Staying in charge of large holes

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by Ed Robertson

Drilling options abound for holes around 76.2 mm (3 inches) and under in diameter, but get tasked with holemaking with diameters at 228.6 mm (9 inches) and up, and your thinking is the first thing that has to expand.

Ideally, among the easier ways to handle large-diameter holemaking is to work with forgings that have the large-diameter holes pre-existing. Then the task actually becomes hole finishing, with an end mill mounted on a vertical or horizontal machining center semi-roughing and finishing the hole diameter.

With weldments or solid workpieces, other options must be considered. Depending on the size and volume of necessary holes, choices include using spade drills for pilot holes and finishing the diameter with end mills using circular interpolation; higher-diameter indexable-insert drills; and adjustable bridge tools on boring machines.

Many shops make large-hole drilling and finishing a task to be outsourced, as holes north of 1016 mm (40 inches) in diameter become the purview of specialty shops. But for holes say up to 270 mm (10.63 inches), indexable drill systems may be worth considering. Major advantages of indexable-insert drills are increased productivity, reduced costs, and better versatility. The use of carbide inserts brings drilling close to the machining rates possible with turning and milling. Higher cutting speeds permit drilling holes substantially faster than HSS twist drills and even faster than carbide spade drills, which can notably lower overall holemaking costs. 

Potential productivity is also increased because the almost flat lead angle of indexable-insert drills results in a shorter feed stroke before cutting, compared to

twist or spade drills that have point angles. Indexing the inserts does not change their positions and the tool length, thus any tool resetting costs are eliminated.

And the use of low-cost, essentially throwaway, inserts with multiple cutting edges eliminates regrinding costs. The multiple cutting edges available provide savings from not having to replace the entire tool.

Indexable-insert drills can be used as non-rotating tools for applications on lathes or other machines or as rotating tools on drilling machines, machining centers, and other machine tools. The machines used, however, must be rigid, in good condition, and have ample speed and power capabilities.

Caution and common sense must always be exercised, particularly with holemaking on lathes, as indexable drills will produce a slug or disc when breaking through the workpiece when through-holes are necessary. When the drill is stationary and the workpiece is rotating rapidly, centrifugal force will turn the slug into a projectile, making shielding and effective safety procedures a must.

Some indexable drill units have the capability, when used on suitable machines, to perform boring as well as drilling operations. For example, mounted on the cross slide of a CNC lathe, some tools can be moved radially outward to drill holes larger than the tool diameter or make a boring pass, thus improving the accuracy and finish of the hole.

Modular Systems

Modular drilling tools feature a modular head that combines a pilot drill in the center with adjustable cartridges on each side that contain general-purpose or finishing indexable inserts, depending on the operation. Different grades and geometries in inner pockets versus outer pockets optimize chipbreaking action and insert tool life. Systems are available for drilling holes in steel, cast iron, ductile iron, stainless steels, and nonferrous materials.

Features include straight or taper through-coolant shanks and optional extensions or reducers depending on necessary hole depth. Common-sense rules dictate using caution if extension or reducer shanks are necessary, as creating excessive lengths when combining components or exceeding three joints in an assembly invite performance problems.

The pilot drill is installed and set first and can be adjusted to accommodate changes in machining conditions or length alterations due to resharpening, Usually an axial screw at the rear of the drill head is used for adjusting. Once the appropriate length is established, a set screw on the side of the drill head secures the pilot drill in position. Inner and outer cartridges can then be loaded and adjusted for the required hole diameter.

Troubleshooting Tips

For pilot drill cracking, verify that the tool is centered correctly and readjust the machine, if necessary. Also check the clamping accuracy of both the tool and workpiece for possible runout. 

If insert cracking is the problem, checking clamping accuracy is also a good suggestion, as well as evaluating a tougher insert grade.

Excessive insert wear can be corrected with a number of adjustments, including increasing coolant pressure and volume and using a coated pilot drill. Other options consist of reducing speed in the area of 20% or investigating a more wear-resistant insert grade.

Testing chipbreaker insert geometries can help optimize chip-management issues, as well as increasing cutting speed while reducing feed, both in the area of 20%. Increasing cutting speed can have the additional benefit of increasing chip evacuation. Increasing coolant volume and pressure can help here as well.

 Building Bridges

For getting up to really big holes, say up to 2,205 mm (86.8 inches), for large turbine housings in the aerospace or energy industries, shops can build adjustable bridge tools with fine-boring heads and use standard shell mill adapters to run them on shop equipment. Basically, if the part fits in the machine, holes can be made.

Constructed of aluminum for reduced weight, bridge tools consist of slides and cartridges for precise diameter adjustments without the need to resort to custom tooling. Roughing heads are highly robust and able to employ negative turning inserts, while standard fine-boring heads can achieve adjustments of 0.01 mm (0.0004 inches).

Handling large holes does not necessarily have to be a matter of outsourcing or acquiring custom solutions. Expanding your thinking can be the first step in expanding your options.

Ed Robertson is a contributing editor and manufuacturing journalist based in the Detroit, MI, area.

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