Model Engineers’ Workshop – July 2019

70 http://www.model-engineer.co.uk Model Engineers’ Workshop

a very fi ne pillar fi le. Finally fi nish the
indexing plate to size. All components for
drill grinding and slitting saws are seen in
photo 7.

Setting plate
The lip of the drill being ground must be
parallel with the bottom face of the collet
block, an easy task with the setting plate,
fi g. 3, It’s made from 10mm aluminium
with a piece of 3mm steel fi xed on the
top to allow clearance for the ER20 screw
cap We now fi t a bar 10mm in the collet
and measure the gap between it and the
plate, add 5mm and that’s the exact centre
height. Machine the block of steel to this
exact height being careful to maintain
sharp edges. I fi xed mine with a single
countersunk screw from underneath so that
it can be angled to the most convenient
position. The second block is used to set the
projection of the drill, useful when you are
grinding a number of drills so avoiding too
much adjustment needed to get the drill
up to the grinding wheel. The photographs
and captions will now show the grinding

process. The setting plates are shown in

photo 8 while 9 and 10 show the setting

procedure. Set the rotating head to 59

degrees and angle it upwards by 10 degrees

for primary clearance. Grind the fi rst facet,

back off and repeat for the second edge,

photos 11 and 12.

To produce the clearance angle, without

using the wedge, angle the tilting bracket

a further 15 degrees to 25 total and grind

away using a lot of cut until one edge

begins to get close to the centre of the drill,

then it pays to take your time as if this facet

is just a little too long one has to return and

re do the primary clearance. We are aiming

to get all four facets to meet at an exact

point. If the drill is badly damaged, it pays

to start freehand grinding on the bench

grinder, as the cutter grinder does not like

you taking big cuts, and in some cases it is

necessary to reset the lips level in the jig if a

lot of material has to be removed.

Why an angled collet block?

The original concept was to just invert

the block and the secondary angle would

be presented to the wheel with only the

lateral in feed needing to be changed.

However due to the peculiar 3D geometry

of operation on this machine, there are so

many different things at different angles

which resulted in the secondary clearance

was always being tapered. It completely

removed the primary clearance at the

periphery of the drill before all the facets

met in the centre. I was to say the least

very dejected as simply flipping the block

over was a primary design consideration

at the planning stage. After a few months

I looked closely at a drill and the penny

dropped. Out came the angle setting

blocks, and I found that if the bottom end

of the block was pushed out by 5 degrees

Close up of setting the drill, showing how the
raised part of the jig ensures clearance for
the collet nut.

Load the universal plate onto the grinder

rotating plate and set it at a horizontal

rotation of 59 degrees and a vertical one of

10-12 degrees angled upwards. Now adjust

the position, switch on and grind the fi rst

primary clearance. Note the setting on the

feed dial on the main bar, back off , index the

drill and repeat for the other lip to the same

feed setting. Check with a powerful eye

loupe that any damage has been removed

from both lips.

The incomplete primary clearance, the

damage still needs removing here. You will

now understand the reason for the separate

removable block for examination!

Using the wedge, here the block sits on its angled face.

The fi nished drill, all facets meeting in the

centre.

10 11 12

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