STUBAI HSS- Kugelschneider/Ballcutter - 14 and 19mm
|Starting cuts||Hollowing||More hollowing||Sharpening|
The two major techniques of wood turning are CUTTING and SCRAPING. A CUTTING technique uses a tool with an acutely sharpened edge with a means of support close behind the edge. The cutting tool is presented to the work so that the support contacts first and the tool is then rotated or raised so the cutting edge contacts the work and cuts. Importantly the support is used to control the depth of cut of the cutting edge and remains in contact throughout the cut. An unsupported cutting edge will typically uncontrollably dig into the work. In this respect the tool rest of the lathe is a secondary support for the cutting edge; the primary support is the support close behind or around the cutting edge.
When shear or skew is added to this equation, by angling the cutting edge so that it is not cutting at right angles to the rotating wood, the resulting worked surface will be extremely smooth. This is a goal of the process, a surface smoothly cut with little or no need for further smoothing such as sanding.
The most obvious examples of this in turning are with regular cutting tools - skews or gouges - where the hollow ground face offers a cutting edge, a clearance behind (the hollowed out face of the bevel) and the supporting heel of the bevel which provides the depth of cut control (and a bonus option of putting a burnish on the wood).
A SCRAPING technique is akin to that used on metalworking lathes. A much less acutely ground tool (as little as 5 degrees undercut) is presented against a surface on or below centre with a level or negative handle angle. The burr on the top of the grind (a result of the grinding process on a bench grinder) aids the scrape.
Scrapers can be put in shear (they are then termed shear scrapers) and whilst scrapers can, in some circumstances, offer a glassy, highly polished cut off the tool this is unfortunately the exception. Most scraped surfaces will offer a poorer quality of finish than cut finishes.
A CUTTING technique is then, more often than not, the most desired process.
Much deep hollowing uses scraping techniques because of the difficulty of supporting a cutting edge when working deep inside a piece. The challenge has been to achieve a cutting technique deep inside a bowl, end grain pot or other closed vessel.
The three major classes of tools which have offered this to date include ring gouges, hook tools and shoed cutters.
Ring Gouges typically arise from Sheffield and can be seen as closed up hook tools with a symmetrical grind. They suffer from being lightly built and are often attached only by a swaged rivet. In use they require a high degree of precision in their presentation to the work with little room for error. Their cutting edge support provision is poor. They are difficult to hone or sharpen.
Hook Tools have a tradition based in mainland Europe. They rely on the cross sectional shape of the blade to offer support for the cutting edge. In use they also require a high degree of skill and precision in their presentation to the work with little room for error. They do not lend themselves to mass manufacture so are relatively rare. Their cutting edge support provision is poor. They are also difficult to hone or sharpen.
Shoed Tools employ a non cutting half ring or shoe which is usually adjusted by a set screw under and behind the typically U shaped cutter. The tool is presented with the shoe offering control with initial contact and then depth of cut. These tools suffer from clogging of the chip exit gate and work best with some non resinous species with optimum moisture content. They suffer, as does any tool with a cutter or shoe attached by a screw, from more chatter and are relatively light weight. They are not very robust. In extreme cases they will load up with shavings and snap off.
The Kugelschneider Ballcutter
It is made from high speed steel so will stand up to hard treatment. The cutter is machined from one piece with a massive non flexing shank so does not suffer from the chatter common with lighter built, bolt on tips or shoes. It will not snap off if overloaded.
It is massive and in use it is much more forgiving than any of the above devices.
Its skill requirement is probably the lowest of all the devices in this class. Catches and dig-ins are far less likely because of the degree of support offered the cutting edge. Used correctly with the handle level or slightly high any catch will likely kick out of the cut and not be disastrous.
Its ability to cut in shear is good and no less than other options.
It can be honed with a diamond plate or Japanese water stone slip and sharpened with a rotating stone wheel designed for chain saw teeth.
Many turners will use this tool as a finishing cutter after hogging out the majority of the piece with heavy scraping action using less expensive and more readily sharpened tools. This heavy scraping action frequently, with cross grain face pieces such as closed form bowls, leaves a pecked surface where the two opposed patches of end grain are notoriously difficult to cut cleanly; so difficult, in fact, that closed bowl forms are often too much trouble and more open forms therefore predominate. The ballcutter is so effective it allows closed form pieces to be put back in the repertoire.
The replaceable tips design saves money in the longer term as only the tip will need replacing.
- Until a high level of skill is attained always start work from the centre and work out towards the rim.
- As in any text book cutting action, be sure the ball behind the cutting edge (the support) makes contact first and is maintained in contact throughout the cut. This will also allow, if desired, a burnish on the wood.
- For inboard work on anti clockwise rotating work, start the ballcutter with the cutting edge almost vertical to the wood and now turn the tool clockwise with the cutting edge into the wood and turn to the outside of the workpiece. Do not turn the ballcutter edge past a horizontal position.
- The tool is designed to work handle level or with a slight handle high angle and skewed approx. 20° to your body. If used handle low a catch is more likely.
- For best results, and where appropriate and possible, introduce shear into the presentation of the cutting edge.
- The hole on the top provides for the efficient ejection of wood shavings.
- Periodic honing with a diamond plate or Japanese waterstone slip will defer the need to grind.
- When honing is inadequate the ballcutter has to be ground and sharpened with a slim grinding wheel no thicker than 3.2mm. A chain saw grinding wheel is perfect.
- Carefully preserve the cutting edge geometry by taking care the whole edge is in contact with the grinding wheel. A very light grind will usually suffice.
- Removing the tip from the tool shank and sharpening by hand adds sensitivity to the sharpening process.
- Do not sharpen the ball on the outside, only the inner lower cutting edge.
- After grinding polish away the burr with an appropriate slip stone.