Sharpening Drill Bits

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Sharpening Drill Bits

Making drilling holes easier

These days, bench grinders are so incredibly cheap that every home workshop should have

one. As we covered in Using Bench Grinders, theres 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.

Lets take a look at how its done.

Easy, huh?

Firstly, dont 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, its 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.

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.

Terms

At this stage Id 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. Dont be shy go off and get that drill!
http://www.autospeed.com/cms/A_110703/article.htmlhttp://www.autospeed.com/cms/A_110703/article.html


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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.


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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 longaxis of about 12 15 degrees. This is called the clearance angle.

Here it is on the 12.5mm drill bit....


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...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.


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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 grinders

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 movementsare 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.


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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 youll see

that as you do this, it moves forward and away from the grinding wheel. However, if yourotate 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.


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Assessment

When learning to sharpen drill bits, dont 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 beforeproceeding further.

Visual check

A properly sharpened drill bit should have equal cutting angles, equal length cutting edges and

equal angle lip clearances.

Test Hole

Drill a hole through a scrap piece of steel. The drill should not chatter, squeal or stick, and

shouldnt 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).


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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.

Problems

So what if you have sharpened your drill bit and then there are problems? The next stage is to

consult this table:

Problem Cause Fix

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

Slow penetration

Squeaking or squealing

Cutting edges dull

Lip clearance insufficient behind

one lip

Re-grind

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.


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Back Cut

Its 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, dont forget that these cuts can also be ground.

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.


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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.


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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!

Sharpening Drill Bits

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