The Sketch Shows the Basic Setup on a gundrilling machine and is pretty much self-explanatory. A very important factor is that the workpiece be in intimate contact with the gundrill bushing at the start of the drilling cycle. If not, you run the risk of bell-mouthing the hole and damaging the drill tip.

The setup shown is for rotating tool and stationary workpiece. There are two variations which might also be called 'basic'– rotating workpiece/stationary tool and counter rotation, where the drill rotates at the appropriate RPM and the workpiece rotates at a much lower RPM in the opposite direction. The two latter approaches are sometime used when drilling assymetrical parts, using counterbalancing, but are more often employed for cylindrical parts with a concentric hole, such as gun barrel blanks, due to the fact that these two approaches normally have a tendency to produce less hole drift. However, most present day gundrilling machines are based on the approach shown in the sketch, since it is quite adequate for the majority of applications.

Not depicted is the intermediate whip guide(s) which can be mounted to prevent shank whip (ballooning) when the unsupported length for a given RPM is exceeded.

A Few Words About The Basic Machine– Gundrilling machines are built to an extreme degree of precision. All of the good things we've talked about here, like tight hole tolerance, good surface finish and minimal hole runout are only achievable with the correct tooling, a good setup, and an accurate machine in acceptable working order.

We've visited clients with a list of problems as long as your arm, then upon hitting the shop floor we come upon a machine that used to be a precision unit– twenty-five or thirty years ago! Their reply to this is usually that they can't afford cost of maintenance or a rebuild. A lot of people in this situation start out trying to run recommended parameters and end up with a bin full of broken tools. Then one day they finally realize they'll have to slow the parameters to save the tools; and that's when they lose their productivity and run out of capacity. Result– "Catch 22"!

These same shops also usually have a drill grinding fixture that has taken on the appearance of a rusty boat anchor. (Enough said.)

Spindle Drive– Older machines usually had a single or two-speed motor and a set of individual pulleys or multiple sheaves to get as close as possible to the optimum rpm. This often forced a compromise where you had to compromise optimum parameters because you didn't have the right pulley combination; and backing off the rpm means backing off the feedrate.

Thankfully, for many years now machine builders have been using electronic speed controls which are capable of accuracy within a couple of rpm, and the only time you might have to touch a sheave is to switch from high to low range. (Watch your step though– some of those old mechanical relics are still kicking around!)

In any event, the spindle and its motor are normally contained in a housing which is mounted to the ground ways of the machine. Then the holes in the chip box are bored from the spindle for near perfect alignment.

Feed System– feed is controlled by a servo or stepping motor which drives the spindle housing by means of a precision lead screw.

Coolant System– A conventional gundrilling coolant system consists of the following:

a. A two-stage pump or two separate pumps- a high-volume low-pressure pump plus a high-pressure pump. The high-pressure pump can’t generate sufficient flow, so it must be fed by the high-volume pump. The low-pressure pump runs at 30-40 psi with a flow rate around 30 gpm. The high pressure pump is normally capable of 1800 psi or higher.

b. A holding tank or sump, which catches the return flow of coolant and holds it for the pumps to recirculate.

c. A filtration system which consists of a coarse stage which removes chips and the larger fines, followed by cartridge filters which remove particles down to 5 micron size. Some shops operate with 15 or 20 micron filters, which reduce filter cost and changeovers, but at the expense of wear on the precision pistons of the expensive high pressure pump.