The Framers Forum

Can anyone help?
I cannot seem to get a perfect join on almost every moulding I use.
It looks like the wedge from the underpinner is opening up the join but why would it do that.
I'm using a brand new minigraf with soft wood wedges.
Any advice would be greatly appreciated.

Ronseal multi purpose "natural" wood filler.

Ronseal multi purpose "natural" wood filler

Also, I'd check the vertical fences on your underpinner. Does it only happen when you put the outer wedge in? Are you stacking them?

It's not that difficult to understand. When you insert the wedges the wood and the wedges are trying to occupy the same space, but they can't some thing has to move. If the wedge is too close to the outside of the moulding, the thickness of the wood between the wedge and the outside of the moulding does not have enough resistance to resist that outward pushing force created by the wedge being inserted. The same thing happens when you drive a nail in to a piece of wood too near the ends. Being to near the end the reduced amount of wood between the nail and the end of the wood is not enough to resist splitting and the wood splits.

Why does it do that? Because it is the line of the least reistance! You can knock a nail in to a piece of wood without splitting the wood, it comes with practice, experience and knowing the right place to drive that nail in to the wood and of course it's the same with where you place that critical wedge that nearest to the outer edge. A thinner piece of wood between the wedge and the outside edge will not resist the force that is trying to push that piece of wood outwarda as well as a thicker piece of wood caused by the placement of that wedge being further from the outer edge. If the force require to compress the wood enough to allow space for the wedge being inserted, is less than the force required to deflect the wood outwards and open the gap, by doing so everything will take the line of least resistance and compress the wood enough to make space for that wedge by compressing the wood instead.

It's all simple physics and mechanics. It's no mystery at all, but just takes a little thnking about cause and effect. Once you realise what is happening and you need to do is change the position of that critical wedge and the problem goes away. It's simple and you can prove it to yourself but moving that wedge over enough to see it happen for yourself. Simple when you see it! It's also always worse in that outside position, when trying to stack wedges in that position, because there is less sideways force keeping the wedges one behind the other and the wedges can more easily find the way round the side of the other wedge and force the corner outwards as a result.

Pretty much life as we know it with narrow mouldings and underpinners.

Inserting v-nails closer to the inside will improve things, but the frame will be weaker.

hafa2uk wrote: ↑Wed 19 May, 2021 6:24 pm gap.jpg
Can anyone help?
I cannot seem to get a perfect join on almost every moulding I use.
It looks like the wedge from the underpinner is opening up the join but why would it do that.
I'm using a brand new minigraf with soft wood wedges.
Any advice would be greatly appreciated.

Do you pin the sight edge first or the outer edge?

That's an interesting question! Why does it matter which wedge you put first? I'm just wonder if there is a reason why. I used to get this problem a bit in earlier years with certain mouldings. I never did find out why, but I started using a little L shaped spacer in side the fences and the inside of the spacer was lined with thin abrasive paper, so that the rebate clamp stopped the outside of the mouldings moving apart. I does not much matter to me these days, because I do so many hand finished frames and any small gaps a the corners are hidden by the hand finishing.

It was just a thought as I was taught to always nail the outer edge first. It probably doesn’t matter but for me I reversed the position on my underpinner so I pin from the back of the machine and find I exert most pressure pulling towards me and get a good first pin (or two) into the outer edge and then work towards the sight edge. I know everyone will have their own techniques and there are a variety of underpinners out there. I suppose too that perfectly pinned joins require perfectly cut mitres.

Mmm, that's interesting! I was taught to put the first wedges in to the tallest part ofthe moulding. I'm not honestly very sure whether it makes much difference or not! At one time I was sometimes joining stacked mouldings toether by pinning through one moulding in to another, somewhere along the line I stopped doing it. It was not a conscious decision, I just forgot all about it! I often put a lot of wedges in to particular mouldings, becuse some moulding pull together better if you do this.

Many of my older mouldings which I mostly bought as job lots, join together better than many of the newer ones! I bought a lot of mouldings from a lady ex-framer and she had lots of stuff from Euro mouldings and Renaissance mouldings, These were always a joy to cut and join, there never created any difficulty for me when mouldings were being joined. Perhaps some things just look better with hindsight. I really don't like joining many of these mouldings produced in the far east, the wood often is of such low density that the wedges just don't pull the corner joints together properly. I have stopped buying, or using such mouldings completely..

I would not want anyone to think that I don't necessarily get gaps in corners that I need to fix, because I still need to at times! Obviously, I can hde some of these gaps inside stacked moulding frame, because they can't be seen. Outward swelling corners was also part of why I bought a bench top belt sander, cleaning up and swellings at the corners is particularly helpful when making stacked frame mouldings. I also often fill, sand and smooth corners on frames as it's really quick and much easier much of the time and as much of my work is hand finished anyway it makes sense to do so anyway.