Here's a master class in hand sharpening drills.
Subject: Re: drill sharpening FAQ?
From: Robert Bastow <teenut@home.com>
Date: Mon, 11 Oct 1999 07:59:04 GMT
Intro snipped.....
The drill was ground, freehand, on the FACE of the wheel (not the flat
side)...care being taken to keep the POINT angle as equal as possible
on both sides..I'll tell you how to do THAT in a moment..
Lets do that now in fact..
Jim, You are dead right about not being able to grind a drill without
mechanical help! Well here's how you create your own "6 Million Dollar
Bionic Darex" ;^)
Let's assume we are going to sharpen a 3/8" diameter, 2MT shank
drill..it is about 8" long (these figures are arbitrary..I just want
every one to have the same mental picture of what I am describing. We
approach the wheel, which has been dressed on its face, dead straight
across with no grooves..(Ve SHOOT anyone ve catch putting grooves in ze
drill wheel!!..No Pity..No Prisoners..Ya! Verdampt!)
(Sorry)...
The drill shank is held firmly in the RIGHT hand...ALL the movement and
control is imparted by the RIGHT hand. For the purposes of drill
grinding, the left hand could be...with benefit..a LUMP OF CLAY!!
It is from this "lump of clay" that we fashion the Bionic Darex".
Place your left hand thumb and finger tips LIGHTLY together..Relax the
other three fingers aand let them naturally curl against the palm of
your hand. Let the drill flute drop into the vee between thumb and
fore finger and let the tip of the finger "Find" the curve of the flute
where it fits comfortably. The tip of the thumb rests on the sharp
junction ot the land and the flute, about an inch back from the drill
tip.
Now...SQUEEZE HARD!!! YOUCH!...I said it would be easier if it were
clay! 8^) Lift the drill from your fingers...see the GROOVE?...Drop
the drill back in..it locates within a thou or two! Magic?..Bionic at
least! Squeeze again to set the groove. You have created a customised
drill guide that fits better that that on any machine ever built! You
can relax your grip now..feel how smoothly the drill will ride back and
forth, guided by the groove you have created for it.
Place the knuckles of your left hand, LIGHTLY on the ginding wheel tool
rest, and swing the drill shank, from left to right (using ONLY your
right hand) and push the drill lengthways though that groove in your
fingers back or forth using the groove to make the drill twist or
"rifle" in your fingers. Do NOT move your left hand in any way..it is
made of clay remember!
UNTIL....
A) The drill axis is "eyeballed" to be at half the required point angle
to the wheel face...You can scribe or chalk reference lines on your
grinder benchtop to help you line this up..at least untill it become
almost second nature.
B) The drill axis is dropped JUUUst below horizontal. This will ensure
that your soon to be ground drill lip will start with a "smidgin" of
cutting clearance.
(Ideally, and certainly for a beginner, the grinder rest should be set
dead radially to the wheel center and about half the drill diameter
below the true center of the wheel)
C) The two cutting edges of the drill..the straight, sharp bits, formed
by the junction of the flute and the back face (the only bit you
grind), should be horizontally disposed..with the edge uppermost on the
side closest to your left hand..the othe sharp bit of course, pointing
downwards (Jeeze this would be a lot easier with a sketch pad)
This I will call the SET or START position!
NOW, move your left hand for the first, last, and ONLY time during th
is whole exercise. GENTLY ease the cutting edge towards the spinning
wheel, carefully maintaining all the angles and orientations of the SET
position..until the cutting edge is JUST shy of touching the wheel. If
you listen carefully you will hear the tone of the entrained air,
whistling through the narrowing gap. You will hear a subtle but
distinct change of tone JUST, I mean Just...a couple tenths of a thou
BEFORE the edge touches the wheel. STOP!!! FREEZE!! DO NOT MOVE!!
Now, press the knuckles of your lump of clay..sorry, your left hand
FIRMLY down onto, into and around the grinding rest..establish a
"Groove" on the back of your hand as well as between your fingers.
We are now ready to grind, Your left hand locked to the drill and
grinding rest is otherwise quite relaxed..letting the drill slide,
twist and tilt wherever your right hand and the groove in your fingers
tell it to go.
The actual grinding is a bit of an anticlimax.
You have previously studied a new drill point, you have read about
clearance, and cutting angles, and rakes and......
With the RIGHT hand in control, gently, kinda, lean forward... bending
or squeezing your arms hands and body..rather than actually moving
them..untill you take up that last couple of tenths and the wheel
begins to cut. Let it cut..don't force it, and dont' rush it..it
really won't hurt anything if you take a full minute Per pass per
face. YOU and your "Bionic Darex" are totally in control of that drill
and the wheel..Forget the times when, close to panic, you swung the
drill wildly past the wheel, hoping to get "the dirty deed" over with
as quickly as possible.
Take your time, enjoy the moment, THINK about the shape you are trying
to generate. Just the one face is left to "Interpretation"...every
other aspect,angle, facet, what have you...Has ALREADY BEEN TAKEN CARE
OF!! and is locked in place under your control!
The right hand should perfome a "Lower Quadrant sweep" for want of a
better term..An observer behind you would see your hand move from about
17 minutes past the hour on a clock face, to roughly 25 minutes past.
But it isn't a smooth arc of a circle, more a sector of an elipse..You
see, as your hand starts to drop slowly, you are also rotating the
drill in "the groove"..the first third of the turn needs to maintain
that very slight clearance angle on the cutting edge, and not increase
it too rapidly.
You need the clearance to cut..But too much at that point will WEAKEN
the edge, and cause the drill to snatch and chip...So the first part of
the rotation is ALMOST but not quite, just as though you were grinding
a straight cone point on the end of your drill. Only as you approach
the second third, does your right hand start to noticably drop..kinda
"Catching Up" on the rotary motion...increasing the clearance as it
does.
In the last third of the rotaion the right hand drops quite
rapidly..Thogh not enough to catch the OTHER drill lip on the
wheel..that lip is coming around quite rapidly by now.
Above all, take your time, if it helps, move the drill one degree at a
time, and think ahead what shape or angle the next degree of cutting
face needs...Remember, you have control, and IT ain't going nowhere
'til you decide.
After a pass on one face, flip the drill in your "Bionic Darex" DO NOT
MOVE THAT LEFT HAND!!, return to SET position and repeat, the pass on
the other face.
Having done a couple of passes on each face..it is now time to check
the results on our homemade "Optical Comparator"
(Sorry Jim I couldn't resist!!) ;^)
Rest the center hole in back end of the drill shank, on the center
point of the "Comparator" and use, first one and then the other drill
lip to scribe a light line on your whitewashed (OK Blue or red dyed)
surface.
You will readily see if the lines coincide..if the lips are even..or
not, as the case may be.
Lets assume they are..Now look directly DOWN on the end of the drill to
check the clearances. HUH? How can you check radial clearance by
looking it staight in the face? Surely you need to look at it
sideways?
Well no you don't...for once all thos interacting and confusing angle
and faces and clearances are going to work together in YOUR favor and
make what could be a tricky bit of metrology..quite simple. While we
are looking at the end of the drill, we will also check that the POINT
ANGLE is correct too!!!
(Ok guys, leave quietly..teenut has finally lost it!!)
No really, trust me. IF you look straight down on the point of a well
sharpened, standard drill, you will see the two cutting edges, joined
by the CHISEL edge which crosses over the web of the drill The angle
fromed by the chisel edge to each cutting edge, should be ABOUT 50
deg...anywhere between 40 and sixty is ok for a first attempt. (I can
hear the purists and theorists screaming and lighting up their flame
throwers) But believe me, get it in that ball park and your drill will
CUT. If the angle is too steep..you don't have enough
clearance...negative clearance will give you an angle event greater
than 90 deg. Too MUCH clerance and the angle will appear too shallow!
While looking at the end, check the point angle, How? Look down the
axis of the drill at the cutting edges. Are they straight? If so,
your point is pretty close to the right angle (As designed for that
drill, by its manufacturer when he set the helix angle and the cross
section of the flute) If the edges appear CONCAVE the point is too
flat and if they appear CONVEX, the point is too "Pointy"
If your drill passes all these tests, which take but a second or two to
perform, THEN IT WILL CUT..pretty close to size, without chattering,
chipping, overheating, wandering or seizing. I guarantee it!
Hey, thats a pretty good start for the first drill you ever ground!
All it takes now is a bit of practice for it to become second nature
and almost as easy with a little 'un or a big 'un!
Hey guys!
My apologies for "goin'on" but If it helps just one person to pluck up
the couragre and go hand sharpen his (or Her) first drill, by hand...
Then I hope you will bear with me.
It is late, I am tired and I am not even going to proof or spell check
this,
'night all
teenut
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