Gear finishing

Hard Tooth Surface Scraping: 7 Essential Steps to a Stable Process

This page covers the hard tooth surface scraping process for hardened gears after heat treatment: how the cutting edge removes stock, what stock allowance survives grinding, and which gears should skip the process. Written for gear engineers, process planners, and buyers who need to judge whether a shop can hold Ra 0.8–1.6 μm on a 58–62 HRC flank.

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58–62 HRCRa 0.8–1.6 μm±0.005 mm12-hour quote

Hard tooth surface scraping setup on a hardened gear flank

On this page

1
What matters before you start
2
How hard tooth surface scraping removes material
3
Which gears suit hard tooth surface scraping
4
Machine, tool, and workholding requirements
5
Common defects and what causes them
6
Step by step: running a hard tooth surface scraping pass
7
Hard tooth surface scraping compared with grinding and honing

Key takeaways

What matters before you start

Stock allowance decides everythingLeave 0.10–0.30 mm per flank after heat treatment. Under 0.05 mm, the edge skates and burnishes instead of cutting.

Rigidity beats spindle speedA 0.02 mm vibration at the work arbor shows up as a wavy flank and a 2–3 μm profile error.

Cutting speed is low on purposeCarbide hobs on 58–62 HRC steel run at 60–120 m/min. Pushing past 150 m/min shortens edge life fast.

Not every gear qualifiesModules under 1 mm and narrow face widths below 10 mm rarely pay back the setup cost.

Mechanism

How hard tooth surface scraping removes material

Hard tooth surface scraping is a finishing operation that runs after heat treatment, when the flank is already at 58–62 HRC. A carbide hob or shaping cutter, coated with TiAlN or AlCrN, takes a shallow chip off the hardened flank. The edge does not push the metal aside the way a grinding wheel does. It shears it. That difference is why a scraped flank usually comes out with a compressive surface layer instead of the tensile layer a grind can leave behind.

The cutting edge enters the gap between two teeth and rotates in sync with the work. Each tooth of the cutter removes a slice of the allowance. Because the chip is thin, the cutting force stays low, and the heat generated goes into the chip rather than into the part. That keeps the flank from tempering back below its target hardness.

This is not a roughing process. It is a correction and finishing pass. The stock you leave for it should be small enough that the cutter never sees a heavy load, but large enough that the edge bites instead of rubbing. In practice that window is narrow, and most process failures trace back to the allowance being outside it.

1
Edge geometryPositive rake, small chamfer, sharp edge radius under 10 μm.
2
Cutting modeShear, not abrasion. Chips are continuous and thin.
3
Heat pathHeat leaves with the chip, so the flank stays hard.

Applicability

Which gears suit hard tooth surface scraping

Hardened gears show up in vehicle transmissions, marine drives, construction machinery, and wind turbine gearboxes. In those assemblies the gear runs at high load and high speed, so flank finish and profile accuracy drive noise, efficiency, and life. A scraped flank at Ra 0.8–1.6 μm cuts sliding friction compared with a ground flank at Ra 1.6–3.2 μm, and the profile correction that comes with the process reduces transmission error.

The process pays back best on medium and large modules, roughly 2–10 mm, where the cutter can reach into the gap and the flank area is big enough to justify the setup. It also suits gears with a shoulder or a flange that a grinding wheel cannot clear without undercutting the root.

It is the wrong call on small modules. Below 1 mm, the cutter body gets too thin, the chip load per tooth drops into the rubbing range, and edge breakage climbs. Narrow face widths under 10 mm have the same problem: the cut is short, the edge spends more time entering and exiting than cutting, and the cost per part never pays off.

1
Good fitModule 2–10 mm, face width above 20 mm, 58–62 HRC.
2
Marginal fitModule 1–2 mm, or gears with an interrupted flank.
3
Poor fitModule under 1 mm, face width under 10 mm, soft gears below 45 HRC.

Setup

Machine, tool, and workholding requirements

The machine has to be a dedicated gear cutting platform, not a general machining center. You need a synchronized work spindle and cutter spindle, a rigid arbor, and enough stiffness that the cutter does not deflect under the tangential load. A machine that flexes 0.02 mm at the arbor will print that deflection straight into the flank as a profile error.

Tool choice comes down to hob or shaping cutter. A hob is the default for external spur and helical gears. A shaping cutter is the better pick when the gear sits next to a shoulder, when the helix angle is steep, or when the gear is internal. Either way, the coating matters: AlCrN holds up better than TiAlN above 100 m/min because it resists oxidation at higher temperatures.

Workholding is where most shops cut corners. The arbor has to seat on a clean, burr-free bore, and the clamping force has to be enough to stop rotation but not enough to distort the bore. A 0.01 mm bore distortion from over-clamping shows up as a 0.015 mm lead error on the flank. Check the bore after clamping, not before.

1
Arbor runoutKeep under 0.005 mm at the gear seat.
2
Cutter runoutKeep under 0.008 mm on the hob arbor.
3
CoolantHigh-pressure oil, 20–40 bar, aimed at the cut zone.

Troubleshooting

Common defects and what causes them

A wavy flank pattern almost always traces to vibration, not to the cutter. Check arbor runout, clamping force, and cutter balance in that order. If the arbor indicates clean and the clamp is light, look at the machine foundation and the spindle bearings. A worn bearing can add a periodic error that repeats every work revolution.

Burnishing instead of cutting means the edge is rubbing. The usual cause is allowance under 0.05 mm, but a dull edge or a cutting speed above 150 m/min will do the same thing. Measure the chip. If you are getting dust and powder instead of thin chips, the edge is not cutting.

Lead error that grows along the face width points to workholding distortion or thermal drift. Check the bore after clamping and let the machine reach thermal equilibrium before the first cut. A machine that has been idle overnight needs 30–45 minutes of warm-up before it holds the lead tolerance.

1
Wavy flankArbor runout, clamp force, spindle bearings.
2
Burnished surfaceAllowance too small, dull edge, speed too high.
3
Growing lead errorBore distortion or thermal drift during the run.
4
Edge chippingChip load per tooth below 0.03 mm or an interrupted cut.

Procedure

Step by step: running a hard tooth surface scraping pass

1
Verify hardness and allowanceCheck flank hardness at three points. It must sit in 58–62 HRC. Measure the remaining stock per flank with a gear measuring center. Target 0.10–0.30 mm. If it is under 0.05 mm, send the gear back for more grinding stock rather than running the scrape.
2
Mount and indicate the gearClean the bore and arbor, then seat the gear. Indicate the bore runout at under 0.005 mm. Clamp, then re-check. If the bore moved more than 0.01 mm, reduce clamping force and re-seat.
3
Set the cutter and syncInstall the hob or shaping cutter and indicate runout under 0.008 mm. Set the axial and radial feed per the module. Confirm the electronic gearbox sync before the first cut. A sync error of one tooth shows up as a deep gouge on the first flank.
4
Run a first-article cutTake one pass at 60–80 m/min with a light feed. Measure profile, lead, and pitch. Adjust the profile correction and helix correction from the measurement, not from the drawing. Do not chase the last 2 μm on the first part.
5
Hold the cutting parametersFor 58–62 HRC steel, run 60–120 m/min and 0.5–1.5 mm axial feed per work revolution depending on module. Keep the chip load per tooth above 0.03 mm to stay out of the rubbing zone. Watch the chip color: light straw is fine, blue means the edge is too hot.
6
Monitor edge wearInspect the cutting edge every 20–30 parts under a toolmaker microscope. Replace or re-coat when flank wear reaches 0.15 mm. A worn edge raises cutting force and pushes the flank into a wavy pattern that no amount of correction will fix.
7
Inspect and releaseRun a full inspection for profile, lead, pitch, and surface finish. Confirm Ra 0.8–1.6 μm on the flank and check for edge breakage at the tooth ends. Release only after the report matches the drawing tolerances.

Process choice

Hard tooth surface scraping compared with grinding and honing

Use this table to pick the finishing route for a hardened gear flank.

Process	Typical Ra	Best for	Watch out for
Hard tooth surface scraping	Ra 0.8–1.6 μm	Module 2–10 mm, shouldered gears	Needs stock 0.10–0.30 mm per flank
Profile grinding	Ra 0.4–0.8 μm	Tight profile and lead tolerance	Grinding burn, tensile surface layer
Gear honing	Ra 0.2–0.5 μm	Noise reduction after heat treatment	Slow cycle, limited correction range
Shaving (before heat treatment)	Ra 0.4–0.8 μm	Soft gears before hardening	Cannot correct distortion after hardening

When to run it, when to skip it

Run hard tooth surface scraping on module 2–10 mm gears at 58–62 HRC with 0.10–0.30 mm of stock per flank and a face width above 20 mm. Skip it on modules under 1 mm, narrow faces, and soft gears. If your gear sits on the edge, send the drawing and we will tell you which finishing route holds the tolerance at the lowest cost.

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FAQs

Questions engineers ask about hard tooth surface scraping

Can hard tooth surface scraping replace profile grinding?

Not entirely. Scraping corrects profile and lead within a limited range and leaves a compressive surface layer. Grinding holds tighter profile tolerance on small modules and narrow face widths.

For gears that need both tight tolerance and a compressive layer, some shops run scrape first and grind second. That adds a setup but spreads the correction across two processes.

How much stock should I leave after heat treatment?

Target 0.10–0.30 mm per flank. That window lets the edge bite without overloading the cutter.

Below 0.05 mm, the edge rubs and burnishes. Above 0.40 mm, cutting force climbs and the flank can chip at the tooth ends.

What hardness range does the process handle?

The practical range is 58–62 HRC. Below 55 HRC, the material tends to smear and form a built-up edge. Above 64 HRC, edge wear accelerates and tool life drops sharply.

If your gear sits outside that range, check whether the heat treatment can be adjusted before changing the finishing process.

How does the surface finish compare with honing?

Honing reaches Ra 0.2–0.5 μm, which is finer than scraping. Scraping lands at Ra 0.8–1.6 μm with a compressive layer and a larger correction range.

If your only requirement is noise reduction on an already accurate gear, honing is the better pick. If you also need profile correction, scraping does more work in one pass.

What causes a periodic error on the flank?

Periodic error usually comes from the work arbor or the cutter arbor. Indicate both and check the clamping repeatability.

If both arbors indicate clean, look at the machine spindle bearings and the gear teeth count. The error period tells you which rotating element is responsible.

Can the process run on an internal gear?

Yes, but only with a shaping cutter. A hob cannot reach an internal flank.

Internal gears need a rigid cutter arbor and a short overhang. If the overhang exceeds four times the arbor diameter, deflection will show up in the lead.

Send your gear drawing for a process review

We review module, hardness, allowance, and face width, then tell you whether scraping, grinding, or honing fits your gear. Quotation and free DFM analysis within 12 hours.

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