Thread: How would you cut these mortices?

Originally Posted by qwertyu

... In regards to epoxy - west system epoxy is designed for coating boats right? Is that still ok for structural gluing like this? Just use something like this to thicken it?

Aerosil - 1 Lt - Filler for Polyester & Epoxy Resins (FREE FREIGHT) | eBay

Never heard of aerosil, so no comment.

The original WEST System of yacht building consisted of building the curved hull surfaces from thin strips of timber laminated together with epoxy and then coating everything in yet more epoxy. WEST is an acronym for Wood Epoxy Saturation Technique. WEST System epoxies are designed for constructing boats - for gluing, for filling and for coating. Do you know any structures as highly engineered as a yacht hull?

The WEST book explains the merits of the variuos fillers. I use:

• #417 - Microspheres, as fine as talcum powder, good general purpose filler,
• #406 - Colloidal silica, imparts anti-slump properties so that it does not run on vertical surfaces,
• #sawdust - actually sander dust, collected from my ROS - my go to filler.

I see - are fillers ever used to add strength? like stones in concrete or is just just to add bulk or reduce slump in epoxy

Originally Posted by qwertyu

I see - are fillers ever used to add strength? like stones in concrete or is just just to add bulk or reduce slump in epoxy

When I was making boats (quite a while ago now) and using BoteCote epoxy there was available ‘microballoons’ and gluing fillers.

The microballons were spherical and added no strength, in fact I think they weakened the product. They were used mainly for fairing. Their main attribute was that they made the product easy to sand - say for fairing racing hulls.

And the gluing fillers (forgotten their name) were tiny slivers of glass or wood fibres. They were meant to be much longer then they were wide - and yes it was claimed that they greatly enhanced the strength of the glued joint due to their properties.

I think this is the modern equivalent of the gluing fillers: https://www.timbecon.com.au/bote-cot...MaAiRmEALw_wcB

BoteCote was a good Australian product and vey affordable.

This link is perhaps the simplest way to explain the difference between "fillers" and how they affect joint strength.

Epoxy Filler Selection Guide for Marine Grade WEST SYSTEM Epoxy

I think this discussion is becoming side-tracked about "epoxy resin" technology. The term "epoxy resin" is used to describe a vast range of "reactive prepolymers and polymers which contain epoxide groups" essentially curing resins for simplicity. The formulations of which have been tweaked to suit numerous applications & uses in industry, as embeding resins, adhesives, fairing fillers, industrial coatings, etc.

Some of those applications require quite specific properties, and in this instance as an adhesive it requires bond strength, sufficient working time, reasonable cure time, and other desirable "workability" properties such as viscosity (low slump for vertical joints or 'runniness' for some applications) etc. The addition of "other materials" to the resin / hardener mix can modify the properties of the resin, some beneficial, however there may be a compromise in other properties.

Therefore, a high strength, easy sanding adhesive is unlikely as is an easy sanding, high strength filler. Compromises!

Originally Posted by qwertyu

I see - are fillers ever used to add strength? like stones in concrete or is just just to add bulk or reduce slump in epoxy

Fillers never add strength - they just thicken the brew. If you want to increase strength then you add fibre - glass fibre, carbon fibre, kevlar, spectra, etc. As one wit said: "Timber is God's carbon fibre."

Microballoons are a hollow, lighter and more expensive option to microspheres. Target market is to yachtsmen who want to spend $1,000 to save 10 kgs. They are horrible things to work with as they do not want to mix with epoxy - so light that they skip around on top.

Ive been doing a bit more research and I figure Ill post this here for my own reference and others too

west system recommends an oversized hole when using hardware of about 6mm but it can be up to 2x size of the hardware. eg 5mm bolt or threaded rod then 11mm hole. That gives 3mm of epoxy around the screw

when gluing one should wet the holes/surfaces with unfilled epoxy prior to filling with filled epoxy

for structural applications/non-sag silica is suitable. there is colloidal silica (west system 406) and fumed silica (aerosil and carb-o-sil). there seems to be mixed in what people prefer but I read that fumed silica has better non-sag

I think I will try a mix of fumed silica and wood flour. I think pure wood flour will be too dark and the silica is white which will lighten it

In regards to making wood flour, what grit should I use on my ROS? Can I use a belt sander at all? I have some 180/240 grit for my belt sander which I never use

Originally Posted by qwertyu

Oh big brain thinking! seems so simple when you say it like that haha



Thanks Ill have a look through that

In regards to epoxy - west system epoxy is designed for coating boats right? Is that still ok for structural gluing like this? Just use something like this to thicken it?

Aerosil - 1 Lt - Filler for Polyester & Epoxy Resins (FREE FREIGHT) | eBay

Double the hole diameter seems excessive.

Epoxy needs a minimum 0.5mm-1.0mm gluleine thickness, depending on the epoxy/manufacturer. Personally, I'd drill a 10mm hole for an 8mm threaded bar. Don't know the reason for the double diameter - might be a reason I haven't thought of.

Good summary, qwertyu, pretty much agree with all that you said.

Pre-wetting with epoxy is recommended for virtually all jobs. Sometimes, to aid penetration, I will thin it with MEK or acetone - this gets it into the grain of the timber.

Haven't used fumed silica - colloidal works fine.

Originally Posted by qwertyu

... In regards to making wood flour, what grit should I use on my ROS? Can I use a belt sander at all? I have some 180/240 grit for my belt sander which I never use

I collect the dust from both orbital and random orbital sanders. Whatever job I am doing, if I think the dust is good, I put it in a jar.

I find that I get the best dust - like talcum powder, from 100 - 300 grit paper. If the grit is less than 100 grit then the sander dust is often contaminated with pieces of sanding grit (aluminium oxide) that comes off the sand paper. If the grit is finer than 400 then you get very little dust so it is hardly worth keeping.

Originally Posted by eddie the eagle

Double the hole diameter seems excessive. ...

Double the diameter was based on a lot of engineering testing about 50 years ago. It is all about spreading the load.

Like all rules of thumb it can be more precisely targeted for specific substrates.

Originally Posted by GraemeCook

Double the diameter was based on a lot of engineering testing about 50 years ago. It is all about spreading the load.

Like all rules of thumb it can be more precisely targeted for specific substrates.

My trade too - epoxy needs glue line thickness to get shear strength as you hinted at. I'd go with the manufacturer's recommendations, bur it's probably excessive. Must away, on pre-xmas spring clean. Can't respond for a while

I suppose now that we have worked out the technique - do you think its stronger than traditional mortise and tenon/floating tenon?

Another thought I have, since drilling holes is a pain and making templates for a few joints can also be a pain, could you use a domino to make the mortise and put 2x threaded rods next to each other?

Originally Posted by qwertyu

I suppose now that we have worked out the technique - do you think its stronger than traditional mortise and tenon/floating tenon? ...

With the massive dimensions of the sofa, there is so much "excess strength" that this question becomes largely irrelevant for this design.

... Another thought I have, since drilling holes is a pain and making templates for a few joints can also be a pain, could you use a domino to make the mortise and put 2x threaded rods next to each other?

A large part of the aesthetics of this design iis the massive scale and the exposed joinery. Change this and you would really change the ethos of the design.

If you did use bolts or threaded rods, why would you also need dominos?

Hang on, what sofa are we talking about?

I suppose my question is more in general - which is stronger - M&T or threaded rod+epoxy in oversized hole.

When it comes to domino - I dont mean put dominos in - just use the domino machine to cut the holes since its so easy/fast compared to using a drill press/drill. But one would just have to use 2x threaded rods in each hole

Originally Posted by qwertyu

Hang on, what sofa are we talking about? ...

This one; the one in the OP's original post #1. That is what the thread in about.

Sofa - Pine.jpg