Trouble shooting CNC drilling

Tooling & Production, Jul 1998

High speed drilling with solid carbide tools with CNC machining centers can produce less than satisfactory results for a lot of different reasons. Here are some guidelines for trouble shooting them from Precision Twist Drill (PTD), Crystal Lake, IL, based on PTD's experience with its PHP solid carbide drills. Drills feature computer-designed tool geometries and advanced PVD TiN coating for optimum results. Good drilling results can be negated, however, if machines are not rigid, holders (collets) aren't accurate, tools are overtightened, too much coolant pressure and/or volume are used, or radial drilling machines or manual drill presses are used. Here's how trouble shows itself and what you might do to correct it:

* Drills break at the start of a hole. Causes include holder (collet) runout, bad piece part holding, parts moving during the drilling cycle. The machine is bad or the point is bad. Or possibly the drill overhang from the holder nose is too long based on drill depth. * Drills break at the middle of the hole. Chip packing and drill breakage can result from too high a feedrate. Peck drilling may be required due to the diameter-to-drill depth ratio. Coolant may be improperly directed, or coolant pressure and/or volume may be wrong. Crooked holes break drills, too. Verify drill rigidity, check point centrality and chisel centrality. Also make sure that the part is flat on drill entry by reducing feed at engagement into part and/or by spotting hole prior to drilling.

* Chips along the cutting edge may mean that cutting speed is too high or be caused by vibration or chatter from machine or piece part that is not rigid. Reduce drilling rates. Also, point cutting edge may be too sharp. Reduce point clearance angle or change axial cutting edge rake. Enlarge hone (K land) width. Abrasive wear indicates that point clearance should be reduced. Other solutions: enlarge hone width; reduce feedrate; increase backtaper on OD. * Excessive outside corner wear can result from cutting speed that is too high for material being drilled. Lower spindle rpms; change or adjust coolant; and add corner break or radius.

* Drilled hole is oversized and/or crooked. Excessive play in drilling may call for machine repair or replacement. Drill geometry is bad. Check runout of drill, point centrality, or look for an improperly ground point.

* Problems with piece parts may result from the drill not entering from a flat surface. Reduce feed on engagement. Check for bad part fixturing that is causing movement. If the hole is crooked, verify drill rigidity, check point centrality and chisel centrality. Part not flat on drill entry. Reduce feed at engagement into part and/or spot hole prior to drilling. Use of a non-contact presetting machine is a must in a high speed rotating machine process. Non-contact presetters reduce the occurrence of chips on the cutting edges when setting the tool offset distance. It also allows monitoring and statistical documentation of tool runout in the holder. Tool overhang should be kept as short as possible for applications. Tools should never be chucked over the flute because chips will pack into the holder, reducing holder life and accuracy. Finally, proper handling and storage of carbide cutting tools are required to eliminate the variety of causes resulting from damaged cutting edges.

PTD's PHP high performance drills are available in a short series (PHP41), standard length series (PHP21), and with a coolant hole option (PHP41/21) for high speed drilling of carbon, alloy, and stainless steels; cast iron, titanium, and other exotic materials. Precision Twist Drill, Crystal Lake, IL, circle 211