Clarification on platen design

[JonEP]

I am gearing up for a June build, basically aiming to reproduce the "new standard" design. I'm doing some prep work, ordering parts, etc., and find that I have some questions about platen design and materials. Forgive me if they are answered elsewhere.

1. It appears that a non-glass platen is easier to build than a glass platen, and does not present a problem for the image quality?

2. Dan describes the platen on the new standard as "acrylic," I believe it is extruded acrylic. (Is this the same as the "doublestrength glass" purchased at Scheel's hardware?). Dan then posts that 1/8" acrylic is too thin, and flexes, and Mathue, who seems to have some experience with these materials, notes in a post that indeed 1/4" extruded acrylic (FF) is better than 1/8", and that for Mathue's build the platen

has been fabricated from 3/8" GP cast acrylic (Chemcast, though Cyro would be better). All joints were pin glued, routed, wet sanded and then polished with red and then white jeweller's rouge.

I also note that Antoha-spb describes a platen built from "optic-grade plexiglass," and in another post suggests that the "Plexiglass shop made me a platen with a perfect 90 deg. angle for just ~$20." Based on all of this, I'm leaning towards trying to find a shop that does plexiglass/acrylic work, in the hopes that I can arrange to have them create a platen of 90-degrees out of 3/8" cast acrylic (my understanding is that 'cast' acrylic is higher quality than extruded acryllic). I like the idea of having the acrylic platen have its own integrity (ie., from being molded together). My thought is that it might be possible to have the shop pre-drill any holes in the acrylic, so that I don't mess that up. Does that ring true to those of you who have built scanners already?

Thanks for any and all comments!
-Jon

[daniel_reetz]

a few brief thoughts from my mobile fon:

don't go too thick with acrylic, you'll lose the gutters of your book, and your scanner will not work with smaller books.
rob had trouble with 1/4" stuff, IIRC.
acrylic shops can easily bevel plastic for your platen.
"doublestrength" is not acrylic, it's basically 3mm glass.

glass vs non glass is basically about cleaning and rigidity. acrylic tends to scratch more easily than glass, and is less rigid. glass is harder to work with, but harder/more rigid generally and i have come to like it better, in spite of the many problems it has.

[Antoha-spb]

I actually don't remember the brand of the plexiglass i bought. Shop salesman told me it is widely used to make stands and holders of advertising materail. My platen is 4 mm thick. Holes are easy to make with an electric screwdriver and a sharp, metal-grade drill (avoid putting much pressure when drilling!). Plexiglass is rather fragile, so mounting bolts are to be enforced with wide shims. Below is the sketch i made for the plexiglass shop



(notes below the picture: all edges are rounded, thickness = 4mm, transparent colourless material, one piece)

By now i scanned a few thousands of pages through this platen. It has some scratches and one 1 cm long crack, but still ensures good scan quality for books. In fact, given it's price plexiglass platen is an 'expendable part' of the scanner.

[daniel_reetz]

Nice stuff, as usual, AntohaSPB. One tip for drilling plexiglass with a metal drill bit -- use a dab of dish soap or glycerin on the tip and go slow to prevent cracking.

[JonEP]

Thanks to you both. I think I am going to attempt to mimic, in a more amateur form, the glass platen created by Edvin (shown here).

I might even try to imitate some of his work on the counterweight. I'm interested in the discussion about where to put rails and weights (ie., he suggests putting rails at the front sides of the platen). I notice that other builders have been satisfied with a single point of connection between the platen apparatus and the sliding mechanism (ie., on the back column)--that seems to be the approach on the New Standard design. No problems there with uneven weight/pressure on the book the way Edvin described?

[Tim]

1. It appears that a non-glass platen is easier to build than a glass platen, and does not present a problem for the image quality?

I thought of and subsequently saw a build using picture frames as the glass portion of the platen. Since my primary goal is extreme cheapness for anything I don't have on hand, I picked a couple matching good size picture frames up from the local habitat for humanity restore and plan to use those. They fit the extremely cheap goal, being just a couple dollars. They seem very easy to use since they have decent wood frames that I can use to attach into a 90 degree angle. Just pop off one edge of each picture frame and part of the work of making the platen is done.

Are there any downsides to using two pieces of glass that are not joined at the vertex? Should the edges be prepared in any way or should they just be set at a 90 degree angle to make a perfect corner?

[Pagnol]

I bought two ready-cut sheets of Plexiglass (enhanced acrylic glass, according to its manufacturer) with the dimensions 260 mm, 320 mm, and 2 mm (thickness) in a German shop for 1,63 € each. Great price, isn't it?

I've already fallen in love with this material and I'm going to build a second scanner that consists of nothing but acrylic glass.

[Antoha-spb]

I've already fallen in love with this material and I'm going to build a second scanner that consists of nothing but acrylic glass.

=== me too, i already made a beta-drawing of all parts

[Mathue]

in the hopes that I can arrange to have them create a platen of 90-degrees out of 3/8" cast acrylic (my understanding is that 'cast' acrylic is higher quality than extruded acryllic). I like the idea of having the acrylic platen have its own integrity (ie., from being molded together). My thought is that it might be possible to have the shop pre-drill any holes in the acrylic, so that I don't mess that up. Does that ring true to those of you who have built scanners already?

Thanks for any and all comments!
-Jon

Cast is more uniform than extruded, has a harder surface and it's melting temperature is about 20℉ higher.. Basically Cast acrylic is cast between two sheets of VERY thick glass that have a high degree of surface uniformity⚗, I forget how many thousandths tolerance the glass is. Extruded as you might imagine will visually show signs of the high pressure extrusion. Now, whether this really makes a difference, I don't have enough time with actual scanning to say yay or nay. So far I've not seen issues with page gutters with 3/8ths, but I'm sure 'pulp', i.e. -cheap- printings might.

Chemical welding or glueing of the acrylic will indeed make for far greater rigidity. If you know exactly where you want the holes, the fabricator will certainly be able to drill them for you, or you could get a narrow rake drill and drill it yourself.

http://www.tapplastics.com/shop/product ... 1350500993

[Mathue]

I bought two ready-cut sheets of Plexiglass (enhanced acrylic glass, according to its manufacturer) with the dimensions 260 mm, 320 mm, and 2 mm (thickness) in a German shop for 1,63 € each. Great price, isn't it?

For retail precut that is not too bad (I don't know whether you got FF or GP acrylic).

Ideally if you could find a local reseller that deals just in acrylic (Which is what Plexiglas is) and similar plastics you'll get a better price. 2mm works out to be a little over a 1/16th of an inch. I don't expect this will work all that well for a platen material, it will flex too much. 6mm would be a better choice which in inches in around 1/4.

Searching the internet I'm not having much luck finding a plastics only retailer in Germany, but that is probably due to me searching using English.

This URL may or may not be of any use to you. http://www.list-of-companies.org/German ... lic_Sheet/

Don't forget that Daniel is now leaning toward using actual glass and since he has FAR more experienced with building these things I wouldn't dismiss his findings.

There are trade offs for each material.

Acrylic, FAR more shatter resistant, lighter, easier to cut, drill and bond.
Glass, less flex for the same thickness and distance, near perfect optical quality, very hard to scratch.