A Single Acme Cutter...Generating is EASIER!

This operation has not been tested sufficiently to guarantee consistant results. Machining
Cutting a 60o thread with a proper setup involves only cutting essentially on a single, straight edge.

But traditional methods of cutting Acme threads have been somewhat more painful. If you were to try to cut them in the fashion of 60o threads, that is, generating the trailing flank by feeding a form tool at 14.5o, the tool will be cutting on a corner formed by the "root" on the form tool and the leading flank.

While there are tricks to break the two corner chips apart, you would still be taking a wider cut at every stage than with a 60o thread.

Usual methods to avoid this have been to cut a square thread (not always fun) first (thereby forming the root), and THEN generating and/or forming the flanks.

Totally Generated

You could avoid most of these problems and gain the advantage of needing NO square tools, and only ONE cutting tool for a wide range of Acme threads if you could generate not only the trailing flank, but also the root.

Taking a right-hand thread for example:
First, suppose you were willing to use a 15o flank angle as used in the metric trapezoidal threads, instead of the standard 14.5o flank angle.

Then suppose you could feed a 30o tool along the right-hand flank at 15o, generating that flank much as you would use a 60o tool at 29.5o for a 60o thread.

Then, when you reached full depth, suppose that you could feed longitudinally to generate the root, and at the same time form the left-hand flank of the thread.

In such an example, you would always be cutting PRIMARILY (except for the nose arc and maybe a small flat), on a single, straight, edge!

How could you manage to feed a tool thus with a standard setup??

First, you will need to set the compound at 30o, NOT 15o, and have that slide retracted a sufficient amount.

Second, for each cut up to the depth of thread, you will need to feed both the compound AND the cross slide in an equal amount. This combination will result in an effective feed angle of 15o.

Third, AFTER reaching the full depth of thread and for each cut until the root is generated, you will need to feed the compound IN twice the amount of longitudinal feed desired, and the cross-slide OUT (remembering to take up slack unless using anti-backlash), one-half the square root of 3 (or about .866), times the compound feed. This will result in an effective feed parallel to the axis, or 90o to the crossfeed.

Each cut should be calculated and the cross and compound settings written in a list before beginning, and then each pair of settings can be crossed off after each cut. Rigid setup and medium light feeds are likely to be critical.

2nd Edition
AM101 2011-2012