DIY Precision Lawn Mower Blade Sharpener

Recently, when it came time to replace my old mower blades, I looked for DIY lawn mower blade sharpener ideas on YouTube. I wanted a design that wouldn’t take up much storage space, but that could be quickly & easily setup and used. I found one design that I thought had possibilities, so I decided to make one. Here are links to the original by Clean & Green Lawn Service LLC & a modified version by Michael Krailo.

To have the blades sharpened “professionally” locally it costs $19, about two thirds the initial cost of new blades (as of June 2022). I grew up on a farm and we always sharpened our own blades--and I still do to this day.

Most of the nuts and bolts for this project I had on hand already, so I was able to keep my new purchases around $26. I was able to pick up an inexpensive angle grinder ($9.99) from Harbor Freight with a “Super coupon”, three foot of the aluminum square tubing ($10.69) and a horizontal hold down clamp ($5.26) from Menard’s. The grinder needed for this project can be small because you don’t need very much grinding power to sharpen mower blades. Plus, I already had a grinding flap disc from one of my other grinders.

Note: I was asked why I chose a flap disc for this project rather than a solid hard disc. A flap disc produces a stronger cutting edge than a solid disc. For example, a flap disc "rounds over" the cutting edge and leaves more metal; a flat file creates a "strait" edge; a solid disc results in a weaker edge because it "grinds out" metal to create an edge.

The sharpening process usually involves three light passes per side and then I check the blade balance. It typically takes me under ten minutes to sharpen and balance a set of blades (two).

Using the videos, I first built a prototype and found that there was too much rotational “play” even when the joints were moderately tight. I eliminated this problem using ¾” aluminum square tubing in place of the ½” steel square tubing used in the original video. I chose aluminum rather than steel for a couple of reasons: lighter weight and because I planned to store the sharpener in my garage (no rust from humidity changes).

I normally store my sharpener in an upright position (Figures 1a, 1b & 1c), so the space required is roughly 10”W x 12”D x 15”H (Figures 2a & 2b).

Note: A short video was added on 8-10-22 as requested, as well as a Figure 9 with two screenshots from the video.

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- (1) 2' of ¾” aluminum square tubing

- (1) horizontal hold down clamp

- (1) Inexpensive 4 ½" angle grinder

- (1) 4 ½” 60 grit flap disc (metal) (used what I had on hand)

- (1) 4”x4”x12” piece of fence post (used what I had on hand)

- (1) ¼" x 4" lag screw (used what I had on hand)

- (4) 1” x 1/8” lag screws (used what I had on hand)

- (10) #9x2” deck screws (used what I had on hand)

- ¾”x16”x9” plywood (used what I had on hand)

- (1) ¼" x 2” bolt

- (9) ¼" washers

- (1) ¼" lock nut

- (1) M8-1.25x60mm all threaded bolt

- (3) M8-1.25 coarse hex nuts

- (2) 8mm washers

Tools

- miter saw (recommended)

- 1/8” (for pilot holes), ¼" & 5/16” drill bits

- protractor to measure blade profile

The vast majority of lawnmower blades come from the factory with a 32 degree bevel (profile). However, it is important to verify your blades' profile before building this sharpener. Even an error of two degrees will be noticeable in the end results. Most dealerships recommend a blade profile somewhere between 30 & 45 degrees, so… If your profile measurements are off slightly, your first sharpening may take a bit longer since you will be changing the blade profile slightly. When I used a protractor (Figure 3) to measure my blade profile it was 32 degrees.

Starting with the 4”x4”x12” piece of fence post (base), I cut a 32 degree profile on one end with my miter saw (Figure 4a). In the YouTube videos they cut their other ends to 32 degrees & 40 degrees respectively (“backward slant angle” - Figures 4b, 4c & 4d). After trial an error, I decided 10 degrees was more comfortable for me when sharpening.

Tip: After building a prototype, I noticed that the bevel profile angle wasn’t correct when I tested with the angle grinder. I discovered this was due to my miter saw gauge being off by almost 2 degrees around the 30 degree marks. I corrected this by using my shop protractor to set the proper angle (32 degrees) before sawing the 4”x4”x12” base.

From the prototype I decided that there was too much rotational “play” even when the joints were tightened moderately. I eliminated this problem using ¾” aluminum square tubing (Figures 5a thru 5d). I chose to use aluminum rather than steel due to weight and because I planned to store the sharpener in my garage.

For the articulating “arms” I cut two 12 inch sections of ¾” aluminum square tubing and drilled ¼” holes in three of the ends and drilled a 5/16” hole in the last end. After “deburring” the drilled holes and cut ends, I joined the two sections of square tubing using a ¼"x2” bolt, three ¼” washers and a ¼" lock nut (Figure 5c). I used a ¼”x4" lag screw and four ¼" washers to mount the square tube arm to the 4"x4"x12” base 2 ½” down from the top (Figures 5a & 5b). The articulating arms were attached on the right side of the base (Figure 5a).

Before mounting the tubing to the grinder, I attached a baseboard to the bottom of the base (used when clamping sharpener to a bench/table). The baseboard was a ¾”x7”x9” piece of surplus wood I had on hand and I used five 2” deck screws. I then mounted the square tubing to the grinder using a M8-1.25 x 60 bolt, two washers and two M8-1.25 coarse nuts and placed a M8-1.25 coarse nut on the bolt end that screws into the grinder (Figure 5d).

Next, I created the mower blade clamping base using a ¾”x5”x6”, a 3/4”x2”x6” hold down clamp mounting board and then mounted the a horizontal hold down clamp with four 1/8”x1” lag screws (Figure 6c).

Since newer mower blades are “stamped” into their final shapes during manufacturing, the cutting edges/ends usually are not parallel to the blade center (Figure 6a). The mower blade clamping base will usually need to be slightly rotated counterclockwise when mounting to the 4”x4”x12” base (Figures 6c & 6d).

I clamped a new mower blade on the mower blade clamping base (Figure 6b) and positioned on top of 4”x4”x12” base to determine the correct centering the blade bevel with the center of the grinder wheel. I drew a line on the 4”x4x12” base (to align afterwards) and then set the grinder down. I finished by mounting the mower blade clamping base using five 2” deck screws (Figures 6c & 6d).

I hope you find this Instructable useful.