FIXING EXCESSIVE DISHING ON FACING CUTS.
This is best not undertaken lightly, because it involves re-machining the male dovetail on the saddle so the dovetail is more nearly perpendicular to the Vee groove beneath.
First task was to establish the extent of the dishing. For the sake of rigidity, the cross slide and topslide were locked solid, and a facing tool was held in the original toolpost rather than my QCTP. A test cut was taken across a short 80mm diameter piece of cast iron. The dishing was measured by mounting a dial indicator (plunger type; the accuracy of the dial of a finger DTI depends on the angle between the finger and the item being tested) on the cross slide and noting the deflection as it was run from the centre to the far side of the workpiece, and halving the result. This showed that the centre was down by 0.135mm over the 40mm radius. The dovetail on top of my saddle is 160mm long, so both sides of the dovetail would need to be skimmed in a sort of wedge shape to a depth (on the headstock side) of virtually nothing at the front to 160/40 x 0.135 = 0.54mm at the back and vice versa on the tailstock side. I decided that 0.45 or 0.50mm would be safer, leaving a bit of concavity but definitely avoiding convex facing, which would leave me worse off than I was before.
The job was done on my Dore Westbury miller with a 60° dovetail cutter. Setting the saddle up wasn’t easy; nothing was particularly parallel or square to anything else, and the only available datum faces were the two horizontal sliding surfaces on the top. Eventually, I got things set up so that those surfaces were level, with the saddle skewed so that there was a variation of 0.47mm along the length of the dovetail. For this measurement, the plunger indicator was used against the edge of a parallel which was pushed up against a true round bar tucked into the dovetail.
The results were pretty satisfactory. Another test cut on the same piece of cast iron showed the dishing had reduced to about 0.02mm or 1 thou, which is acceptable.
While reassembling the cross slide and gib to the saddle, I realised that the gib was twisting slightly as it was adjusted. No wonder adjustment had always been a compromise between no tight spots on the one hand and no free play on the other. On examination, the gib dimples were conical, and had probably been formed with a drill bit. The M4 adjusting screws were threaded right up to the end. Using a sharpie pen on the dimples revealed that they did not properly coincide with the ends of the screws, causing the strip to deform as things were tightened up. I adjusted the position of the dimples with a 4mm slot drill, which gave them flat bottoms. Using my Perris lathe, I faced the ends of the screws and turned the end few threads down a little. This cured the problem, and the cross slide now works smoothly.
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First task was to establish the extent of the dishing. For the sake of rigidity, the cross slide and topslide were locked solid, and a facing tool was held in the original toolpost rather than my QCTP. A test cut was taken across a short 80mm diameter piece of cast iron. The dishing was measured by mounting a dial indicator (plunger type; the accuracy of the dial of a finger DTI depends on the angle between the finger and the item being tested) on the cross slide and noting the deflection as it was run from the centre to the far side of the workpiece, and halving the result. This showed that the centre was down by 0.135mm over the 40mm radius. The dovetail on top of my saddle is 160mm long, so both sides of the dovetail would need to be skimmed in a sort of wedge shape to a depth (on the headstock side) of virtually nothing at the front to 160/40 x 0.135 = 0.54mm at the back and vice versa on the tailstock side. I decided that 0.45 or 0.50mm would be safer, leaving a bit of concavity but definitely avoiding convex facing, which would leave me worse off than I was before.
The job was done on my Dore Westbury miller with a 60° dovetail cutter. Setting the saddle up wasn’t easy; nothing was particularly parallel or square to anything else, and the only available datum faces were the two horizontal sliding surfaces on the top. Eventually, I got things set up so that those surfaces were level, with the saddle skewed so that there was a variation of 0.47mm along the length of the dovetail. For this measurement, the plunger indicator was used against the edge of a parallel which was pushed up against a true round bar tucked into the dovetail.
The results were pretty satisfactory. Another test cut on the same piece of cast iron showed the dishing had reduced to about 0.02mm or 1 thou, which is acceptable.
While reassembling the cross slide and gib to the saddle, I realised that the gib was twisting slightly as it was adjusted. No wonder adjustment had always been a compromise between no tight spots on the one hand and no free play on the other. On examination, the gib dimples were conical, and had probably been formed with a drill bit. The M4 adjusting screws were threaded right up to the end. Using a sharpie pen on the dimples revealed that they did not properly coincide with the ends of the screws, causing the strip to deform as things were tightened up. I adjusted the position of the dimples with a 4mm slot drill, which gave them flat bottoms. Using my Perris lathe, I faced the ends of the screws and turned the end few threads down a little. This cured the problem, and the cross slide now works smoothly.
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