So many times we need to make matching holes to bolt something on. You purchased something to mount on your trailer or your workbench or some other project, but now you need the exact aligning holes to bolt it on. There are obviously many ways to accomplish it, but when this option is available, here is a super easy way to drill matching bolt holes perfectly every time. And, the best part – you do not have to measure much.
First, let’s look at the classic way with measuring and marking. It is easy and straightforward, but has several steps with a lot of room for error.
The Classic Method
For the sake of illustration, we will use a simple example of mounting a taillight. The light comes looking beautiful, and the website even has some dimensions for the holes. (Image from etrailer.com).
So, we lay it all out on the steel, with measurements for each hole, then following instructions from this post about drilling, we punch and drill each hole. Hopefully we did the math right as we set it all up to drill matching bolt holes for attachment.
But, what if we get one of the holes a little off? What a hassle. Go back and drill the holes a little larger till we can make it fit. Or, work to elongate a hole to make it fit. Or, maybe use this technique to force things to fit. Any way we do it, it is a hassle and it turns out less than what we wanted.
For situations like this, there is a simpler way.
Drill Perfect Matching Bolt Holes
This is the easy way — when the situation allows you to do it. I am sure many of you know and use this method already, so I apologize if the presentation is old news. However, if you do not know it, then read on. It is a great time saver and it helps you get just the right alignment as you drill matching bolt holes.
The method works when you can put the two pieces together easy enough for drilling. The method does not work when the parts are obnoxious in size or some other thing that precludes the methods below.
The essence is locating the new holes by using the new piece as a template for drilling the matching holes. There are 3 ways this works.
Method 1 – Direct Drilling
This is the easiest, but it also has some limitations. With the direct approach, we drill matching bolt holes by using the item to attach as the template. We simply clamp it at the desired location (you might have to measure for the first hole), then drill through using the holes of the existing part. Here is a video as the explanation:
This follows right along with the concept of quick and easy fixtures. For many applications, this works great, but the limitations are:
- Drill size. For larger holes, see Method 2 below.
- Soft or thin guide material. If the material you are intending to guide with is thin (too thin for the drill edges to contact as you begin to drill), the drill may wander. If the material is soft (like the plastic shroud of the trailer taillight above), then the drill may damage the soft guide. For both these cases, use Method 3 below.
Method 2 – Indirect Drilling
As a simple modification of Method 1 above, if the hole is large, then we must drill in steps. For instance, if the holes you need are larger than you want to drill through with a single pass.
Mounting a trailer tongue hitch is an example. We want 1/2″ bolts, but we do not want to push a 1/2″ drill through a steel beam by hand. So, we start with a small hole, then make it bigger and bigger with successive larger drill bits. (Like 1/8″ then 1/4″ then 3/8″, then finally 1/2″.) Drilling in steps makes big holes much easier.
Anyway, the technique starts like Method 1, using the large drill bit, but only deep enough to leave a good center mark for a guide. Switch to a small drill bit, drill through, then progress larger and larger from there.
While only the initial drill point mark is truly guided by the existing holes of the part for mounting, that mark sets the hole location for the smaller drills to follow. This is why we call it indirect drilling. Either way, it is another possibility to drill matching bolt holes.
The example shown in the image is perfect for this. It is the mounting plate for the I-Beam at the top of the Gantry Crane leg. We want those holes to match, of course, so using one mounting plate to guide holes in the other plates as well as the beam makes everything line-up. (In case you want them, here are the I-Beam clamps shown.)
This indirect method is also perfect to drill matching holes to tap. Tapped bolt holes are smaller than the clearance holes, so we start with the drill point, then end at the proper tap drill size. In the case of threads, it is extra important that the holes match as well as for tapping the treads straight.
Method 3 – Transfer Marking
When the above methods do not work – either because of the awkward nature of the part you are attaching, or some other reason – we can use transfer marking. The two ways to accomplish it are: 1) with a pen; 2) with a transfer punch.
Sometimes the best you can get is a pen in the hole to draw the position onto the mating part. (Felt tip markers like SHARPIE work best.) When you do, make sure the pen is straight up and down as you trace it. You will then need to eye the center and mark it with a punch to drill. This technique is often the best for thin sheet metal parts.
Another way is to mark the center of a hole with a “transfer punch”. Shown in the image, transfer punches are not super common for DIY, but nice in these circumstances. Select the right size punch (largest that will fit the existing hole), then use a hammer to indent the center of the hole into your new part. Do this for all of the holes while the part is clamped in the correct location. This will “transfer” the locations of the holes so you can drill matching bolt holes.
The process with a transfer punch accomplishes basically the same thing as Method 2 – Indirect Drilling above, except the center is put there with a punch instead of a drill.
A Practical Tip For Matching Bolt Holes
In the text above the focus is on drilling matching bolt holes, preferably without measuring. It helps the end process to be more exact. But why do we need them exact?
That may sound like an obnoxious question, but think about it. For most situations, the the exactness of the hole is really only to assist the holes to line up to put the bolt through. If that is the case, then all the holes can be off a little, and it does not matter as long as all the bolts go in. Right?
One mistake of many DIYers is attempts at accuracy. We have a 3/8″ bolt, so we want to drill a 3/8″ hole. If the holes are really 3/8″ (0.375″) diameter, then the position must be super accurate. If, on the other hand, the holes are 1/32″ oversize (0.406″), then there is a little room for inaccuracy in the hole placements, and it will still work. This becomes more important when there are several such holes.
As a general rule, I almost always drill holes that are 1/32″ oversize. On bigger bolts, even up to 1/16″ oversize. This helps assembly a lot. Even when I am drilling holes using one of the methods shown above, I tend to make the holes just a little big for easy assembly.
I say it is a mistake of DIYers because I have seen, and done it myself, where I end up in a bind because bolts in the holes are too tight.
That Is How It Is Done
I use these techniques all the time in my shop. It is not that I dislike measuring and marking for drilling holes. However, if we skip the measurements, then we are more likely to have the right holes in the right places. Make the job as simple as possible, and the process as robust as possible.
Also, I like it when things just bolt up nice rather than having to fiddle to make things fit. These techniques come in really handy when doing big projects where a lot of bolting is required – like building a gantry crane, or like bolting a trailer frame. The method to choose depends on the situation – including what tools you have available.
Where possible – and I know it is not always possible – use a drill press to create the holes. With a drill press, there is so much more control to drill matching bolt holes. Just a parting word of advice.
Good luck with all your drilling projects.
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