how to get a screw into a drillhole behind the material

I should have emailed this to the TV show "Ask this old House" for tips.
The problem: the problem is that I have some old cement shingles, some
of which are cracked and where I place a aluminum sheet metal over the cracked shingle. So I pull out the 2 old nails and I cut a piece of sheet metal to fit but the problem is that I need to get the 2 new screws centered over the old drillholes.
The best I have come up with is to get painter's tape which is semi-sticky and cut two pieces that are small and cover the screwhole with the sticky part facing outward so that when I place the sheet metal the tape will stick to the sheet metal centered on where the new drill hole must be made.
I need a method that precisely lines up the old drill holes with the new drill holes to be made.
Anyone have a better method?
Archimedes Plutonium www.iw.net/~a_plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Sometime ago, I had a similar problem. What I did was get a very large pieces of brown wrapping paper, tape it very firmly above the area in question so that I can lift it out of the way without repositioning it. Then I use a large sewing needle to locate the 3 holes. Satisfied that I had the correct holes, I marked the paper with a flow pen and lifted the paper out of the way. I positioned the replacement block where it belonged and dropped the wrapping paper over it to show me where the 3 holes were and marked the block. It worked for me!
Jim Y

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jim Y wrote: <snip drivel>
take this stuff over to alt.building.construction. thanks.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

This one is not that hard.
Just measure the distances, taking care to do it accurately.
It is easy to measure to the 32cnd of an inch, and that is plenty close.
You have to have a place to start measuring from. Your shingles probably have uneven edges. So take a fine marker and create very small dots or x's on surrounding shingles. Measure from there to the screwholes. Put your sheet metal down and measure again.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Sat, 30 Oct 2004 14:47:20 GMT Jim Y wrote:

Thank you very much. That is just what I needed.
In the past whenever I ran into this problem I would "triangulate the hole center". I would put the edge of the covering material where the hole was and run a line upwards. Then I would measure the distance from the bottom of the underlying piece to the hole center. It gave accurate enough hole centers but this method is lousy if the materials are cumbersome and when multiholes are needed.
So I was thinking of someway of inventing what I call a reverse screw where I have the holes and screw into them some special screw that has a very pointed tip or a marking point and then lay the covering material and these special screws then mark the center of the new holes to be drilled into the new covering material. Trouble here also is that some materials are awkward for such a method.
So I am really looking for a method that is almost universal in application and is easy and not time consuming. Jim may have spotted the universal method in the idea of making a template and once the template is at hand to imitate the old-surface as much as needed and then put that template onto the new-surface to get the exact hole centers.
Jim's method is superior to mine because it is more universal.
Hole triangulation is okay for non-precise needed jobs. But where precision is needed and where multi-holes and awkward materials and irregular shapes of covering make it difficult then the universal means is to make a template.
I was thinking of an invention of something I call a "behind the wall pencil" where you screw in this special screw that leaves a mark or hole into the new covering. But I see that such is not as universal as Jim's idea of a template.
So thanks, it is a template that will solve this problem.
Archimedes Plutonium www.iw.net/~a_plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Sat, 30 Oct 2004 14:47:20 GMT Jim Y wrote:

Yes, thanks to the above poster, I am able to think up a universal template for all applications where eyes are needed in back of a material. So we need a template. But instead of a wrapping paper and needle to find the hole, let us make the entire template itself as see-through. So we need some on the dimensions of the new material itself.
So the best that I can think of is sheet of clear see through plastic of the same thickness as the new covering material. My sheet metal is pretty thin but I can get some plastic that is almost as thin as the sheet metal. So what I do then is with the plastic see through place it over the object and with a marker mark the hole and I do not need a needle since I can simply see the holes.
Now comes the tricky part. Do I drill a hole into the plastic to transfer it to my sheet metal covering or can I transfer those markings onto the new covering and leave my pastic template undamaged without holes?
Anyone have further advice?
Archimedes Plutonium www.iw.net/~a_plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Archimedes Plutonium wrote: (snipped)

Thinking about this problem some more I suppose a mix of methods would work well. Using both the plastic see-through as a template and the triangulation of leaving two marks 90 degrees apart where the 2 lines meet as the center of the hole. I suppose one can use just a piece of stiff paper to mark where the center of the hole and the bottom edge and then the center of the hole and the horizontal edge and triangulate where the new hole to drill.
Also thought of a device shaped like a long U. Flexible enough to move the two tops together. I am going to attempt a ascii drawing and hope it does not get mangled in the posting.
p1 -- --p2 | | | | | | | | | | \ / ------
where I labeled p1 as the point inserted into old hole and the new material slips into the U shaped curve and then you simple press the other point of p2 against the new material and mark where the new hole is to be drilled and where p1 and p2 have adjustable sockets so that they fit most any size of holes to get a rather accurate placement of the exact center.
And whilst dreaming up this new gadget to place new drill holes that coincide with old drill holes I realized that this device is akin to a device that is used all the time-- calipers to measure exact small distances.
So maybe I need nothing more than various size calipers where I put one arm at the exact center of the old hole and the other arm at a well defined edge and then transfer that data to the new material and then do a horizontal as well as a vertical caliper measure which should triangulate the new drill hole of the new material.
Also thought of a sort of plastic swiss cheese template with holes almost everywhere and with a magic marker mark the hole or holes of the old material and then make marks from the old material of where the template is supposed to match the new material then simple make a pencil mark in the holes designated.
Archimedes Plutonium www.iw.net/~a_plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.