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This article was first published in 2008.
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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.
Easy, huh?
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.
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Good
quality drill bits are expensive – very expensive in large sizes. But once you
can competently sharpen drill bits with a bench grinder, you can buy lots of
your drills secondhand, massively dropping the purchase prices.
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Terms
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.
Sharpening
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.
Assessment
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).
How
Accurate?
A
1941 book I have on drilling and grinding makes the following statement:
While
it is worthwhile acquiring the experience in grinding twist drills by hand, it
should be noted that this would not be tolerated in any engineering works where
accuracy is required: the drills would be properly ground in the tool room or
proper drill-grinding equipment would be available.
Therefore,
if really accurate holes need to be made (eg for a small diameter tap), it is
best to buy a brand new drill bit for the job.
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Problems
So what if you have sharpened your drill bit and
then there are problems? The next stage is to consult this table:
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Problem |
Cause |
Fix |
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Swarf emerges unevenly
Drill point appears to wobble, drill press
shakes
Internal shoulder at base of blind hole
Oversize or rough hole |
Off-centre point angle – unequal cutting lip length or angles |
Re-grind |
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Slow penetration
Squeaking or squealing |
Cutting edges dull
Lip clearance insufficient behind one lip |
Re-grind |
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Drill grabs |
Point angle too sharp
Lip clearance too great |
Re-grind |
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.
Back
Cut
It’s
not mentioned in the textbooks but many large diameter drill bits have a sharply
angled back cut behind the leading lip. In my experience, such a back cut can
radically improve drill performance, so when sharpening large drill bits, don’t
forget that these cuts can also be ground.
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Different Materials
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.
Conclusion
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!
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