Mounting lathe tools at center height / operator error?

I have frequently read "Always set the tool at center height".>>

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I also found in a modern textbook a formula for setting a lathe tool. It said: increase the height of the tool over center by 1mm for each 25mm increase in diameter if the work diameter exceeds a certain minimum. Another author wrote, "reduce the tool below center when the work diameter approaches a smallish diameter". Sounds like the same advice from a different vantage point.>>

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I recently turned a 75mm long by 25mm dia. leaded steel work-piece with a tool ground as for General Purpose Turning (as per G.H. Thomas’ Manual). The tool was sharp and I first took a facing cut and adjusted the tool to eliminate the “pip”. The area to be turned was only 25mm long and the work extended 30mm from the 3-jaw chuck. All was going well, but after reducing the diameter to about 13mm, I experienced the tool cutting irregularly and the required pressure on the cross-feed had increased. After checking that the tool was still sharp and still on-center (using a 150mm rule), I raised the tool and it completely stopped cutting. Then I lowered the tool and it began to cut again. AND, the tool was still very sharp. I took a light facing cut and the tool left a “pip” typical of a tool below center. But it WAS now cutting nicely, albeit below center.>>

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I should mention that my lathe is a heavy 10”x24” with the tool mounted in an EMCO brand Quick Change Tool Post. I have checked the adjustment of the spindle bearings and they are tight. No detectable spring in the carriage/saddle. If the tool is cutting, the work turns parallel.>>

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I generally use the facing-off technique to set the tool height. I figured that this put the tool at center height if there is no “pip”. Now, I am thinking that cutting on the side of the tool while facing, is not the same as cutting with the end tip of the tool, as when turning.>>

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When I ask experienced lathe hands how they set their tools, most have suggested facing-off until no "pip" and declaring that this provided a “dynamic” on-center tool height or they suggest using the 150mm ruler. There is a certain amount of indifference in their response as though no one else has these sorts of problems. Must be “operator error”. And indeed, it may be.>>

You do not say Tony to what accuracy you mean by centre height, precision not being guaranteed. I deliberately set my lathe tools a couple of thou low as by doing this I can be sure they are not above. If above then they will start to rub at smaller diameters.

At very small diameters, say 1mm, of course centre height is more critical, but even more critical that it is not above centre.

A few thou will have only minimal effect on the cutting angle presented to the workpiece, so can be ignored.

Haroldl

Edited By WALLACE on 14/07/2011 19:25:47