Cutting Small Tubes With Hand Tools

While working on a few antenna projects recently I cut a lot of ⅝", ½" and ⅜" aluminum tubes. This is a subject I touched on briefly in an article on cutting pipe square. Making a straight 90° cut of a round object is not as easy as it might seem. Lucky for us, extreme accuracy is unimportant in the construction of HF yagis. An ugly cut is usually hidden inside the next larger tube, where it is invisible and soon forgotten.

The 3 most common ways to cut small tubing:

1. Pipe cutter
2. Hacksaw (with guide)
3. Band saw

When I first moved to this QTH with a plan to build many antennas, I considered purchasing a band saw.  They're wonderful machines that make clean and reliable cuts with little effort. New ones aren't cheap and used saws come with risks do to their age and maintenance record.

Many of us have a table or power saw that might seem suitable, but they are not. The blades are not designed to cut metal and the rotation speed is far too high. Aluminum in particular requires a slow speed saw. If you could get around these obstacles, do you really want aluminum shrapnel flying out of the back or damaging the saw? Those tools are designed to cut wood, not metal.

I chose to delay the purchase of a band saw and used hand tools for cutting metal. I never did buy that band saw and now that the construction flurry of the past several years has abated it hardly seemed worth it now. Cutting large pipe, tubes and plates with hand tools takes time and can be tiring. My time as a retiree is not a scarce resource and exercising the arm muscles has benefits. Others would choose differently.

Tool quality

To save yourself a lot of grief, do yourself a favour and buy high quality tools. Do not go by price alone since that is no guarantee of quality. Consider yourself fortunate if you have an acquaintance with metalwork expertise who can point you in the right direction. Otherwise you need to know what to look for in a tool.

It's time for a story. When I was in my early 20s I took a bicycle making course from a master frame builder. I loved bicycles and cycling and I wanted to learn more. This was back in the days when the best bicycle frames were made of high strength, thin wall steel. There were only a handful of reputable makers of high end steel frame components -- my choice was Columbus SL.

The course was held in a community college where they had classrooms equipped with enough hand tools and gas welding equipment for all of us. However the instructor urged us to buy our own high quality hand tools. Of course not everyone did, saving expense by using whatever was in the tool room.

When one student ran into difficulty cutting a (very expensive) oval tube, the instructor proceeded to demonstrate the correct technique with the shop-supplied hacksaw. It could not cut in a straight line. The hacksaw frame twisted and the blade warped no matter how much tension was applied to the blade. In an angry outburst he nearly hurled the saw across the room. 

He calmed down and turned the incident into a lesson. He carefully explained the flaws in the tool's design and how it would damage the work. He rooted through the tool room and found one hacksaw that he decided was adequate. He demonstrated the difference.

It's been many long years and I have not forgotten that lesson. I was careful to buy good tools even when I was loathe to spend the extra money. When you consider the pain you encounter with poor tools there really is no such thing as a cheap tool! Some of my best tools seem to last forever and continue to work well despite their heavy use. One example is my hacksaw, pictured above.

That is not an expensive tool. However it was pricier than many others. Although the logo is that of a popular Canadian brand, that is only the branding. I don't know the manufacturer. There are others like it to be found if you shop carefully.

When I bought it I inspected the structure and then performed a few simple tests. Firmly grip the handle with one hand and place your other hand on the back bar. First, try to rotate the back bar. The hacksaw should twist very little and instantly rebound when you release the pressure. If it moves more freely, the backbone or its mechanical bond the with handle and back bar are inadequate. Second, try to pull the back bar towards the handle. It should strongly resist and not behave like an accordion.

The final test may be difficult to perform in a store. Install a blade and bring it up to a high tension. If you can't achieve that it should be rejected -- the structure twists or accordions, or the tensioner is weak or is uncomfortable for your fingers. Next, try to twist the blade with your fingers. If you have adequate tension you will find it very difficult to twist. Check that the blade is vertical and straight.

My hacksaw passes these tests. I've owned it for close to 20 years and it continues to perform well. Longevity isn't easy to test, but it is easy to achieve with a good tool that is not misused or abused. The only maintenance I have to do is to change blades when they wear out.

The common hacksaw blade length is 12". Don't buy a smaller hacksaw because you'll find it difficult to cut plate and large pipe. Don't skimp on blade quality. The better blades are well worth the premium price. I stick with 18 teeth/inch for cutting steel and aluminum. A finer blade has little benefit for antenna work, in my experience, although it can help to start the cut without the blade skipping sideways.

The width of a hacksaw cut is no narrower than the blade's maximum width. A perfect cut isn't possible so expect it to be a little wider. I measured the cut on the tube slit at right at 0.03", using the above 18 teeth/inch blade. When you cut a 12" tube into two equal pieces and file the edges clean, each will be slightly less than 6". I've never found this to be a problem in antenna work. For finer work, position the hacksaw blade to the outside of the measured cut line.

I use a flat file to trim the small lip created by the cut. The blade primary removes material but it also pushes some to the side. A round file removes the lip and debris inside the tube. I use a small triangular file to clean the cut edges of tube slits, inside and outside.You want smooth surfaces to avoid cutting yourself when handling the cut tube and to ensure good mechanical and electrical performance when telescoped inside a larger tube.

On to the next tool. I have two pipe cutters, one small and one medium size. They are not precision tools. Care is needed to ensure clean, square cuts. This might be surprising to some. After all, you have a blade and wheel in fixed alignment rotating around the tube. What could possibly go wrong?

In a picture below you can see several tubes cut with the pipe cutter shown at right. If you look very closely you'll notice that some of the cuts are not 90°. The obliqueness is small but it's there. Again, that isn't important for antenna work. It has to do with tool design and how it's used. Even a good pipe cutter can be mishandled to cut poorly.

The tool looks simple enough.. A sturdy frame supports a roller and a round blade. The blade is thin and wedge shaped but harder than the metal to be cut. You open the gap to fit the tool over the tube and then pull down the blade until it contacts the tube, while ensuring that the roller is flush to the opposite side of the tube. You spin the tool and periodically draw the blade inward until it pushes through the tube wall.

In a perfect world this would result in perfect cuts. The world isn't perfect and neither are tools.

The blade and its body spin on a removable axle (to allow for blade replacement) between the 'C' arms of the tool body. The imperfect fit for both allows for play. The worse the tool or due to long service the greater the amount of play. My pipe cutters are of no better than moderate quality. Spending more didn't seem worthwhile since the cuts are often hidden and aluminum alloys are soft compared to other metals. The blades can last a long time when only used for cutting aluminum tubes.

Aggressive force on the blade will cause the blade to tilt at an angle. It may track in a circular pit or it may wander or spiral. I've found it very easy to trace a spiral when cutting PVC pipe since the soft plastic "grabs" the tilted blade. It's important to start the cut with light pressure no matter the material.

Despite being careful the cut might not be square. Can you tell from the picture? Expand it to full size and the tubes with a bad cut will be easier to identify. 

The other thing you should notice is the profile of the raw cut of the tube posed in the pipe cutter. It is far was than what you get with a hacksaw or band saw. While it may be easier to cut a tube with a pipe cutter, there is more filing to be done afterward.

First, the cut is not vertical. It has the same profile as the wedge shaped round blade. A flat file on the open end of the tube (for both tube halves) is needed to remove the large angled projection.

Second, the cut is not as clean as a hacksaw because the pipe cutter does not remove material. The material pushed aside by the blade piles up to form a substantial lip. It is high on the outside of the tube and shallow on the inside. Both lips should be filed flat so that the end of the tube is corrected to its original diameter. You can see the filing marks on the set of tubes above. Don't skip this step or you may have difficulty telescoping it into the next size larger tube, and even if you do the mechanical and electrical performance will suffer. 

A quick sweep of a round file is usually enough to clear the lip on the inside of the tube. For the outside lip I use a flat file while rotating the tube. Be careful not to file into the tube surface while you abrade the lip.

A pipe cutter works like a butter knife. It pushes the soft butter aside rather than removing it, or splitting it like the ways a chef's knife cuts pliant vegetables. Metal tubes are too rigid for cutting with a knife.

Does it matter?

As I alluded to earlier, pretty cuts are rarely important for antenna work. Once it's in the air no one will notice. You might think that it'll bother you but it won't. You'll soon forget, and you'll forget even sooner the more antennas you build.

What matters is safety and performance. Improperly finished cuts will draw blood when you handle the material. Even for the clean cuts of a band saw I take a few moments to remove imperfections and the small ridges at the inner and outer tube edges. For mechanical and electrical performance we want maximum surface contact where tubes overlap. The ridge formed by a pipe cutter prevents that. For those of us in cold climates, an exposed ridge at the element tip can increase ice buildup and delay sloughing off when the sun comes out.

So, yes, it matters. It's worth the small investment of time and effort to clean the imperfect cuts of a pipe cutter or hacksaw. Or you can invest in a band saw. Then you will spend the time that you saved to keep it in good working order and spend more money on those long flexible blades. In my opinion, buying and learning to use good quality hand tools is the right choice for most hams. If you're the rare exception, by all means invest in a band saw.