Drilling a round hole in sheet metal

The problem…

I am pretty certain that the reason you are reading this page, is because like me you have on at least one occasion tried to drill a hole into thin sheet metal using a standard twist drill only to find the shape of the finished hole to be less than round. The hole may look a little like the picture below. See warning and disclaimer at the bottom of the page!! not round The hole above is a “special” kind of round known as “equilaterally curved”, strangely enough if you measure through the centre of the hole at any point it will measure the same in every location and assuming the drill is sharpened correctly it will be the correct size. For example if you used a 10mmØ drill the hole would measure 10mm when measure through the centrline, but as you can see it is far from round. If you are an experienced engineer you may have noticed that the smaller the drill, or the thicker the plate the less the effect is, and that if you drill a small enough hole it will be round, the reason for this is simple, and can be found below…. TRIVIA – did you know that 50 pence and 20 pence pieces are also equilaterally curved, but both have seven sides so are heptagons. if you don’t believe me, measure one through the centre line with a digital calliper in a few places….

The reason…

Simple mathematics, the drill point is longer than the thickness of the material being drilled, causing the central part of the drill (the web) to become unsupported and “wobble” off axis, however because a hole is already present the drill can only wobble within the confines of the hole it is drilling. At this point most people say, well use a short drill and clamp the work piece down securely, the problem with this is, you will still get an out of round hole…..

The Solutions….

OK, so we know the problem, but not how to overcome it?? there are a few methods that will fix the problem, you can either modify the drill to “encourage” it to drill a round hole, which would be great if you had a bench grinder to hand and didn’t mind changing the geometry of one of your prized drills or you can use the tried and tested method below that requires no modification to be made to you precision cutting tool. My preferred method for achieving a round hole in thin sheet metal is so simple its amazing, and one you would never consider trying, Not only that, but I can say from personal experience that this tip works for holes up to 32mmØ in sheet metal as thin as 1.2mm (18swg). Get to the point I hear you say!!!!……. To drill a round hole in thin sheet metal, take a piece of cloth for example an corner of an old tea towel cut into squares, an old flannel or anything you don’t mind cutting into squares and throwing in the bin once you have your nice round hole. For holes up to 19mmØ (¾”) I would suggest starting with a piece approximately 100mm (4″) square, and fold it in half, and then half again, obviously a bigger piece is require if you are drilling bigger holes. If you do not have any spare cloth to hand, it is sometimes possible to use a good quality paper towel, but cloth is recommended.

  1. If possible, secure the work piece to the machine bed, or suitable piece of wood, or proceed with extreme caution if you are using a hand held drill. (The results will be the same in either case).
  2. Drill a small pilot hole in the required location (3mmØ)
  3. With the drill spindle stopped,locate the folded cloth centrally over the hole and fit the large diameter drill to the machine (no need to use further pilot holes if you use the correct speeds and feeds)
  4. Advance the drill through the thin sheet metal, it may be difficult to get the drill to start cutting, but a gentle force should be used until the cloth has been pierced, taking care not to distort the sheet metal, the drill will then advance normally through the metal, but the resulting hole will be both round and, assuming drill geometry is not miles out, the size will be spot on.

WARNING – To avoid personal injury, Care must be taken when using this tip because the cloth will become entangled in the drill, take care that all loose clothing and body parts are well out of the way of the cutting area.

Disclaimer – All workshop tips and advice offered on this site is done so in good faith, I can take no responsibility for personal injury arising from anyone using the advice given. PLEASE FOLLOW NORMAL HEALTH AND SAFETY PRACTICES AND USE PERSONAL PROTECTIVE EQUIPMENT AS REQUIRED, YOUR SAFETY IS YOUR RESPONSIBILITY!!