What if the hole is too deep and the twist drill bit breaks?
Deep hole drilling is hard, and this article outlines the various techniques required, including peck drilling, parabolic groove drilling, custom g-code cycles, and gun drilling or BTA drilling.
Most CNC operators know that peck drilling can help drill deep holes. Some use high-performance drill bits, such as parabolic groove drills.
But it turns out there are a lot of lesser-known techniques that can help you successfully drill deep holes.
How Deep is a “Deep Hole” and How to Choose a Processing Method
Most tool manufacturers consider any hole deeper than 3 or 4 times the diameter of the twist drill bit to be a deep hole. There are various spiral grooves with fancy geometries, such as parabolic groove drills, that can help you drill deeper, but they also have a limit.
Pecking the Drill Bit
Peck drilling cycles are common on most CNC machines. When using peck drilling, the twist drill is periodically retracted a distance to allow for chip breaking and clearing. There are many different peck drilling cycles, and the deeper you drill, the more often you peck and the further you retract. The most important thing to remember when using peck drilling is to avoid trapping chips at the bottom of the hole. If there are chips at the bottom, they will interfere with the twist drill’s ability to drill, resulting in premature dulling of the drill and a poor surface finish.
When peck drilling, consider pausing the retraction after a slight retraction of 0.025mm. Allow the twist drill to rotate a few turns to pull the chips out of the deepest part of the hole. Also, never pull the drill out of the hole if you are returning to the process. Doing so will make it easier for the coolant to push the chips back into the hole.
Parabolic Trough for Deep Holes
Twist drills with parabolic grooves help evacuate chips when drilling deeper than 7 diameters. Using a parabolic groove drill, you can drill deeper, 20 diameters, compared to only 7 diameters.
The Role of Coolant and Chip Breaking
The biggest obstacle to deep-hole machining is the chips:
- How do we get them out without clogging?
- How do we prevent them from compromising the surface finish of the hole?
The choice of tool, process, and coolant all play a role.
Certain types of drills have inherent advantages for deep hole machining. For example, parabolic flute twist drills have changed the geometry of the flutes to optimize chip evacuation in deep holes. Gun drills and BTA drills are both targeted at the needs of deep holes, especially chip evacuation.
Coolant is critical in the chip evacuation process. The best way to do this is to deliver coolant at the tip of the tool with the highest possible pressure. High-pressure coolant directly to the tip of the tool can generate considerable force to push the chips upward and out of the hole.
The delivery system in the spindle delivers coolant under pressure into the drill hole along the length of the drill. This helps to flush the chips upward from the bottom of the hole, which helps with deep hole drilling.
Spindle coolant holes in a twist drill.
The peck drilling cycle is primarily concerned with chip breaking and chip evacuation. Each pack typically breaks chips. Long thin chips stick everywhere and are difficult to evacuate. Compact chips can be evacuated more efficiently from deeper holes. The deeper the hole, the more often the twist drill must peck to ensure the chips remain compact.
Also, the retraction distance is important. Further retraction helps pull the chips out of the hole. However, the drill must retract further, which slows down the speed, and care should be taken not to fully retract the drill out of the hole. An open hole causes chips to be flushed to the bottom and must be evacuated again.
A custom deep hole cycle uses a custom g-code to optimize the pecking strategy at each stage as the hole gets deeper.
Customized Deep Hole Drilling Cycle
Unfortunately, canned drilling cycles are often limited when drilling very deep holes. What we need is a custom deep hole drilling cycle.
How do custom cycles differ from regular canned drilling cycles?
First is the peck strategy. It is important to be able to start with a small peck and only peck more as the hole gets deeper. Full retraction too early before the hole gets deeper is a wasted motion that does not serve any purpose. Ideally, you want to drill a few diameters deep without pecking, then start pecking. The frequency of pecking needs to increase as the hole gets deeper. The nature of the pecking must also change depending on the hole depth.
We start with a little short retraction – just enough to break the chip. However, as the hole gets deeper, longer and longer retractions are needed because we not only need to break the chip, but also to facilitate chip extraction. Finally, we want to be able to prevent the twist drill from completely backing out of the hole so that the chips are not flushed back into the hole.
Second is the feed and speed of the pecking. As the hole gets deeper, we can reduce the feed rate and spindle speed. It is not necessary to do this until the hole reaches the limit depth, but it will help a lot once we reach that depth.
Finally, any time we are writing a custom cycle, we need to consider whether to use rapid or feed rate for pecking and retracting from the hole. This is the reverse motion that doesn’t cut anything, and we have the potential to save more time than with a fixed drill cycle that keeps all motion at a feed rate.
By using custom g-code to do all of this, we can waste as little time as possible on pecking while ensuring that when the going gets tough we can do something to keep the tool running without breaking. While fixed drill cycles vary from CNC to CNC, few CNCs have the flexibility to control all of these variables.
Gun Drilling: The Deepest Hole Yet
The techniques described so far will get you all the way to a depth of about 20 diameters. After that, we need a whole new approach.
