; the radial relief on peripheral teeth is checked by rotating the cutter against the handbook’s Table 1.
- Secondary clearance of 3 to 5 degrees behind the primary land is ground the same way at the larger angle.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set up a tool and cutter grinder to sharpen a milling cutter
- Check that the clearance ground suits the feed, depth and material being cut
- Check relief and rake angles with a dial indicator
Who it is for
- Toolroom machinists
- Cutter grinders
- Machine shop supervisors
What you get
- Clearance angle needed by the cut
- Wheel or tooth-rest offset for the set-up chosen
- Offsets for all four set-ups
- Indicator drop for the relief on side and end teeth
- Indicator drop for the rake angle of the tooth face
Interpreting the result
- Grind at least the clearance the cut needs: a land that rubs heats the edge and wears it quickly; much more weakens the edge.
- Offsets grow with the wheel diameter above centre and with the cutter diameter below centre; measure both after dressing and resharpening.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 838–843. Topic reference; no source tables or recommended settings are reproduced.
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