Gear Cutting Machine Tool Care

⚙Care and inspection

• Decide when to sharpen (regrind) the hob by the amount of wearHob

  Usually decided by flank wear of the cutting edge. For coated hobs, it is economical to sharpen at 0.1–0.2 mm of flank wear. Wear increases rapidly once it exceeds about 0.3 mm. Where crater wear is the problem, use 0.03–0.04 mm as a guide if the wear is close to the cutting edge, and 0.05–0.08 mm if it is far from it

  If the timing is missed, even with small flank wear, the cutting edge chips from crater wear, leading to abnormal wear (Nidec Machine Tool)^[1](1 source, for reference)

• Grind only the hob's rake face, and grind all flutes by the same amountHob

  Grind the rake faces of all flutes until the worn marks disappear from the tops and flanks of the hob teeth. Continue grinding to the flute bottom to blend it in. Match the most worn flute and grind all flutes by the same amount (do not grind more on only the heavily worn flutes). Dress the wheel before finishing and spark out (Moderow)

  The hob is made so that the correct tooth profile appears when the rake face is ground radially (Nidec Machine Tool). Grinding more on only some flutes makes flute spacing uneven and produces tooth profile errors (Moderow). Ground correctly, it can be sharpened repeatedly to the end of its life without harming gear accuracy (Moderow)^[2][1][3]

• Remove runout when mounting the hob on the sharpening machineHob

  Nidec Machine Tool requires mounting runout on the sharpening machine to be within 10 µm. Hob runout during grinding is the most common cause of flute spacing errors, arising from a loose or eccentric arbor, non-parallel collars, an over-tightened nut, and runout of the grinding machine spindle (Moderow)

  If sharpened with runout, the amount ground changes every half revolution, and the runout (indexing error) is ground into the hob. A hob sharpened on a bent arbor becomes an eccentric hob even when correctly mounted on the hobbing machine (Cluff, Liston). Shops that make precision gears sometimes center and sharpen the hob while it is still on the hobbing machine arbor (Cluff). Wear of the index plate and pawl also causes flute spacing errors (Moderow)^[1][2][4][5]

• Keep the rake angle (rake face orientation) as originally designedHob

  Grind a hob with 0° rake (radial) radially. Grind a hob with a rake angle at the same rake angle (the rake angle and rake offset are marked on the hob's end face or in the drawing). So that the rake face does not become convex or concave, grind helical-flute hobs with the conical side of a dish wheel rather than the flat face of the wheel (Moderow)

  If the rake face deviates from radial, the pressure angle is off; if it becomes convex or concave, the tooth profile is off. In Nidec Machine Tool's example, a 3° rake angle error throws the pressure angle off by about 10′, and 0.1 mm of radial alignment error (convex or concave) throws the tooth profile off by about 6 µm. Moderow says a deviation of 0.016 from radial gives roughly 0.001 of profile error (about 1/16). Negative rake (grinding too much of the upper side of the tooth) makes the gear's tooth tips thin and roots thick, and the profile chart tilts to one side. Positive rake is the opposite (Liston, Moderow). Grinding a helical-flute hob with a wheel dressed with a straight dresser makes the rake face convex (Liston)^{[1][3][5][2][6]}

• Do not cause burns or cracks in sharpeningHob

  Before infeed, find the highest flute and touch it lightly before feeding (if runout or uneven spacing is combined with feed, some flutes get heavy grinding and heat up). Grind a clean finished surface (Moderow)

  When grinding heat softens the cutting edge, wear speeds up sharply, and small cracks grow at the tooth root until the tooth breaks (Moderow). Nidec Machine Tool lists grinding cracks from sharpening as one of the starting points of hob chipping, and says that for pinion cutters too, grinding burn and cracks trigger chipping of the cutting edge. The cleaner the sharpened surface, the more parts can be cut per sharpening (Moderow). For pinion cutters too, a rough sharpened surface shortens life (Nidec Machine Tool)^[2][7][8]

• After sharpening, inspect flute indexing (flute spacing) and radial alignmentHob

  A method presented by Nidec Machine Tool that needs no dedicated measuring machine: (1) For flute spacing, mount the hob on a straight mandrel, set it on two center stands, place two lever-type dial indicators with 0.002 mm resolution at the center of the tooth on the rake faces of two adjacent flutes, and zero them. (2) Advance one flute at a time, and record the other indicator's reading for every flute when the hob has been rotated until one indicator reads zero. (3) The maximum absolute difference from the mean of the readings is the single pitch error, and the difference between the maximum and minimum of the running sum of those differences is the cumulative pitch error. For radial alignment, zero the indicator on a gauge block at the same height as the center height, rotate the hob until it reads zero near the tooth bottom, then record how far it moves toward the tooth tip (if the rake angle is not 0°, lower the gauge block by the rake offset)

  Sharpening errors cannot be seen with the naked eye, so using the hob without measuring cuts bad gears (Moderow). Test numbers 5–7 of JIS B 4355 (gear hob accuracy) give the measurement methods (as Nidec Machine Tool explains: flute spacing is 5-6, radial alignment is 7). In Nidec Machine Tool's example (16-flute hob), single pitch error is 4.7 µm and cumulative pitch error is 9.8 µm. For radial alignment, a reading that is positive going from the tooth root toward the tooth tip is called "rising toward the tip"; if you reverse the direction of measurement the sign reverses and you will correct in the wrong direction, so be careful^{[9][6][2][10]}

• Check the hob arbor, collars, and outboard supportHob arbor

  Check arbor runout (bending), foreign matter in the center hole, clearance between the hob bore and arbor (enlarged hob bore, worn-down arbor), parallelism of the clamping collars (spacers), and misalignment or wear of the arbor's outboard support bearing

  Cluff lists as causes of hob mounting runout: insufficient arbor centering, insufficient centering of the hob on its hub, a bent arbor, clearance between bore and arbor, non-parallel collars, and misalignment or wear of the outboard support bearing. Nidec Machine Tool also lists an oversize hob bore as a cause of tooth profile waviness^[4][5][7]

• Choose the hob shift method to suit the purposeHob

  Nidec Machine Tool's comparison: shift every cycle (every part; wear is spread out but shift time is added to cycle time), intermittent shift (every few parts; cycle time is short, but abnormal wear if the interval is too long), one-pitch shift (one hob pitch at a time; not affected by hob runout so tooth thickness and profile are stable, but abnormal wear if the interval is too long and shorter life if too tight), and multi-shift (repeating the one-pitch shift while changing the start position; wear is spread out, but runout may change tooth thickness and profile). For direction, one-direction shifting, in which a new cutting edge always comes to the generating side, is preferable for accuracy (at the end, the life ends or the hob returns to the original position; returning takes time). For long-life tools, reciprocating shift is also acceptable

  This is to use all the hob's cutting edges effectively and spread wear uniformly along the whole length of the hob (Nidec Machine Tool). If the flute lead (inclination of the helical flute) is wrong at sharpening, the hob tapers from end to end and gear dimensions change as the hob shifts (Cluff)^[11][12][4]

• Decide what to do about the rake face coating of resharpened hobsHob

  Grinding the rake face removes the rake face coating. Nachi-Fujikoshi separates its hob types into a specification used only with resharpening (performance without a rake face coating) and a specification that is recoated at every resharpening (fully coated). Gleason has a service that strips the old coating and recoats, and Liebherr also has resharpening and coating services

  The suitable coating and resharpening specification differ with the machining method (oil-based, water-soluble, or dry) and productivity (Nachi-Fujikoshi)^[13][14][15]

• Use the tool manufacturer's resharpening and reconditioning servicesHob

  Gleason says it resharpens HSS and carbide hobs to DIN grades or its own specifications and returns them to like-new quality, life, and performance. It also offers a reprofiling service (rebuilding the tooth profile) that uses up the tooth length and repairs damaged parts. Liebherr also resharpens and coats gear cutting tools

  If you do not have a sharpening machine and measuring equipment in house, sending it out makes it easier to maintain accuracy^[14][15][13]

• Decide when to sharpen pinion cutters by wear widthPinion cutter

  Wear of the cutting edge is usually greatest on the trailing side tooth flank. It is economical to sharpen when the wear width there reaches 0.2–0.3 mm

  The side relief angle on the tooth flank of a pinion cutter is small, usually about 2°, so welding to the flank tends to occur together with flank wear. If the sharpening timing is missed, flank wear grows and promotes welding (Nidec Machine Tool)^[16](1 source, for reference)

• Grind only the pinion cutter's rake face and protect the rake angle and sharpening anglePinion cutter

  Grind only the rake face (the rake angle is usually 5°). Helical pinion cutters are sharpened one tooth at a time, so accuracy is harder to keep stable than with spur cutters. The sharpening angle of a helical cutter is not necessarily perpendicular to the cutter's helix angle. On a cutter with a chamfer, if the grinding position shifts, the chamfer shape goes off. Do not cut in too much at once when resharpening

  A rake angle error becomes a pressure angle error in the tooth profile. Too much infeed when resharpening causes chipping (Nidec Machine Tool). A rough sharpened surface shortens tool life, and grinding burn or cracks trigger chipping of the cutting edge (Nidec Machine Tool)^[16][17][8](1 source, for reference)

• Send shaper cutters and skiving cutters to the manufacturer for resharpeningPinion cutter

  Gleason offers a resharpening service that returns shaper cutters and power skiving tools to like-new quality, and Liebherr also offers resharpening and coating of gear cutting tools

  To maintain accuracy and life, it is surest to have them sharpened by the party who knows the tool's design^[14][15]

• Regrind the flanks (tooth profile) of shaving cutters and keep a record of the regrindingShaving cutter

  A shaving cutter's flanks are reground on a dedicated grinding machine. Gleason returns resharpened cutters with record documents and a regrinding chart

  If the serrations (the grooves forming the cutting edges) are shallow, the cutter can be sharpened only a few times and its life is short (whether the shallowness is a left/right difference, a tooth tip/root difference, variation around the circumference, or shallow overall) (Nidec Machine Tool). Mitsubishi Heavy Industries (now Nidec Machine Tool) introduced a cutter that extends the number of parts per sharpening with a high-rigidity design and surface treatment (in the company's chart, parts per sharpening are 2,440 for another maker's cutter, versus 4,280 for the high-rigidity design and 5,080 for high-rigidity design plus surface treatment). Shaving is a finishing operation before heat treatment; after heat treatment, gears are ground (Nidec Machine Tool)^{[14][18][19][20]}

• Do not bump shaving cutters when carrying or handling themShaving cutter

  Handle them so the teeth are not chipped in transport and handling. Check for chips before use

  Causes of chips present before use include grinding cracks from tooth grinding, cracks during heat treatment, and damage in transport and handling (Nidec Machine Tool)^[18](1 source, for reference)

• Manage wear of skiving cutters, and resharpen and recoat themSkiving cutter

  Gleason offers resharpening and coating services for power skiving tools, and a system that resharpens the cutter inside the machine to eliminate stops for cutter changes. Liebherr also makes skiving tools and offers resharpening and coating services

  Skiving by principle has a large cutting load, and tool life has been an issue (Mitsubishi Heavy Industries technical review). In the tests in that technical review, the number of parts machined is compared at a wear limit of 0.3 mm (one example, as a guideline)^[21][15][22]

• Restore the shape of grinding worms by dressing, and send CBN plated tools out for reconditioningGrinding worm (threaded wheel) and dresser

  Dressable threaded wheels are dressed at a set interval. Plated CBN grinding worms and diamond/CBN plated dressing tools are reworked on the base metal and replated by the manufacturer (Liebherr and Gleason reconditioning services)

  Loss of shape of the grinding tool becomes tooth profile and lead errors as is. Nidec Machine Tool has released gear grinding machines with a wider wheel to lengthen the dressing interval (ZE series, 160 mm wide)^[15][14][23]

• Replace worn arbor and collet parts and maintain the accuracy and rigidity of the fixtureWorkholding fixtures (arbors and collets)

  Replace wear parts such as collets, segmented collets, and clamping springs. Inspect and adjust them regularly (Gleason)

  Poor accuracy or insufficient rigidity of the workholding fixture causes lead (helix) errors, and poor mounting accuracy of the cutter or workpiece causes runout of the tooth space (Nidec Machine Tool)^[24][25][17]

• Set and manage tool lifeTools in general

  Decide the number of parts cut per sharpening (or up to the end of the shift) and do not use the tool beyond it. If the machine has a function that detects abnormal load during cutting and stops, use it

  The Japan Machine Tool Builders' Association recommends tool life management to maintain cutting conditions, and an abnormal load detection function that stops machining the moment abnormal load is applied to the cutting edge (a worn or chipped tool is a source of heat and fire). One method is to treat the hob as at end of life when it reaches the end of its shift range (Nidec Machine Tool)^[26][11][19]

÷Hob shift amount and effective shift length (Nidec Machine Tool's simple formulas)

1. Shift amount ℓ (mm) = Mn × π × z ÷ (cos γ × i). Mn is the hob's normal module, z is the number of starts of the hob, γ is the lead angle of the hob, and i is the number of flutes of the hob. It is the amount by which the cutting edge that serves as the generating center is moved in turn to the position of the cutting edge in the next flute
2. Hob allowance length a ≈ π ÷ 2 × m
3. Cutting edge working range K = k + k0 (k is the working length of the cutting edges on the hob periphery, and k0 is the tooth profile generating range from the generating center (one side))
4. Effective shift length S = hob blade length l − (K + 2a)

Shift amount

ℓ = Mn × π × z ÷ (cos γ × i)

Tooth profile generating range (one side)

k0 = Ca × cos β ÷ (tan αn × cos τ) (Ca is the hob addendum, αn is the tool's normal pressure angle, β is the helix angle of the gear, and τ is the hob's mounting angle)

Effective shift length

S = l − (K + 2a)

Worked example: conditions (shift amount)

Mn 1.5, γ 3°, 3 starts, 16 flutes

Worked example: result (shift amount)

ℓ = 1.5 × 3.14 × 3 ÷ (cos 3° × 16) ≈ 0.88 mm

Worked example: conditions (effective shift length)

Gear m2.25, pressure angle 14.5°, 25 teeth, helix angle 30°, outside diameter 70 mm; hob outside diameter 90 mm, 2 starts, blade length 130 mm (climb hobbing). From the charts, k = 23, k0 = 13

Worked example: result (effective shift length)

K = 36, a = 3.5, S = 130 − (36 + 7) = 87 mm

The shift amount formula is not one that gives optimal values for optimizing wear or stabilizing accuracy (Nidec Machine Tool's caution). The working length k of the peripheral cutting edges changes with the gear's diameter and helix angle, the hob's outside diameter and lead angle, and the hobbing method, and strictly it is calculated by the tool manufacturer. Many NC hobbing machines calculate the shift range automatically^[11][12](1 source, for reference)

▦How tooth thickness decreases when the hob mounting angle is changed (Nidec Machine Tool's example: m2, pressure angle 20°, hob outside diameter 80 mm)

As a countermeasure to flank scratches and tearing in dry hobbing, the mounting angle is sometimes offset 15′–1° from the correct one. Tooth thickness decreases by that amount, and if infeed is held back to compensate, a chamfering hob gives a smaller chamfer at the tooth tip. Theoretical formula for tooth thickness reduction: ΔW = mn × {sin 2γ × (Δε)² ÷ (2 tan αn)} × (g ÷ sin³γ + Z); chamfer reduction (radial): ΔC = ΔW ÷ (2 tan αn) (mn is the hob's normal module, γ is the lead angle, Δε is the mounting angle error, g is the number of starts, and Z is the number of teeth. For helical gears, put Z ÷ cos³β in place of Z). Depending on the workpiece tolerance, a design change may be needed^[27][28][3](1 source, for reference)

÷Calculating flute spacing error of a hob (from measurement with two dial indicators)

1. Mean of indicator A readings recorded at flutes No. 1 to n: Avg = (No.1 + No.2 + … + No.n) ÷ n
2. Difference for each flute S_k = Avg − No.k
3. Single pitch error = maximum absolute value of S_k
4. Work out the running sum S1, S1 + S2, S1 + S2 + S3, … in turn; cumulative pitch error = its maximum − its minimum

Mean

Avg = ΣNo.k ÷ n

Single pitch error

max|Avg − No.k|

Cumulative pitch error

max(Σ_{j≤k} S_j) − min(Σ_{j≤k} S_j)

Worked example: conditions

16-flute hob (Nidec Machine Tool's measurement example)

Worked example: result

Single pitch error 4.7 µm, cumulative pitch error 9.8 µm

At the end of the measurement, return indicator A to the No.1 flute and confirm that it returns to the initial 0. Graphing the values shows the single and cumulative errors (Nidec Machine Tool). The standard for the measurement method is JIS B 4355^[9][10]

▦Example of hob sharpening tolerances (standard of the Metal Cutting Tool Institute, US. AA and A grades extracted from the table in a 1992 article, in units of 0.0001 inch)

Values are the total indicator reading (TIR). 1 is 0.0001 inch (2.54 µm), so for example 10 is about 25 µm. Module m is a guideline converted from diametral pitch DP as m ≈ 25.4 ÷ DP. The tolerance on flute lead depends on blade length: for grade A, 10 for blade length up to 1 inch, 15 for 1–2 inches, 25 for 2–4 inches, 30 for 4–7 inches, and 50 above 7 inches. The current Japanese standard is JIS B 4355 (modified adoption of ISO 4468; the 2020 edition of ISO is current, with seven grades 4A–D), and the values here are not a substitute for it^[2][4](1 source, for reference)

÷Lead of the helical guide for pinion cutters (checking whether it can be shared)

1. Check the cutter's normal module mn, number of teeth Zc and helix angle β
2. Lead L = π × mn × Zc ÷ sin β (or L = π × cutter pitch circle diameter ÷ tan β)
3. If another workpiece/cutter gives nearly the same L (within the lead error tolerance), the cutter tooth count is an integer, and the helix hand is the same, the same helical guide can be shared

Lead

L = π × mn × Zc ÷ sin β = π × Dc ÷ tan β (Dc is the pitch circle diameter of the cutter)

Worked example: conditions

mn 2、Zc 40、β 20°

Worked example: result

L = 3.1416 × 2 × 40 ÷ sin 20° ≈ 735 mm (estimate)

The formula and the conditions for sharing come from Nidec Machine Tool's material. The example figures are an estimate using this formula^[29][8](1 source, for reference)