![]() 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. This is a screw in a tapped hole.Here’s a Little Machine Shop chart for metric and inch tap and clearance drill sizes. Now your hole should be tapped and perfect, run your bolt or machine screw through and enjoy the fruits of your labor. Here is an image of a taper tap also known as a starting tap on the left, followed by a plug tap, then a bottom tap. A bottom tap will bottom out on a hole when tapping, the only downside is you cant start tapping with a bottom tap since it has no taper. The taper tap is used to start the thread in your hole but you should continue tapping with the plug and bottom if necessary.Ī plug tap has more cutting threads with less of a taper than a taper tap, this can get the job done but if you have a blind hole you might want to finish off with a bottom tap. When tapping a hole you there are three types of taps to consider, a taper, plug, and bottom tap. Turning the tap clockwise one rotation Turning the tap counter-clockwise half a rotationĪs you are tapping the hole you can use cutting oil when dealing with metals such as steel this will help make your tap last longer, remove chips more efficiently, and smoother and accurate threads on your workpiece. This will remove the chips as you are creating new threads and will make it easier to tap your hole. Once you made a few turns go counter-clockwise half a turn after every full turn. When you feel the tap cutting into the metal check that your tap is straight and perpendicular. Slowly turn the tap in a clockwise motion for a few turns while maintaining the tap in a straight position. ![]() Now you are ready to tap your hole, using a tap handle make sure your tap is secured inside and the tap is straight and perpendicular when entering the hole. ![]() Clean your work area of chips and debris. Drilling on an angle can break your drill bit or ruin your end result. Once you are ready to drill make sure you are drilling onto your workpiece straight down and not on an angle. This will make sure you start drilling in the correct position and prevents slipping of the drill bit. Generally speaking, you should mark the spot on your workpiece you are going to drill with a center punch. In this example you can see we want to use a 10-32NF threaded machine screw, we will need a 10-32NF tap, and to use this tap we need to drill a hole with a #21 drill bit, the closest fraction being 5/32″. You can also use this Metric drill and tap chart. You can use this SAE drill and tap chart. ![]() To drill and tap a hole on your workpiece you’re going to need to know your drill bit size and tap size for your hole.
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