Drill bit size for tapping holes chart3/12/2024 Especially if you make it out of something harder, like cast iron, so repeated use doesn’t wear one side or another to knock you off target. Once you’ve made the jig with the guides and marks you’re read to use it freehand, just as if it was a Kreg. The Rockler one, you can see on AKlein’s YouTube channel, comes with a built in clamp for the base, so that you can lock the material in place. Mark the measured angle on the side, then follow it with the expected Drill Bit in either a Drill Press or a Drill Guide, like the Milescraft or Rockler ones. ![]() This isn’t exactly something I will want to do free-hand with a handheld drill. There’s another challenge – building a jig or otherwise using an R-sized drill bit to accurately enlarge the 0.262″ hole. 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. 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. This will be my first time tapping 80/20 materials for M10 casters. ![]() There’s also a bit of trial and error involved. If 8.6mm provided too much engagement, and 8.8mm too little, there are also 8.7mm drill bits.įor 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%. I can find them for close to the same price as 8.50mm or R letter-sized drill bits.īut the R-size drill bit is pretty close to the recommended hole size too, somewhat validating the “close enough” approach. Looking online, there are 8.6mm drill bits. This agrees with what many online and reference charts say, leading me to trust the calculator.įor 70%, it recommends an 8.636mm hole size. Checking first for 75% engagement, the calculator says the cutting tap hole size should be 8.5386mm, which rounds down to 8.5mm. ![]() Let’s use nominal dimensions, which would mean 10mm for the major diameter and 1.5mm for the metric thread pitch (length between threads). Little Machine shop lists major and minor diameters, but only for imperial tap and drill sizes. Kennametal has an online calculator that only needs to know the major diameter and pitch. It would provide slightly less engagement than 75%, and should be easy to find at industrial suppliers.įor 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. An R drill bit is approximately ~8.61mm, which would provide slightly less engagement than 75% – which follows closer to 80/20’s example.Īlright, so R it is – that’s close enough. There’s 8.80mm and S, which I’d assume would be around 62.5% engagement. There are not many options that are slightly larger than 8.50mm or R. What if I want to drill a hole for a tap to cut M10 threads with 70% engagement? 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). (When looking at reference charts online, it’s a good idea to double check with other references.) This is according to the Little Machine Shop chart, or others’. 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″. I have a letter drill bit set, but if not I’d have to buy either size. There’s no fractional equivalent close enough. (I have also seen at least one chart list Q as the closes imperial drill bit size.) The closest imperial drill bit size is R, which is 0.339″ or ~8.61mm. ![]() 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. Here’s a Little Machine Shop chart for metric and inch tap and clearance drill sizes.
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