A. Cratering is an erosion of the Flat just below the full length of the outside angle. It's caused by spindle speed too low or feedrate too high. For a period of time (until the cutting edge dulls) the eroded gullet may act somewhat like a chip breaker. But, as the gullet grows, it finally erodes up into the cutting edge, resulting in bad chip formation and tool breakage.

Manufacturers in high-volume applications (such as the auto industry) may overfeed their tool past the point where the cratering has formed, then discard the tool prior to a point where cutting edge deterioration causes tool breakage. This procedure– though not considered 'good practice'– may justify itself in terms of max. predictable output with minimum downtime.

B. Corner Breakdown is caused by heat from excessive spindle speed and/or insufficient coolant flow. Once the corner breaks down, the margin starts to rub and hole size gets tight.

C. Built-up Edge (sometimes called 'welding' or 'pickup'), is usually caused by incorrect coolant. The four prime requirements of a good gundrilling coolant are: removal of the chips as they form, cooling, lubricity and anti-welding characteristics. The anti-weld requirements for a gundrilling coolant are much more critical than for more conventional machining.

Built-up edge causes oversize and irregular holes. The built-up edge is actually material from the workpiece which 'welds' itself to the cutting corner and acts like a boring insert and enlarges the hole size. Because the welded material is very hard and brittle, it breaks off and the hole goes back to its correct size; but then the buildup and breakoff reoccur in a cyclical manner.

The resultant hole has alternating shiny and dark rings. The uneducated eye interprets the dark rings as 'good hole' and the shiny rings as 'glazing'. Quite to the contrary, the 'glazed' rings are good hole and the dark rings are where you were 'boring' rather than drilling. Section the hole and you'll find that the dark rings are 'rough as a cob' and the shiny ones are a high RMS gundrilled finish.

Many times CNC people become frustrated with their first gundrilling encounters, finding that they get rough holes and can't hold size. Mostly the problem is that they used their existing grade of coolant, which didn't have the proper anti-weld chemistry for gundrilling. Happily, a lot of coolant manufacturers have become attuned to this need, and now offer grades which meet this requirement.

D. Pickup on Margin (or 'smearing') is a different condition than "C." above. It can be caused by incorrect coolant or wrong tool contour, but is more likely the result of restricted coolant flow and/or low coolant pressure causing excessive heat and friction. This condition appears on the margin right below the cutting corner and may be 1/8" or so long. It appears as a discoloration on the carbide and resembles a plated surface with some of the plating worn away.

E. Wear on Margin can be caused by misalignment of tool to bushing or incorrect backtaper on the tool.

F. Burn on Outside Angle is caused by excessive RPM or insufficient coolant flow (or low coolant pressure).

G. Chipping– Conditions causing chipping are pretty much self-explanatory.

A WORD OF CAUTION!

It should (and must!) be noted– all of the above conditions (with the possible exception of built-up edge) must be completely removed when regrinding the drill tip. Tools with built-up edge often clean themselves once the contributing causes are eliminated. But... before resuming production, inspect tool for overall damage and assure it's consistently running 'clean', and that your hole now has acceptable finish.

If you catch any problem early on, you'll remove .005– .010" in regrind. Should you allow the problem to go unattended, removal could amount to 1/8" or more ( a loss in tool life of 12 or more regrinds).