This is perhaps a very dumb question for this group.
I am constructing an electronic appliance on a 10" x 6" diecast
The problem is that the holes are never on center to where I start
them. I use a prick punch to get a nice accurate counter sink mark. I
then use a 1/8" bit to drill the pilot. Here's where the problem
sometimes begins. The pilot holes are off, sometimes by 1/32". Then, I
enlarge the holes to the desired size using a step bit(unibit).
Sometimes, what starts out being a well centered pilot hole, ends up
being a larger hole off center by 1/16" or more. I can understand how
a pilot hole can be off because of the drill bit flexing, but not the
Any idea what causes this? Sorry if that sounds stupid. What can I do
to drill nice centered holes?
Thanks so much
Are you using a handheld drill? Sounds like it. The first thing is
to use a milling machine if possible, or a mill-drill if not, or at
least a good quality drill press. Second, you need to clamp the box
rigidly (I'd use a vise but you can clamp it to the table) and locate
your hole precisely with respect to the axis of the spindle. Then
start your hole with a stiff tool (many guys use a "center drill"
but you can also use a "screw machine drill" or if you can pick up
a centerpunch and if you can centerpunch accurately, you can start
it with a small drill) and drill just a little bit. Then, while the
part is still clamped, put in your first drill bit and drill it
through, then move to your unibit. - GWE
Pretty much what Grant said. If you really want good results and aren't
familiar with, nor have access to, a drop spindle mill or a mill drill, you
can achieve excellent results with a drill press by laying out your hole
locations, then picking up the cross lines with a wiggler held in the chuck.
Once you've established proper location, it's important to clamp the item
being drilled so it remains on location until you have drilled your hole.
Hoping to do that by hand holding is just introducing more error.
You'll get all kinds of advice regards the use of center punches, but
personally, the only place I recommend their use is when drilling by hand,
or drilling in concrete. A lot of error is introduced to hole location
when you strike a center punch, error that is easily avoided by not using
the punch. By using a center drill, or a spotting drill, you don't need
the punch. To be quite frank, I spent considerable time working in
commercial machine shops and never saw them used for drilling holes. In
fairness, I should state that my experience is aero-space related, where the
amount of acceptable error is generally quite low. I realize that there
are industries where center punching holes is likely the accepted way to
To contrast with Harold and Grant, I have never owned a mill and do fine
work. For my lathe project, I had to mount a 1/2 x 4 x 36" slab of steel to
an aluminum beam:
I did was clamp the plate to the bed positioned as I want it, mark the
outline (outside, and in the holes) with a Sharpie, remove the plate, mark
where I want the holes (in the thickest part between the sides and the
webs), centerpunched them and drilled 13/64" on the drill press. Then I
clamped it back on in the same place (using the outline to line it all up)
and centerpunched the hole locations on the bed casting. Then I drilled
these 13/64", deep enough to be threaded for the screws. Then I enlarged
the holes in the steel plate to 1/4", which proved to be a little tight for
the error in the holes, so I went to 9/32" and finished up by countersinking
the screws as they are flat head type and have to be below the surface.
Now having read Grant and Harold's replies, you might think the words
"centerpunch" and "error" kind of follow each other naturally. BS. I only
use a centerpunch because the dimple centers the drill nicely. Better than
having it skate around an uneven surface and end up a smidge too far to off
to be useful. No, I don't have a center drill either, and I don't know if
my drills are split point (I think they are though).
The error in the location of the holes is because I didn't always get the
angle right when I centerpunched through the holes in the plate. To reduce
error I kept those holes small (13/64, same size as the 1/4-20 tap hole, in
case I have to do it the hard way and use the plate as a drill guide for the
bed holes. By drilling them seperately I saved some effort), just large
enough for the centerpunch to pass. I could've just as well used the
sharpie and marked the holes' location on the bed that way, but decided
against it as it's a larger dot. I'd end up punching it anyway because it's
soooo much more convienient to have the self-centering dimple.
Really big ANYWAY, finally for something to apply to your situation. If the
centerpunched mark is on center and your drill is square and centered, it
should all come out fine. If the mark is centered, I'd suspect your
technique - are you not drilling evenly or squarely? Too light or too heavy
pressure? Dull drills? Can you clamp the part in place and use its holes
for location while drilling? Or add one screw before drilling the rest?
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
What Grant said, but if you have to use a hand drill, be aware that side
force on the drill motor will walk a drill sideways especially in a material
like a zinc diecast box wall.
The side force can be associated with pressure on the pistol grip, which
will cause the drill to shoot high, or side pressure from holding the drill
motor with the hand that's not pulling the switch, causing the drill to walk
left or right.
A good method is where a minimum amount of downward (feed) force is applied
to the pistol grip (because the grip isn't inline with the drill), and if
the second hand is used for downward pressure, it's placed at the rear of
the drill motor (the heel of the hand pressing directly down the centerline
of the drill.
If you need to use a hand drill in a position where you're pulling down on
the drill (a situation where you aren't behind the drill motor), grip the
drill motor securely near the chuck for the hand that's providing the feed
pressure (instead of trying to apply the pressure from the pistol grip).
If you're using a drill press or hand drill, a stubby-short pilot drill
chucked to the flutes is about as reliable as it it gets. There are a lot of
brands, but I've been using stubby 1/8" Hanson #30108 Screw Machine Length
135 degree split-point drills that are commonly used for drilling sheetmetal
for pop-rivets. The flutes are about 3/4", and the entire length is 1-3/4".
These drills have provided reliable performance for a couple of decades.
These are so short that it's unlikely that they'll flex when the applied
pressure is in line with the drill. They will drill off the mark if the
drill isn't held perpendicular to the workpiece.
Some practice holes can be tried in a similar material. The layout work
should include crossed layout lines so you can observe what's taking place.
For thin work material it's generally best to have some backup material,
wood or other scraps.
A prick punch is good for layout, but a more blunt center punch leaves a
better mark for starting a drill. The wider upset is better at locating and
holding the drill point.
Your pilot drill is too big and it doesn't sound like you are using any
kind of cutting fluid.
Try using a 3/32" drill to pilot and then your unitbit.
Cutting fluid keeps both cutting edges clean so the drill cuts the same
on both sides.
I am assuming you are using a hand held drill motor, but this will
work with a drill press too. When you lay out the holes scribe lines
that are about 1/2 inch long. Then use your prick punch and follow
that with a centerpunch to get a dimple deep enough to keep the 1/8
inch drill from wandering. Now before you use that 1/8 drill check
that the center purch dimple is really dead on the center of the
scribed marks. If it isn't slant your centerpunch and move the dimple
over so it is exactly centered.
Now if you are using a hand held drill motor, you really need a guide
to make sure the drill in perpendicular. This has been idscussed
before here. Try searching using google groups and searching just in
RCM for drilling and mirrors. If I am having to use a hand held drill
motor, I will drill a hole thru a block of 2 by 4 using a drill press
and use that as a guide to make sure the hole is at 90 degrees to the
Okay now drill your pilot hole about a 1/8 th to 1/4 deep. Stop and
see if it is exactly centered. If not use something to cut some metal
away on the side that is closest to the center. Now drill with a
drill just big enough that your rat tailed file will go thru the hole.
Still not centered? Use that file to move the hole over toward the
center. Now drill with a bigger drill and repeat until the hole is
full sized and perfectly centered.
Don't quite understand what I am saying. Go to your library and look
for some books on metalworking with hand tools. I think you will find
the same description with pictures.
I do a *lot* of chassis work. Here's how I do it - and if you
follow this plan you can spot holes to within a few thousanths.
First off if you really care about where the holes are going to
go, put some blue on the material. You can get blue dykem but
I just use a blue (or any other color for that matter) sharpie
pen. Then lay the holes out with a scriber so there are lines
to guide your eyes. Center punch the locations at the intersecions
of the lines.
Now stop here and *look* at the marks. If I'm doing very accurate
work I will use a microscope - but for most cases a simple
loupe or magnifier will work. If you look close the punch
mark will *not* be at the center of the intersecting lines.
Some might be, but most won't. Take the centerpunch and drift
the mark so it's as close as you can get to the intersection
of the lines.
Then go ahead and drill the pilot hole - but *not* using a
1/8 inch diameter drill. That's my *finsh* size for some
holes! Depending on what you're drilling, the tap drill
size for a 2-56 screw (I think this is a number 50 drill)
is a good pilot hole for most electronic chassis work.
A drill press works well for this but you can use a hand held
drill if you are carefull. If you happen to snap off a small
drill in thin sheet metal it's not much of a big deal, you
typically pick out the remains by hand.
Now go back and look *again* at the pilot hole in relation
to the intersecting marks. If it's not right on center,
use a slightly larger drill and drift the hole to get it
I never did like unibits for chassis work. I much prefer
to grind my own sheet metal drills with a center, that
pick up the pilot hole.
Examples of sheet metal drills:
Typically you won't do much better than +/- five thou by eye for
hand-drilled holes of that sort. But if you have a microscope
or a good loupe you can get down below that. The smaller the
hole you are trying to get, the smaller the pilot hole you need.
I just got done mounting a bunch of micro connectors at work,
which involved laying out hole patterns on circuit board material
with a dial caliper used as a scratch gage. The pilot holes
I was using for those were 0.020 diameter - and I started them
under a microscope, by hand with the drill in a pin vise. Once
the hole even *starts* on center, the drill will pick it up
The key in all this is to do the QC step at each point along
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
I rest my case. When you understand how to properly drill holes without
the use of center punches, you can drill 1/4" holes in two different plates
and use 1/4" bolts to assemble the items without enlarging the holes to
compensate for error, in part due to the center punch itself. It's not BS,
it's good workmanship. Only you can say what level of quality you prefer.
Sounds to me like this gentleman prefers a higher level.
O.K. That is a start, at least. I have some suggestions:
1) If you *do* use a center punch, you want to support the
metal right behind the area to be punched. A chunk of brass rod
held upright in the vise would serve for this. Otherwise, there
is a good chance that the metal will flex, and the point of the
center punch will bounce and make a second dent off-center.
This is even more likely if you are using an automatic center
punch (the spring-loaded style).
2) The ideal way to use a center punch is to start with scribed
lines (using layout die to make them easy to see), and a "prick"
punch -- a center punch ground to a much sharper angle, to start
marking the hole. Then shift to a standard center punch, and
make a good dent there. There are lots of sizes of center
punches, and you want one which will leave a large enough dent
to properly guide the drill bit.
3) A center drill is the next thing to use. It is short and stiff,
so it is unlikely to walk while you drill deep enough to accept
the full width of the "chisel-point" on a standard drill bit --
or better, use a drill bit with a split point which has no
chisel point. I would suggest that this bit should match the
smallest step of the Unibit to give proper guidance, while not
depending on the Unibit's own drill point, which is somewhat
4) The use of a vise bolted to the drill press table will help keep
things from shifting as you drill -- but the drill press itself,
if a typical import, will have a lot of slop in the quill, so it
can move off center even with the workpiece properly clamped on
center -- thus the need for a center drill hole to guide the
drill bit, and/or a split point which does not walk when
starting a hole the way a chisel point will.
In case you are wondering what I mean by a "chisel point", look
closely at the point of one of your larger drill bits (so it is easier
to see). You will see that you wind up with two flats (perhaps slightly
curved, depending on the sharpening pattern) which meet at the center of
the drill bit, providing a straight line across the center. This is the
A split-point drill bit has two or three planes of sharpening on
each flute, and they join in a sharp point at the true center, not a
chisel point, so they start much better -- either in a center punch
mark. or even without one.
My drill press is a medium sized bench top unit. The spindle travel is
maybe 2.5" - 3". That presents a problem. Once I have the wiggler
chucked and centered on my work piece, how do I remove it and chuck a
bit without moving the table? Even large free standing presses have
maybe a 4" spindle travel. That seems to present a problem of removing
and replacing bits without moving the table and losing your position.
Am I missing something here?
I almost wondered if the post was tongue in cheek. WRT the use of punches,
what do you think of using the type that are made to transfer hole locations
from particular diameter holes? I'm at a loss for the proper terminology as
usual, but they fit rather snugly in a matched diameter hole. If the
template is thick enough they stay pretty close to perpendicular.
I recently snagged a layout jig table on ebay but the damn thing must weigh
500 pounds so it's been sitting on my trailer a little too long. It has a
1/4" drill motor that can be located precisely over a plate for making
templates. I'm hoping it will facilitate some more efficient, accurate
layout once I get the hang of it.
Those are transfer punches.
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
No, you're not missing anything. Having limited stroke on a drill press is
a definite negative. I solve the problem by using a mill with a knee.
Large gang drills also have considerably more stroke, but aren't often found
in home shops.
Depending on the size of the drills you intend to use, you may be able to
move back and forth between the wiggler and the drills without moving the
table. One of the tricks I've used is juggling the length by which I chuck
the wiggler and drills, which often will allow a greater range without the
need to move the table. If not, there are other ways you can do
approximately the same thing. If you're using relatively short drills and
the length of the wiggler gets in the way, any small pointed object will
serve. It may not run true at the point, but even when it inscribes a small
circle, you can usually center it well over the cross lines you have
scribed. Touching the piece after locating it often will show any deviation
from the desired location, so the off center circle it describes lets you
know that you are not well centered. The key word here is small. If the
point runs out drastically, it will serve no useful purpose. That's the
beauty of wigglers, which can be set to compensate for any runout.
I understand the problem well, and it is for that very reason that I have so
much contempt for mill drills, which yield, essentially, the same problem.
A little creativity on your part may have you using this system, which is a
very good way to locate your holes aside from using dials (or a DRO) on a
mill. The degree of precision can be staggering if you can layout your
lines with a height gage instead of a scriber and scale. You can work
easily to +/- .003" if you have good vision.
Nope! Serious as a heart attack!
As Jim says, they are transfer punches, and while they work quite well, my
shop does not have a set, although there has been a time or two that I sure
as hell could have used them when I was actively machining for gain. Die
makers used to use them extensively. When applied properly, they are quite
accurate. Hand held, hand sharpened punches, on the other hand, to me
are worthless if you're trying for perfection, a curse I happen to possess.
With the advent of CNC machining, tool rooms that have the task of matching
holes are now likely doing it all by CNC. Perfect fits every time.
That sounds like a device that was probably used for making templates for
use on something like a Weideman turret punch press, where you go to
location on your part and punch given configuration holes. They used to be
used extensively in precision sheet metal shops. Depending on how it
locates, it may, indeed, facilitate your layouts. If you have a link that
shows the device, or a picture, I'd like to see it. Very interesting.