Thin is in when cutting metal

In last month’s column, I noted chip thinning is one of the key concepts programmers and machinists should understand. Presented here is further discussion about how chip thinning can enhance productivity.

Chip thinning, at first glance, can be a daunting concept to grasp because it involves complex geometric relationships and intimidating calculations. This often causes machinists and programmers to avoid learning how to take advantage of the concept.

All images courtesy Savannah Machinery Works

Figure 1. A portion of a challenging part at Savannah Machinery Works.

A good programmer will typically calculate the spindle speed using a published formula, such as sfm ÷ tool diameter × 3.82, and then use the rpm to calculate the feed rate, using the equation rpm × number of flutes × chip load per flute. This tried-and-true method is certainly acceptable and usually provides adequate machining performance. Once completed, a program typically goes to the shop floor and a machinist adjusts the speed and feed to achieve the desired results.

Having a good starting point for the speed and feed is critical to success in any machining operation and a knowledgeable programmer can usually get close on the first try. But there are times when the setup does not provide the necessary rigidity to support conventional methods, the part geometry makes conventional machining techniques a challenge or the workpiece material is difficult to machine. These scenarios can cause chatter, tool breakage, part movement and poor surface finish.

The first reaction of experienced machinists to these problems is to reduce the speed, feed or both. Slowing the operation is not necessarily wrong but it increases cycle time.

Understanding how to take advantage of chip thinning can counter the learned response of reducing cutting parameters to combat difficult situations. The major cutting tool manufacturers publish chip thinning calculations in their technical literature and some have simplified the calculations into tabular form based on the ratio of tool diameter to DOC. Nonetheless, chip thinning can be tedious to apply when dealing with numerous or complex toolpaths.

The advantages of chip thinning are so great that several CAM developers offer software packages that capitalize on its application. But it is possible to take advantage of chip thinning without expensive software if programmers and machinists understand the concept and approach it creatively.

Figure 1 on page 38 shows a portion of one of Savannah Machinery Works’ more challenging parts. Although the shape is not complex, the tolerances are not tight and the material is only moderately difficult to machine, it was difficult to achieve the desired results with conventional machining.

Figure 2 shows how a 12mm-dia., 4-flute, carbide endmill initially entered from outside the pocket, moved to the middle and began a canned pocketing routine. This method caused chip packing in the flutes, flute chipping and part movement in the chuck. Our first response was to reduce the cutting speed and feed rate to overcome these issues, resulting in an agonizingly long cycle time.