Harold Hall

Workshop Processes


With the ratio being 100:1 even a very small increase in the diameter being cut will have an appreciable effect on the depth of the bore. The top slide should therefore remain as originally set and the diameter of the bore increased, first by using the cross slide, and then when close to the required dimension by winding the top slide back.


However, there is a potential problem with this approach that must be taken note of even though the likelihood of it surfacing is very remote. As the top slide is being wound back to increase the diameter being cut the pressure on the cutter will be attempting to take up the backlash and as a result increase the diameter by more than has been set. Even so, providing that the top slide is not set to move very freely it is very unlikely that this will occur at such minute depths of cut but if in doubt, use the slides lock to stiffen up the movement temporarily. However, do not adjust the lock when very close to the diameter required as using the lock is likely to make changes to the diameter being cut with the result that the bore could become oversize.


Continue to increase the bore diameter using the cross slide and when within say 0.004" (0.1mm) hone the edge once more. If you do this whilst on the top slide place a piece of cloth on the bed to catch any particles from the hone, removing the cutter may be safer but you will then have to reset the saddle stop.


When the bore is close to the required diameter, as indicated by the short reduced diameter portion of the gauge, now is the time to increase the depth of cut by winding the top slide back and increasing the diameter in very small steps. Aim to get the bore a very close but not tight fit on the gauge. If the gauge becomes a tight fit make an extra cut without having placed on any more depth, this is particularly important if you are machining the bore with a boring tool having a thin shank, that on the right of Photograph 5 is definitely preferred for the task.


Having wound the top slide back to arrive at the required diameter the inner portion of the bore will be undersize. To overcome this, remove the saddle stop and hand feed the tool in just once more to bring the bottom of the bore to diameter, taking care as the tool contacts the end of the bore.