Mind the Tool Center Height When Turning Small Diameters
Let’s use the Pythagorean Theorem to see how small, turned diameters can vary from nominal when your lathe’s tool center height is off.
Louis Trumpet with Vallorbs Jewel Company in Bird-In-Hand, Pennsylvania, wrote a piece that reminds us how small turned diameters can vary from their nominal dimension when a lathe’s tool center height is off. What follows is what he sent to me (which I’ve only mildly edited).
From Louis: Very small, precision turned parts may have several different diameters, making for complications here and there. For example, when turning these small parts, you may notice that when one diameter is on the nominal dimension while others may be off by several tenths. At first, you might think this is caused by different tool pressure at the different depth of cuts, but that’s unlikely. To be sure the problem isn’t mechanical, check for backlash, flex in the machine/toolholder and the fit of the material to the guide bushing (when using a Swiss-type lathe). If everything checks out, and you’re still experiencing a differential, the likely culprit is that your turning tool center height is off. The following example shows how being off center can affect the diameters being turned.