The Lead Accuracy and Straightness of Acme Screws

For the CNC builder and buyer, the lead accuracy, tolerance, and straightness of acme screws are very important considerations. The following section will cover these aspects of lead screws.
Lead Accuracy

Lead accuracy refers to the differential of the theoretical and actual linear distance travelled by the nut based on the lead. Lead accuracy is given as deviation per linear measurement. For example, the lead accuracy of a lead screw may be 0.005 inch/foot.

Lead accuracy = 0.006 inch/foot
Lead = 0.25 inch

So if the acme nut travelled 1 linear foot or 48 revolutions the deviation would be plus or minus 0.006 inch. The actual length travelled could be 11.994 inch or 12.006 inches. You may like to check out this simple acme screws accuracy calculator.

For estimating the machines capability, it is important to know and understand the lead accuracy of the acme screw. Sometimes the lead accuracy is not given and must be acquired by contacting the manufacturer. Each manufacturer has standards for lead accuracy tolerance.
Lead Screw Straightness

Lead screw straightness refers to the tolerance of straightness of the screw. Straightness is also given as deviation per length, such as 0.001 inch/foot or 1 mm/meter. The straightness of a lead screw is important for maximum rpm capabilities.

It is important when handling lead screws to always keep them supported evenly as possible to maintain the straightness. In general, quality rolled and milled lead screws of the centralizing type will maintain a 0.010 inch/foot tolerance. However, this is again up to the manufacturer. Ground lead screws usually hold a higher tolerance than rolled or milled, as low as 0.001 inch/foot tolerance.

If you where end machining the acme screw, you must support the lead screw throughout its length in order to maintain the straightness while milling. If you have the end machining done elsewhere, request that they straighten the lead screw after the machining is performed.
Continue to: Life and Efficiency of Lead Screws