This section covers drilling. A metallic snap sounds like a broken tap in a machine shop. The snap mean you didn’t add enough cutting fluid, used too fast of a speed, or your drill bit was to small for the job. A broken tap is far easier to fix before the broken tap sits inside a hardened steel block.
To do this correctly, you need to know how the drill size relate to thread diameter. And yes, you are drilling more than just hole. You’re removing a certain amount of material. This will determine whether you get solid threads or ones that breaks off when you turn a fastener.
How to Drill and Tap Threads Correctly
The provided paragraph does not contain these specific measurements for Unified Fine, Unified Coarse, or Metric threads; it only explains how to use a chart to correlate different thread type to their required drill sizes. This shows how the drill size for each thread type relates to the amount of thread engagement. This is based off a goal of seventy-five percent thread engagement, which is what most machinists aim for. The reason that this percentage isn’t made up is that it still yields almost ninety percent of the maximum strength of the thread without creating so much cutting torque as to snap the tool.
If you drill with a smaller hole you get one-hundred percent engagement, which leads to disaster as the material violently tries to resist your attempts at cutting it down. Drilling too large results in only fifty percent engagement making tap easy to install but also leaving threads so weak that they can be stripped out instantly on soft materials such as plastic or aluminum. It’s important to use the proper drill size and tap type.
Don’t assume you only need one tap that works in everything. The three most common types of taps is taper, plug, and bottoming. A taper has a long chamfer good for starting threads on a through-hole where you can chip the shavings out the other end. A plug tap is a good do-it-all tap with a shorter chamfer different than a taper, for both through- and blind holes. A bottoming tap doesn’t even have much chamfer at all, its purpose is to finish the thread right up against the bottom of a deep hole. If you try to start a thread with this tap, it break immediately because it has no guidance to get to center.
A spiral point tap pushes chips forward in the exit pathway. This increase production when used on a through-hole job. A spiral flute tap is an auger-like tap that lifts chips out of a blind hole, preventing them from packing into the tap and locking it up.
Speed and lubrication are determined by material science. People who only tinker in their hobbies tend to gloss over this subject since “all metals are the same,” right? Nope! Stainless steel must be tapped slowly and using lots of heavy sulfurized oil; you cannot pause or hesitate during a cut, otherwise it will work-harden instantly. Aluminum is different, fast speeds plus loads of light oil or kerosene are needed to avoid soft chips welding onto the flutes. Then there’s cast iron which defies convention: it needs to be tapped dry due to its graphite content acting as a self-lubricant. Adding any fluid turn it into abrasive sludge, destroying not only the surface finish but also the tap itself. Titanium demands rigid setups plus extreme caution when using heavy tapping oil; it heats-up quick and gums-up taps in no time flat.
The trick to successful threading is understanding the physics of removing chips. It doesn’t matter if it’s fine metric pitch or coarse threads; success come from not forcing it, but trusting the tool to operate correctly at the correct speed. Break the chips by turning back half a turn each complete revolution. Always keep the tap perpendicular to the surface and always use the correct cutting fluid for whatever material you’re working on.
These tricks might seem like too much at the time. However, they would of saved you hours of head scratching later when you attempt to remove a broken piece with a chemical solution or extractor. You don’t want nice looking holes with ridges inside them; you want clean threads that hold. A good tap cut is quiet, rhythmic, and reliable, which results in a part that fit together exactly as intended without the fear of sudden fracture.
