Toolpost Grinding

External grinding using an angle grinder.

For external grinding I tried using a Bosche angle grinder, mounted using a 10mm socket head screw passed through a piece of 1/4" bms plate into the threaded hole for the handle. The plate was then clamped by the tool holder. Motor speed was controlled by a speed controller unit, adjustable from about 1/8 to full speed. Initial results were decidedly poor, the surface finish looked like something a rat had chewed on. The problem was vibration, and the source was using a tatty grinder disk which was not running concentric, and vibration within the 90-degree geared head. The latter could be minimised (but not eliminated) by reducing speed. The former was sorted by using a new disk carefully set up. Experiments to get the disk to run true included trimming the edge by running a low speed whilst holding an old stone from a surface grinder against it. Some improvement was made but it still wasn't great. Finish on the workpiece improved to a barely tolerable level, but I'm sure more could be done if I could get the disk to run really true with no vibration. Perhaps a better quality angle grinder would not suffer from the inherant vibration problems.

Internal grinding using high-speed motor.

Internal grinding was more successful and I used a compact motor which was originally a polytron (lab cell-masher), with a small Jacobs No.0 chuck attached to the spindle. A convenient threaded hole allowed a 1/2" rod to be attached to the side for mounting on the lathe. I've never seen a 'Dremel' but I guess the motor would be a similar size - perhaps a bit more powerful (but I didn't use it at full power anyway). Speed was again controlled through the speed controller, and this was really essential. Flat-out the thing rotates at 30,000 rpm and makes a noise you wouldn't believe - neighbours came out looking for low flying aircraft! I guess I ran it up to about half-full speed. Standard small stone bits as used in die grinders (with 1/8" shanks) were used, and my favourite shape was a 1/4" cylinder. I successfully ground the jaws of an old lathe 3-jaw with this setup and the finish was good. Very light cuts were taken using a very slow hand feed with the chuck revolving in slowest back-gear. The stone held up well with no appreciable wear.

Surface grinding with a 4" disk from an angle grinder.

Whilst on the subject of grinding, perhaps I should mention a method I use to obtain a 'surface ground' finish without the aid of a surface grinder. I made up an arbor to accept a 4" x 1/4" abrasive disc designed for an angle grinder, this being run on my drill/mill at it's fastest speed (about 2500 rpm - which isn't really fast enough but it works). These discs are tough - designed as they are to withstand considerable abuse - and are far less likely to shatter in use compared to standard grind wheels. Neither do they shed grit everywhere and are very long lasting. It is necessary to arrange for the disc to cut just on the outer 1/4" or so of the disc's flat surface in order to control the cut better. Unfortunately the discs I had were slightly convex, and so to achieve this concavity the surface was turned (!!) in the lathe. This is not a particularly pleasant job, cover up the lathe well and clean up thoroughly afterwards. The cut was made using slowest backgear (which prevents abrasive dust being thrown everywhere) with a carbide tool. The tool needed sharpening about half a dozen times before the job was complete. To be exact, the finish on the work is not quite flat but is very nearly so, a slightly deeper cut is made on the leading and trailing edges of the work. However, the extent of this depends on how many passes you are prepared to make as finer and finer cuts are taken, and it's not too much trouble to get a mirror finish that looks like a gauge block. It's easy and quick to remove 10 or 20 thou which is more than sufficient for finishing jobs. The largest job I have tackled this way was a block of bms about 4" x 2". The finish is pleasantly smooth and clean and adds that bit of class to your home produced tooling.

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