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It doesn’t happen often around the house, but sometimes you need to tap new threads into metal. When drilling the initial hole before tapping, it needs to be smaller than the tap but large enough that the tap will cut the edges. One of the most common sizes is a quarter-inch with 20 threads per inch, so you will want to know what size drill bit for 1/4″ 20 tap.
The concept of tapping threads is relatively simple, but in practice, it can get complicated.
- If the hole is too small, the tapping will extend too far into the center making it difficult to screw the bolt into the hole.
- If the drilled hole is too large, the bolt will not mate well enough with the threads.
There can be exceptions to this rule when it comes to the hardness of the material being drilled, which may make a difference with the tap drill size you will be using.
The standard tap drill is 75% for average or softer materials, but you would may 50% for harder materials that need a tighter connection with the screw or bolt.
Harder materials may include steel, stainless steel, and iron. For softer materials you are looking at aluminum, plastic, brass, and more. If you are not certain, be sure to check the manufacturer’s instructions for the correct size tap drill to use in your particular use case.
When tapping harder materials, you may want to use the finer threading of 50%. This can change your drill bit size – the recommended size drill bit for 1/4″ 20 tap when using a 50% tap drill is 7/32”.
Here are different drill sizes when you have different threads per inch for 1/4″ screws:
|Screw Size||Threads Per Inch||Drill Bit Size|
|Drill Bit Size|
As you can see, there are some commonalities, but also some differences in what size drill bit to use when taking threads per inch into account. In general, the higher threads per inch, and the more fine the threads are, the larger (slightly) your drilled hole that you will be tapping should be.
Other Considerations for Drilling 1/4″ 20 Tap
When you drill any type of material for tapping, be sure to decide beforehand whether you will be tapping with 50% or 75%. This may change depending on the hardness of the material, so if you are able to drill through a scrap piece first, that may give you a better idea of how you should tap.
How to Drill for Tapping
When you have decided on which size drill bit to use, you will then need to move on to the drilling process. When drilling into metal you will want to mount that material with at least two clamps to reduce the chance of the metal spinning and possibly injuring you. Wear safety goggles and consider thick gloves as well.
You will first need to mark the area to be drilled. Use masking or painters tape not only to leave a nice area to mark, but also to help keep the step drill bit in one place. Using a center punch with a hammer will help as well by leaving a divot for the bit to rest in. If drilling into a thin or flexible material, be sure it has solid backing so you do not deform it.
Make sure your drill is completely perpendicular at 90 degrees to the material you are drilling. Any angle created may create problems with both tapping and screwing in bolts. If possible, use a drill press or a guide for your handheld drill. Use cutting fluid and regularly sharpen your drill bits, make sure to do your best to keep the drill bit from getting too hot.
How to Use a 1/4″ 20 Tap
The purpose of tapping is to create a threaded hole for a screw or bolt to enter, so always have the bolt ready for a visual comparison that you are headed in the right direction.
After the initial hole is drilled, you will first want to de-burr the edge of the hole and remove any sharp edges. Additionally, adding a chamfer at this point will make the tapping go easier. It is easy to chamfer with specialized drill bits, but you can use normal drill bits as well.
Be sure to clean out the chips, dust, and other debris that may have accumulated in the hole or around it.
When setting up the tap, it helps to use a 90° tap guide. When we add so much downward pressure to the hole and tool that we are working on, it is easy to lean to the side, ending up with a crooked tap. Use the guide and take your time.
One manner of tapping threads into a hole is called “pecking.” This process involves going forward a short distance, and then backing off. It helps keep your tap or drill bit from overheating or even breaking under the extreme torque used to get through metal.
The most common form of pecking involves turning a full turn of the tap in, and then a half turn out. To be even more careful, you could go a full turn, remove the tap completely, clean out the hole, go in two turns, and repeat until the hole is fully tapped. This takes quite a long time but will create the most even thread tapping.
Selecting the correct size drill bit for a 1/4″ 20 tap depends on the coarseness of the threads you will be tapping, which is in turn based on the hardness of the material you will be drilling through. Make a decision beforehand and test with scrap material if available so you are able to have the right tools on hand.