Motorsport Fabrication Fundamentals: Tapping Holes
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Tapping Holes
12.07
| 00:00 | - A common operation we'll need to complete when building components for motorsport fabrication is tapping or threading holes in order to accept a suitable fastener. |
| 00:10 | On face value, this is a relatively simple task but there's actually quite a lot to understand and it is easy to go wrong here which can then be quite difficult to recover from. |
| 00:19 | To start, let's take a high level view of what we're doing. |
| 00:23 | We begin by drilling a suitable sized hole in the workpiece and follow this up by utilising a specially designed cutting tool called a tap which is wound into the hole by hand in order to remove material from the hole and form the threads. |
| 00:37 | There's a lot of subtle aspects that play into this though which also will affect the finished product. |
| 00:43 | First up, we need to understand what thread and what size we're working with. |
| 00:47 | That might sound like an obvious statement but it's more confusing than you might think. |
| 00:52 | While there's mainstream thread such as UNC or UNF which are imperial thread standing for United National Coarse and United National Fine, or metric threads which are defined by the fastener diameter and thread pitch, for example, M10 x 1.5m, means the threaded hole has an outside diameter of 10 mm and there are 1.5 mm vertically between adjacent threads. |
| 01:16 | There's also more unusual threads like BSP and NPT which stand for British Standard Pipe and National Standard Pipe respectively. |
| 01:25 | There's also some more obscure and less used threads such as Whitworth which you're unlikely to need to worry about. |
| 01:32 | The reason that I mention this is that in some instances, if you're working with an unknown fastener, it can be difficult to tell exactly what the thread is based solely off a thread gauge in a set of digital callipers which we'll look at as we go through this module. |
| 01:46 | In most instances though, we'll be choosing the fastener for a particular task so you'll know with confidence what the thread is. |
| 01:53 | If you're not in this situation then the first place to start is by measuring the outside diameter of your fastener and the easiest way to do this is with a digital vernier calliper. |
| 02:02 | This is a pretty simple task and the only two tips I'd give you are to make sure that your callipers are zeroed when closed before taking a measurement and to make sure that you use the wide, flat path of the calliper's jaws which will then ensure that you're measuring the true outside diameter of your fastener. |
| 02:19 | The next trick is deciphering what your measurement means. |
| 02:21 | If we're considering an M10 x 1.5 fastener as we just discussed, it's the outside diameter of the threaded hole that's 10 mm. |
| 02:30 | Obviously we need a little clearance between the outside diameter of the threads and the fastener so you can expect to measure 0.1 to perhaps 0.2 mm smaller when you're measuring the fastener. |
| 02:42 | Perhaps 9.8 to 9.9 mm in this case. |
| 02:45 | This is where things get a little tricky with metric versus imperial fasteners. |
| 02:49 | For example, a 3/8th UNC fastener is the closest in terms of diameter to M10. |
| 02:55 | The difference in metric terms being 9.5 versus 10 mm. |
| 03:00 | 0.5 mm is still a pretty clear difference but when we get down to some of the smaller sizes it can be a little harder to decipher. |
| 03:08 | Next we need to define the thread and this can actually help you sometimes if you're stuck deciding if you're dealing with an imperial or metric fastener diameter. |
| 03:15 | The way to check the thread pitch it using a thread gauge which will usually come with a tap and die set or can be purchased separately. |
| 03:22 | These have a range of metal templates that match the thread pitch for a given thread. |
| 03:27 | There are a couple of things to keep in mind when using these though for reliable results. |
| 03:31 | First of all, it's important to make sure that you're dealing with a clean fastener. |
| 03:35 | This won't be an issue if it's new but if you're dealing with an old or used fastener, dirt or debris trapped in the root of the thread can affect your ability to properly use the thread gauge. |
| 03:46 | Brake clean and a stiff bristled brush will make quick work of cleaning the threads though. |
| 03:50 | Next we need to understand how to actually use the thread gauge. |
| 03:54 | This involves placing the relevant gauge along the length of the fastener so that the gauge extends into the thread roots. |
| 04:00 | It's very obvious when the thread gauge is a poor match for the fastener's thread pitch however once you get close to the correct pitch and particularly with some of the imperial and metric pitches that are close, this becomes a little bit trickier. |
| 04:13 | A good way to visualise how well the thread gauge matches the thread is to hold the fastener and gauge up to a light source. |
| 04:20 | This will magnify any mismatch because you'll be able to see light bleed past where the gauge isn't contactin the thread completely. |
| 04:28 | When you have the correct pitch, there should be essentially no light visible. |
| 04:32 | The next step is to drill the hole that will be the basis of our thread. |
| 04:36 | The diameter of the hole however is critical as this will form the root of the thread so the diameter needs to accurately match the root diameter. |
| 04:44 | A good rule of thumb for metric fasteners is to subtract the thread pitch from the fastener diameter to arrive at the correct drill size. |
| 04:51 | For example, our M10 x 1.5 thread would require us to start with an 8.5 mm hole which is simply 10 mm minus our pitch of 1.5 mm. |
| 05:02 | We've included a PDF guide to this module that will provide you with the recommended drill sizes for a given thread. |
| 05:09 | In some instances, we get into a situation where the theoretically ideal drill size doesn't exist or we may not have easy access to. |
| 05:17 | Let's take for example an M8 x 1.25 mm thread. |
| 05:21 | Using our rule of thumb, this means our drill size should be 6.75 mm which may not be that easy to find, depending on where you are in the world. |
| 05:30 | Generally, in this situation we'd step to the next largest size which would be 6.8 mm, which should be a lot easier to source. |
| 05:39 | If you're tapping a softer material like aluminium, you could step the other way and use a 6.7 mm drill as the tap will still be able to clear the additional material and will result in a complete thread engagement. |
| 05:51 | The reality however is that in most situations, stepping up to the next available drill size is absolutely fine. |
| 05:58 | Once the hole is drilled, it's also advisable to use a counter sink drill bit to remove any burrs resulting from the drilling operation as well as to make it easier to lead the tap into the hole. |
| 06:08 | In turn, this makes it much easier to get the tap started. |
| 06:12 | When it comes to the taps themselves, there are a variety of options here too. |
| 06:17 | In general, taps are available in three styles. |
| 06:20 | We start with the taper or starter tap which as its name suggests has a tapered point that extends up the first 8-10 threads. |
| 06:28 | This style of tap is designed to make it easy to start tapping the hole and it also helps to centralise the tap, ensuring the finished thread is perpendicular to the workpiece. |
| 06:39 | Next we have the bottoming tap which has almost no taper at all. |
| 06:43 | This is designed for threading blind holes as it allows the hole to be threaded almost all the way to the bottom. |
| 06:49 | The downside of the bottom tap is that the reduced taper makes it almost impossible to get the tap started and usually these taps are used after the thread has begun with a taper tap. |
| 07:00 | Last we have the plug tap or intermediate tap which falls somewhere in the middle. |
| 07:04 | These will usually have a taper that extends between 3-5 threads up the tap. |
| 07:09 | They're easier to start than a bottoming tap but they won't thread to the bottom of a hole. |
| 07:14 | Terminology also tends to differ between suppliers and countries and you may hear the plug tap referred to as a second tap. |
| 07:22 | In some instances, the term plug tap is also used to refer to a bottom tap so it is worth understanding exactly what you're buying. |
| 07:30 | There are also different flute designs available. |
| 07:32 | Most tap and die sets will include what is referred to as a straight flute which refers to the hollows between the threaded portions of the tap. |
| 07:40 | These are designed to collect the swarf from the tapping operation and it's no surprise that in this style of tap they are straight, running along the length of the tap. |
| 07:49 | On the other hand, spiral flute taps are also available where these flutes form a spiral shape that wraps around the tap. |
| 07:56 | The difference is in the way the swarf is cleared. |
| 07:59 | With a straight flute tap, the swarf is pushed down towards the bottom of the tap which requires the tap to be completely withdrawn when tapping blind holes so that the swarf can be cleared. |
| 08:09 | A spiral flute tap on the other hand pushes the swarf up and out of the hole which is better for use in a blind hole. |
| 08:15 | Spiral flute taps also don't require the backing off operation during tapping which we'll get into shortly. |
| 08:21 | Let's now have a look at the process of tapping a hole. |
| 08:24 | In this case we're tapping an M10 x 1.5 mm hole through a piece of steel plate. |
| 08:30 | We've drilled the hole to 8.5 mm using a drill press to ensure that the hole is perpendicular to the surface and we've lightly chamfered the top of the hole using a countersink bit in the drill press. |
| 08:41 | Since this is a through hole rather than a blind hole, we can simply choose a taper tap which will make the process nice and easy. |
| 08:47 | We can now insert the tap into a tap wrench or handle and give the tap as well as the hole a light coat of cutting oil. |
| 08:54 | This lubricant is an essential part of achieving a sharp and accurate thread and it will also help extend the life of your taps. |
| 09:01 | Now we an place the tapered end of the tap into the hole and begin to gently turn the tap. |
| 09:05 | If you're dealing with a conventional right hand threaded hole, then we'll be turning the tap clockwise which is pretty self explanatory. |
| 09:13 | The only time we need to be mindful of this is on the relatively rare occasions where we may be using a left hand threaded tap. |
| 09:19 | Particularly with a tapered tap, it should require relatively little effort to get the tap started and just light pressure from our fingers will be sufficient. |
| 09:28 | Once we have the tap started and we've rotated it by one to two turns, it's a good idea to remove the tap wrench and visually inspect that the tap is perpendicular to the workpiece. |
| 09:38 | We can do this by eyeing up the tap compared to the workpiece in two planes at 90° to each other. |
| 09:44 | This will help ensure that the tap is truly perpendicular in both planes. |
| 09:48 | As the thread progresses, initially it can be worth performing this check a couple of times to make sure the tap isn't beginning to wander off course. |
| 09:56 | The actual tapping action involves turning the tap one to 1.5 turns clockwise before backing the tap off a half turn in the opposite direction and then repeating the process. |
| 10:07 | This backing off action breaks the swarf free so that you don't end up breaking your tap off in the hole. |
| 10:13 | For blind holes, you'll also need to remove the tap completely once or twice and clean the swarf from the flutes as well as from the bottom of the hole. |
| 10:20 | This will prevent the sward building up beneath the tap which will prevent the thread extending to the base of the hole and this can also risk the tap breaking off in the hole. |
| 10:29 | Extract the tap and clean the thread any time you feel the resistance to turning the tap increase. |
| 10:35 | With our through hole example, once the thread has been tapped the whole way through, the resistance on the tap will basically drop to nothing, indicating that the tap is no longer cutting threads. |
| 10:45 | We can now extract the tap and clean down the tap as well as the hole. |
| 10:48 | This is particularly important for blind holes as debris can remain trapped at the base of the hole. |
| 10:54 | Compressed air is an easy way to remove any remaining swarf. |
| 10:58 | If you're tapping a blind hole then the correct tap will depend on the depth of the hole. |
| 11:02 | If the hole is deep enough then we can still start the tapping operation with a taper tap before swapping to a bottoming tap to complete the task. |
| 11:10 | If the hole is too shallow then you may need to begin the thread with an intermediate tap before switching to the bottoming tap. |
| 11:17 | This does require a little more care to ensure the hole is tapped perpendicular as the intermediate tap doesn't tend to self align as much as a tapered tap. |
| 11:26 | If you're dealing with a very shallow blind hole then even the intermediate tap may have too much taper to get the thread started and it is possible to begin tapping with a bottoming tap. |
| 11:35 | These offer very little self alignment and make getting the thread started very tricky, particularly on harder materials like steel. |
| 11:43 | A little trick here is to use your drill press and insert the tap into the chuck. |
| 11:47 | You can then align the tap with the hole and turn the chuck by hand until the tap is started in the hole. |
| 11:53 | The support of the drill press here ensures that the tap remains perpendicular and once the first few threads are cut, you can then remove the tap from the drill press and complete the hole using your tap wrench. |
