Brush Blade for a Weed Trimmer

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.

Comments

[Moltroub]

I do have a question. 3’ is a good distance from any running blade, but does the blade throw things at the operator like a line trimmer? I was just wondering if you could fabricate a shroud to prevent broken saw teeth or debris from flying at the operator?

[Phil B]

I apologize that I did not see this earlier. Nothing is thrown by the blade. There is not enough torque to throw anything.

[stautzen]

as an engineer i will say that used properly as the author describes it there is no danger here. for many reasons. but think about the the difference in torque between a skill saw and a weed trimmer and the leverage you have holding it on a 3 foot lever arm. and i appreciate the tip on spark plug socket. saved me a lot of time.

[Phil B]

I used the saw blade on my weed trimmer yesterday. The blade bound a couple of times. It stopped spinning immediately. There was no torque or deflection of the boom. It was a lesson to me to feed the blade into branches and trunks more slowly.

[Phil B]

Thank you. I am glad the spark plug socket worked out well for you. When you are not trimming growth the socket is still fully available to you for removing or installing spark plugs. That is a bonus.
Unfortunately, the plastic bump knob does not hold up well when tightening a circular blade in place. If you have a Troy-bilt machine similar to mine, you probably got an additional reverse thread mounting bolt. That works. Or, I made a steel bump knob that holds up well against the demands of using it to mount the blade. I did an Instructsble on that. Welding is involved.

[spiderstones]

You really good at everything....Rick

[Phil B]

Thank you. I have had interesting problems to solve.

[zxkn]

PLEASE DO NOT DO THIS MOD!

I don’t mean to offend the author, it was very ingenious and well explained Instructable.

But it is NOT SAFE!

Aside from the previously mentioned safety issues, all modern circular saw blades are tempered carbon steel. If you happen to hit something hard, you can break off one or more chunks of steel, turning them into the kind of shrapnel that is produced by a grenade or a pipe bomb.

Please don’t turn your weed wacker into a pipe bomb!

[Phil B]

I would not make or use it if I thought it not safe. Go to YouTube and there are a couple of videos using a saw blade on a trimmer very safely, even if they are more aggressive with them than I would ever care to be. Do you say my brush blade is not safe because of an actual incident of which you have firsthand knowledge, or is your judgment based on theorizing?

[zxkn]

Im extrapolating from both personal experience and theoretical knowledge,

I spent a lot of time using circular saws in my yournger years working in residential construction. Took a lot of unsafe shortcuts to get the job done quicker.

Now I’m a practicing engineer, and know better. Fortunately I always managed to avoid serious injury.

What happened to me was with a circular saw that I had taken the blade guard off of. Using one hand to lean out off the side of a scaffold while the other was stretched out trying to nip a 2x4 that didn’t get cut off right. (Didn’t want to move the entire scaffold for one cut). It was one of the old school saws with the toggle lock for the trigger. Ended up dropping the thing on a pile of rebar wile it was still running, shattering the blade.


While considerably greater forces were involved in my situation, I still urge extreme caution. Those circular saw blades are a very brittle temper. Not at all like a hand saw, or reciprocating saw blade.

Even if you went thru the trouble of annealing the blade to eliminate that threat, for every one person like you who is careful enough to use it, the next five are not. So I still wouldn’t suggest it.

[Phil B]

Your unfortunate circular saw experience is not an analogous situation. Go to YouTube and search for “weed trimmer bush blade.” One videio is by a guy in the USA who needed to remove brush from a fence line. The other is a forest worker in Sweden. Both are using Husqvarna trimmers and both are using carbide tip blades with larger diameters than I use. (Part of my rationale is that a smaller diameter blade has less potentially dangerous torque by virtue of its shorter radius.) In both videos the operators are cutting branches up to 3 inches in diameter. They buzz through them like a hot knife in soft butter with no kickback and only fine dust from the blade.

Engineering is a wonderful discipline. I once wanted to become one. I am reminded of an older gentleman I once knew. He did many very technical things without even a high school education. Once a group of engineers at the company where he worked had developed an idea. He happened to look over their shoulders and told them their idea would not work. That did not please them, but he was right.

[zxkn]

I will still stand by my assessment of the safety risk.

And sadly yes, there are far too many engineers that suck at engineering. Once had to argue with a co-worker who wouldn’t believe me that en electric motor had stationary stator windings in the outside housing, he was convinced it was just a chunk of solid steel. Sadly academia caters to engineering students who can can perform complex differential equations in their head but don’t understand how a screwdriver works. I see it time and time again.

CAD programs are making it worse too. Kids seem to think that if it works in the 3D model it will work in real life. Which is how you get a 4-40 cap screw supporting a 200lbs weight

[Phil B]

The first three minutes of this video is very typical of my experience with a blade for cutting light brush with a weed trimmer. Notice nothing unsafe happens during the video. I have to say I do not cut things out in front of me, only at ground level.

https://m.youtube.com/watch?v=8N42hy1cqMk

[Jhonbaker]

Just a thought - I read through the comments and looked into the commercially available attachments used for this purpose - they are substantially different. What is good about this idea is the claim that releasing the trigger applies a brake to the blade - I hope this is true as I've interpreted it. However, think about this if you intend to use this idea - when using a table saw or circular saw - if the blade and wood manage to jam it throws the wood across the room at high speed - I've been hit by a small chunk of ply across the room and it left a deep bruise and small laceration beneath my protective clothing - In this case the "wood" is attached to the ground so the thing that will get thrown is the tool with the blade attached. Not a safe thing to happen with or without a blade guard. I don't think it would be an issue for the designer with what he is cutting but do not try to cut more substantial items that are attached to the ground - this is why the commercially available items are designed the way they are. I'm not sure the motor is strong enough to throw something like a good tablesaw - not a test I want to perform.
Over all it is a good idea - Many of us fellas and a number of ladies have surely thought of it but didn't go the extra steps to realize the idea - It is quite a clever way of attaching the blade - not sure why you need finer cuts on brush - but sure.
EDIT: I am glad someone did this - I've always wanted to do it.

[Phil B]

I use a finer tooth blade because I believe it cuts less per tooth than a coarser blade and is less likely to bind or kick back. From experience cutting steel with a disc on an angle head grinder, the position on the blade where it contacts the material to be cut has a big effect of any tendency to push back. I have cut things with a very small stalk with this and found no sensation of pushback at all. As I said, I do not force the blade, but let it cut at its own speed. Since posting this I stumbled onto a video at YouTube showing a man fitting a slightly larger blade to a weed trimmer and using it to cut limbs 3 or more inches in diameter from trees. There was no sign of binding or pushing back. I do not know that I would care to cut larger things like that. I think such larger things might overload the driveshaft. Remember, too, that the factory string cutter guard is still in place, although that is not prominent in the photos. I think if you tried what I have done you would find your fears about binding and kickback are unfounded.

[Jhonbaker]

I don't disagree with your conclusions and am a little surprised and pleased that there is someone out there cutting larger branches with such a device. My concern was losing control of the tool if it kicked back and the tool flying. I did see the guard in the last photo - or part of it anyway. I did mention or intended to that the motor might not be powerful enough to kick back with enough force to cause much damage and certainly not to the user - I more wondered about safety for others in the area. I might have to fashion one and and test its limitations!

[Phil B]

I looked and found two videos at YouTube in which guys attached a circular blade to a weed trimmer. Both used larger diameter blades than mine. The users were also cutting sbove their heads. I do not like using a blade power tool above my head.

A circular electric saw used in carpentry can kick back, but a weed trimmer does not seem to catch and kick. I would think the centrifugal clutch might also slip.

I would always want to pay attention to the cut I am making, lest the weight of a branch would pinch against the blade. It is better if the pieces separate and lean away from one another as the cut progresses.

In the YouTube videos limbs separated and fell as if butter cut by a hot knife.

A large concern for me when attempting this was to have a solid mount for the blade that remains centered without wobble.

[Phil B]

At YouTube search for CIRCULAR Saw blade on TRIMMER???+TEST and How to turn your weed eater into a tree eater! A search for “weed trimmer brush blade” turns up some additional videos I have not yet viewed.

[Jhonbaker]

Will do! Thank you. The more I think about it the less concerned I am about kick back. While there are substantial differences between this and a chainsaw - it is similar in enough regards and I've never had issue with a chainsaw kicking back and losing control. The table saw kick back is dangerous partially because of the fence which creates a trapped portion allowing it to build pressure and release with more energy than without the fence. So, all my concerns are abated. Thank you for the conversation and inspiration!

[Phil B]

I am glad to have been of help. I found a video today showing a guy in Sweden who put a circular carbide blade on a Husqvarna trimmer and demonstrated cutting down a tree with a diameter almost 12” using only the trimmer. He cut a wedge out of one side. He could have used the space created by removing the wedge to cut even farther into the tree trunk, but succeeded in felling it by cutting at the sides and back of the tree. He admitted he would normally have used a chainsaw. He did have a few provlems with kickback. I simply want to cut an occasional woody stalk my trimmer string will not handle.
I do not yet understand all I wish to understand, but many of these people buy blades at eBay. They seem to be a kit in which there also are washers to center the blade on the shaft below the bump head for the string.