05-14-2011, 07:30 AM
OK. Sorry to have been remiss in posting about this, but I have been working on it.
Here's where I am.
*I'm using standard (but chosen to be the most even grain and least cracked and spliced) 1/4" (6mm?) 3 ply, BC sanded exterior plywood. Costs around 18 dollars a sheet at the Home Depot. The test shield, I domed, as said before, but it was unsuccessful in some ways, and very irregular. You can't simply start at one spot and go around the shield with a series of screws. The sides are no problem. The top and bottom are a little more difficult, but they can be bent. The problem areas are at 10, 2, 4 and 8 o'clock. These have to bend in two directions at once, and are not interested in cooperating. It was about 2 3/4" in depth.
*I resoaked it and domed it again, a little deeper, and still a little irregularly, but by then around 3 1/4". I was encouraged enough to continue.
*The third soak (still on the 1st board) I was determined to get the full 4". I did, but the plywood broke in two or three places, one of them pretty badly, so I put the screws in that area through a scrap piece of plywood laid on the surface, like an ad hoc splint, spreading the pressure over a wider area.
That was a great discovery. I used that method from then on to help press down any high spots on the surface from. Looks like a bunch of bandaids all over the place, which I guess it is. In the end, that test shield was a lumpy mess anyway, but I did get the 4" depth. The edge is so full of screw holes it's not stable any more. I set it aside, and looked at it many times over the next several days before I proceeded, thinking about how to prevent the problems that I'd had with it.
* *
I have determined that it's not reasonable without some kind of a steamer unit to get the plywood to bend that far, and it would take a 3 dimensional 2 piece press to make the wood bend and stay and three men and a strong boy to press it down into place. That's all beyond my equipment/ability/pocketbook at the present time. The big boys are able to take a flat sheet of plywood and bend it into spiral rocking chair struts and runners, office chair arms and so forth, from triple the thickness I'm using, so I know it's possible, just not by me. I'm sure they don't use the cheap plywood like the rest of us do, either.
So by working out a set of "concentric" ("epicentric?") ellipses stacked up to press against the shield from the under side, I was able to settle on about 2 1/2 to 2 3/4 inch dome, which is how they'll be. These elliptical braces under the shield help to prevent places where the shield dips instead of domes, and by backing off the deeper dish, the surface irregularities are reduced.
It is domed enough that it looks clearly "not flat", and by using a cross-piece handle, the more flexible side-to-side arch can be held to the desired depth and stabilized. I'll use a 1 x 4 cut down to the right arch and glued and nailed to the face to maintain that lateral curve. I think that will solve the problem, and make a handle that's built into the shield, plus giving the added advantage that it will have the handle removed a bit from the plane of the shield, hopefully around the center of gravity, so the back of the carrier's hand won't bash against the wood. The vertical seems to be stiff enough without any help.
When daylight comes, I'll measure out the curve and band saw some cross brace/handle pieces. I now will have 3 elliptical domed scutum blanks!
There are quiet a few screw holes in the shields, of course, most of them around the edges. But I will run the sander over both sides to knock down the splintery places, then fill the holes with wood putty and sand again before putting on the fabric.
I cut two sheets of plywood into shield sized ellipses way back at the start, believing I could make the project work, or if it didn't work, at least I'd have 3 more flat shields. By using a system of putting screws widely spaced around, but not all the way screwed in, then tightening one a little, then another, then adding a scrap wood splint here and there, then tightening more screws a little at a time, adding another splint, adding a screw where the wood began to bow, I achieved a domed shield by the third one of the "production blanks" that is nearly regularly arched all around. I'm pretty proud of it. If that sounds tedious and aggravating, well, that's because it is. Now, if I can figure out how to get the cloth to go on smoothly on the front face without wrinkling, I'll be in the happy place.
The first blank, the test copy? I decided it had too much experience and history involved in its life to toss away, so I got a nail and a string and cut a 32" circle from the middle part. It was sort of domed already, but I put it back in the water for a while, then bent it over a circular piece of 3/4" plywood fastened to a piece of 2 x 6 for height. That makes about a 2 1/4" center height. I screwed it down to another board, and will use that for a parma. Were parmae domed? Hard to say, but we'll be the first kids on the block to have one that is. I'll bet others will be jealous.
When all is dry, and the handle blanks are cut, I'll put up some pictures. Whew! It's a relief to find that my mental gymnastics were not all for nothing. Robert V: Thanks for that formula for determining the nail distance and string length. That was a great help, especially when I had to make those other ellipses, and make them the same proportions as the shield blank.
Here's where I am.
*I'm using standard (but chosen to be the most even grain and least cracked and spliced) 1/4" (6mm?) 3 ply, BC sanded exterior plywood. Costs around 18 dollars a sheet at the Home Depot. The test shield, I domed, as said before, but it was unsuccessful in some ways, and very irregular. You can't simply start at one spot and go around the shield with a series of screws. The sides are no problem. The top and bottom are a little more difficult, but they can be bent. The problem areas are at 10, 2, 4 and 8 o'clock. These have to bend in two directions at once, and are not interested in cooperating. It was about 2 3/4" in depth.
*I resoaked it and domed it again, a little deeper, and still a little irregularly, but by then around 3 1/4". I was encouraged enough to continue.
*The third soak (still on the 1st board) I was determined to get the full 4". I did, but the plywood broke in two or three places, one of them pretty badly, so I put the screws in that area through a scrap piece of plywood laid on the surface, like an ad hoc splint, spreading the pressure over a wider area.
That was a great discovery. I used that method from then on to help press down any high spots on the surface from. Looks like a bunch of bandaids all over the place, which I guess it is. In the end, that test shield was a lumpy mess anyway, but I did get the 4" depth. The edge is so full of screw holes it's not stable any more. I set it aside, and looked at it many times over the next several days before I proceeded, thinking about how to prevent the problems that I'd had with it.* *
I have determined that it's not reasonable without some kind of a steamer unit to get the plywood to bend that far, and it would take a 3 dimensional 2 piece press to make the wood bend and stay and three men and a strong boy to press it down into place. That's all beyond my equipment/ability/pocketbook at the present time. The big boys are able to take a flat sheet of plywood and bend it into spiral rocking chair struts and runners, office chair arms and so forth, from triple the thickness I'm using, so I know it's possible, just not by me. I'm sure they don't use the cheap plywood like the rest of us do, either.
So by working out a set of "concentric" ("epicentric?") ellipses stacked up to press against the shield from the under side, I was able to settle on about 2 1/2 to 2 3/4 inch dome, which is how they'll be. These elliptical braces under the shield help to prevent places where the shield dips instead of domes, and by backing off the deeper dish, the surface irregularities are reduced.
It is domed enough that it looks clearly "not flat", and by using a cross-piece handle, the more flexible side-to-side arch can be held to the desired depth and stabilized. I'll use a 1 x 4 cut down to the right arch and glued and nailed to the face to maintain that lateral curve. I think that will solve the problem, and make a handle that's built into the shield, plus giving the added advantage that it will have the handle removed a bit from the plane of the shield, hopefully around the center of gravity, so the back of the carrier's hand won't bash against the wood. The vertical seems to be stiff enough without any help.
When daylight comes, I'll measure out the curve and band saw some cross brace/handle pieces. I now will have 3 elliptical domed scutum blanks!
There are quiet a few screw holes in the shields, of course, most of them around the edges. But I will run the sander over both sides to knock down the splintery places, then fill the holes with wood putty and sand again before putting on the fabric.
I cut two sheets of plywood into shield sized ellipses way back at the start, believing I could make the project work, or if it didn't work, at least I'd have 3 more flat shields. By using a system of putting screws widely spaced around, but not all the way screwed in, then tightening one a little, then another, then adding a scrap wood splint here and there, then tightening more screws a little at a time, adding another splint, adding a screw where the wood began to bow, I achieved a domed shield by the third one of the "production blanks" that is nearly regularly arched all around. I'm pretty proud of it. If that sounds tedious and aggravating, well, that's because it is. Now, if I can figure out how to get the cloth to go on smoothly on the front face without wrinkling, I'll be in the happy place.
The first blank, the test copy? I decided it had too much experience and history involved in its life to toss away, so I got a nail and a string and cut a 32" circle from the middle part. It was sort of domed already, but I put it back in the water for a while, then bent it over a circular piece of 3/4" plywood fastened to a piece of 2 x 6 for height. That makes about a 2 1/4" center height. I screwed it down to another board, and will use that for a parma. Were parmae domed? Hard to say, but we'll be the first kids on the block to have one that is. I'll bet others will be jealous. When all is dry, and the handle blanks are cut, I'll put up some pictures. Whew! It's a relief to find that my mental gymnastics were not all for nothing. Robert V: Thanks for that formula for determining the nail distance and string length. That was a great help, especially when I had to make those other ellipses, and make them the same proportions as the shield blank.
M. Demetrius Abicio
(David Wills)
Saepe veritas est dura.
(David Wills)
Saepe veritas est dura.
