How I Picked a Tap Drill Size

I’m planning out a project where I’ll be using a new and unfamiliar tap size, which means I’ll need to do a figurative supply run.

After sorting out the taps I’ll need – OSG spiral point plug and spiral flute bottoming taps, it was time to sort out my drill bit needs.

I’ll be reusing some older 80/20 t-slot framing extrusions – 15-series (1.5″ x 1.5″) in either standard or light and smooth profiles. Both have 0.262″ through-holes, which can be tapped straight away for 5/16-18 thread.

But, I need the end holes tapped to M10 x 1.5 threads for use with swivel casters. This means I’ll need to enlarge the holes a bit, a task that is made a little easier with the light and smooth profiles.

The easiest option would be to find some 5/16-18 threaded-stem casters, but I’m very much set in my brand preference when it comes to indoor casters.

Looking at online thread engagement tables – here’s one by OSG – we can see that 0.262″ would be 70% engagement for 5/16-18.

I have seen general recommendations of 50% for steel and 75% for aluminum. 80/20 sizes their 15-series extrusions for 70% engagement without additional drilling needed before tapping, and so it seems safe to stick with that.

Some engineering references recommend 60-70% engagement in general. Sometimes it takes some trial and error to sort out which is best for an application. I have found that optimal thread engagement can depend on different factors.

For a moment, let’s say we want to go with 75% engagement.

Here’s a Little Machine Shop chart for metric and inch tap and clearance drill sizes.

To prep a through-hole for an M10 x 1.5 tap, that chart says I’ll need an 8.50 mm drill size for 75% thread engagement in non-ferrous materials. That’s around 0.3346″.

The closest imperial drill bit size is R, which is 0.339″ or ~8.61mm. (I have also seen at least one chart list Q as the closes imperial drill bit size.)

So, I can go with 8.50mm or R. There’s no fractional equivalent close enough. I have a letter drill bit set, but if not I’d have to buy either size.

If I wanted to tap ferrous materials such as steel, 50% thread engagement would mean a 9.00mm drill bit size, or T as the closest imperial letter gauge drill bit size, which is 0.358″. This is according to the Little Machine Shop chart, or others’.

(When looking at reference charts online, it’s a good idea to double check with other references.)

Okay – so 75% engagement for M10 would mean an 8.50mm or R (~8.61mm) drill bit size, and 50% engagement for M10 would mean 9.00mm or T (~9.09mm).

That was pretty easy.

The Close Enough Method

What if I want to drill a hole for a tap to cut M10 threads with 70% engagement?

There are not many options that are slightly larger than 8.50mm or R. There’s 8.80mm and S, which I’d assume would be around 62.5% engagement. That seems too loose for aluminum.

It seems best to stick with 8.50mm or R. An R drill bit is approximately ~8.61mm, which would provide slightly less engagement than 75% – which follows closer to 80/20’s example.

Alright, so R it is – that’s close enough. It would provide slightly less engagement than 75%, and should be easy to find at industrial suppliers.

A More Accurate Approach

For the most accurate sizing, you’ll need to consult an engineering reference (such as the Machinery’s Handbook) for the proper equations to use and do some math by hand. I won’t cover that here.

Kennametal has an online calculator that only needs to know the major diameter and pitch. Little Machine shop lists major and minor diameters, but only for imperial tap and drill sizes.

Let’s use nominal dimensions, which would mean 10mm for the major diameter and 1.5mm for the metric thread pitch (length between threads). Checking first for 75% engagement, the calculator says the cutting tap hole size should be 8.5386mm, which rounds down to 8.5mm. This agrees with what many online and reference charts say, leading me to trust the calculator.

For 70%, it recommends an 8.636mm hole size.

Looking online, there are 8.6mm drill bits. I can find them for close to the same price as 8.50mm or R letter-sized drill bits.

But the R-size drill bit is pretty close to the recommended hole size too, somewhat validating the “close enough” approach.

If 8.6mm provided too much engagement, and 8.8mm too little, there are also 8.7mm drill bits.

For my application, the casters will be threaded at least an inch into the aluminum extrusions, which means that I’m not too concerned as to whether the thread engagement is 68% or 71% instead of exactly 70%.

There’s also a bit of trial and error involved. This will be my first time tapping 80/20 materials for M10 casters. I have previously only tapped 15-series extrusions for 5/16-18 mounting bolts, where the built-in through-hole is already sized for 70% engagement.

I’m not concerned about how things turn out here – I’m sure the R drill will be fine for my application – but I thought it would be interesting and potentially helpful for me to share the steps and resources I used to land on a drill bit size.

There’s another challenge – building a jig or otherwise using an R-sized drill bit to accurately enlarge the 0.262″ hole. This isn’t exactly something I will want to do free-hand with a handheld drill.