Harold Hall

Workshop Processes


Bars with HSS Tips.

Another method having interchangeable tips is one with a steel shank that holds a small piece of high speed steel at the end on which the cutting edge is ground. Whilst a very satisfactory method it will not work at very small diameters. Photograph 3 showing a commercial set, the smallest which works down to  8mm when the cutter is at 90° to the shank but as much as 10mm when at 45° and ground for machining a blind hole.


The photograph shows  there are four sizes in the set, though you may find it possible to purchase individual sizes. Each bar provides the facility for a tool bit at 90° to the bar at one end (as shown by the front bar) and at 45° to the bar at the other (as shown in the second bar). The larger bar uses square tool bits. The bars are 150mm long and will bore to a depth of at least 75mm with an average cut.


The problem with this type  is that room has to be provided for the clamping screw being the reason why the cutter is so far from the end and as a result increases the minimum diameter considerably when the cutter is at 45° so as to bore a blind hole.


This problem is overcome by the tool in Photograph 4 showing a shop made version in which the tool is clamped using a rod down the length of the shank, again this will not do much better than 8mm minimum. However, it is an economic way of producing very useful boring tools for larger diameters. A ball should be placed between the end of the clamping rod and the cutter to secure it.


Solid Boring Bars.

These can be high speed steel or carbide and it is in this range that we really get down to the diameters that the enquirer obviously had in mind.


Solid carbide tools are made in a very wide range of bore diameters from as small as 1.25mm diameter through to 12mm diameter, maybe larger, all sizes being available in a range of bore depths, typically 4 to 10mm for the 1.25mm  one. Photograph 5 shows a carbide cutter with a minimum bore diameter of 2.8mm and a depth of 18mm. Whilst they are not overly expensive at the smaller sizes, they then become very much more expensive as the size increases. Whilst ideal for the job the expense makes them only worth considering at a size where the workshop owner finds it impossible to grind a suitable tool from high speed steel, but is required for an essential task.

Boring tools with HSS inserts.
Boring tool, with HSS tip, shop made
Boring Tool, very small, solid carbide.