Look, we’ll be straight with you — most of the time, 75% thread engagement is overkill. That may not please the overachievers and perfectionists among you, but it’s science, people (well, physics, more specifically). Usually, between 55-75% is going to do you just fine, reducing the strain on your equipment for near-equal quality. Now, that’s a bit of a range to aim for, so there are a series of other factors you need to take into consideration when determining proper thread percentage for a project. Nailing the correct Tap and Drill combo is a bit of a mathematical pain in the touche, but will deliver results you can really get behind.
First, by increasing the clearance between your tap hole and tap diameter, you’re maximizing your machining efficiency. The higher your thread percentage, the more pressure your tap is under, with very minimal gain — especially after 60%. Less pressure and work = less tap wear, which is always a plus.
Being careful about your thread percentage is also going to protect your screw threads. A too-large or too-small tap drill is going to create shallower threads, or possibly even cause some stripping.
Beyond that, less pressure during machining is also going to improve your internal threads. A hole that is too small (or a tap that is too big for the hole) causes too much pressure during machining. This increases your risk of binding, breaking, or tearing as the tap tries to remove too much material at once.
These explanations might make it sound like a lower thread percentage is always a good thing, but when you’re cutting softer materials, that higher percentage is actually preferable. A softer material demands deeper thread cuts to ensure proper capture and hold. This is especially important in situations where you might have fewer threads to bear the burden, like when cutting thinner sheets. Luckily, softer materials also make it easier to cut a higher percentage with less or equal pressure on your tools. For reasons like this, it’s important to carefully calculate the thread percentage, and match your tap and drill skillfully. Ideally, you want maximum thread capture while also keeping machine strain and tool wear to a minimum.
If you’re feeling a bit twisted up about the math part of it, use this PDF version of our handy formula chart that you can download and print out.