just glue, just screws, or both?

[Florin Andrei]

(newbie question)

As a hobby, I'm doing small projects with plywood, usually not much bigger than a bread box. Previously, I used both wood screws and glue at the same time to assemble the parts (drill holes for the screws, paint the edges with glue, then assemble them with wood screws as usual), with excellent results - solid construction that withstands a lot of damage. I never used dowels, dados, or anything like that, just straight square edges.

The problem was, I could never align the pieces down to the last mm, the parts were usually off by 1 or 2mm because I could never drill the screws in with enough precision. Another problem, because I would usually use epoxy resin for gluing the parts, I had to work pretty quickly before the resin would solidify, and that had a tendency to create problems.

I'm making a square box now, around 10" each edge, using 1/2" baltic birch ply. This one needs to be very precise and very square (it will be the cradle for a telescope tube), so I'm not using screws to assemble it. I just glued the edges together with West System 105 epoxy resin, and then clamped it down. So far so good, all the pieces are aligned very well, and it's perfectly square. Looks great. In a few hours the epoxy will cure to solid.

My questions is: after the epoxy has cured, should I drill some wood screws, let's say 2 for each edge? Would that provide additional bonding strength? My goal is to make this cradle as strong as possible, given the material I'm using.

More general question: When assembling pieces with straight edges at 90 degrees, do I actually get any additional strength if I use both glue and wood screws, as opposed to using either just glue or just screws?

[John A langley]

I think your post is going to get a lot of different replies. And I think I might get a little criticism for mine. If I were going to use screws I would use something thicker than half inch but it is too late. I've glued a lot of baltic birch drawers together butt joint using Titebond II and a couple of pins nails. On a few occasions I have had to take a drawer apart. I've actually pulled the vaneer off the drawer side. Baltic birch has absolutely no voids in it and the end grain accepts glue very well. Seeing that this is for a telescope tube you might have room on the inside of the box to reinforce your joints.I do not know anything about the epoxy resin you are using.Maybe somebody will chime in on that.

[James Baker SD]

I am also a bit worried by the application--i.e. telescope craddle. Is it a big scope, say a 16" Dob? I am not so worried about the joint coming apart, but your square becoming a parallelogram when the weight of the scope is applied, especially if you tilt it to match your latitude. Triangular braces, cut with as perfect a 90 degree corner as you can make, glued into the corners to keep the craddle a square would be more value than screws at that point. If you want to add screws, consider sinking them into your corner braces.

James,
(also a woodworker and amateur astronomer)

[Florin Andrei]

No, it's a small scope, 6" mirror, 8" tube. It's not a very heavy thing.

Yes, there will be corner braces, at least on the bottom, to keep the cradle square, and to seat the tube nicely in the cradle (the inner edge of the braces will be rounded to match the tube). I'm following the Stellafane design pretty closely:

http://stellafane.org/tm/dob/mount/cradle2.html

[Mike Henderson]

I'd add the screws because plywood doesn't always glue well, especially when gluing to an edge of the plywood.

Mike

[HANK METZ]

Supplement the interior bracing with exterior corner irons, an exoskeleton.

images.jpg
- Beachside Hank
The use of nails in fine furniture is to be abhorred- drywall screws are preferred.

[Richard Dragin]

Glue and screw will be the strongest combination although either one or the other would probably be enough. I would also recommend against traditional wood screws which have a tapered shaft (also known as cut screws) because they are weaker than rolled screws. If you are shopping at the BORG newer style deck screws will be the strongest they carry. The zinc plated wood screws are a waste of money. A straight pilot bit with counter sink is also required.

[doug faist]

Another $0.02. When I was starting out with a Cra?sman table saw and no money, I made a lot of furniture, boxes, tables, etc., with Baltic Birch. I used only glue because I didn't have the patience to pre-drill, countersink then cover the screw holes. I have yet to have one of those pieces fail; even through two kids and six moves.

Obviously, glue AND screws would be stronger, but, all told, but with your corner blocks, probably not necessary.

Doug

[Marion Abken]

I think just gluing would be fine... plywood especially... endgrain & facegrain glues good!

I would expand your joinery to adding the Rabbet Joint... [1/4" x 1/2" for your BB 1/2"]
actually (1/2 the thickness ~1/4") x (the thickness ~1/2").
Modify your part dimensions accordingly.
When you clamp & glue, the Rabbets will keep the joints in place and square.

[Sam Murdoch]

I think just gluing would be fine... plywood especially... endgrain & facegrain glues good!

I would expand your joinery to adding the Rabbet Joint... [1/4" x 1/2" for your BB 1/2"]
actually (1/2 the thickness ~1/4") x (the thickness ~1/2").
Modify your part dimensions accordingly.
When you clamp & glue, the Rabbets will keep the joints in place and square.

This is a +1 to Marion's response. West is a very secure adhesive. The plywood will fail long before the glue ever does. Some joinery in the future as Marion suggests will be even more secure. Now that you have a nice square and glued box you could - just for fun - add some decorative fastenings - screws with fancy heads, or little pegs of another species, for e.g.

Another future option , though another step, is to use an 18 gauge or smaller pin with a nailer to hold the parts square, after which you can add screws for strength. This is a great application for a Festool Domino, but... $$

[John A langley]

Sam makes a good point about the dominoe. I've used it when I've made some really large drawers but I used 5/8 baltic birch because I thought 1/2 was too think for even the smallest dominoe. And Marion makes a good point for the rabbit joint. The reason I did not suggest anything other than what you have is because you had already built the box. Doug is right about a glue joint on baltic birch being durable.

[Mike De Luca]

Forget the screws. I'd probably glue in some inside corner blocks between the 45º corner braces and call it a day.

Instead of the individual blocks shown in the picture, I'd run a continuous strip between the front and back braces, epoxy them to the sides as well as the back of the braces. The blocks will keep the box square and it will be solid as a rock.

If you need clearance for the adjustment screws, rip the strip at a 45º angle along its length.

l_Clamp-11.jpg

CradleGlue.jpg

[Florin Andrei]

Many good replies, I'm learning a lot, thank you all.

If you are shopping at the BORG newer style deck screws will be the strongest they carry.

Pardon my ignorance - what's the BORG?

I would expand your joinery to adding the Rabbet Joint...

For my next project I'll start thinking about rabbet and/or dado joints. I did it once before, it didn't seem difficult, not sure why I didn't do it this time.

Instead of the individual blocks shown in the picture, I'd run a continuous strip between the front and back braces, epoxy them to the sides as well as the back of the braces.

I was actually thinking to do that anyway, regardless of any other strengthening measures I may take, at least for the top side if not for the bottom too. Seems simple enough.

[Stan Krupowies]

Many good replies, I'm learning a lot, thank you all.



Pardon my ignorance - what's the BORG?

Big Orange Retail Giant - aka Home Depot but also refers to Lowes and sometimes Menards

[David Giles]

Screws are for people that don't own clamps.

Seriously, do a test and report back. Make three test pieces using a 90 degree butt joint just like your project. One with screws only, one with glue only and one with both screws and glue. See how hard they are to break apart. Then you will intuitively know forever more about the relative strength of screws and glue.

My personal experiments indicate that glue alone without any clamping pressure is amazingly strong.