Choosing the right grinding wheel is crucial for properly sharpening your mower blades. A grinding wheel that is too coarse or too fine can lead to inefficient cutting, excessive vibrations, and blade damage. This article will go over the key factors to consider when selecting a grinding wheel for mower blade sharpening.
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Grinding Wheel Grit
The grit size of the grinding wheel refers to the size of the abrasive particles on the wheel. This is typically specified by a number, with lower numbers indicating coarser grits and higher numbers denoting finer grits. For sharpening standard mower blades, a grinding wheel grit in the range of 36 to 60 is generally recommended.
A coarse 36 or 46 grit wheel is ideal for aggressive stock removal and re-establishing a blunt edge. The coarse abrasives will bite into the steel rapidly. However, the finish from a coarse wheel will be rough. Finishing with a finer grit wheel will be required to refine the edge.
A medium 60 grit wheel provides a good balance of cutting speed and edge finish. Heavily damaged or nicked blades can be restored with a 60 grit wheel. The edge finish will also be reasonable enough for most mowing applications.
Finer 80 grit or higher wheels are suited for light stock removal and polishing the edge. However, the cutting action is slower compared to coarse and medium grits. Very fine grits are best reserved for the final finishing strokes or delicate edge tools.
Grinding Wheel Grade
The grade refers to the hardness and durability of the grinding wheel. Harder grade wheels maintain their form better during sharpening operations. However, they cut less aggressively. Softer grade wheels cut faster, but wear down more quickly. Common letter grades are as follows:
- A – Very Soft
- B – Soft
- C – Medium
- D – Hard
- E – Very Hard
- F – Extremely Hard
For mower blade sharpening, a medium grade wheel in the C to D range provides a good balance. The wheel is hard enough to keep its shape when grinding high strength mower blade steel, while still cutting reasonably fast.
Grinding Wheel Bond
The bond refers to the materials that hold the abrasive particles together to form the grinding wheel. The bond type affects the wheel’s cutting action and durability. Common bonds used in grinding wheels are:
- Vitreous – Glass-like bond that is very rigid and hard. Provides an aggressive but cooler cutting action.
- Resinoid – Synthetic resin bond that offers controlled cutting action with good durability.
- Rubber -Flexible bond suited for rapid stock removal but requires frequent dressing.
- Shellac – Natural resin bond that gives a very cool cutting action.
- Silicate – Inorganic bond that is extremely durable but cuts slowly.
For mower blades, vitreous and resinoid bonds are good options. Resinoid bonds are more forgiving for novice sharpeners. Straight vitreous bonds cut aggressively but require more wheel dressing. Either bond will work well for maintaining mower blades.
Grinding Wheel Size
Standard grinding wheel sizes range from 4 inches to 7 inches in diameter. Smaller wheels offer greater maneuverability and are less intimidating for beginners. Larger wheels provide higher grinding power and allow longer edges to be sharpened in one pass.
For mower blades, a 6 inch diameter grinding wheel is a versatile size that can accommodate different blade lengths. Miniature 4 inch wheels can also work for single bevel blades. Larger 7 inch wheels are best suited for wide angel grinds on oversized commercial mower blades.
Grinding Wheel Speed
The recommended maximum speed of the grinding wheel depends on the wheel diameter and construction. This speed rating will be printed on the wheel for reference. Exceeding the rated speed can lead to wheel breakage. Grinding wheels are typically rated for speeds between 4000 to 6000 surface feet per minute (SFPM).
Bench grinders will often have a 1725 RPM motor. At this speed:
- 6 inch wheel – 3450 SFPM
- 7 inch wheel – 4000 SFPM
- 8 inch wheel – 4500 SFPM
So 1725 RPM bench grinders are well matched for 6-8 inch diameter grinding wheels. Always check that your grinder speed matches the wheel rating.
Grinding Wheel Composition
There are two main types of compositions used in grinding wheels – aluminum oxide and silicon carbide. Aluminum oxide is a versatile material capable of grinding high strength steels. It is commonly used in all types of grinding wheels. Silicon carbide is extremely hard and sharp. However, it is more brittle than aluminum oxide. Silicon carbide wheels are best suited for grinding carbide, ceramic, plastic, and non-ferrous metals.
For grinding tool steel mower blades, an aluminum oxide wheel is the best choice. Aluminum oxide will cut the blade steel efficiently while maintaining its form for consistent results.
Balancing and Dressing
Proper wheel balancing and dressing are also essential for safe and effective grinding. The grinding wheel should be balanced prior to use to prevent vibrations at higher speeds. Dressing restores the cutting edges of grinding wheels as they are worn down. Dressing is typically done with a single point diamond dresser.
Wheels should be dressed when cutting action seems to slow or edges burn from a dull wheel. Dress moderately and err on the side of under-dressing. Over-dressing quickly shortens wheel life.
Wheel Storage and Care
Take measures to protect wheels from damage when not in use. Store wheels upright and fully supported to prevent chips and cracks. Do not allow wheels to rest on damp surfaces or exposed to temperature extremes for prolonged periods. Inspect wheels for damage prior to each use. Discard cracked or otherwise defective wheels immediately.
Grinding Wheel Recommendations
Based on the criteria discussed, here are some good all-around grinding wheel options to consider for mower blade sharpening:
| Grit | Grade | Bond | Diameter |
|---|---|---|---|
| 36 or 60 | C or D | Vitreous or Resinoid | 6″ or 8″ |
A 36/60 grit x C/D grade x vitreous/resinoid bond x 6/8″ diameter aluminum oxide bench grinding wheel will serve most mower blade sharpening needs. Softer grade wheels in the B/C range can also give good results for less experienced sharpeners.
Using a Grinding Guide
Using some form of guide can greatly improve grinding wheel results. A guide provides a consistent sharpening angle and evenly grinds both sides of the blade. Simple shop-made guides or commercial aftermarket guides are available.
Guides are highly recommended to achieve symmetric grinds. Freehand grinding takes considerable skill to master. Let the guide do the work of maintaining the angle so you can focus on the process.
Grinding Technique
Here is a general step-by-step method for grinding mower blades:
- Secure the blade firmly in a vise.
- Attach the guide and set the desired bevel angle.
- Position the blade perpendicular to the wheel with light contact.
- Turn on the grinder and gently move the blade side to side.
- Use even steady passes from outer edge to center to sharpen. Add water to cool.
- Grind until original bevel is reestablished and edge is sharp.
- Repeat on the opposite side using the guide to match the bevel angle.
- Hone the edges lightly with a fine grit stone.
Remove the minimum metal necessary to sharpen. Follow the existing bevel to maintain blade geometry and balance. Take your time and do not overheat the edge. Multiple light passes are better than heavy grinding.
Conclusion
Choosing the ideal sharpening wheel for mower blades depends on selecting the proper grit, grade, bond, diameter, speed rating, and composition for the job. An aluminum oxide wheel in the 36-60 grit range, C-D grade hardness, vitreous or resinoid bond, and 6-8 inch diameter is recommended. Used with a quality grinding guide at the appropriate speed, a wheel with this specification will efficiently sharpen mower blades.
Proper wheel dressing, balancing, storage, and usage technique are also key to achieving a sharp, long-lasting mower blade edge. Investing in the right grinding wheel makes the difficult task of sharpening mower blades much more achievable for the home shop mechanic.