FTM 190 – Drilling and tapping – Part I

Part I of a series on drilling and tapping. How to choose the right size drill bit for the thread/pitch combination of the tap.

This edition of the Fastener Training Minute with Carmen Vertullo was originally published July, 20th 2023 as “Everything you ever wanted to know about blind rivets” during episode 190 of Fully Threaded Radio.

Well hi everyone, this is Carmen Vertullo and the Fastener Training Institute with he Fastener Training Minute coming to you from Carver Labs in beautiful El Cajon California.

Today we’re going to talk about tap drills, that is the drill bit that you use to prepare a piece of material to receive a tap which will cut threads in it. We’re not going to be talking about tapping nuts per se which is a completely different type of animal, or talking about the drills and taps that you use in a typical machine shop environment. And if you are a supplier of fasteners out there in the world, you might be selling people drills and taps as well. This is going to be a series of Fastener Training Minutes because there’s a lot to know, and I won’t skip anything. So when we come back I’m going to tell you a few basic things about drills and taps, and  the drills that we use for tapping holes.

Well welcome back everybody this is Carmen Vertullo with the Fastener Training Minute, talking today about tap drills. That is the drill that we use to drill a hole ready for tapping. Now typically you have to choose a drill size that is the right size to give you a tapped hole that will give you the strength of the mating part and so on, and the resource for getting that information is going to typically be a chart. We have charts called drill bit and tap charts, and if you’re selling drills and taps you should be providing these charts to your customers, preferably for free. But some of them are pretty good and they could be sold. So I would just start by saying if you don’t have that resource of a drill bit and tap chart let me know, send me an email, and I’ll try to get you a good resource for a tap drill chart.

Well as you know in our fastener world, we have two basic categories of fasteners when it comes to dimensions: the inch fasteners and the metric fasteners. and when we’re selecting a tap drill for an inch fastener we would go to the chart, and we would see all of the various sizes and pitches for the inch fasteners on it. For example ,I’m looking at a chart right now, and let’s say I wanted to drill and tap a hole that was 1/4-20. I look at the chart and it says 1/4-20 and that says #7. Well what in the Hell is #7? Well in the drill world, we have three or four different ways to measure the size of drill bits. One way is by the fractional size,  and other way is by a decimal number. A third way is by using the gauge system, which is either a letter or a number, and that’s where the #7 comes from. And the fourth way is the size in millimeters, or metric. And those all have various crossovers, and we can usually find the perfect drill, or one that’s close enough.

So if I go from my chart here that said I should use the #7 for the quarter twenty drill bit, I just go to the tool chest and I get out of number 7 and I use it. But what does that give me the exactly? Well what it gives me based on the recommendation, is 70% of full thread and a hole that will give me the appropriate minor diameter for the thread. In other words, it will gauge properly. In some cases we might want to use a larger drill bit, and in the Machine Shop environment we do that for two reasons: one, it’s not a critical thread, two, perhaps it’s a very difficult material to tap, and three, we can drill and tap faster in the CNC environment with less tap breakage when we go to a slightly larger size. But how big a #7 drill bit?  So the same drill and tap chart that has the tap size on it also has a drill bit size equivalent chart. And if I look for my #7 drill bit on here I’m going to find that on the chart right now. It is .201 which is pretty close to a 13/64 (witch is .203), which we probably could get away with if we had to. So the tap drill size could be one size up or one size down. Generally with one size up you can be safe.

Now the interesting thing about the tap drill bit size is that it’s derived by taking the nominal size of the hole which is a 1/4 inch, and subtracting the thread pitch. Now for a 1/4-20, the pitch is 20 (threads per inch). That’s not much help because we have to derive the actual decimal pitch size, which would be 1/20 which is 50/1,000. And if we take .250 (which is a 1/4 inch), and subtract 50/1,000 from it, we get .200, and that’s why we land on a #7 at .201 being the best tap drill for the 1/4-20.

If we go into the metric world, that illustrates itself even more significantly. Because the pitch of the metric thread is called out in distance between the threads in millimeters. So if for example, we look look at an M10 thread with a pitch of 1.5 and  we take 10 mm and subtract 1.5 mm from it, we get 8.5 mm. That is the drill bit size for a metric M10 x 1.5, and it’s very straightforward.

So that is just one small little piece of information regarding drills and taps. We’re going to continue this conversation next time, and we’re going to talk about things like what are the various kinds of taps that we use in the Machine Shop environment, what taps do we use when we’re hand tapping, what else do we use when we’re tapping blind holes? What kind of tap do we use when we are using the CNC Machinery? What is a roll-thread tap? and we probably will delve into the process of making threads in nuts as well.

Well that’s enough on tap drills for now.  I hope you learned something. Thanks for listening. This has been Carmen Vertullo with your Fastener Training Minute.

 

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