Subject: Drilling a series of accurate holes

From: [email-masked] (Jens)
Date: Sun, 28 Jan 1996 00:04:08 GMT

My apologies for this very long and verbose post that follows. If you
are pressed for time you might want to ignore it ... I had to explain
things though.

Hi,
I am re-hashing and old problem that I had before. 
I am trying to set up a jig to drill up to 75 holes at equal spacing
with an exact (within 0.01") match between multiple parts drilled with
the 75 holes. (hope that explains it).
I currently use a jig that involves drilling hole 1 based on
measurement, then the work piece moves up and a pin is inserted in
hole 1 in order to drill hole 2. The distance between the pin and the
spotting drill is the distance between all of the holes. My problem
with this setup is that each hole depends on the accuracy of the
previous hole. The actual distance between the holes is less important
than that all holes on all parts line up with each other. With the
setup I currently have (even though it's evolved quite a bit) the
individual errors that add up between the holes can really mess things
up. It would be ok if the errors were random since over that many
holes this would even out but the errors do not appear that random
with the end result that between part a and part b there could be a
good  1/2" difference in the location of the last hole.
I contemplated life (and a number of other important issues) and
decided that there is no way that I can drill accurate holes with the
current (or any other) setup that relies on the previous hole to
locate the current hole. I also don't want to use another jig to drill
maybe 10 longer spaced holes and then drill all the in-between holes
and thus reducing total drift.
So, it seems the way to go would be to have a multi-stop gauge that
locates every hole separately with the errors not adding up. The
obvious way seems to be a pre-drilled gauge (every hole measured
accurately) and a stop pin moved from each hole to the next as a hole
is drilled. This is all nice and dandy but when you have a long part,
the time that it takes to leave the drill press, walk to the end of
the gauge, move the pin and walk back to the drill press really adds
up. Somebody in this world must have thought up a nifty way of doing
this step drilling accurately without having to walk back and forth
(at a reasonable cost - ie cheap).
Maybe some of you geniuses out there might have a thought on this. I
am starting to have a problem seeing the forest cause of all the trees
...... I have just pondered this waayyyy too long now.
Thanks for your input and sorry for the looooong post.
Regards 
Jens

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From: [email-masked](Kevin Richardson )
Date: 28 Jan 1996 05:01:59 GMT

In <[email-masked]> [email-masked] (Jens)
>Thanks for your input and sorry for the looooong post.

You may want to make a couple of small checking gauges to assure hole
positioning.  This consists of nothing more than two dowel pins in a
piece of steel.  The first gauge would have one dowel your finished
hole diameter and a smaller dowel.  This way you can check your hole
positioning before you drill your second hole to size.  Second gauge
would have both dowels same size-(finished hole size).  You will
achieve accuracy within a couple of tenths and eliminate accumulated
error.

Regards,
Kevin

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From: [email-masked] (Jens)
Date: Sun, 28 Jan 1996 07:02:53 GMT

Kevin, thanks for the input but I am having trouble understanding what
you mean. The holes I need to drill, lets say 75 in stock 'a' and 75
in stock 'b' need to be accurate between 'a' and 'b' but spacing
between hole 1 and hole 2 on stock 'a' could be out without any real
consequence as long as spacing between hole 1 and 2 on stock 'b' is
out by the same distance. The major concern beyond that is time taken
to drill since this is a production situation. I do not have time to
measure each hole before I drill it. It needs to be a fluid motion
drill,move stock, drill etc.
How would the gauges work in that sort of situation. Also, how would
accumulated error be eliminated because you are still referring to a
previously drilled hole. So even if you are out by 0.005" per hole
(this being about the best possible accuracy from a drill press to my
understanding) there is a potential discrepancy of + - 0.375" or in
the worst case hole number 75 on stock 'a' could be off by 0.75" from
the 75th hole on stock 'b'.
Could you pleas elaborate.
Thanks
Jens

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From: Peter O'Leary <[email-masked]>
Date: 28 Jan 1996 22:05:27 GMT

>are pressed for time you might want to ignore it ... I had to explain

Jens, no need to apologize; after all, this stuff is detail oriented, so 
we need the details!

>the 75 holes. (hope that explains it).

I think I remember a previous post on this same subject a few months back 
(was it you?)
The challenge you have is much like the problem of advancing a strip of 
material through a progressive die, though you have the additional factor 
of trying to locate from a drilled hole.  The drilled holes vary in size 
and position enough for you to accumulate errors.

Why not try the following?  Make a jig with a top plate having three 
holes, accurately sized and equally spaced.  You would be well off to put 
hardened drill jig bushings in all three holes.  The first hole would be 
for the drill, and the second and third for pilots.  Make the pilot holes 
and the pilots somewhat (1/32") larger and put a shallow taper on the 
ends of the pilots.  This way the pilots will make up for slight 
differences in hole size.  The reason you should use two pilots is that 
they will tend to average out errors in the spacing between the holes.  
If you really want to get fancy, mount the pilot pins in a plate and 
actuate the plate from above with a toggle (de-sta-co style) clamp from 
above to engage the pilots.

Just a thought.
--
Pete O'Leary

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From: [email-masked] (Jens)
Date: Mon, 29 Jan 1996 01:19:01 GMT

I guess this question had to come up sooner or later. Although I have
never worked with a full sized pattern I envision several problems and
because of these I have avoided the whole issue. One is cost (at $5
per bushing things add up fast) especially since I have to deal with
multiple spacings which means I would need to make up a jig for each
set of drill patterns. My current setup allows me to easily change the
hole distance. The other big issue is space/usability. The material
being drilled is 1/2" or 3/4" square tube up to lets say 7 ft long. By
the time a master pattern is clamped onto the blank, things are
getting mighty awkward to move around on a 12*12 drill press table.
Yes, I could build supports but things could really start to become
involved. 
The other thing I forgot is that I drill different sizes of holes (but
not in the same part) which means that for one hole spacing I will
need two set of masters.
Am I imagining all these problems or is this a realistic way of
looking at the whole thing ?

Jens

[email-masked] wrote:

> why aren't you just using
>a full size master drilling template for all the parts? That way every
>part will have the same pattern of holes, IE the pattern in the master
>template.
>
>Gary
>
>

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From: [email-masked] (Jens)
Date: Mon, 29 Jan 1996 01:19:05 GMT

>I think I remember a previous post on this same subject a few months back 

Yes it was ... but unfortunately the result of the discussion that I
constructed is not meeting my needs.

>and position enough for you to accumulate errors.

right

>they will tend to average out errors in the spacing between the holes. 

What kind of accuracy would you figure I would see between hole 75 of
workpiece 'a' vs the 75th hole on workpiec 'b' ?

>Just a thought.
Thanks, any thoughts are welcomed ... you never know what little thing
switches/triggers on the 'aha' light in my brain. :)

Regards
Jens

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From: [email-masked]
Date: Sun, 28 Jan 96 17:51:03 PDT

> Could you pleas elaborate.

Apparently I don't understand the problem, but why aren't you just using
a full size master drilling template for all the parts? That way every
part will have the same pattern of holes, IE the pattern in the master
template.

Gary

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From: [email-masked](Dale A. Keltie )
Date: 29 Jan 1996 04:52:16 GMT

In <[email-masked]> [email-masked] (Jens)
>looking at the whole thing ?

Jens - 

I'm not altogether sure I've got this thing pictured right, and I've
made certain assumptions (a trait I've been told recently is one of a
multitude of flaws in my character) about your setup that may not be
entirely correct but. . .

Any error in your current fixture would, as you said, be accumulative
but should be relatively consistent from part to part.  The only
mechanism I can think of that would produce the part to part errors
you're getting is inconsistency in the manner in which the fixture is
applied.  If I have this thing pictured correctly, the center of the
reference hole in the fixture is fixed, with a dowel pin, at the center
of the previously drilled hole and the new hole is drilled through a
2nd. hole (and drill bushing, more about that later) in the fixture. 
With this arrangement, and assuming the fixture is held flat against
the stock, there seems to be only one degree of freedom remaining which
could produce the part to part error; rotation of the fixture about the
Z axis or, in this case, the dowel pin.  I would use a 2nd. dowel pin
in the fixture to pick up one edge of the stock. If the fixture is held
with the dowel pin firmly against the edge of the stock (and, again,
the fixture is held flat against the stock), I don't see other sources
of error that wouldn't be random and therefore non-accumulative.

With respect to having to drill different size holes using the same
fixture; one way to handle this, provided the required hole sizes
aren't too much different, is to use slip fit renewable type drill
bushings and simply install the appropriate size as needed.  Another
possibility is to use your fixture to spot drill only, and do the
actual through drilling without the fixture.  This may contribute a
small, but random, error.

Incidentally, all of the above assumes perpendicularity of the entire
arrangement including, in the case of tubing, the hole in the far side.
This can be a real problem with tubing, more so in the case of slender
drills; the far side hole can be almost anywhere the point of the drill
can reach.  The problem can be alleviated somewhat by using as short
and stiff a drill as possible (e.g., a screw machine type).

Hope this is of some help, and Regards, Dale

An aside: It would sure be convenient in this newsgroup to have a means
of
transferring small drawings sometimes.  Yet another nebulous thought
lurking
in this alleged mind.