RPM= 3.82 X SFM/d*Feed Rate (in./min.)= IPR X RPM

If Maximum Unsupported Length is excessive, an intermediate support must be provided to correct the condition. If this is not possible, feeds and speeds (RPM and Feed Rate) must be proportionately reduced sufficient to eliminate the possibility of shank whip.

Coolant flow rate in GPM is important, but as long as there are no abnormal restrictions it is an automatic resultant of coolant pressure and tool size. The single oil hole in most gundrill tips is about 1/4th. the diameter of the tool. (Some tips have a kidney-shaped oil hole and some have two holes, both which allow a greater flow rate.) Coolant flow for a large tool is high and pressure is low. Conversely, a small tool requires higher pressure, but the flow rate is lower. Example: for 3/4" dia tool– 400 psi. and about 10 gpm; 1/4" dia. tool– 925 psi and about 2.2 gpm.

If you're using a CNC, define feed in IPR (inches/rev.) rather than in./min. so that your feed rate will automatically increase/decrease proportional to changes you may have to make in your rpm.

Two good sources for parameter tables:

We recommend The Gundrilling Handbook, which devotes an extensive chapter to the whys and wherefores of the subject, including a full set of parameters, tool life data, and enough background information to get you up and running in a fast, troublefree manner. Another source is the Machining Data Handbook. Additionally, some of the Gundrill Tool manufacturers include tables on their websites, although they don't always take material hardness levels into account as we do in the Handbook.

Your final, optimum parameters will depend on many factors, such as material being drilled (as well as its hardness level), available coolant pressure, unsupported length considerations, part configuration, and machine type, to name a few!

Next, consider your basic manufacturing philosophy ("speed vs. comfort"). If you have "the need for speed", you'll run faster parameters and trade off lower tool life (and maybe quality) for higher productivity. If you're producing quality parts in a high-end market, such as instrumentation or medical appliances, you'll probably be willing to sacrifice some productivity in favor of quality. And– if you luck out– you can have it both ways! You'll find more info on the "speed/comfort" subject in the Fine Tuning section of the site.

Once you reach what you think is your optimum set of conditions, go back and 'read' your tool wear– you may discover that you need to make a few final tweaks in your settings.

Any Questions, Give Us a Call!

Or better yet, order your copy of


The Handbook includes complete Parameter tables
for a host of materials and hardness levels
as well as Coolant Pressures, Flow Rates,
Tool Life, and Max. Unsupported Length
plus everything else you'll need for
successful Gundrilling.