Shown here is a monster screen gantry, with a 22" screen mounted to a gantry with a 1.5m (5 foot) maximum reach.

The gantry is self balancing so you just shift the screen to position and leave it there, no mean feat with a 5kg screen!

The screen is mounted in an aluminum surround for the modern, minimalist look.

What you need:
Screen, any LCD should do the trick, most have mounting hardware integrated on the back now which would save a bit of time
Screen cables, standard cables won't have the reach, so get some extensions
Various bits of wood/metal (how I have made it is by no means the only way) I used 12mm MDF sheet, 12mm aluminium plate, and 38mm aluminium pipe
Hardware (bolts, nuts, wingnuts) as required
Steel cable, this is very important, other things won't cut it, and will just stretch or break
Paint

Skills you need:
Beginner design skills, this instructable shows you how to calculate the balance lengths, the rest is just getting them to fit together
Medium engineering skills, drilling, tapping, turning, and sheetmetal work were all used in this build

Tools required
Sockets/spanners etc
Drill and drill bits
Taps
Tin snips
Woodworking equipment; I used a cnc router but a jigsaw or tablesaw will get you there just as well, but with a little less design flair
Lathe; optional but helpful for making up pulleys
CNC mill; again optional, but makes the mounts for the screen, and the main pole a lot easier.
Paintbrush

It is possible to make up for a lack of skills or tools by adjusting the design, just take care to think it all through, as the most expensive bit is the most at risk too!

Warnings:
Screens are heavy (mine would happily remove toes if it fell), so test your gantry before putting anything valuable under it
Some screens will require mains voltage to run out along the gantry, take special care to ensure the cable cannot be pinched or damaged by the moving parts

Step 1: Screen Preparation

Screens do not come with a mounting point at the top, but this project needs one! It must be bolted through running horizontally, and parallel to the screen. It also must provide enough friction to allow tilting of the screen.

Most screens will have a series of holes in the back you can bolt into. Attach a plate to these, and screw some plates which locate the screen pivot in the correct position. Although I made a complete surround for my LCD panel, I still screwed plate to the back to get the requisite pivot point. You can see this in the bottom of the second picture; the part shown in the top half will come later...

Step 2: General Dimension Calculations

For this step you must first have your screen weight and height, I shall go through the calculations using mine as an example, 5kg and 300mm.

Decide on your maximum span, in my case I wanted 1.5m.

Because the screen sits basically level with the lowest pivot point of the main gantry, the secondary gantry must be 300mm shorter to allow the screen to sit flat.

Secondary gantry length = (Span - Screen height) / 2 = (1500 - 300) / 2 = 600
Main gantry length = Span - Secondary gantry length = 1500 - 600 = 900

The most critical set of dimensions relate to the wire balancing setup, this ensures the screen balances when extended, rather than collapsing. I've made the generalization that the forces acting on the gantry are directly upwards, and solved it for the fully extended situation. However, when the gantry is mostly folded up these forces will be a lot different due to the wire angles; but in this situation the joint friction should overcome those effects.

Wire distance Main is how far along the main gantry the wire pulley is located. This distance governs the leverage ratio, and therefore wire tension of the gantry. I chose a 5x leverage ratio to give a 25kg wire tension, this also means for a 200mm move of the wire through the pole, gives 1m height change of the screen.

Wire distance main = Span / Leverage ratio = 1500 / 5 = 300mm

Wire distance secondary is similarly calculated, but uses the secondary gantry length instead of the span.

Wire distance secondary = Secondary gantry length / leverage ratio = 600 / 5 = 120mm

This design works best if the wire lift forces are as close to vertical as practically possible, so the height of the pole, and height of the center vertical bars should be a large as possible. Now all that is required is the mechanical design to fit these dimensions and optimizations.

The wire routing runs through three pulleys, to its fixed mount on the secondary gantry. The second picture shows the routing (yellow) and location of pulleys (blue circle) and fixed point (blue cross). The pulley system allows the gantry to extend or retract at the same height without significantly changing wire lengths or tension.

Step 3: Mechanical Design

This is where your creativity can take off, I'll give you a quick run through of my design to outline the principals and what is required from the components, but feel free to use alternate materials and designs to your advantage!

The support pole I used is 40mm 4mm wall aluminum tube, with three main pivots bolted where required. The pole also has a height setting ring, and Shelf plate, which allow it to spin on my desk.

The screen is kept level by the parallel bar linkage and can spin due to a bolt going vertically through the final vertical plate. The screen can tilt due to a high friction mount on the top of the screen.

The gantry plates and link bars are all made with two sides, and set a distance apart to ensure the gantry will not twist or fold. Various spacers were machined to ensure the plates were far enough apart.

The parts list:
Support pole
Shelf plate
height setting ring
3x main pivot plates
2x main gantry plate
2x secondary gantry plate
2x long link bars
2x short link bars
2x center vertical bars
1x end vertical plate
1x end spin hook

Not shown in the drawing are the pulleys, spacers, and bolts.

more detail into how each part works and goes together will be given later in this instructable

Step 4: Create Components

I'm going to kind of gloss over this step a bit, as there was a lot of work, but it was all pretty simple. Also I have access to some cool CNC gear, so a how to for that wouldn't be used by most anyway.

For the gantry plates and link bars I machined them out of 12mm MDF on a CNC router, then painted them (this took much longer than machining them!). The gantry plates had 1.6mm aluminium sheet added to the back and top of them after initial testing showed too much flex; hereafter gantry plates refer to both the MDF, and aluminium.

The aluminium parts were mostly machined on a CNC mill, with a few 'by hand' operations for drilling/tapping etc.

The spacers were just turned up by parting off an appropriately sized bit of tube at the right lengths.

The pulleys were turned out of aluminium round bar, the pitch diameter is 20mm.

Here's a video of the CNC router in action anyway, still a few bugs to work out, but good enough for gantry plates!

Step 5: Mount Support Pole

The support pole must be mounted securely, to take the torque of the screen at full extension. The mount must also allow the pole to rotate.

With a desk shelf these constraints were very easy to satisfy, I simply used a hole saw to cut through the bottom, and attached a plate to the shelf. The plate was only required because I could not access both levels with the hole saw.

On top of the shelf plate sits the height setting ring; two cap screws secure this ring to the pole to ensure it does not slip through the desk. Directly on top of the height setting ring sits the first of the three main pivot plates.

Once these parts are assembled, give it all a firm push/pull just to make sure it is strong enough, and be sure to twist the pole to make sure it is not binding somewhere. Both of these things are a lot easier to fix now than later!

Step 6: Screen Mount

The screen mount consists of two components (three if you count the screen); the end vertical plate, and the end spin hook.

The end vertical plate mounts to the short link bars and secondary gantry plates which keeps it vertical at all times, the bolts hold aluminum bosses firmly against the flat bar to make axles, which allow the gantry plates and link bar ends to rotate freely.

An 8mm hole was drilled down the center of the end vertical plate to mount the end spin hook. The spin hook consists of an L shaped bracket, and a 150mm long 8mm bolt; a lock nut and internal thread holds the bolt to the L bracket. The bolt is free to spin so the screen can be faced in any direction. On the end of the spin hook is an 8mm hole through which the screen is mounted, spacers were added so tightening this bolt increases the friction of the join, allowing the screen to be tilted.

Step 7: Main Pivot Plate Assembly

The three main pivot plates must be mounted to the support pole. These were machined with undersize holes, and slits so they would naturally clamp the pipe. Slip these into place by prying the slit open with a screwdriver to enlarge the hole.

Once all three pivot plates are in place, ensure they are also parallel. Next drill through the plate and pipe with a tap sized drill bit. Remove the pivot plates and drill through with a clearance fit. Tap the support pole. I used M4x12 button head cap screws for this.

Reassemble and put bolts in the new holes.

Step 8: Main Assembly

The majority of fasters used on the gantry and link bars are tightened with wingnuts, this is to allow adjustable levels of joint friction. In all instances just do them up moderately tight to start with and adjust as necessary later.

Bolt the main gantry plates to the lower support pole main pivot.

Bolt the long link bars to the mid support pole main mount.

Bolt the center vertical bars to the other end of the long link bars; remember the spacers and wire guide pulley.

Bolt both pairs of gantry plates to the lower end of the center vertical plates, again, remember the spacers.

Bolt the short link bars to the middle of the center vertical plates.

Bolt the short link bars, and secondary gantry plates to the end vertical plate.

You now have the main moveable assembly sorted, test it out by moving the screen around, and ensure it can move in the directions it should (postition, spin, tilt) while staying horizontal. Next is the pulley/wire system...

Start by bolting the fixed end to the secondary gantry plates.

Next bolt pulleys onto the main gantry plates, and top main pivot plate on the support pole.

Route the wire over the center vertical bar pulley, under the main gantry plate pulley, over the support pole pulley, and down through the support pole.

Step 9: Pre-testing

It is now time to test the wire balancing system!
This may require two people, or a higher shelf to rest your screen on. In either case raise the screen, draw sire through the bottom of the support pole and tie it off (I knotted it around a bolt). This temporarily secures the other end of the wire.
The screen should now be able to be moved and positioned anywhere in a single height plane. Take care when first positioning it as it may drop significantly while the cable takes up slack.

Test many different positions, and check the gantry for any flex. My initial design using only the 12mm gantry plates allowed too much flex. The addition of 1.6mm aluminium plate on the inside of all gantry plates fixed this.

Step 10: Cable Winder

With the screen now free to move in a single height plane, we are left with three options:
Leave it at only a single plane.
Add a counterbalance weight.
Add a cable winder.

For my application raising and lowering the screen is a must, so only the later two were seriously considered. The counterbalance weight would need to be 25kg, and although it would make it easier to position the screen, a compact enough weight to unobtrusively fit under my desk proved too elusive; and a cable winder was made.

The winder consists of a rotating bit of threaded rod, held captive top and bottom, two guide rods, and a threaded centerpeice, which moves up or down when the threaded rod is rotated. A nice handle was made up for easy rotation. The whole assembly fits down inside the main support pole, and is held by the same screws as the top main pivot.

To make this you need three discs that just fit inside the support pole. 3 holes must be drilled in identical places through the discs, the center disc must have large holes for the guides, and a tapped center. The threaded rod must be turned down on both ends so it is held captive by the outer two discs. The guide rods are press fit into these discs. The upper disc must be notched to allow the cable to feed through. The center disc has an extra hole for the wire to be threaded through and tied off on the other side.

Raise the center disc to the top before tieing off the cable, this means the screen will lower when the slack is taken up, but will be raised as the center disc pulls the cable down into the support pole when the handle is rotated.

Drill holes in the top disc to receive the bolts holding the top main pivot plate to the support pole, the stop the whole assembly rotating as well as keeping it from being pulled out of the support pole.

Now your screen gantry is mechanically ready to go, give it a test to ensure winding it up and down works properly.

The final step is to route the screen cables and turn it on, I just zip tied them to the gantry plates, on the outside to avoid pinching.

Step 11: Job Done!

As seen in the pictures, it can sit on the desk shelf as normal, or be raised, extended, to wherever the user desires!

In my case, the first use was to watch some "Game of Thrones" from bed, a most epic show for a most epic creation I think :D