Drilling Rifle Barrels
This letter is a response from Dan concerning an inquiry about potential problems involved in deep hole drilling. It complements the article on making rifle barrels.
I received your message concerning the problems associated with deep-hole drilling.
There are several causes for a deep-hole drill to wander while it is drilling. As you mentioned, variations in steel hardness or consistency can cause the drill to wander off course. Normally the material is spun and the drill does not rotate while drilling. If there is any imbalance in the material at high RPM’s centrifugal force can cause the material to bow. In a case like this it is possible to drill a straight hole while it is spinning but when the spinning stops and the force diminishes, the hole becomes arced.
Also, it is important to have the proper cutting geometry ground on the drill tip, which is normally carbide. A dull drill can cause problems too. The carbide tip must be soldered onto the shaft so that it is in-line with the axis of the tool. If it cocked off at some odd angle it won’t drill straight. Likewise the initial startup must be centered in the work. The tip must be guided with a snug fitting drill bushing. While a long drill is starting, it must not be allowed to flex in its center or chatter. Our drills are typically 40″ long and they must be held with a whip guide to prevent side to side movement. After it is in the hole for a ways the hole acts as a good whip guide.
Another cause is insufficient chip removal, causing the chips to plug up in the hole. Cutting feeds and speeds must be within an acceptable range too. Generally larger diameter holes are easier to drill and can be done so faster. The 17 caliber barrels are the most difficult to drill, much more so than 22 caliber.
The machine doing the work must be stable and rigid and with good precision bearings too. The cutting fluid must be suited to the type of material and pump pressures regulated properly to clear the chip.
These are a few of the causes that come to mind that can cause a crooked hole. There are probably others too. I hope that this has been of some help.