A Word About Boring Tools

Boring precision holes presents a unique set of problems. The conditions under which this operation is done differ considerably from conditions encountered in conventional turning.

Tool overhang is that portion of a boring bar or tool that extends out of the boring head or tool holder--the unsupported section. To determine the amount of overhang, we take the ratio of the diameter of that bar to the unsupported length of the overhang. This diameter-to-length ratio is of prime importance in a boring operation. The force required to deflect a boring bar decreases by the cube of its length--or, for each diameter the bar is extended, its resistance to deflection is decreased approximately eight times. This certainly is reason enough to keep the overhang to an absolute minimum.

In addition to bar overhang, it would be worthwhile to note this same condition applies to the work piece.

To offer a rigid tool with a reasonable boring depth, Criterion tools are designed at approximately a 4.5:1 ratio. This, we believe, is a workable ratio that should produce excellent boring results. In working with a boring tool in a Criterion Boring Head we therefore suggest the following:

Tool geometry must be considered one of the most important aspects of tool performance. Clearance and rake angles are more critical on a boring tool than on a turning tool. Information on redial and axial rake angles as well as side and front clearance angles can be obtained from charts published by most of the carbide manufacturers. Consult these charts and select the correct geometry for your job--taking into consideration the type of material to be machined, as well as the nature of the part.

When using throwaway inserts, you may find you must compromise between what is truly the best tool geometry and what is available. Some of the new N/P inserts now on the market bridge this gap rather successfully. It would pay to consider them.

Tool geometry, at the point of cut, is equally important in both high-speed steel and carbide tools. With carbide, of course, we can substantially increase the RPM and S.F.P.M. (surface feet per minute) and we can bore a more precision hole. Tools made of solid carbide have the additional advantage of stiffness (three times that of steel). This is most desirable when boring deeper holes, or where no deflection can be tolerated. Greater care, however, must be exercised when using carbide to prevent chipping. Mishandling when not in use can often be more destructive to a tool than actually using it. In use, two of the most common causes of chipping are:

Another important aspect of tool geometry is the lead angle. The three most commonly used are:

Positive or zero leads are used when boring to a shoulder, while negative leads are used only for a "through" bore or boring into a relief at the bottom of a bore. Although a negative lead tool will create greater pressures at the point of cut, it will produce a better finish while maintaining greater tool life.

In addition to the tool geometry at the point of cut, we must concern ourselves with the remaining portion of the tool. This, of course encompasses the shank configuration.

In a previous section we discussed tool overhang and now we must consider how we can best design a tool to minimize the problems that overhang creates. The greater the weight at the unsupported end of the tool, the more it will be induced into resonance. This, of course, causes chatter. To ensure against this, the ideal design is a tapered shank with the smallest diameter near the cutting edge. If the cutting diameter will not permit a tapered shank, design the bar with the same diameter running the full length of the bar. Under no conditions should the diameter increase in size towards the cutting edge. This just adds additional unsupported weight in the most critical area.

In order of importance regarding the elimination of chatter, we consider the following:

Assuming each of these factors have been properly selected, you must at all times, whether boring conventionally or into deep holes, make certain the tip face of the tool (cutting edge) is on the exact centerline of the hole.

In the drawing "B", we see the correct position for efficient performance--good clearance and free cutting. On drawing "A", clearance is excessive under the boring tool, while top rake is negative--a very undesirable condition. In drawing "C" we note we have lost all clearance on the boring tool.

In addition to better tool performance when the cutting edge is on centerline, we also achieve another vital function. Because tool advancement is measured by graduated lead screws that are usually in increments of .001 or fractions thereof, accuracy can only be obtained with the cutting edge on exact centerline.

Chip control is always of prime importance when boring and especially so when going into a blind hole. Because this problem is difficult to completely eliminate, we must again resort to compromises. Bearing in mind our earlier suggestion that we must use as large a bar as possible, we should not plug up the hole to the point where it interferes with chip disposal. Along with good chip disposal, we shall also achieve better coolant flow. A through bore presents a somewhat better condition in that it permits good chip clearance and adequate coolant flow even when using a larger diameter bar.

The problem coolants present is much the same as that encountered with chips. The difference being, with coolants we are trying to get the fluid in the hole and the chips out.

When possible, the best condition is to flood the bore with coolant. This considerably lowers the temperature at the point of cut and at the same time helps "wash" the chips from the hole. Spray mist (coolant and air) is another good method of applying coolant.

However it is applied, coolant should be used whenever the material or the machine permits it. When selecting it, look into those brands that include some lubricating as well as cooling properties.

When holes of extreme depths over 5 to 1 ratio are to be bored, we must take into consideration all the things we have discussed, plus the use of a special boring bar that eliminates or substantially reduces "chatter". Criterion manufactures such a bar under the name Cridex that reduces the resonant vibrations induced by the cutting action. A large mass element, located in the working end of the tool, is spring loaded against a suitable bearing surface and supported concentrically within a cavity. This is a friction damping principle and has proved extremely successful up to ratios of 12 to 1 or greater.

It would be well to remember that whether conventional or deep hole boring is done, elimination of chatter can best be accomplished by proper speeds and feeds and tool geometry. In deep holes, look also to the built-in damping element.

Overhang		Diameter to length ratio or length over bore.
Surface Feet Per Minute		(S.F.P.M.) The revolutions per minute (R.P.M.) of the tool or work piece. The speed at which the work and the tool pass each other is usually measured in "feet per minute."
Feed		The axial movement of the cutting edge through the bore--usually measured in inches per minute or revolution.
Depth of Cut		Amount of material removed from one side of the work piece in each pass.
Formula for Determining S.F.P.M. & R.P.M.		S.F.P.M. = .262 x dia. x R.P.M. R.P.M. = S.F.P.M. .262 x dia.