Lawn Care
DIY Reel Mower Sharpening: Backlapping vs Professional Grinding
The Scissor-Action Mechanics of Reel Mowers
Unlike rotary mowers that rely on high-velocity impact to tear through grass, reel mowers utilize a precise scissor-action. The rotating helical reel blades sweep across the stationary bedknife, shearing the turfgrass cleanly. According to turfgrass research from the University of Minnesota Extension, this clean cut minimizes cellular damage, reducing moisture loss and significantly lowering the risk of fungal pathogens entering the leaf tissue.
However, this mechanical advantage demands exact tolerances. When the cutting edges degrade, the mower transitions from shearing to folding and tearing the grass. Frayed turf tips turn brown within 48 hours, giving the lawn a dull, whitish cast. Proper reel mower sharpening is not a single procedure but a tiered maintenance strategy dictated by the specific wear pattern of your cutting unit.
Diagnosing the Cut: Dullness vs. Misadjustment
Before applying abrasives, you must confirm whether the cutting unit requires sharpening or simply mechanical adjustment. The standard diagnostic tool is the paper test. Hold a standard piece of 20-lb copy paper and push it across the entire length of the bedknife while the reel is engaged.
Diagnostic Warning: If the paper cuts cleanly at the ends of the reel but folds in the middle, your reel is likely suffering from "hourglassing" (deflection wear) or the bedknife has a dip. Backlapping will not fix physical deflection; this requires professional spin grinding or bedknife replacement.
- Clean cut across entire length: Cutting unit is sharp and properly adjusted.
- Paper folds or tears uniformly: Blades are dull; requires backlapping or grinding.
- Cuts at ends, folds in middle: Hourglass wear or bedknife dip; requires professional spin grinding.
- Intermittent folding: Bedknife clearance is too wide; requires adjustment, not necessarily sharpening.
Method 1: DIY Backlapping (Routine Maintenance)
Backlapping is an abrasive lapping process designed to maintain the micro-edge between the reel and bedknife. It is a routine maintenance task, not a restorative one. According to the USGA Green Section, regular backlapping extends the time between costly professional grinds by maintaining the mating surfaces of the cutting unit.
Tools and Materials Required
- Abrasive Compound: Clover Lapping Compound (120-grit for standard maintenance, 80-grit for minor nicks, 220-grit for final polishing). Cost: $15–$25 per tin.
- Backlapping Kit: Includes a drive gear adapter and a nylon application brush (e.g., Toro or Oregon brand kits). Cost: $35–$60.
- Power Drill: Variable speed, capable of low RPM (300–500 RPM).
- Solvent: Mineral spirits or brake cleaner for post-lapping cleanup.
Step-by-Step Backlapping Procedure
- Secure the Cutting Unit: Lift the mower and block the wheels. Ensure the reel can spin freely without contacting the ground or safety guards.
- Attach the Drive Adapter: Remove the reel drive gear cover and attach the backlapping adapter to the reel shaft.
- Apply the Compound: Using the nylon brush, apply a thin, even bead of 120-grit lapping compound along the entire length of the bedknife.
- Run in REVERSE: Attach your drill to the adapter. Critical: You must run the reel in reverse (opposite to its normal mowing direction). Running it forward will sling the compound outward and risk cutting the brush. Run the drill at 300–500 RPM for 3 to 5 minutes.
- Reapply and Repeat: Apply more compound as it is squeezed out. Continue until the reel spins smoothly with a consistent, high-pitched "hissing" sound, indicating uniform metal-to-metal contact.
- Clean Thoroughly: Wash all compound off the reel and bedknife using mineral spirits. Any residual abrasive left in the cutting unit will act as a grinding paste during your next mow, accelerating wear.
Method 2: Professional Spin Grinding (Restorative Fix)
When backlapping fails the paper test, or when visual inspection reveals rounded edges, deep nicks, or conical wear, the cutting unit must be spin ground. Spin grinding utilizes a specialized grinding stone that matches the exact rotational axis of the reel, restoring the factory helical geometry and removing microscopic hourglassing.
Professional shops use heavy-duty machinery like the Foley Belsaw or Nebraska Blades spin grinders. Attempting to grind a reel freehand with an angle grinder will destroy the helical pitch, rendering the cutting unit useless.
Cost and Maintenance Matrix
| Method | Estimated Cost | Frequency | Best Use Case |
|---|---|---|---|
| DIY Backlapping (120-grit) | $5–$10 per session (amortized kit cost) | Every 20–30 hours of mowing | Routine edge maintenance; minor dullness |
| Professional Spin Grinding | $85–$150 per cutting unit | Annually or every 150 hours | Severe dullness, nicks, hourglass wear |
| Bedknife Replacement & Grind | $200–$350 (parts + labor) | Every 3–5 years | Bedknife dip, extreme relief angle loss |
| Full Cutting Unit Replacement | $400–$800+ | Rare (catastrophic failure) | Bent reel shaft, irreparable bearing damage |
Setting the Bedknife: The Make-or-Break Final Step
Sharpening the reel is only half the battle. If the bedknife is not set to the correct clearance, the mower will still tear the grass. Modern turfgrass engineering favors a "clearance cut" over a "contact cut" to reduce friction, heat buildup, and fuel consumption.
The Feeler Gauge Adjustment Protocol
You will need a set of brass feeler gauges. The target clearance for most residential and commercial reel mowers (such as the Toro Greensmaster or John Deere 220-series) is between 0.001 and 0.003 inches.
- Loosen the Bedknife Screws: Slightly loosen the mounting bolts holding the bedknife to the bedknife bar. Do not remove them.
- Establish Contact: Turn the adjusting screws at both ends of the bedknife until the reel makes light, uniform contact with the bedknife across its entire length. You should hear a faint "clicking" sound as each blade passes.
- Introduce Clearance: Back off the adjusting screws exactly one-quarter to one-half turn. Insert your 0.002-inch brass feeler gauge between the reel and bedknife.
- Verify the Drag: The gauge should slide through with a slight, uniform drag. If it passes through freely, the clearance is too wide. If it won't fit, the clearance is too tight.
- Tap and Torque: If the clearance is uneven, gently tap the bedknife bar with a brass or dead-blow hammer to seat it against the adjustment screws. Once uniform, torque the mounting bolts to the manufacturer's specification (typically 15–20 ft-lbs) in an alternating star pattern to prevent warping.
Pro Tip: Check the Relief Angle. The bedknife must have a relief angle (usually 15 to 20 degrees) ground into its back edge. If this angle wears flat, grass will pinch between the reel and bedknife before the cut occurs, resulting in a ragged edge even if the blades are razor-sharp. If the relief angle is gone, the bedknife must be replaced or professionally reground.
Frequently Asked Questions
Can I use valve grinding compound instead of Clover lapping compound?
While automotive valve grinding compound works in an emergency, it is not recommended for long-term use. Automotive compounds often use silicon carbide abrasives that can embed into the softer bedknife metal, causing accelerated wear to the harder reel blades. Clover compound uses aluminum oxide, which is specifically formulated for the metallurgy of turf cutting units.
How often should I backlap a manual push reel mower?
Manual push reel mowers (like the Fiskars StaySharp Max or Scotts Classic) operate at lower RPMs and typically cut higher residential turf. Backlap these units once at the beginning of the spring season, and again mid-summer if the paper test indicates folding. Over-backlapping manual mowers will prematurely wear down the reel diameter.
Why is my reel mower leaving uncut strips of grass?
Uncut strips, often called "ribbing" or "washboarding," are rarely caused by dull blades. This is almost always a result of an improper ground-to-reel speed ratio. If you are walking too fast for the reel's RPM, or if the drive wheels are slipping, the reel does not make enough cuts per square foot to shear the grass cleanly. Slow your walking pace or check the drive belt/gear tension.
