Drilling holes using a pilot hole 1

[LennyG]

When drilling large holes (one inch or larger), when is a pilot hole required? What size should the pilot hole be?

 

[MechEng13]

A lot of the desicion of when to drill a pilot hole depends on what kind of horsepower you have to drill with, and whether you running the part on a CNC machine or running it manually. As for the size of the pilot hole, most of the toolmakers I have dealt with say that you want to run the pilot at a little less than 50% of the final hole size. Of course if you are drilling on a manual machine you may have to run multiple pilots depending on your available horsepower.




Alan M. Etzkorn
Product Engineer
Nixon Tool Co.

http://www.nixontool.com

 

[griffengm]

Rule of thumb:
Pilot should be same diameter as width of point of finish drill. Any smaller requires more HP and invites wandering; any larger increases wear on localized areas of lip and invites over feeding.

Griffy

 

[BillPSU]

Horsepower is the least of your worries when using either a large diameter twist drill or spade drill. Available thrust on the machine is usually the contolling factor. Most CNC machine do not have sufficient thrust for these large diameter drills, 1.5" and bigger. The z-axis drive motors normally attempt to drive the drill at the commanded rate but are unable to meet requirement and feed rate decreases, slide lag increases and if the hole is deep enough the z-axis overload may pop. I normally used a .50" pilot hole for most of these cases because this diameter of hole removes enough material to accomodate the web of the large drill reducing the thrust requirement tremendously. Manual machines pose different problems.
Radial drills would have the arm deflect away from the workpiece and at breakthrough would unload into the drill. Causing high feed rates and the possibility of chipping or breaking the drill. Smaller drills were of insufficient capacity and damage to feed mechanism or stopping the spindle has occurred.

 

[ubrales]

All the above ideas are good - I would like to add to the suggestions. Since the final drill will follow the pre-drilled hole, it is a good idea to pre-drill (pilot) using a 2-fluted slot drill (looks like a milling cutter). Slot drills will provide truer hole locations as compared to twist drills.