Problems with the hole often get machinists to blame the material. In my experience checking the drill and machine run-out will almost always show the root cause.
There are many ways that a drill can cause problems for a machinist. Many times we look for evidence on the drill itself- chipped corners, edges, or margins. Sometimes the evidence is on the workpiece. And often that means a call to the material supplier.
But just because the clues appear on the material doesn’t mean that the material is the cause of the problem. In drilling, there are four clues that say “check drill run-out and toolholder alignment and rigidity.”
Four Clues
- Vibration
- Tapered Holes
- Oversize Holes
- Eccentric Holes
Drill run-out will cause excess vibration when drilling. Run-out can also affect the concentricity and roundness of the hole. Run-out can result in the hole becoming elliptical, tapered, and affect tolerances needed. A savvy operator checks drills for run-out before putting them in the machine. And a savvy set up technician always checks for run-out in the toolholder or chuck when starting a job.
Toolholder or machine caused
However, the cause of the run-out may not just be a bent drill- the tool holder, chuck or spindle may also be to blame. In addition to indicating the drill bits outside the machine, check that the drill chuck and machine spindle is running true.
Length can be an issue
Finally, make sure that the drill is inserted to the proper depth in the chuck, and that the chuck is not overextended. I went on a claim for steel that would not “drill straight” to find a 3/16” drill held in the biggest Jacobs style chuck I had ever seen being held on a # 4 Morse taper. The entire assembly was nearly the length of my forearm, and swinging around on a short cycle time Acme job, the drill never hit the next part on center due to the vibration and lack of rigidity. A more appropriate chuck installed at a shorter length solved this “material problem.”
When the workpiece shows the evidence of the problem, I humbly suggest checking the drill run-out.
Photo credit