Referring to The Sketch–
1. Horizontal Machining Center– Take away the chip box and the gundrill bushing and you have the horizontal machining setup.
2. Lathe, Screw Machine or CNC Turning Center– This will look very similar to the horizontal machining setup, except that normally the workpiece is rotating and the gundrill is stationary in a tool holder.
3. Vertical Machining Center– You guessed it! Rotate the sketch 90 degrees counterclockwise.
Caution– Once you determine the length of drill you need for your application, assure that you have adequate clearance in your machine setup.
Coolant System– You'll require high pressure to do a reasonable drilling job. Many later CNC’s have high pressure coolant systems installed at the factory. They will tend to be in the 1000 or 1500 psi range. In such cases, follow the filtration recommendations of the manufacturer.
If you attempt to install a high pressure unit on an older machine, proceed with caution. It’s conceivable that you could blow the seals on the machine; so consult the original equipment manufacturer.
Another consideration is whether your machining center has ‘coolant thru spindle’. If it doesn’t, you can use a special ‘live’ gundrill holder (known as a ‘coolant-induced’ holder) which allows the coolant to flow thru the holder into the hollow shank of the gundrill. These holders are a boon for limited production, but they aren’t always adaptable to a high production environment.
Also, what size holes do you need to drill? 1800 psi will allow you to drill a very small hole- probably much smaller than you need. 1500 psi will cover you for a .080" dia. hole, and 1000 psi will let you drill somewhat smaller than 1/4” dia. If you’re going to drill a 3/4” dia, you can get by with 400 psi. If you’re limited as to what pressure you can generate, you may still be able to drill small holes within reason by adjusting your parameters downward.
Coolant– Your CNC won't necessarily be running a coolant that's suitable for gundrilling. The anti-weld characteristics required for gundrilling are much more critical than for most CNC machining. This is important! If you're considering trying a gundrill, check into your coolant capabilities. Your manufacturer/supplier probably has an alternate choice that is acceptable for gundrilling as well as conventional machining. For more on this important subject, see "The Right Coolant".
Starting the Hole– A gundrill bushing is very seldom practical in a CNC operation. Accordingly, you have to make your own "bushing" by machining a pilot hole in the workpiece with a center drill or end mill (using circular interpolation). The hole ideally should be toleranced to nominal drill diameter +.0010–.0000", preferably tighter, but never undersize. if you get an acceptable hole and have minimal drill breakage, you're home free! The depth of the pilot hole must be sufficient to bury the outside corner of the gundrill without the tip touching the bottom of the hole.
Sequence– Following this sequence of operation should keep you out of trouble:
1. Index gundrill into pilot hole slowly with spindle turning at bare minimum speed. Stop index just short of contacting drill tip with bottom of pilot hole.
2. Initiate coolant flow.
3. Ramp up rpm and feed slowly until you have a bite into the material. Then ramp up to specified parameters. For the drilling sequence, always set your program to read inches/rev. rather than inches/min.
4. At bottom of hole, stop spindle and then shut off coolant prior to retraction.
5. Pat youself on the back, trying hard not to break your arm!
