Harold Hall

Workshop Processes

However, when  attempting a very accurate final diameter, it may be necessary to infeed the tool a second, and maybe third, time before setting on  more depth.  The reason for this caution can best be explained by an example.


Going back to our setting of depths of 0.25mm resulting in a deflection of  0.05mm. If at the final stage a further 0.08mm depth is set on, as measurement of the diameter indicates, the tool will actually be requesting 0.13mm to be removed (0.08mm plus the deflection of 0.05mm). As this is only about half of the depth of previous cuts the deflection will be less than 0.05mm resulting in more than 0.08mm being removed and the hole therefore oversize.  When nearing the required diameter  do take account of tool deflection by feeding the tool in a second , maybe a third and fourth time before measuring and setting on an increased depth using the cross slide.    


3. A rather more obscure reason for tool deflection occurs when a bore is being made that is near the cutters minimum. In this case with some cutter shapes, such as those shown on the previous page, there is little room for the swarf being produced and it is carried over the tools shank with the rotation of the workpiece. This can then be forced between the cutter shank and the bore being made resulting in deflection of the cutter, if you have not firmly secured the cutter I have known the tool to be physically moved.


This problem is most likely with blind holes in which case the swarf should be removed as far as is possible after each cut, perhaps even removing the tool a number of times during each pass.


4. Occasionally, you may find that the tool is refusing to cut cleanly even though  it would appear that it should do. This occurs when a bore close to the cutters minimum is being attempted and often appears to start satisfactorily but soon ceases to cut properly when perhaps no more than 1/2mm deep. This is because the secondary clearance of the cutter is making contact with the bore, which may be difficult to see.


Rather than looking for a smaller cutter, or grinding more from the secondary clearance of the cutter being used, raising the tool within the bore will frequently overcome the problem as it increases the clearance between tool and bore. In the same way that a knife tool will work well below centre height a boring tool will work when above.


Finally, for guidance on producing the small boring tools seen on the previous page, see my pages on forming small boring tools. Other small tools are also illustrated.