A patch of grass on our back property line does not get mowed when the neighbor bales hay because of a couple of trees and a culvert. My wife wants the area trimmed so our property looks neater. Growing in the midst of the tall grass are some plants with woody stalks one-half inch or so in diameter. I wanted to add a saw blade to our trimmer so I can cut those woody stalks before I cut the grass with the string trimmer head.

Our trimmer is a Troy-Bilt 25cc 17 inch machine with a gear head straight boom. I know there is a commercial attachment for cutting light brush, and I have used one I borrowed. I decided I would prefer the cut of a finer tooth small diameter circular saw blade. This Instructable tells how I mounted the blade.

Materials

6 1/2 inch 90 tooth saw blade for flooring (5/8 inch arbor hole)
5/8 inch spark plug socket
Two fender washers

Tools

Digital caliper
Metal lathe
File
Wrench

Step 1: A Natural Fit for an Adapter

A 5/8 inch spark plug socket I already have makes a very good adapter for the saw blade. No welding is necessary, although I did use a metal lathe to put a 5/8 inch shoulder on one end of the spark plug socket. In the absence of a metal lathe, a drill press and a grinder or a file could be used to reduce the diameter for mounting the saw blade.

This spark plug socket was almost exactly the length needed for the blade mount. I did trim the length a little on the lathe for a better fit. The 5/8 inch hex socket fits perfectly over a hex fitting on the spool shaft. And, most of the shaft is 3/8 inch in diameter. It fits perfectly inside the 3/8 inch square end for the socket wrench and keeps it centered on the shaft.

The second photo shows another view of the mounted blade.

Step 2: Machining

I machined a shoulder onto the end of the socket that snaps onto the ratchet. (First photo)

I wanted the socket to be very close in length compared with the spool shaft so the retaining bolt could catch well. I removed about 1/8 inch from the open end of the socket. (Second photo)

I can still use the socket as a 5/8 inch deep well socket when I am not trimming brush with the string trimmer.

Step 3: Hardware

I found I need two washers to fit under the retaining bolt to keep the saw blade firmly on its shoulder on the spark plug socket. One with a hole slightly larger than 5/8 inch goes next to the blade. Then comes a washer with a hole about the same size as the retaining bolt. (The retaining bolt is actually reverse or left hand thread. I may buy another at a local store specializing in fasteners, or order an extra spool knob for convenience. I had thought I could just use the spool knob, but the bolt pulled out of the plastic knob and left the saw blade loose on its mount.)

Step 4: Concerns and How to Use

Safety--

The commercially available brush cutting blade for this weed trimmer uses a blade slightly larger in diameter than my blade. That blade is not at all sharp. It has four large cutout sections for four bumps that approximate teeth. Both my blade and the commercial blade could cause injury, if someone carelessly moved a foot or a hand too close to the blade. In any kind of normal use, the hands and feet are automatically more than three feet away from the blade in both cases. Also, when the trigger for the gasoline is released, the blade automatically stops.

My blade runs very smoothly. As long as I accelerate gradually and let the saw do the work I do not believe I am stressing the bearings or the drive shaft.

Use--

While the blade is far away from any grass or brush, smoothly accelerate until the blade begins making a singing sound. Hold the engine at that speed.* Rotate boom to the side so the blade is at an angle. Move the blade into contact with stalks of the brush to be cut. Let the blade cut at its own speed. Do not let it slow while cutting. It cuts quite quickly. Release the gas trigger when the stalk has been fully cut.

*If the speed is too slow, the spinning trimmer head tends to wrap with long grasses. Keeping the speed up limits this tendency. The blade begins to make a singing noise at about half of full open on the throttle.

UPDATE: As you can see in the comments below, several expressed strong concerns about safety. This video from 30 seconds in to 2:50 in from the beginning is very typical of my experience with a brush cutting blade on a weed trimmer, except I have limited myself to cutting things at ground level. There are a number of videos at YouTube in which people cut brush with a blade on a weed trimmer. Most users are more aggressive and cut much larger things than shown in this video. Notice the operator is always a very safe distance from the blade and the machine is never even close to out of control. Amazon offers brush blades and adapter washers for use on weed trimmers, usually for Stihl or Husqvarna. I was not aware of the videos or of the conversion kits until well after I posted this Instructable.