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Here's an example, which will hopefully make it more clear why the
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small error of the dials is usually insignificant: Suppose you want to turn a 1/2" diameter rod to .475" in dia. You start by making a shallow turning pass to get a reference surface. Let's say you measure it with your dial caliper and it is now .497" dia. so you need to remove .497-.475" or .022. So you will need to advance the cross feed by 11 divisions (reducing the radius by .011 will reduce the dia by .022). To be safe, you advance by 9 divisions, then stop and check the diameter again. Let's say it is now .477. For your final finishing pass you advance the cross feed by 1 division reducing the diamer by .002 to .475. The error of the dial for this small adjustment is negligible - less, in fact, than the inherent precision of the lathe. The only time the error would be significant is if you advance the dials by a full turn or more without rechecking the diameter by measuring. Suppose you needed to remove, say .240 and counted off 3 full turns of the dial (.120) without taking another measurement. Since the error for each full turn of the dial is .040 - .03937 = .00063, the total error would be .00189 or nearly 2 thousandths. In practice, as you approach your desired diameter, you would stop the lathe and measure the actual diameter, then turn the dial by just a few divisions as needed to reach the final diameter. With careful technique you should be able to get the final diameter accurate to .001". --- In 7x12minilathe@..., Frank Hoose <fhoose@y...> wrote:
Don't be overly concerned about true inch leadscrews. |
