What Filler??? |
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snowflake
Old Croc Joined: 29 December 2004 Location: Bristol Status: Offline Points: 3118 |
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in some ways a screw is a liability, in other ways it isn't - depends on the material and the joint used.
in theory a screw will concentrate the stress in the joint at the point where the screw is - this then becomes a point of failure. this only holds for materials that are like theoretical materials eg plastic it is a bit more complicated with materials such as ply as they have layers and grain. because the layers in ply are glued together over a large area, the glue that they use is often not that strong and there are small imperfections in the layers. the failure mode is often that one of the pieces delaminates. this happens particularly with 90 degree butt joints or rebates joints. In this case screws can help by holding the laminates together. I did some tests a few years ago with different joints. the strongest joint was one where both pieces were cut at 45 degrees and the cut ends glued together with a biscuit in the joint. no screws but every layer in the wood was glued to the other piece of wood in the joint and could not delaminate. all that said, screws are a quick and convenient way to hold things together while the glue sets. self-drilling, self-countersinking, non-corroding, smooth shank screws are what you want. no more than one every six inches is enough.
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ceharden
The 10,000 Points Club Joined: 05 June 2005 Location: Southampton Status: Offline Points: 11776 |
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In order to get a good glue joint the two pieces need to be held tightly together. Screws are good for that.
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70,s hero
Young Croc Joined: 14 December 2014 Location: bristol Status: Offline Points: 637 |
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when I made my cabs I looked at different fillers and used the car filler as it sets faster and is finer to sand , that said PVA with sawdust is ok, I think that if a plug lifts then it is unlikely its down to the rust on a screw, more likely movement in the joint and the screw lifting and pushing it out over time unless the filler was put in on a dusty dry joint. Plastic padding sticks like you know what and i have never seen it come out, it doesn't shrink and to my knowledge is non porous.
Looking at the screw scenario and stresses, if I were to screw two pieces of wood together and then use a nail bar and hammer to force them apart, the usual result is that there is a portion of wood left with the screw in it fixed to the other part. The argument that less screws are stronger is IMO contrary to excepted engineering principles (for wood) This tells me that the more screws that are used the better, whilst the forces are transferred to the screwed fixing upon failure of the joint, the joint remains intact around the screw fixing. If you have ever broken apart a cabinet, you will see this. When I constructed my cabs I used screws every 50 mm ( a bit OTT I know) but the application demanded structural integrity. In effect the further that the lateral forces are transferred in to each pane the batter. After talking with a structural engineer friend, I decided to half rebate each section which increased the contact surface area considerably and removed the lateral stresses upon all of the major joints. The upshot is that in reality, you could fix and glue a batton internally which is a really good way of stabilizing a 90 degree joint. Bona make an excellent filler, usually used for refinishing wood floors, going to try that next time for convenience.
Edited by 70,s hero - 08 March 2017 at 10:28pm |
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Top banana
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I-shen Soundboy
Registered User Joined: 21 December 2016 Location: Big Smoke Status: Offline Points: 375 |
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Agree - if you use screws, use a lot of them.
The clamping force of an individual woodscrew is several orders of magnitude less than a clamp, so consequently is only effective over a relatively small area. I'd contend a couple of sash clamps could out-clamp a box of woodscrews. Using screws and corner batons internally is another great idea that avoids having to fill screw holes. Or, if you absolutely must have external screws, don't countersink them, give them a cup washer, like on the removable back of old guitar cabs - although nowadays you can get solid stainless machined cup washers, colour anodised if you like, rather than the old pressed brass ones. I've got a couple of boxes covered in these raised silver screw heads: Very Metal. |
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SouthwestCNC
Young Croc Joined: 27 November 2015 Status: Offline Points: 830 |
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No matter the filler, filler doesn't swell with the ply when paint is applied. Without applying filler to the entire cab filler will always be visible. Plug the counterbored screw holes with wooden plugs is the only way to make invisble with paint.
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cravings
Old Croc Joined: 30 January 2007 Location: Ireland Status: Offline Points: 7441 |
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i feel like i notice filler shrinking over time rather than swelling.
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Shortrope
Young Croc Joined: 08 July 2013 Location: Ireland Status: Offline Points: 1232 |
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A lot of the time it's actually the timber swelling.
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My Tinnitus is coming along nicely!!
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SouthwestCNC
Young Croc Joined: 27 November 2015 Status: Offline Points: 830 |
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JonB67
Young Croc Joined: 22 April 2016 Location: UK Status: Offline Points: 1376 |
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So is it better to just glue and clamp?
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SouthwestCNC
Young Croc Joined: 27 November 2015 Status: Offline Points: 830 |
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Not without biscuits, dowels/dominos, or rebates imo. A glue only butt jointed box will not withstand multiple drop tests like a screwed box will.
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I-shen Soundboy
Registered User Joined: 21 December 2016 Location: Big Smoke Status: Offline Points: 375 |
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Screw and glue is the simplest and cheapest way to go.
I'd only advise more esoteric/traditional methods if you're getting into your cabinetmaking rather than knocking out half a dozen suicide boxes. Anyhow, if you've spent a few days laquering and polishing a cabinet (as opposed to ten minutes tuff-cabbing), you're far less likely to subject it to multiple drop tests |
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SouthwestCNC
Young Croc Joined: 27 November 2015 Status: Offline Points: 830 |
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Yep its horses for courses. There are branded touring cabs that are just pinned and glued, There are also youtube vidoes of them being pushed off the top a stack resulting a flat packed cab in a matter of seconds. Edited by SouthwestCNC - 09 March 2017 at 7:27pm |
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