Thomas Industrial Library

Pulley grooves are special - and specific - application grooves, quite common in many power transmission industries. Their sole purpose is to drive a belt. While most pulley grooves are cast with a high degree of precision, many others are machined from solid material. Pulleys made of steel or cast iron have more resistance to wear than their aluminum counterparts. Wear decreases the pulley efficiency, particularly for small diameters.

Production machining of pulley grooves on CNC lathes involves inserts of specific size and shape, designed exclusively for pulley grooves. Many cutting inserts for pulley grooves are made of ceramics and similar compounds and can be used at extremely high spindle speeds in aluminum, steel or cast iron (typically without coolant). The machining involves somewhat lower cutting feedrates, yet still yielding superior performance.

Pulley grooves are also grooves with an taper and are similar to taper grooves in many respects. Unlike the taper grooves described in the previous section, pulley grooves are formed to machine the groove in one plunge-in pass, using a dedicated insert. A standard square insert can also be used, along with multi-contour machining, but this method is highly inefficient and may be justified in one-time applications only. Groove angles of pulley grooves can be quite numerous, depending on the application, but 34 _ , 36 _ , and 38 _ angles are fairly common.

A typical pulley groove is shown at right, using Imperial dimensions. Note the dimensioning method used - the depth and the bottom width are not given, as they are not necessary. Top width and the groove angle determine these dimensions - for programming, they have to be calculated. Another dimension that is not given is the bottom groove radius - the insert determines its size, no special programming is necessary. Some older drawings specify the top corner with a note 'file break all sharp corners' . This method is not acceptable on CNC lathes and corner breaks have to be calculated to be included in the part program.

The most important part of programming V-pulley grooves (grooves in the shape of a letter V) is to guarantee the groove width dimension, specified at the top (0.6 in this example). By selecting the proper insert, all other dimensions fall into place, and the whole effort boils down to a simple plunge motion along the X-axis.

Insert Selection

Although numerous in numbers, pulley grooves represent rather a narrow area of practical applications. V-belts that will be used on these grooves are made according to certain standards, so it only makes sense to manufacture grooving inserts that can complement this standard. The selected insert illustrated at right is real - it has been taken from a tooling catalogue - only relevant dimensions are shown.

Depth Calculation

The program can be written once the groove depth is known. In order to calculate the groove depth, the insert selected must have an overall width greater than the groove width. In the example, groove width is 0.6 inches, overall insert width is 0.696 inches, and the bottom width is 0.154 inches (over sharp corners). Keep in mind that in any tapered groove, the groove depth influences the groove width (at the top) and vice versa. The goal of this calculation is to establish programmable grooving depth that will result in 0.600 groove width (top width).

Groove corner radius at the bottom is not important for the calculation (not shown).

The actual grooving depth will be 0.6476 inches, programmed as 8.0 - 2 x 0.6476 = X6.7048 .

Tool Setup and Program

Typical setup for this type of insert (angles at both sides) is to the middle point of its width. It may take a little more effort at the CNC lathe to complete the actual setup, but alternatives are even less practical. Having the middle of the insert as the command point, it is easy to position the tool, as pulley grooves are generally dimensioned to their center (middle).

Writing the program is simple, as only in-out motions are necessary for each groove.

(T03 = SPECIAL CERAMIC PULLEY INSERT - MIDDLE SETTING)

N51 (G20) T0300

N52 G97 S4000 M03

N53 G00 X8.2 Z-0.5 T0303 M08 (START POSITION FOR THE RIGHT GROOVE)

N54 G01 X6.7048 F0.125 (HIGH SPEED - SLOW FEED PLUNGE CUT)

N55 G00 X8.2 (RETURN TO ORIGINAL DIAMETER)

N56 Z-1.188 (START POSITION FOR THE LEFT GROOVE)

N57 G01 X6.7048 (HIGH SPEED - SLOW FEED PLUNGE CUT)

N58 G00 X8.2 (RETURN TO ORIGINAL DIAMETER)

N59 X200.0 Z100.0 T0300 (TOOL CHANGE POSITION)

N60 M01

Pulley groove programming is one of the rare occasions where the insert dimensions determine certain aspects of the program data.