On my old 40.5 they used a piano hinge. But it was stainless steel, and a little ugly. The thing about mounting piano hinges, is you need to get a lot of things right, and there isnt a lot of room for error. And because there are so many screws, it is hard to do freehand.
Most of the time piano hinge installation is fixtured, or at least well planned into the design.
There is an additional difficulty with installing the hinge on the chart table, insofar that both leaves of the hinge are on hidden faces, and they also butt face to face when installed. ( think of the hinges on the front door of your house)
Sometimes piano hinges can be used, like on a cupboard door, where the hinge will only close to 90 degrees.
So, because the hinge is closing face to face, no fasteners can protrude above the hinge leaf face. If it did, the hinge just wont close.
This means we need to do 2 things:
Firstly, the hinge has to be mounted with flat head wood screws.

They are very tricky to use in a situation where the hinge has to close up fully.
The screw heads have to be staggered, because if you dont, the opposite screw heads will interfere with each other (even though they are recessed)
Now, unlike the door of your house, these hinges will be installed blind
Heres what I mean by that.
When you open your front door, you can see the hinge screws.
Likewise, when the carpenter installed it, he was able to drive the screws into both the door jamb and door easily, cause he could see both surfaces.
Not so with your chart table.
When you are going to be installing the hinge, you wont be able to see either surface. This makes things tricky.
The last thing that makes this a show-stopper for most people is that there is a lot of alignment going on to get this perfect.
There is side to side, up-down, and fore aft. Mess up any one, and the whole thing will bind up.

I caution that it is a bit tricky to do this job and have it end up looking perfect.

Here is the method that worked for me:

On the hinge, scribe a line which is half the thickness of the leaf (once you cut off the beveled part) Thus starts just below the major diameter of the pivot point of the hinge pin. So, on a plastic hinge you will have a metal rid going through a circular support area. The outside of this plastic circular area is the swing point.





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So, this scribed line is where the screws are going to be mounted.
Mark a point approximately 1 from one end , and mark off hole locations at 1 intervals.
You have 2 leaves on the hinge. You need to do the same on the other leaf, but the holes must be staggered by half the pitch. This is so that the screw heads dont interfere with each other.
By the best tool to use for all of this is a digital vernier. You can buy a cheap one for $20
So, you now have 2 sets of holes marked on the hinges. One set should be at approximately half the table thickness once installed .
Note that this plastic hinge stock is only available in 12 lengths, so you will need about 3 lengths overall.

On a drill press, drill and countersink for #6 flat head screws.




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Take your time with this, double checking that the screw holes are going to be on the center of the hinge lid thickness.


Put some thin double sided carpet tape on the edge of the hinge lid.
Lay out the hinges on the end of the hinge lid, sticking them to the tape. Make sure it all looks good, and trim the last hinge to fit.
with a dremel, drill 1/16 pilot holes into the lid at the screw locations.

Remove the hinge from the sticky tape, and remove most of the sticky tape, just leaving enough to hold the hinge very lightly back in place again.
By this, I mean just have a few 1 lengths of carpet tape on the table lid. You want the hinge to easily separate from the tape.

Carefully stick the hinge back on, ensuring that the holes line up.

Now, empty out your chart table. Get a whole bunch of old cds .
Stack up in 4 locations inside the chart table cds until you have a calibrated surface to put tge lid down on, which exactly matches where it needs to be. (you can use wood for riser blocks as well, and use the cds to get the meeasurement. Dead on.

Now, put a strip of double sided sticky tape on the forward edge of the table where the other hinge leaf will mount. Slowly and carefully slide the lid forward, until you stick the other leaf to the tape.

Now, very carefully with a knife, separate the hinge from the lid where it is lightly stuck on, keeping it firmly stuck to the forward tape.

Remove the lid, and use the dremel to drill the pilot holes for the other leaf.

Now, here is a neat way to easily drill all the holes in the hinge without a ton of measuring, etc.
Make up this little block out of plexi.




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Once you drill one hole in the hinge, you can use this block to locate the next hole.
Youll need a spare 1/8 drill bit.
You drill the first hole with 1/8, then use the spare 1/8 bit to locate the index jig.
Then drill through the second hole.
Then leapfrog the block to the next location
Hope that explains it.

To mount the hinge, mount it on the lid first. Then with one or 2 people holding it up in place, put on the opposite face screws.

Black plastic hinges
I bought mine in Toronto from a large industrial place called Piedmont Plastics.
These hinges need to be modified in 3 ways
The leaves are too wide. You will need to trim them to width on either a band saw (preferred) or table saw. Use a fence, and take your time
Holes for #6 stainless flat head screws need to be drilled and countersunk
The hinges come in 6 lengths. One of them will need to be trimmed shorter, depending on your chart table width



Here is the hinge. It has an overall height of 1.75. The small angled edges need to be trimmed off.





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This is the final dimension you need on either side. Make sure you cut these really Straight.





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Measure 1/4 from the edge of the hinge on each side. This will be the centre line for the holes you are drilling





Basically this is what you are going to do. Drill holes on a staggered pattern.




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There is a very accurate and easy way to do this.


This is a closer view of what the stagger looks like.




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Now: 2 things....
The holes we are going to drill initially are going to be pilots only. 1/8 is a good size for this

So, at one end of the hinge, drill the 1/8 pilot very carefully on both leaves.

Now, get a small scrap of plexi, and make the following jig;




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This is an indexing jig. It will allow you to drill this hinge in 10 minutes perfectly with your eyes closed.
You need a spare 1/8 drill bit.
Once you have made the jig, open one of the end holes just a little bit, by wiggling the drill bit around while you have the drill turned on.

Ok. So take the spare 1/8 drill bit, and insert it into the slightly bigger hole.
Now, line up the other hole with the centre line on the hinge leaf. Drill the next hole. Repeat by walking the jig down the hinge.

The screws you are going to use for the hinge are #6 flat head Robertson. They have to be Roberston. Reason is you are working blind when you install them, and you can load Robertson screws onto a screwdriver, and they stay put




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Of course you will need a Roberston screw driver. Do NOT get a multi head screwdriver where you just load in the tips. You need a proper screwdriver with long thin shank. The multi drivers are too fat to fit into the chart table crevice.

Once you have the hinges all made, remove the chart table lid from the table.
Remove the existing Holywood hinges.

So, on the plastic hinge you will have a metal rid going through a circular support area. The outside of this circular area is the swing point. When the hinge mounts on the lid, it needs to have the circular part of the hinge just proud of the lid top.



Here is how to line this all up
You will need some double sided carpet tape. Not the stuff with foam in it... The really thin stuff with adhesive on each side.

Put a strip of the tape on the full width of the front face of the chart table lid. (Where the hinge will mount.)
Remove the piece of release paper from the sticky tape.
Now, very, very carefully stick the hinges to the tape, in the exact position they are going to be installed. Take your time with this.
The small pilot holes on the hinge should be at approximately half the thickness of the table top.

Make sure it all looks good, and trim the last hinge to fit if necessary.

With a dremel, transfer the 1/8 pilot holes into the lid at the screw locations.
Only drill the hole about 1/4 deep.

Remove the hinge from the sticky tape. and remove all of the sticky tape from the lid. Now use a 7/64 drill all the holes to about 3/4 depth.
Now, cut 1/2 lengths of the sticky tape. On the hinge lid, mount 1 piece of the tape just at the end of where the hinges were. So, as an example, if you only had 2 hinges on the lid, youd mount four 1/2 pieces of stick tape.

Next,,, time to do some drilling.
You need a drill bit 9/64. This is clearance for #6 screw.
On the hinge leaf which is mounting on the chart table lid, drill all the holes 9/64. Then (with a good quality countersink), counter-sink the holes so that the screw face is just flush. I highly recommend a drill press for doing this.
Once you have the hinges drilled and counter-sunk, carefully remove the release paper from the small strips of double sided tape.

Then, screw the hinges on to this face, only on their 2 end screws.
They will now be held on by the sticky tape, and the screws. Press on the hinge to make sure the tape is sticking well.
When you stick this hinge back on, ensure that all the holes line up.

Now remove the screws

Now, empty out your chart table completely. Get a whole bunch of old paperback books, and a stack of cds .

Stack up in 4 locations inside the chart the books until they are just lower than the height of the Bottom of the table lid. Now, use individual cds to shim the height up, until you have a calibrated surface to put the lid down on,

You want the table top to be exactly level, and at the precise height to meet the forward edge.

Now, put a full length strip of double sided sticky tape on the forward edge of the table where the other hinge leaf will mount. Slowly and carefully move the lid forward, until you stick the other leaf to the tape.
Now, very carefully with a knife, separate the hinge from the lid where it is lightly stuck on, keeping it firmly stuck to the forward tape.

Remove the lid, and use the dremel to drill the 1/8pilot holes for the other leaf about 1/4 deep. Then drill them 7/64 about 3/4 deep.

Now, pull the hinge off the sticky tape. Drill and countersink all the holes on this hinge leaf for the #6 screws.

To mount the hinge, mount it on the lid first. Then with one or 2 people holding it up in place, put on the opposite face screws. You will need to load the Robertson screws onto the screwdriver , and then screw them in

It should be perfect. Good luck!!

TA Da!!!