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 Custom Deep Hole Drilling Cycles CNCCookbook's G-Code Tutorial

How Deep is "Deep"?

Most tooling manufacturers regard any depth that is more than 3 or 4 times the diameter of the twist drill as a deep hole. There are various fancy geometries, such as parabolic flute drills, that help you to go deeper, but they will also have a limit. Most will recommend that you switch to parabolic flutes at hole depths that are about 4x to 6x diameter. After about 18x to 20x, even parabolic flute twist drills start to give out. BTW, G-Wizard Calculator will tell you as part of its hints when you need to adopt a Parabolic flute drill as well as when you may need to start using a peck drilling cycle.

So what do you do if you need to drill and even deeper hole? What if you're following those recommendations and still breaking twist drills?

The good news is that it is possible to drill such holes and the bad news is that it is a bit of a Black Art. What you need to help you through it is a Custom Deep Hole Drilling Cycle.

Introduction to Custom Deep Hole Drilling Cycles

Unfortunately, canned drilling cycles often have limitations when it comes to drilling very deep holes. What's needed is a Custom Deep Hole Drilling Cycle.

What's different about a Custom Cycle versus the normal Canned Drilling Cycles?

First is the pecking strategy. It's important to be able to start out with a small peck and only go to bigger pecks as the hole gets deep. Doing the full retraction early before the hole has gotten deep is a waste of motion that serves no useful purpose. Ideally, you want to plunge without pecking to a couple of diameters or so and then start pecking. The frequency of the peck needs to increase the deeper the hole is bored. The nature of the peck has to change too based on the hole depth. We start out with a little short retraction--just enough to break the chip. But, as we get deep, longer and longer retractions are necessary as we need to not only break the chips but facilitate chip extraction. Lastly, we want to be able to prevent the twist drill from retracting entirely clear of the hole so that chips are not washed back down the hole.

Second is our feeds and speeds. As the hole gets deeper, we benefit by reducing the feedrate and spindle rpm. There's no need to do this until the hole reaches a threshold depth, but once we're at that depth it helps a lot.

Lastly, as long as we're programming a custom cycle, we want to consider whether to use rapids or feedrate speeds for the peck and retraction from the hole. This is the reverse motion that isn't cutting anything, and we have the potential of saving more time versus canned drilling cycles that keep all the motions at feedrate speeds.

By using custom g-code to implement all of these considerations, we waste as little time as possible on pecking while ensuring that when the going gets tough we're doing enough to keep our tool happy without breaking. While Canned Drilling Cycles differ from controller to controller, very few will offer the flexibility to control all of these variables.

G-Wizard Conversational CNC's Custom Deep Hole Wizard

G-Wizard Conversational CNC is a set of Wizards that go with the G-Wizard Editor, our g-code editor and cnc simulator. These wizards let you answer a few simple questions and then they generate g-code based on your answers. It's faster and easier than most CAM programs and helps you get the job done faster. One of the Wizards we've included makes it easy to create Custom Deep Hole Drilling Cycles such as we've been discussing. Here's a screen shot of the Deep Hole Cycle Wizard:

The Custom Deep Hole Cycle Wizard...

You can see all of the capabilities we talked about are built into the Wizard. In addition, the default values chosen by the Wizard represent the Best Practices we uncovered in researching deep hole drilling. You can start from those best practices and customize the Wizard to be either more or less aggressive depending on your application. It will then generate the g-code needed to get the hole drilled.

What could be easier?

Exercises

1. Fire up G-Wizard Editor and check out the Conversational CNC Wizards. Go through the process of creating a Custom Deep Hole Drilling Cycle.

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Next Article: G96: Constant Surface Speed Programming

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