# Tips for drilling two directly opposite holes in a square steel tube with a step bit and no drill press?

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.

• "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. Commented Apr 14, 2021 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? Commented Apr 14, 2021 at 17:19
• @CarlWitthoft you're right. I meant 20mm on each side, fixed. Commented Apr 14, 2021 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. Commented Apr 14, 2021 at 20:15
• 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
Commented Apr 14, 2021 at 20:32