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First off, I apologize if this has been asked before, I searched for similar questions before posting but couldn't find one.

I want to drill a 10 mm hole straight through a square steel pipe (20 mm sides, ~1.5-2 mm thickness). The limiting factor is that I'm using a step bit, so I can't just drill through both sides at once (because then I'll end up with a much larger hole on one side), and that I don't have access to a drill press, especially with the Corona situation.

Other than trying to do a really good job when measuring where to put the holes (with a combination square or something), are there any tips for aligning the holes as close to perfect as possible? I'm trying to DIY a derailleur hanger alignment tool for my home bicycle workshop, and the error margin at the end with the holes gets multiplied many times over at the end of the tool, where the measurement is made.

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  • "You're gonna need a bigger^W longer drill bit" . Use some small but long drillbit guided thru the first hole to create a guide hole on the far side. Then use your step bit to open that hole to desired size. – Carl Witthoft Apr 14 at 17:17
  • BTW, I don't recommend using the word "diameter" for a square cross-section, as it refers to the diagonal but may confuse the average bloke. Is your pipe 20mm on a side or 20 mm diagonal? – Carl Witthoft Apr 14 at 17:19
  • @CarlWitthoft you're right. I meant 20mm on each side, fixed. – Victor Stanciu Apr 14 at 20:02
  • It seams that buying a 10 mm drill bit would be the best solution. It would be cheaper than buying a drill guide, or building a jig. – Alaska Man Apr 14 at 20:15
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    Do you have a combination square available with both 90 and 45 degree angles? You can use the 90° side to run a line around all four sides of the square tube. If you do it accurately you should end at the starting point. Then use the 45° angle to mark a diagonal line on one side starting from the first line. Flip the square and repeat to get two diagonals meeting at the center of that side. Repeat on the opposite side of the tube. You can use the 90° angle to run around the tube again through the X's to confirm the locations. – HABO Apr 14 at 20:32
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I've had to do this before to hinge together multiple pieces of universal strut accurately.

If you have to free hand it, precise measurement is your friend. If you can't get a straight cut on the end to measure from, you can use a piece of steel banding or thin cardboard with a straight edge to wrap around the pipe and get a line. For measuring center, measure accurately a tiny bit past center from each side and mark very accurately. Then center a line of your tape between the two points and mark. Measure off each side to ensure accuracy. Mark an X through the point that extends significantly in both directions so you can recover the marking if the part near the hole is destroyed. Then use a center punch to put a dent in the metal for the point of your drillbit. If you don't have a center punch or if the dent is slightly off, use a small drillbit. Start it straight at high speed, then tilt it to drill toward the direction that will recenter it and then straighten back out while drilling at high speed. Check the center and correct again. When you have it correctly centered, drill through. Then switch to a bit large enough to cover the centering flaws and repeat, correcting if the flaws drift it off center. Now you have a straight, accurate pilot hole to use with your step bit. Be warned in metal of uneven thickness step bits tend to drift, especially the type with a single cutting edge. If you are not experienced at drilling perpendicular holes, drill some practice holes and see how they come out. Try a few different body positions until you find one where you can hold the drill stably and you have a good perception of perpendicularity. You can also use a visual aid like a square block beside your drill bit to help your eye but with a step bit I find it easy to see if it's tilted because the back of the cone is a flat face that has to be parallel to the surface you drill.

A layer of masking tape to mark on can really help too. Step bits are not great for this purpose but it is possible. An actual 10mm drill bit will get you a cleaner hole. For the nicest hole freehand, I would drill the pilot holes on both sides, then drill an 8mm hole as straight as I could through the first side and use the pilot hole to carry through to the other side, then do it again with 10mm. For soft materials like aluminum or plastic I'd use 6mm and 10mm. Most of my bits are actually imperial so sizes are rough, but you get the idea.

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  • Missed this: "Square steel pipe"... I'll delete my comment. – P2000 Apr 16 at 14:17
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A Black and Decker Drill Guide #71-055 may be what you need. While no longer made they are available used. I have had mine for 30+ years. The various profiles allow drilling squarely through most shapes. The bonus is if drilling overhead the cup will capture and chips or debris.

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Use a 10mm drill and make sure the piece is clamped so that the drill can be kept vertical or perpendicular to the surface of the piece.

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