What’s So Hard About Deep Hole Drilling?
When a precision machine shop receives an RFQ one of the things they check the print for is deep holes. Deep holes pose a number of machining challenges. Addressing them adds time and cost to the production process, especially when the tolerances are tight. This blog post discusses the difficulty of deep hole drilling and what a machine shop must do to be successful.
What is a Deep Hole?
The definition of a deep hole is one where the depth is more than 10 times the diameter. Thus a ¼” diameter hole becomes “deep” when it exceeds 2 ½” in depth. (Or when a 6mm diameter hole exceeds 60mm.)
Gun Barrels and More
Deep hole drilling is sometimes referred to as “gun drilling”. That’s because one of the first engineered components needing a long, deep, and straight hole was the gun barrel. Today many machined parts rely on deep holes to function. For example:
- Crankshafts and camshafts, where drilled holes deliver lubrication
- Fuel injectors
- Perforating dies (used in the food industry and elsewhere)
- Cooling channels for gas or liquid
- Surgical instruments
- Hydraulic valve bodies and similar parts
The Challenges of Deep Hole Drilling
The four main challenges are:
- Tool runout
- Chip evacuation
Runout refers to the tendency of the drill tip to orbit around the axis of rotation. The amount of runout grows as the drill gets longer and will increase the diameter of the hole.
“Walking” occurs at the point the drill tip contacts the workpiece. If the surface isn’t completely perpendicular, a sideways force will push the drill in that direction, (and a long, thin drill can bend slightly.) This results in the hole being misplaced and drilled at an angle. It may also break the drill. The same can happen if the surface has an as-cast or rough milled finish.
Material cut away at the bottom of the hole has to be removed to make space for the drill to advance. In deep holes these chips tend to wrap around the drill flutes and build up to where they rub against the sides of the hole. This raises the temperature and will eventually cause the drill to seize and/or break.
In most precision machining operations cutting fluid keeps the cutting interface cool. In a deep hole it’s very difficult to get fluid down to the bottom. As a result, the drill tip temperature rises to where it may damage the workpiece or even weld to it.
Best Practices for Precision Deep Hole Drilling
Producing accurate deep holes starts with the machine tool. Best results will be achieved on a CNC machine with a high quality spindle. This reduces runout and provides a high level of control over the drilling cycle. (Note that on round parts it may be better to rotate the workpiece and keep the drill still, or even have both components counter-rotating.)
Next comes the toolholder and the drill itself. The holder must keep the drill on the axis, and the drill must be exactly straight and symmetrical about its axis.
To stop the drill walking, the surface must be prepared. In addition to fixturing the workpiece to put the surface perpendicular, this may include milling a flat pad. Starting with a center or pilot drill ensures the hole is in the correct position but does add another tool change.
The drill itself must be suitable for deep hole drilling. The main requirement is a center hole for delivering cutting fluid to the tip. This will lower cutting temperatures and push out the chips. For longer holes a special-purpose deep hole drill bit will have only one cutting edge rather than two. This again helps with chip removal.
Where through-hole fluid delivery is not possible the alternative is “peck” drilling. This means pulling the drill out of the hole, which pulls chips out with it, and reinserting. Some machine shops like to use a set of drills of progressively increasing length: this will increase cycle time.
A Specialized Precision Machining Capability
Deep holes are unavoidable in many engineered metal parts but producing them is a machining challenge. A precision machine shop, like those that Impro operates, will have the CNC precision machining capabilities to produce these features accurately and consistently. Contact us to discuss your machining needs.