3

I am completely new to wood working and am currently designing a computer desk.

I do not know what length screws to use and the distance between them and dowels. Any other advice would be gratefully received.

  • I am designing in Sketchup
  • Plan on using 18mm plywood
  • Using 6mm screws
  • 3.5mm pilot hole
  • With 6mm dowels

Thank you in advance

EDIT: Was asked to clarify my problem and to provide a picture- What length screws do I need to use to build a computer desk? Also what distance should I have between screws and dowels for best strength?

I have added dimensions and labels to the image hope this helps.

Screen grab with dimentions

There is nothing fixed in stone with the dimensions as yet. I am thinking of adding some drawers just under the top, and will be adding support to the shelves. The cylinder shapes on the bottom left are for making holes in the plan for screws and dowels

  • 2
    A sketch of the desk would be very helpful in answering this question. – Ashlar Jul 25 '16 at 20:17
  • 2
    You need to reveal where you are planning to use screws. Make sure that you are not intending to put screws into the edge grain of the plywood - very little holding power and tends to separate the plys. Dowels - for what purpose? – Ast Pace Jul 26 '16 at 0:13
  • Welcome to Woodworking.SE! In order for our community to provide the best possible answers to your questions, please include more details such as illustrations of your design and the attachment points in question. – rob Jul 28 '16 at 4:18
  • @rob Sorry for to briefness, Is the information added more helpful? – Phoenix830 Jul 28 '16 at 13:05
  • Yes, much better; thanks. If you can also provide a zoom-in of the joints you're planning, that would also help. – rob Jul 28 '16 at 14:13
1

Firstly the big issue for most woodworkers, the question of screwing into the edges of plywood.

You'll see plenty of advice not to do this if you look around, but you'll also see admissions from lots of folks who do it all the same and most will tell you that it works fine if done properly. There are various tips about how best to accomplish this without issues, here's a summary from various sources:

  • Thicker plywood is a better choice when attempting to screw into edges. 18mm or 3/4" is often the minimum thickness recommended so you're good here.
  • Always centre the screws on the thickness of the plywood.
  • Drill suitable pilot holes (note: clearance holes and counterbores are needed in the piece the screw goes through).
  • Squeeze the faces of the plywood with a clamp when driving the screw in to physically prevent it from splitting or bowing. This can be worth doing even if using self-tapping screws.
  • Don't screw near a corner (even if using the clamp trick). Place screws at least 5-6cm from a corner.
  • Pick your screws with care.
  • Glue the plywood butt joint, that way the screws aren't being relied upon for strength but are mostly there to initially hold the case together while the glue dries.

So as per the above you should ideally glue the edges of your ply to the case sides but you may want to avoid that for a few reasons.

  1. the surface plies of modern plywood are often too weak (too thin, and not bonded well enough) to reliably glue to.
  2. it makes assembly much more difficult and frustrating if you've never done it before.
  3. much less glue squeeze-out to clean up when you're done (this is surprisingly difficult to do properly, and even harder on plywood surfaces than on solid wood because of the way the veneer is peeled from the log). However, you can protect adjacent surfaces with tape to prevent the problem in the first place, similar to how it's used here to protect joints from finish.

I do not know what length screws to use and the distance between them and dowels.

Longer is better for strength, but increases the potential for the screw to split or bulge the plywood at the edges. But screw type is arguably more important than length. Basic wood screws aren't ideal for plywood, although they can work OK. Confirmat screws are often recommended for use on plywood due to their superior holding power but common chipboard screws should work perfectly well and are far easier to find, these screws are made to hold in material much less dense than you'll find in the edges of plywood so should hold extremely well in anything better.

Once you've selected your screw it would be advisable to do a few tests screwing into some offcuts of your plywood, because the quality of ply varies so much what one person has had success with does not guarantee success for you, using a different make or type. Test to find out what size of pilot hole works best and whether or not you need to clamp the plywood to help prevent it bowing or splitting — the larger the diameter of pilot hole the less the chance of these problems, but at the cost of lower holding power.

Truth be told the dowels you're planning on using as well will add a lot of strength here, much more than they often do in a joint (in solid wood). If you're buying lengths of dowel and sawing it down yourself you should groove the dowel pegs to aid them being driven home, see previous Question, How can I flute my own dowel or create dowel with similar properties. Also chamfer the leading edges, just as is done on commercial dowels, which can be done with a sharp chisel, in a pencil sharpener (yes, really) or simply by rotating the dowels against abrasive paper.

  • 1
    Thank you for your answer and more so for the excellent tips. I will come back to this when I come to building the desk – Phoenix830 Jul 31 '16 at 23:11
2

I'm assuming that for a desk, you do not want the screws to be visible from the work surface.

If that is true, then you probably want to use a system like the Kreg pocket hole jig, which actually calculates the length of the screw for you based on the thickness of the material the screw is drilling into.

If you are using butt joints (or dado joints) and do not mind having the screw visible, you can have a 25mm (I work in US so please excuse inaccuracies) to 38mm screw to join the two materials together; you can space the screws from 15 to 20cm apart.

There are also some great resources you can refer to, like "The Joint Book" which may be available at your local library, or for purchase at Amazon.com.

You may also want to check out wood joining articles at FineWoodworking.com; an enjoyable site with lots of tips and videos for all levels of woodworker.

  • A butt joint with 18mm stock and a 25mm screw wouldn't be very strong. I'd be using something 45-50mm (with pre-drilling to avoid splitting plys as noted by Ast Pace). – Aloysius Defenestrate Jul 26 '16 at 0:33
  • 2
    The necessary size and spacing of screws is function of the anticipated loading conditions and the type of wood. Without a clue of the design, there is no way that anyone can supply a responsible answer to OP. – Ast Pace Jul 26 '16 at 2:00
  • I do agree that there is not too much strength with the above mentioned design and that without knowing the design, it's a bit of a stretch to provide a good answer; I did work under the assumption that this was a computer desk, and as the trend for computer desks have been to support laptops, I was not too concerned about the amount of load the desk could handle. Now, side to side racking rigidity is another thing, which would be accounted for with the type of support set up at the back of the desk. – Jaime Rios Jul 26 '16 at 11:02
  • I also think we need more detail before this can be Answered properly, unless someone wants to take on the task of covering all possible way the screws might be used :-) Bust just about the laptop thing and the low expected load, the laptop itself will weigh very little of course, but the pressure exerted on the table by the user would be many times this. Couple that with the expectation of leaning against the edge to stand up or sit down and we're into the realm of the loads all tables are normally built to withstand, with the consequent need for strong, stable connections. – Graphus Jul 27 '16 at 6:04
  • Thank you for the replies! I've been offline so was unable to reply sooner. Sorry for the unclear question I wasn't sure what to ask. I also hadn't looked into saving an image from Sketchup yet (I used snipping tool) I will be looking in to that shortly to add dimentions. I have now uploaded a picture, It is not complete but is the main idea. I plan on hiding the screws using a wood plug cutter thatI have already purchased to hide holes in my skirting (goo.gl/9Evksq) – Phoenix830 Jul 28 '16 at 12:50
1

With the addition of your sketch for the design of your desk several other design considerations besides screw length come to mind. Although this answer does not directly address your question it might be helpful.

  • Desktop projection. If you want to sit at this desk, the top should project further than the base. Many desks have an op[en area so that a chair can park under the desk. The raised base on your desk will prevent that and will limit how close to the desk you can sit. Consider projecting the top at least 30cm beyond the base.
  • Shelf support. The shelves on the left lack support at the back and right side. The shelves should have support on all three sides. A back panel and a divider panel in the middle would improve things.
  • Lateral bracing. The cross braces at the back may not be adequate to prevent the entire desk from leaning side to side and trying to fold up. A back panel or skirt at the underside of the top attaching to the top and sides would help keep the desk rigid.

Feel free to ask another question if you want additional clarifications.

  • Thank you, I hadn't considered projecting the top and will now add it. I will also move the plinth back. The image above is just where I got to when I was trying to add pilot holes and didn't know what length to add. The shelves were going to be supported by M6 rods and insert nut and a metallic finish plastic tube similar to what you find on glass tv units. However I am now thinking of using dowel moulding either 15 or 25mm. This was going be on the front right. The vertical panel is a sort of box with the back missing to hide wires and to give support on the right as well. – Phoenix830 Aug 11 '16 at 11:47
  • Lateral bracing - I will be adding a back panel similar to the bottom shelf but also connects to the top. I just haven't decided on the exact high of the shelf yet. I need a gap for the scanner wires or to add a whole (may weaken the support?) Thank you for your much valued input and will add the changes to my design – Phoenix830 Aug 11 '16 at 11:48
  • Sorry it looks like a lot my paragraphs didn't show – Phoenix830 Aug 11 '16 at 11:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.