This reduces heat and wear on the side bore wall. This can cause finish and work-hardening problems, but is a good design for use with multiple materials.Ī lot of long-chipping materials benefit from drills with a concave lip, which causes the extrusion to curl tighter and break against itself. Straight-lip drills tend to take the extrusion from the point that is being pushed into the flute area and cause it to curl and eventually break against the side wall of the bore. The flute shape and angle of the point create the cutting lip shape. Parabolic-fluted tools have a lot of space for chip evacuation and good strength properties, but tend to flex more than standard tapered-web or parallel-web drills. Last, the flute and cutting lip geometry also influence the quality of the hole. The center of the drill mainly extrudes material at low surface speeds until it is pushed into the flute area where it deforms and hardens to the point of fracture at high surface speeds. Helical-point tools tend to work well by reinforcing the chisel edge to resist fracture, and they also center well. Full-split points tend to be the most successful for good hole dimension, as nonsplit pointed drills do not center very well and increase thrust load when drilling. Many different point geometries are created for various reasons, from conventional nonsplit to Racon®, and everything in between. Point geometry can also affect bore dimensions. Straight-fluted drills have a high core strength to resist flexing, yielding a very straight hole, but can cause issues with chip evacuation in long-chipping materials, which negatively affect the hole finish and size. However, the surface area of the tool rubbing against the part during drilling is essentially doubled, creating more heat that can cause other issues if not mitigated with coolant. A double-margin tool usually produces a better hole and more stable drilling conditions. The tool design also has a great impact on hole quality. Solid-carbide drills tend to leave a more cylindrical hole with a better surface finish than HSS or HSS-E tools because of the higher stiffness of the tool material. Stamping, flame or laser cutting, casting, milling, and drilling are common holemaking methods that yield completely different bore conditions.

The first part of the threading process is always creating the hole. It is crucial to look at the tapping process, not just the tool, to achieve a good finished product. A tap failure is usually blamed on a “bad” tool when, most likely, there is nothing wrong with the tap itself it just might not be ideal for the machining conditions. Other factors such as toolholding, workholding, machine condition, and lubrication also can have a significant effect on the tapping operation. Thread mills will usually overcome the issues that can cause taps to fail, making them an attractive option for most applications. Of all the threadmaking processes, tapping seems to suffer the most from this oversight.

When diagnosing problems with internal threading, a machinist often overlooks the size and condition of the hole before threading. Roll form tapping as a chipless threading process requires excellent hole size and quality.

Thread forming tap speeds