Important Information on Mounting Cutters in Power Tools


I obtained my first power tool at the tender age of 10. I have used power tools and machinery ever since and in all those years I have injured myself in every way conceivable and a few ways that really weren’t. I have also destroyed a multitude of power tools and broken every attachment and cutter I have owned…. until recently. Over the years I have cut my finger nail off, set myself on fire, split the end of a finger in two, taken large chunks of flesh out of my hands and leg and come very close to losing my eyes. I say this to point out that through painful and bloody experience I know what I’m talking about.

Now that I make part of my living from selling power tools and cutters I have discovered that several of my stupid mistakes are actually quite common. In the interests of our customers (and their spouses) and the bonsai hobby at large I thought it would be useful to put some of my experience on paper (screen ??). I ask that you pay very close attention to the following. It WILL save you a great deal of money in the long run. I may just save you a trip to hospital too!

The cutters required to carve wood, and bonsai trees in particular, efficiently need to be of very high quality. Anything that will cut wood quickly will cut flesh like thin air. Thanks to health and safety regulations there are no “unsafe” power tools available in the U.K. However by their very nature a “safe” power tool can be VERY dangerous. I know of a guy who literally cut his head off with a chainsaw, on purpose! It goes without saying, although it needs to be said, that using a poorly maintained, worn out or damaged power tool is, at best, a false economy and just might cost you a great deal. NEVER COMPROMISE ON THE MAINTENANCE AND SAFETY OF POWER TOOLS!!

Speaking personally I get sick and tired of every one banging on about health and safety. I don’t believe it’s somebody else’s job to make sure we are safe at all times. This flies in the face of evolutionary progress. My own thought is that, if you are daft enough to cut your face off with a Dremel the human gene pool is probably better off as a result. However the subject of this article is to do with a slightly more technical aspect of using power tools rather than telling you not to clean your ears out with a die grinder.


Mounting Cutters in Power Tools

All of the cutters we sell and indeed all of the cutters I recommend for carving work in bonsai and woodwork require very high speeds to work properly. By “high speed” I mean 10’000 RPM (revolutions per minute) or above. Cutters designed to run at this speed are very special and will not do a good job, or last long, if run at 3000RPM in your average electric drill. In actual fact most of the cutters we offer run best at 15-25,000 RPM. These speeds are easily attained by die grinders and hobby tools like Dremel etc’. The reason for such high speeds is very simple, the faster the cutting teeth pass the wood the more chips will be removed and the easier the work will progress. Also at such speeds the cutting edge hits the wood surface so fast and so often the cutting action is very smooth and very little inertia is passed into the work piece, that’s REALLY important with bonsai! Slow speeds of rotation mean the cutting teeth slam into the work, trying to bite heavy chunks of wood. This means that the tool will become blunt much faster and will also pass a lot of energy into the work piece, if that work piece is a bonsai tree the result will be a lot of root damage.

So, the efficient performance of power carving tools relies upon high RPM. Unfortunately high RPM brings with it some inherent problems. For instance Centrifugal forces that attempt to pull an object traveling in a fixed rotational path away from the center of rotation. As the rotational speed of a tool increases centrifugal force acts to throw it’s mass outwards. Once this force exceeds the tensile strength of the tools weakest part the tool will deform. In practice this usually means the cutters shaft will bend.

In order for a rotary cutter to work efficiently it must spin accurately around it’s axis, resisting deformation by centrifugal force. Doing so means that each cutting tooth will strike the surface of the work equally. This means our tool cuts ‘smoothly’, it means the cut face is neat and clean, the tool will last a long time and our power tool will not be put under undue stress.
Of course it’s not possible to create a tool that has perfectly balanced mass when spinning around it’s axis. Although for all practical purposes we can make tools that work very well within our operating parameters there are tolerances, and this is where our problems really begin. As a tool rotates it’s effective mass increases with speed, it gets heavier. Any imbalance in the tool is magnified and any deflection in the tools axis is magnified. Once this reaches a critical point, again, our tool will bend. Tools that do not rotate evenly create much higher sideways forces and so are more likely to become deformed.

No matter how good the quality of your power tool it’s output shaft will have a small degree of ‘run out’. That is, the degree of tolerance from a perfectly true rotating axis. The further you get from the tools bearings the larger the run out. That’s why it’s not possible to make very long high speed cutting bits. Because of the relatively large run out at their end they would bend very quickly.

Most of this is of little concern if you have purchased good quality tools from responsible manufacturers. But mis-using a tool can be very dangerous and the term ‘mis-use’ is not as self explanatory as it appears. Mounting a tool incorrectly constitutes mis-use and so does running a tool in a worn out machine. So here are my recommendations ….

Never use a machine with worn or noisy bearings or a worn casing or worn motor bearings. Even one or two thousandth of an inch run out at the machine will make a big difference to the mounted cutter.

Never use a worn collet, retaining nut or output shaft.

Always use the correct size collet for your cutter. This is particularly relevant where larger shaft tools are concerned and is the main source of tool failures I have seen. Remember a ¼” is not 6mm (it’s 6.35mm). A quarter inch shaft cannot be safely mounted in a 6mm collet. A 6mm shaft can be inserted into a ¼” collet but the run out will be excessive.

Only ever mount high speed cutters using collets NEVER EVER mount them in a 3 jaw chuck (fitted to some Dremels) The clamping force of a chuck is not sufficient to hold high speed tools in place and they are also very inaccurate and will cause excessive run out.

Always tighten the securing nut fully. If the securing nut is not pulled up sufficiently the tool may rotate in the collet. This will wear the collet, the tool and the output shaft of the machine and will cause excess run out.

Never use distorted tools, these will generate excessive side forces on the machines bearings that will wear out much more quickly.

Never use blunt or dirty tools. These will require extra force to get them to cut. This will lead to excessive machine wear. It will also generate heat that may affect the wear resistance of the cutting teeth. Always keep cutting tools clean and free of resin build up. For an effortless solution see our Cutting Tool Cleaner.

Regularly remove collets, clean them and their mounting sockets with a brass brush. See our Collet Brush Kit.

Always store tools carefully to avoid damage when not in use.


Mounting Cutters in Power Tools

Securing nut, collet and output shaft of a die grinder
Securing nut, collet and output shaft of a die grinder

mounting cutters
The angles shoulder of the collet presses against a similar face inside the output shaft. When the nut pushes the collet into the shaft it reduces the collet bore diameter and clamps the cutting tool in place with 360° of pressure.

mounting cutters
1/4 inch and 6mm collets. There is a .35mm difference in the bore diameter. That's a lot in engineering terms!

A correctly sized collet with cutter shaft fully inserted
A correctly sized collet with cutter shaft fully inserted

A 1/4 inch shaft will not fit into a 6mm collet
A 1/4 inch shaft will not fit into a 6mm collet!

when things go wrong
This is what happens when things go wrong. This cutter was run at 33,000 RPM (much too fast) whilst only secured using a 3 jaw chuck.

As the tool was spinning the chuck was unable to hold it
As the tool was spinning the chuck was unable to hold it. Gradually it slid upwards as the shaft slipped between the jaws. Only 9mm remained in the chuck when the run out reached a critical point and failure occurred.

This tool looks perfectly good but there is a problem
This tool looks perfectly good but there is a problem...

Upon closer inspection scuffing/polishing can be seen
Upon closer inspection scuffing/polishing can be seen and also scarring along the shaft. In use the tool vibrates very badly...

From this picture its very obvious the owner has tried to force the tool into a collet that's too small
From this picture its very obvious the owner has tried to force the tool into a collet that's too small. Because the tool does not fit the collet her has over tightened the securing nut to compensate, leading to the scarriing along the shaft. The tool has less than 10mm inserted into the machine, excessive run out has occurred and the tool has been bent at speed. It's trash. However it could have been worse!

pencil marks show how far this tool can be inserted into a 6mm collet
Pencil marks show how far this tool can be inserted into a 6mm collet (not far enough) versus the correctly sized collet.

Two die grinders fitted with 1/4 inch collet (above) and 6mm collet (below)
Two die grinders fitted with 1/4 inch collet (above) and 6mm collet (below). The tools have 1/4 inch shafts and are both inserted into the machines as far as is possible. In use the bottom tool will fail and could cause very serious injury, damage the machine and ruin the cutter.

Unless you are drilling a hone in a wall, NEVER EVER MOUNT A HIGH SPEED CARVING TOOL IN ONE OF THESE
Unless you are drilling a hone in a wall, NEVER EVER MOUNT A HIGH SPEED CARVING TOOL IN ONE OF THESE. We believe their use on Dremel type machines verges upon negligence.


Graham Potter
Kaizen Bonsai Ltd.
July 2009
The information given in this article is provided in good faith based upon our experiences. We accept no responsibility for actions or consequences arising from it's practical application.
Copyright Kaizen Bonsai Ltd 05/2009