Keeping Bolt Holes Aligned when Drilling and Tapping
This tutorial describes steps for joining two metal plates with multiple screws and keeping the holes aligned for all of the screws. With a milling machine, this would be easy, because you can position the table with precision for both plates. Doing it on the drill press by just doing manual layout and drilling depends on more precise hand work than is reasonable to expect. So we will use some tricks that make the holes line up even if they are not as precisely positioned as they would be on the mill.
The example we use here is just two aluminum plates attached together with four cap screws. By drilling through both plates at the same time and assembling with screws as we go, the screws themselves keep things aligned. Basic concept: for each bolt, there are three steps:
- Clamp the plates together and drill a tap size hole through both plates
- Separate the plates and drill through the top plate with the body size drill. As discussed in the drilling tutorial, enlarging a tap size hole to body size can cause chattering, so the drill should be fed quite aggressively.
- Tap the hole in the bottom plate.
This approach keeps the body hole and the tapped hole on the same axis. This process is repeated for all holes, doing partial assembly along the way.
Of course, if the plates are clamped together securely, it is possible to drill all of the tap holes in one step, but the safest approach is the use the first and second holes to guarantee alignment.