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What Size Drill For 1/8 Pipe Tap?

Are you looking to tap pipe? The first step you must take is to drill the right size hole before tapping threads in your pipe.

But, not just any drill tip will do; you must choose the correct one to ensure accurate pipe threads that will provide a tight seal.

In this article, I will explain what size drill you will need for 1/8 pipe tap. Matching the drill bit to the tap size is only the first step; you need to follow best practices for drilling and tapping to ensure a job well done. I have included some drilling and tapping tips that will come in handy especially if you are new to these pipe cutting techniques.

So, let’s get started!

What Is A Pipe Tap?

What Is A Pipe Tap

Before discussing what size drill is best for 1/8 pipe tap, let’s first find out: what is a pipe tap and how does it work?

A pipe tap is a tool used to create threads inside a pipe during pipe fitting and machining applications. Threading the interior end of a pipe creates a receiving female pipe into which a male pipe is inserted to join two pipes, creating a tight seal.

Also known as threading taps, pipe taps are made from steel alloy or some type of hardened steel, making it harder than most metal pipes.

The design of a pipe tap will vary from one manufacturer to another. Some taps are manual with one end acting as a handle while the other end features cutting grooves. With manual taps, the threading process involves cutting by hand.

More modern tapping tools are built with one end featuring a lug or nut while the other end is used for tapping. This design allows you to fit the pipe tap into another hand or powered tool that helps to turn the tap, making the process less manual.

The tapping process involves inserting the pipe tap into the pipe end, and applying pressure while turning the tap. This is why the pipe tap is built from harder material than the pipe metal; to cut the appropriate size threads inside the thread.

The grooves on the pipe tap have gaps that collect the cut metal, allowing you to discard the waste material as you thread the pipe’s interior.

The most rule of thumb when tapping is to ensure an even entrance to the pipe. If you cut the pipe unevenly, it will be difficult to insert the male end of the pipe and the pipes would be joined at an angle, compromising the seal’s integrity.

What is a tap drill?

What is a tap drill

Now that we’ve understood the basics of pipe taps and tapping, let’s look at tap drills.

Before creating threads inside a pipe using a tap, you must drill into the pipe first using a tap drill. A tap drill is drill is a hardened bit that cuts holes in metal pipes.

When choosing a tap drill, ensure that it is the right size for the tap you will use to make the threads inside the pipe. The right size drill also ensures there isn’t extra metal around the hole that would make it difficult to insert the tap for threading.

Tap drills are sized by diameter and length. The drill bit countersinks the hole, meaning the hole is slightly wider at the top and narrows as it goes deeper. The countersink design accommodates the pipe tap tool, which has a slightly tapered tip. The tapered design allows you to seat the pipe tap before starting the threading process.

What size drill for 1/8’’ NPT Tap Is Best?

So, as I have explained, matching the drill bit size to the particular tap you will use to cut threads is an important first step. This allows you to drill the right size hole, keep the hole clean, and ensure the tap seats properly in the drilled hole.

If you have a 1/8’’ NPT pipe tap, the R type is the appropriate drill bit to use with this tap. If the pipe tap is tapered, the ideal drill size is 21/64mm in fraction form or 0.339’’. For non-tapered or straight pipe tap, use a drill size of 11/32mm or 0.344’’. This delivers 27 threads per inch.

Tips For Drilling The Right Hole For Your Tap

Tips For Drilling The Right Hole For Your Tap

A major mistake newbie machinists make is using the wrong drill size when tapping a pipe. While you can use charts to check the drill size you should use for a certain tap, many of these charts are outdated and were in use when machinists preferred to tap holes with a high percentage of thread height just to be on the safe side.

Also, in the past, many of the drill used for tapping holes were made from cobalt or high speed steel. In contrast, modern machinists now use more accurate carbide drills. Carbide drills are more efficient and deliver higher performance and accuracy than high speed steel drills. High speed steel drills also cut bigger holes than the more precise carbide drills.

The correct tap drill size ensures better tapping outcomes and generally takes the fuss out of making threads in a hollow pipe. Tool manufacturers recommend making the pre-drilled hole diameter larger and applying thread values of between 60 and 70 percent to increase the pipe tap’s efficiency and longevity.

Keep these tips in mind when drilling holes for tapping:

1. Store drilling tips with the respective pipe tap

First things first, once you match drilling bits to their pipe taps, it is best to store them together. This ensures that you use the right size drill for each pipe tap. The correct drill bit ensures you don’t drill extremely large or small holes.

Excessively large holes can cause the subsequent threads to be too shallow. On the other hand, drilling holes that are too small can cause chipping or binding in the tap, forcing it remove more material from the pipe than is necessary.

2. Lubricate, lubricate, lubricate

Overheating is a potentially big problem when it comes to pipe drilling and tapping. The excessive heat can result in drilling holes that are too large or small for the tap size. A simple solution for this problem is to lubricate the drilling application with oil. Not only does oil prevent overheating; it also helps to clean the waste debris, make the drilling action easier, and keeps the bits from making excessive undesirable noise when in action.

3. Start with a center drill

If you don’t have much experience drilling holes in hollow pipes, it is best to use a center drill before using the larger bit you’ve chosen for the 1/8 NPT pipe tap. The center drill allows you to create a small starting hole to guide the rest of the drilling, ensuring that the drill bit drills in the right place.

4. Try pecking to avoid overheating

Overheating is a major contributor to poorly done drill holes for tapping. An additional way to avoid the effects of overheating is to use the pecking technique. Pecking entails drilling into the hole, then pulling out the drilling machine for a bit to allow the hole to cool and to remove the debris. Rotate the drill handle a full turn then reverse it half a turn in the opposite direction. Remove as much debris as possible when you pull out the bit, and remember to oil the pipe surface as well as the bit.

5. Vary the force applied

Be sure to moderate the amount of force you apply when drilling a hallow pipe. Excessive force will damage the drill bit or result in an uneven hole, making it difficult to insert the pipe tap into the hole for threading.

How To Use An NTP Pipe Tap For The Best Results

How To Use An NTP Pipe Tap For The Best Results

Whether you are using a 1/8 NTP pipe tap, which requires an R drill or 21/64mm drill bit, or a 1/16, ¼ etc. tap, you should adhere to some key principles for perfect tapping job.

Here are some rules of thumb for using a pipe tap:

Use high quality taps

If you want high quality outcomes, invest in the best the pipe taps on the market. Use high-speed steel taps from a well-known manufacturer and replace worn taps with new, sharper ones. A dull tap requires extra effort to cut thread into pipes. It can also cause the tap to chip or break while in the hole.

Sit the tap squarely in the hole

For accurate threading, carefully insert the tap in the hole. Ensure that the tap sits perfectly in the hole before starting the threading process. Aiming for a perfect start minimizes wear and tear, breakage, and overheating.

Keep the hole clear of debris

Tapping efficiency improves when you keep the hole clean and free of chips and debris. Pay attention to how the pipe tap feels. Any resistance is a sign of excessive debris that should be cleaned. When this happens, reverse the tap and pull out; this action will clear the chips, preventing the tap from binding and flutes clogging.

Keep an eye on tap alignment

Taps typically come with tap guides that keep the pipe tap straight in the hole for accurate threads. If you are tapping manually, be extremely careful to keep the tap aligned in the whole. With machine tapping the pipe tap is automatically centered but if you are tapping manually, tap misalignment is a real possibility.

Summary: What Size Drill For 1/8 Pipe Tap

Matching the right size drill to your pipe tap ensures efficient and accurate threading. An R type is the right size drill for 1/8 tap pipe, this equates to a 21/64mm or 0.339’’ measurement. Keep in mind the drilling and tapping tips we’ve recommended for the best pipe threading outcomes.

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