TECHNIQUE- TWIST DRILL
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Now we have covered the tools, lets take a look at the technique.
Once you have marked where you need the hole you will need to centre punch it. This will provide a start for the drill bit, preventing it from slipping or skating and ensuring accuracy.Line up the point of the tool with your mark, then hit the punch with a hammer to create a small round indentation. Be careful doing this on thin metal, you may need something behind to prevent it denting a larger area.When drilling metal you should always wear safety goggles. Be aware that the drill could jam in the hole as it breaks through the underside of a larger hole. Keep a good grip on it at all times. Also be aware of any sharp edges or burrs that the holes may have. Burrs can easily be removed using a file.To start the hole you will need a pilot hole. A bit around 3 or 4mm is ideal for a pilot hole. Once the pilot hole is drilled, then change up to a larger bit. If you require a hole of around 12mm, its sometimes easier to use an intermediate size, like an 8mm, especially if you power drill isn’t particularly powerful.It is essential to keep the bit as sharp as possible or there is a chance it will wander or overheat. Overheating a drill bit will damage it beyond repair.Lubrication may well be required especially if you are drilling something thicker than the diameter of the bit, and always on stainless steel.Speed is another key factor for drilling metal. Take a look at the chart below for a guide.If your power drill does not have a variable speed control a good way is to intermittently pull the trigger on and off. This will give it a chance to slow and reach an optimum speed.
If you are drilling a deep hole of into think material you will need to remove the bit from the hole occasionally to allow any swarf to be removed from the hole.
Use these as a basic guideline for twist drills. Your power drills handbook should have the settings for speeds listed assuming it is variable speed.
For Stainless steel use a slower speed than recommended for steel.
It is essential not to run a drill too fast on stainless steel as it will heat up easily and become hard, making it very difficult to drill.
Technique- Hole saws, Step drills & Countersinks
When using a hole saw it will be essential to lubricate it.
Select the required size cutter and screw onto the arbour. The pilot drill should not extend past the teeth of the saw more than the thickness of what you are cutting. If necessary, clamp a piece of waste material to the back. This will also help reduce burring.
Centre punch the drilling position, and begin drilling. The hole saw arbour will be fitted with its own pilot drill. Carefully drill the hole at around 970rpm. Slow the drill down as the teeth are about to come into contact. Hold perpendicular as possible so the teeth cut evenly. Keep going until you are through, don’t be tempted to increase to a higher speed, this will stop it cutting, and simply wear the teeth out.
Cone & Step drills
Again, it is essential to keep these well lubricated.
Depending on the tool you have you may or may not require a pilot drill. Some have them built into the ends.
Centre punch and drill a pilot hole if required.
Begin drilling at the necessary speed (check chart below) the larger diameter step you are using you will need to decrease the speed slightly.
Drill with a firm pressure. As you approach the size you require be careful not to go too far, resulting in an oversized hole.
Lubricating a countersink is necessary, Once you have drilled the required diameter hole use a countersink to produce a chamfer for a countersunk screw or bolt head to sit into.
You will need to hold the it as square to the surface as possible to produce an even hole. A very slow speed is best, somewhere around 300rpm-500rpm and a lot of pressure.
Check the size of the countersink you have made to the screw frequently. Woodworkers carbon steel countersink bits are no good for metal. You will require a HSS one, usually with three flutes. These are prone to slipping in chucks, when you are buying one lookout for one with a hexagonal shank.
For hole saws, cone drills and step drills use this chart as a guide.
|16mm||530 rpm||275 rpm||900 rpm|
|20mm||460 rpm||230 rpm||690 rpm|
|25mm||350 rpm||175 rpm||525 rpm|
|30mm||285 rpm||145 rpm||425 rpm|
|35mm||250 rpm||125 rpm||375 rpm|
|40mm||220 rpm||110 rpm||330 rpm|
|50mm||170 rpm||85 rpm||255 rpm|
|75mm||115 rpm||55 rpm||165 rpm|
|100mm||85 rpm||40 rpm||125 rpm|
|175mm||115 rpm||55 rpm||165 rpm|
Drill produces a very high pitch squeal
Drill Bit becomes very hot
|Drill is running too fast|
Drill bit dulls very quickly
Drill bit becomes very hot
|Lubrication is required|
|Drill bit Wobbles||
Damaged Drill bit
Ensure the Bit is correctly positioned in the chuck
|Drill Bit skates across the metal surface||
Centre punch is too small
Smaller pilot hole is needed
|Drill Bit grabs as it breaks through||
Too much Pressure
Drill is running too fast
Other helpful tips
As your drill bit breaks through the underside ease up on the pressure to help prevent it grabbing.Buy the best cutting tools you can justify. Even if you purchase a full set of low priced unbranded ones, also get yourself a few sizes of good quality bits, a 3mm, 4mm, 5mm, 6mm will come in handy.
A countersink or larger diameter bit can be carefully used to remove burrs from drilled holes.
Article courtesy of Hitch