by Hironori Kondo
A double-sided insert with positive rake angles is your best bet for small part machining
A recent trend in the automotive, aerospace and general machining industries is to produce smaller, more precise parts to reduce weight and increase performance.
If you’re machining small, precision parts on automatic lathes and small sized CNC lathes, you should consider double-sided inserts with sharp cutting edges. These inserts are typically negative with no relief angle. A relief angle is created by tipping the insert pocket at a negative rake angle. The problem with a negative cutting angle is that it increases cutting forces, which is often a disadvantage in machining small parts, components with thin walls or gummy materials, such as stainless steel, low carbon steel, super alloys and aluminum. A negative insert increases the cutting pressure on a part and small parts or parts with thin walls will deflect or deform from increased pressure, whereas gummy materials need to be “sheered” with a positive insert, rather than “pushed” with a negative insert. The negative insert in a gummy material will have a tendency to have a built up edge, where the gummy material sticks to the insert. A positive insert will sheer the material and avoid built up edge.
Positive inserts have relief and sit in the pocket at a neutral or even positive rake angle. This positive angle reduces the cutting force, making it ideal for machining small or thin-walled parts. It’s also best for machining gummy materials.
The problem with positive inserts is that they’re single sided and the cost per cutting edge is generally expensive. The solution is to create a double-sided positive insert so that you get twice the life out of one insert, reducing tooling costs and increasing productivity and cutting efficiencies.
Here’s an example of how a double-sided positive insert benefitted a manufacturer in the automotive industry.
A manufacturer machining a stainless steel (ISO X5CrNi18-9, AISI S30400, JIS SUS304) shaft for an automotive component was using a DCGT type insert on a small sized CNC lathe with wet coolant. The manufacturer selected a double-sided positive insert with the associated toolholder.
Vc = 110 – 150 m/min / External turning
Vc = 10 – 100 m/min / Face turning
f = 0.04 mm/rev / External turning
f = 0.05 mm/rev / Face turning
ap = 0.4 mm
With the DCGT type insert, the manufacturer encountered trouble in chip control. Vibration caused the sudden chipping and shortened insert tool life, resulting in 5,000 pieces per corner. When the manufacturer used the new double-sided positive insert with toolholder, the rigid clamping of the toolholder and the low cutting force prevented the vibration, and high wear resistance prolonged tool life by 30 per cent, resulting in 6,500 pieces per corner. The double-sided insert provided a 2.5 times increase in the number
of machined workpiece per set; 26,000 pieces per insert. SMT
Hironori Kondo is miniature product manager for Tungaloy Corp.