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This is my very first DIY project, I am building an arcade stick:

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enter image description here

Rubber feet will be attached to those 4 holes.

Now I do not have tools for any woodwork at home (will buy once I do start doing more DIY!). I plan on sending the template to a wood CNC workshop (2 dimensional cutting) and they will cut the wood out for me. So careful planning and screw hole sizes are important here. I will be using tapered M4-30/35 (thread: 1.41) screws mostly.

Materials used: Birch plywood, Clear Polycarbonate Top 3mm, Clear Acrylic Bottom 2mm.

  1. When drilling screw holes for acrylic should I make it slightly larger than the actual diameter of the screw hole so that it does not crack? e.g. 4.0~4.1mm. I can only find pilot hole guide for plywood but not acrylic or polycarbonate sheets.

  2. Depth of the screw hole for the birch plywood. Would 3/4" suffice or should I drill same length as the length of the screw since it is hard wood?

  3. I am trying to minimise screws showing. Will this setting be strong enough? I am concerned about 2mm acrylic panel wobbling especially.

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Many thanks in advance.

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    I removed the first paragraph of the question. I understand where you were coming from and this is a fine place to ask about your project. There is an Arts & Crafts currently in Private Beta. I imagine it will open up in a couple of weeks.
    – Matt
    May 3, 2016 at 0:42

4 Answers 4

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When drilling screw holes for acrylic should I make it slightly larger than the actual diameter of the screw hole so that it does not crack?

Very much yes. You're not drilling pilot holes here, you're drilling clearance holes which are completely different. See more in this previous Answer.

Depth of the screw hole for the birch plywood. Would 3/4" suffice or should I drill same length as the length of the screw since it is hard wood?

This applies elsewhere in your proposed build as well, it would actually be much better if the plan didn't call for screws to be driven into the sides of the plywood.

The hold of screws in ply is much better if driven in through the face and not into the edges. The difference is so great that it can be worth the trouble to cut smaller pieces of plywood and then glue them together in a stack to achieve a sufficient thickness. Alternatively you can use the trick posted at the end of the Answer linked to above where you drill a hole and glue in a dowel for the screw to bite into but this may require buying a larger bit (sized to the dowel used) as well as possibly a suitable saw for cutting the dowels flush with the surface. Some sanding will probably also be required to make them completely flush with the face of the plywood.

I am trying to minimise screws showing.

On the sides you can do what's called a counterbore (see this previous Answer) and plug those with wood or fill with wood filler to hide the screw heads.

Will this setting be strong enough? I am concerned about 2mm acrylic panel wobbling especially.

Four screws holding the acrylic in place is very likely not sufficient to prevent it sagging. I think the ideal for this is for a slightly larger piece to be used where all four edges would be set into a groove or rabbet/rebate, similar to how the bottom of many boxes are done. If you do want to secure it with screws a few more should be spaced around the perimeter.

Note: for the top screws holding the polycarbonate not to sit proud of the surface the drilled holes need to be countersunk and if you don't have a combination drill/pilot/countersink bit you will need to buy a countersink separately.

Countersinks are not all very well made and many cheaper ones are not very sharp so you might want to practice doing the countersinks on a scrap of the acrylic to ensure a clean and even result (e.g. oval holes are easy to create by accident). In addition to the possibly unevenness a blunter countersink will begin to heat the plastic and it could actually melt at worst. So controlling the speed of the drill is vital. Also be very careful how hard you press during both the drilling and countersinking.

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  • Hello, thank you for such an insightful answer. May I ask one further question please? The middle birch plywood panel would be 4mm dimension of 330x250 mm. Would it able to handle weight of the joystick and my hand? joystick itself doesn't weigh too much(500g). Should I use 9mm instead? I don't want to use anything thicker because that would start to restrict joysticks movement..
    – zcahfg2
    May 4, 2016 at 2:26
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    @zcahfg2, I can't say for sure but although plywood is inherently strong I don't think 4mm is enough here. 3-4mm plywood is light enough that it can be slightly flexed between the hands. I would think that you'd want to use something where no flex is possible and 9mm should provide this, although 12-18mm plywood is commonly used where strength/stiffness is a definite requirement.
    – Graphus
    May 4, 2016 at 6:30
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  1. When drilling screw holes for acrylic should I make it slightly larger than the actual diameter of the screw hole so that it does not crack?

If you're planning to have the screws thread into the acrylic, I would make them slightly undersize and then tap the acrylic. If the screws just pass through the acrylic (and are threaded into something else), then you want the holes slightly oversized (0.5mm would be good).

  1. Depth of the screw hole for the birch plywood. Would 3/4" suffice or should I drill same length as the length of the screw since it is hard wood?

Are these pointed wood screws or blunt machine screws?

If pointed wood screws, you don't necessarily have to drill as deep as the screw. You would want the holes deep enough that the main body of the screw is pre-drilled (the tapered tip doesn't really need to be pre-drilled).

If blunt machine screws, you'll want the holes full depth (and probably prudent to add 2 mm or so for tolerance of you can fit it).

  1. I am trying to minimise screws showing. Will this setting be strong enough? I am concerned about 2mm acrylic panel wobbling especially.

Not entirely sure what you mean by "wobbling." The acrylic is going to deflect out-of-plane no matter how much you screw it to the frame.

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  • Hello, thank you for your answer. Can you also kindly point out the stability of the box when I screw like that? If I move the case often will the screws come lose and/or breaks the case? If so any ways to improve the stability?
    – zcahfg2
    May 2, 2016 at 22:19
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The other answers seem to have covered your main questions, but there's something you may not have considered - if the boards are to be cut by CNC router then you will not get sharp internal corners (e.g. 90° degree cuts), instead these will have a small fillet (radius between corners) due to the way the router works. The radius of the fillet will be the radius of the cutting bit used. This may be as small as 0.5mm radius but it might affect you so it's worth thinking about. In reality you can probably just clean this up yourself with a chisel or the guys at the workshop might do it for you if you ask.

Also as a more general point, 2mm acrylic is going to be very flimsy - since it's only attached at the edges you may get some warping or wobbling. If it was my design I'd want to use something a bit thicker or add screws to the centre. You could use countersunk screws and finish them flush with the top of the acrylic.

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I am trying to minimise screws showing.

The approach shown by the red screws in your diagram will be visible even if you counterbore and plug.

Furniture block joint

An alternative is to use battens on the insides of the corners and use screws from the inside. Or just glue it if you have (or can buy/borrow) a couple of clamps. This is the sort of joint you see in some old apple crates etc.

Box joint

Another option, since you can get wood CNC cut, is to get them to cut fingers for a box-joint

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