Straight talk about drilling straight, deep holes

CTE Managing Editor Greg Bartlett spoke with Salvatore DeLuca, product manager at Dover, Ohio-based Allied Machine & Engineering Corp., which manufactures holemaking and finishing tools for a range of applications in boring, burnishing, drilling, reaming and threading.

Cutting Tool Engineering: How much of a concern is maintaining straightness when deep-hole drilling?

Salvatore DeLuca: The straightness requirement of a deep-hole drilling operation can vary significantly from one application to another. For example, water lines in a mold will have a much looser tolerance than what you would typically see in a firearm barrel or engine block. Understanding the workpiece material is the key to success. Without the right geometry insert and holder, a deep hole can be nearly impossible to achieve, let alone keeping it straight.

Note the visible perimeter ring created by the initial spot drill and chamfer. Establishing a pilot hole to secure stable entry ensures that the job starts on center. Image courtesy of Allied Machine & Engineering

CTE: What situations most commonly cause problems with straightness? How are they corrected or avoided?

DeLuca: More often than not, we see improper geometry selections being a main contributor to issues. This goes back to understanding your workpiece material and making sure you have the right tool for the job. Some workpiece materials will have a high variance in composition, which can cause the cutting edge to deflect, ruining your chances for a straight hole. Material-specific insert geometry can make a huge improvement. If your tolerance is on the more difficult end, having a guided toolholder is the ideal way to go. Our customers have had really good and much quieter results from our guided T-A drill, nicknamed the stealth drill.

Salvatore DeLuca, product manager at Allied Machine & Engineering. Image courtesy of Allied Machine & Engineering

Also, make machine spindle maintenance routine and be smart when choosing toolholders. If the spindle or toolholder is out of spec, it creates runout, which will be amplified as it travels down the whole tool assembly to the tip. Both of these factors can cause all types of havoc, including but not limited to poor interior surface finish. Spindle maintenance and toolholders that provide an even gripping force will help center the tool during clamping.

Lastly, your choice of coolant affects the tool life, cutting forces, power consumption, accuracy and surface finish. Always check the coolant’s concentration, and don’t forget to listen to your machine. You want to hear a consistent popcorn-type sound instead of coolant sputtering/choking or your machine straining. Watch the load on the machine to see if it spikes inconsistently too. Any of these signs can be a red flag and may result in poor interior surface finish and off-center deep holes.

If it is a high-production job, we recommend that you talk to a coolant provider for advice on the best coolant for your application so you can take full advantage of the application. High-pressure coolant can also help break the chips and improve the ability to evacuate chips faster in deep-hole drilling. This keeps chips from packing or welding to the cutting edge and reduces cycle time by allowing the speed and feed rates to be increased. High-pressure coolant ultimately helps form small, well-broken chips, which can be evacuated more easily.

A guided drill with material-specific insert geometry maintains chip control and provides a superior surface finish in this 9″ hole through 17-4 stainless steel. Even cutting forces help maintain straightness. Image courtesy of Allied Machine & Engineering