Ronald Crowell: Discussion of acme lead screw stock

Ronald Crowell: Facebook Group: Atlas and Craftsman Lathes

Do not get an "ACME threaded rod", get an "ACME lead screw". They are different!! I don't know what McMaster-Carr might send you. Another ACME lead screw provider is Roton, and their pricing is competitive.

. An "ACME lead screw" will have rolled threads with a more precisely controlled runout and machined smooth flat thread lands (meaning the flats on the OD). An ACME "threaded rod" will likely have non-machined rolled threads with the tell-tale rolling pinched lands that are not machined. The differences are readily apparent when from low cost vendors and a decent lead screw is more costly than a threaded rod. In addition, the surface finish of a quality ACME lead screw is almost a mirror..not so with a threaded rod.

. McMaster-Carr does not specify run-out tolerance on their lead screws like a real specialized lead screw manufacturer, but unless you are making long high precision threaded parts that probably won't be an issue for you. That being said, the lead screws they have in 3/4"-8 ACME are available as:

- Carbon steel, thread Class 2G, Grade 2, Rockwell B61; and

- Precision, 1018 steel, thread Class 2C, Brinell 126.

• McMaster does not say if it is a Brinell 500kg or a Brinell 3000kg, but either one is much harder than the Rockwell B61 and will be harder to machine.

• Thread Class 2G is for general purpose, and a Class 2C is Centralizing thread. To explain:

• General Purpose: With clearance on all diameters for free movement, general purpose Acmes are used for a wide variety of parts including all types of vise and jack screws where holding major diameter is not a critical consideration. Three classes of thread fit tolerances are available: 2G, 3G and 4G. Basic Acme dimensions are given in accompanying tables.

• Centralizing: Centralizing Acmes have limited clearance at the major diameter of screw and nut and are used extensively for feed screws. The limited clearance enables a bearing at the major diameter to maintain alignment of the thread axis preventing wedging on the flanks and part sagging. Five classes are used: 2C, 3C, 4C, 5C, and 6C. Classes 5C and 6C are designed to allow limited backlash, though some backlash will be experienced with any of these classes.

•Differences between General Purpose and Centralizing threads are found by referring to the limiting dimensions and tolerances. For example, while a 1/2" diameter, 10 pitch Class 3 General Purpose thread has a .5000" maximum major diameter and .4950" minimum for a tolerance of .0050" - the similar Centralizing Acme has a .5000" major O.D. and a minimum major of .4989" for a major diameter tolerance of just .0011". Tolerances for the pitch and minor diameters are the same for both General Purpose and Centralizing Acmes of the same diameter and pitch.

»»» End of my dissertation.

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