[DsKnees]

I'm starting a new project that I can actually show pictures of, so I thought I'd sort of "blog" the project as I go. Feel free to ask questions or give suggestions.

The project is to make a lap tray to hold the Apple's "magic" keyboard and trackpad. I want to have them in roughly the same orientation as they would be in one of their laptops, because that is what I am most accustomed to.

The reason I am making this is because I like to use my TV as a computer display, and I want a keyboard and trackpad I can use while I'm sitting on the couch. The Apple keyboard and trackpad are roughly the same as what is found on my laptop, so they were the obvious choice.

The prototype I have been using for a few months was made from pink foam board, and I routed it out using a hand held router and a crude jig with sliding gantry. Then I surfaced it with a layer of fiberglass to make it rigid. It has done it's job; it has proven that the general design and concept works.

From the beginning I have been wanting to make this out of carbon fiber. Why? Because why not? But seriously, it is more experience for me with the whole digital design process, and it will give me more experience designing composite parts.

Below is the CAD file I'm working with (Rhino). This is the idea I have been playing with, and at the moment I just want to make the part on the CNC to test it. I am not sure what the correct tolerances are, the correct depth, etc.

There are a few details that may stand out. There is a recessed area in both cavities--that is meant for velcro to hold the keyboard and trackpad down. I thought it would be a good idea, but I haven't really needed it.

Also there is a cut-out on the back edge for the keyboard, and a bit of a ramp along the back edge of the track pad. These correspond to the antennas on both devices. Carbon fiber is conductive, so it will block radio frequencies. So I am expecting that I will have connectivity issues if I do not create clearance in these areas.

Also, I drew a flange extending around the base. I added this because VCarve didn't seem to recognize the vertical sides of my part. So I made a flange larger than the actual work piece, so now the program will remove the excess material around the permieter.

First attempt to machine. I made 3 tool paths, which is the process that has been working really well for me. I start with a rough pass using a 1/2" ball nose cutter, and within ~.050" of the final surface. Then my second pass is a "finish pass" with a 1/8" tapered ball nose, but I use a large step-over and leave 0.010".

When the second pass is done, I coat the part with a thin, fast setting epoxy. I put it on pretty thick, leaving excess resin on the surface, because I want the surface to be saturated. The excess resin machines off on the final pass, so I'd rather have a little extra than not enough.

The final pass then uses the same 1/8 tapered ball nose cutter, but I use a much smaller step over. And I cut right down to the final surface.

For this project, I decided to try using the vacuum table. I thought it would work fine, and I liked the idea that I would not have any trimming when it was done. But unfortunately, the MDF started to curl while the program was running, and the vacuum couldn't hold it down. The part was ruined, but fortunately the bit didn't break. The picture shows the damage--this happened part way through thte 2nd pass.

(I did face off the bottom first, so it it was flat. But like all kinds of wood stock, there can be internal stresses that show themselves when you start cutting them.)

Next step: I want to try to use the vacuum table again. I know MDF is a low quality material, but I want to see if I can make it work. I want to glue up a block of 2 layers of MDF like I had before, but I will make it over sized. And when I machine it, I want the excess material on the perimeter to stay connected by maybe .030". I can still separate it without too much difficulty, and hopefull the extra material helps is stay held down.

 

[old55]

This will be an interesting one to watch.

 

[DsKnees]

Update: I thought this would be pretty easy, but it's turning out to be a bit of an education. So far the MDF material is proving to be unsuitable for this project.

I have made 3 attempts now, and each time the MDF starts bowing dramatically in the thin areas, where one side is the outside surface of the MDF sheet. The first time I did a simple glue-up, gluing 2 pieces of 3/4" MDF together with epoxy. The second and third time I also put a layer of epoxy on the bottom of the block, thinking it would help stabilize the fibers. On these second 2 attempts, I also faced off the bottom to ensure it was flat before clamping it down to the router

All 3 times the MDF has started to bow downward when it starts getting thin. The first two failed because the vacuum wasn't strong enough. And the third failed too, but for a combination of reasons. All 3 are bowed down in the middle dramatically, which is enough to call them failures.

I think I know what is happening. Please let me know if you have any insight. I understand that MDF has denser material on tob and bottom surfaces, with less dense material in the middle. Based on instructions for making a vacuum table spoil board, they say to cut through the first ~.040" on both sides to remove the denser surface layer--which allows the vacuum to pull through the material.

Apparently this top layer remains under stress, and when I thin the material, the surface layer is able to expand slightly, causing the material to warp.

So my question is, is there a way to "fix" this problem? Maybe I could face off the majority of the material on the bottom layer, so the glue joint between the two surface layers of MDF runs through the middle of the part? No surface layers on the sides of the part. In theory having 2 surface layers glued together would cancel each other out.

(For my other, somewhat secret project, I have been using a base layer of HDF, sometimes with fiberglass cloth on both sides of the HDF, to make it as rigid and dimensionally stable as possible. It's gone so well I started to think working with MDF is super easy. Little did I know!)

I'm not giving up. I'm currently gluing up a plywood block. I am assuming that won't give me as many issues.

Another option is to re-design the part so the top surface is flat, and the bottom is sloped. Then I would use a 1/4" birch plywood as the top surface, and maybe MDF below that.

Alternatively I could make it so the tray is the same thickness all the way across, with only the pockets being sloped. That doesn't fix the warping problem, but if simplifies the work holding while testing the different materials and techniques.

Anyway, I'll update as I go. I won't let this one beat me!

 

[TenGees]

Maybe make the cutouts right through and add a hardboard bottom?

 

[DsKnees]

Maybe make the cutouts right through and add a hardboard bottom?

That's an interesting idea, but I'm not sure how to implement it?

One detail that may not be obvious is the pockets are at different depths, so adding a hard board bottom would require flipping the part and machine pockets from the bottom. Both the keyboard and the trackpad are built at a slight angle, which is part of the challenge.

 

[DsKnees]

Alright, so a little success. I hoped to get further along than this, but I guess this weekend I learned more than I was productive.

First, the final result with the keyboard and trackpad. It came off the machien sliightly warped, which isn't hugely surprising given the nature of wood. But this is just a test.

(It isn't carbon fiber yet, but I haven't given up on that part yet!)

After the roughing pass:

After the finish pass:

A light clean up to fix the bottom edges:

 

[DsKnees]

More progress. I made 2 different versions since the last update.

The first version actually has carbon fiber. It is two plies of 1/4" birch plywood with carbon fiber on top and bottom. I had to adjust the design so the tray thickness does not taper. This allowed me to easily cut out the part without cutting through the (mostly cosmetic) carbon fiber layer on top.

I think I will stick to this non-tapered design. The final product is going to be 2 piece molded carbon fiber shell, and using the tapered design will be more difficult to make and assemble.

I am preferring this design right now because I am trying to plan ahead for the assembly process. I know the pockets for the keyboard and trackpad are the same depth, so the easiest thing to do for assembly is to make a back cover that orients itself by touching these two pockets. This will simlify gluing the two halves together, because the two halves will automatically alighn. The more I think about it, the more I realize that this a tricky project to pull off!

Besides allowing for easier assembly, having the pack cover touching the pockets means I can make a glue joint there, which will dramatically stiffen the entire assembly. So I think it is the best option.

The tapered design would be more desirable if the tray is meant for a desk, but it is meant for laptop use/ lounging/ home theater computer. So making the tray tapered doesn't seem to improve it's functionality.

Here is the carbon fiber/ plywood version. (Note that I was lazy in my prep work. I didn't face off the material before laminating the carbon fiber layers on top and bottom, so the blank wasn't perfectly flat. So the vacuum table didn't hold it all that well....so at the last minute it moved and the back edge got chewed up a little. But no big deal; the part I made still serves it's purpose, and I learned a little more about where the limits are when using the vacuum table.)

Some might be interested in how I did the carbon fiber layers? I glued up the blank and the carbon layers in a single step. I'm using my "reference" surface covered with a teflon sheet over it. I place a piece of carbon fiber fabric, wetted out with epoxy, directly on the teflon surface. Then the 2 layers of birch plywood, with epoxy brushed on both sides. Then the second layer of carbon fiber on top. On top of that is a layer of peel ply, a breather fabric, and the vacuum bag.

I didn't do step by step photos, but here is what it looks like with vacuum applied. This was my first try doing this exact method. I'm trying to combine steps to save time. It mostly worked.

Fast forward to yesterday's project. Here I am experimenting with the shape to improve comfort, and I am trying to pin down the pocket depth and overall dimensions. Also I experimented with internal fillet size, because I wanted to know if I could get away with machining with a larger diameter bit.

Anyway, here is the newest version: