Bill Tisdall, Development and Turning Products Manager

Boring bars and their application present a unique problem. We have been taught that a stable process requires us to minimize the overhang of the tools. And yet in boring we are forced to “hang the bar out” to reach the cutting zone. In addition to hanging out the tool, we cannot see what is actually going on in the bore. It can be very frustrating.

First, we must take a good look at our L/D (length to diameter) ratio, chip evacuation, and the method we use to hold the bar if we want an optimized process. Luckily just a few tips can take care of most of our boring woes:

Use the biggest diameter bar as possible. Using a 1.25” diameter bar as opposed to a 1.00” diameter bar will result in a three-fold reduction of deflection.

1. Use the shortest boring bar possible. Above, we reduced the L/D ration by increasing the diameter of the boring bar. The effect is similar when considering a boring bar hanging out 3 times D (this is a very solid set-up) versus a bar hanging out 4 times D (this set-up is hit and miss). Once you are at 5-7 times D a solid carbide boring bar should be applied.

2. When holding a boring bar, make sure that you are holding onto 3-4 times the diameter of the bar.

3. A cylindrical bar, with no flats, held in a split sleeve, is always preferred. The goal is to achieve 360 degree contact around the bar to eliminate vibration. A great solution for this is the Sandvik Coromant Easy Fix system. Set screws can be used with steel bars with flats. Never use set screws on a solid carbide boring bar.

4. Chip formation and evacuation is critical in boring. Choose an insert geometry that will form chips that look like “sixes and nines”. Stringy chips and long pigtails are very difficult for the coolant to evacuate out of the bore. These chips could pack into the bore which can damage the insert and cause problems for automated measuring and part handling systems.

5. Recutting chips in a boring operation is common because some chips are just not evacuated. Sometimes it is better to use an insert in a tougher grade to protect against failure.