These days, bench grinders are so incredibly cheap that every home workshop should have one. As we covered in Using Bench Grinders, there’s a host of uses for a bench grinder, including grinding welds and shaping metal. But perhaps the greatest real-world use is sharpening drill bits.
Let’s take a look at how it’s done.
Firstly, don’t assume that because it looks easy, sharpening drill bits requires no skills and no practice. Sorry to tell you, but that is simply not true.
Especially when grinding small drill bits, it’s very easy to produce ‘cutting edges’ that in fact render the drill useless for any work at all!
However, once you know how to sharpen drills (and especially what to look out for when assessing how good a job you have done), it becomes much easier.
When to Sharpen
Clearly, sharpening of drill bits is normally carried out when they have become dulled. The signs of a dulled drill bit include the need for higher than normal pressure, the generation of excessive heat, squealing, and a lack of cutting progress.
However, drill bits might also need to be re-ground to suit different materials. As they are bought, drill bits have cutting surfaces that are configured for general-purpose work – drilling steel, for example. But for optimal results on materials like plastics, rubber or very hard materials, the drill geometry is best changed from the all-purpose shape. This can be easily achieved by grinding.
At this stage I’d like you to go to the workshop or toolbox and grab a large drill bit – one that still has the ‘factory’ sharpness and end geometry. Don’t be shy – go off and get that drill!
Hold the drill vertically and rotate it until it looks like this. Note the angles that the top edges of the drill bit form to the vertical – here, each face is at 59 degrees, giving an included angle of 118 degrees. This is a typical point angle for a general purpose drill bit.
Here the point angle can be seen for a large (12.5mm) drill bit.
Rotate the drill bit in your fingers until you can look at the ‘ramp’ behind one of the two cutting lips. If the drill bit is held vertically, the edge of the ramp forms an angle with the short axis of about 12 – 15 degrees. This is called the clearance angle.
Here it is on the 12.5mm drill bit....
...and here it is highlighted.
Before grinding a drill bit you must have very clear vision of what you are doing. This means the grinder must be brightly illuminated and, if you wear glasses for close-up work, you should have them on. The grinding wheel should be of fine grit and its face should be flat and square.
The first step in sharpening a drill bit is to grind the point angle.
Stand slightly to the left of the grinding wheel, feet apart. Hold the drill about a quarter of the way along from the point, using the thumb and forefinger. Rest these fingers on the grinder’s tool rest. Use the other hand to hold the drill at its shank.
Hold the drill so that it is horizontal but approaches the grinding wheel at an angle of about 60 degrees. Rotate the drill so that its cutting edge is parallel and close to the wheel. The drill can then be moved forward and the point ground.
The next step is to grind the lip clearance.
Use the left hand to swing the shank of the drill downwards and to the left. These movements are only slight. Remember to keep the right hand supported by the tool rest. As you move your left hand, use your right hand fingers to roll the drill clockwise about a quarter-turn and simultaneously feed the drill forward against the grinding wheel.
Practice doing these motions with the drill you just brought in from your workbench. Use the edge of the desk as the 'pretend' grind stone.
The sequence of motions is: left hand down and leftwards, right hand fingers rotate drill clockwise, both hands move drill forwards. Watch the cutting edge (red arrow) and you’ll see that as you do this, it moves forward and away from the grinding wheel. However, if you rotate the drill too far, or the left hand is not moved towards the left, the opposite side cutting lip (near to the point) will come into contact with the grinding stone – not what is wanted!
When the point angle and lip clearance have been ground for one face, do the other. Make sure that the point angle is symmetrical and centred.
When grinding use only light pressure and frequently pause to let the airstream cool the drill bit.
When learning to sharpen drill bits, don’t sharpen a whole bunch of drills! Instead, sharpen one drill and then try it out on a piece of scrap steel, seeing how well it actually drills holes. To be honest, many of my first attempts at sharpening drill bits resulted in drills that were worse than before they were ‘sharpened’. So you must evaluate the results of your grinding before proceeding further.
A properly sharpened drill bit should have equal cutting angles, equal length cutting edges and equal angle lip clearances.
Drill a hole through a scrap piece of steel. The drill should not chatter, squeal or stick, and shouldn’t need excessive pressure. The swarf (chips) should be continuous and produced in two lines, and the drill bit should be a tight fit in the drilled hole (ie remove the drill from the machine and try it back in the hole).
So what if you have sharpened your drill bit and then there are problems? The next stage is to consult this table:
Another way of finding problems is to again visually examine the drill-bit, looking especially at the clearance angles. As the following diagrams show, by looking at the drill-bit end-on, the actual clearance angles can be assessed.
As we said earlier, if you move away from drilling typical materials like steel, a slightly changed drill bit specification is likely to give better results.
These diagrams show the variations – note that it is primarily point angle that is altered, so the grinding movement that gives the clearance angle is largely unchanged.
Sharp drill bits that have the right shape for the job speed up the work, allow holes to be drilled with greater accuracy, and are less likely to cause accidents. But when you begin sharpening drill bits, take it slowly and carefully, checking the performance of the drill bit and ensuring that correct grinding procedures are being followed. Then, when you have the knack, sharpen all the drills in your tool box!