Underpinning issue

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Underpinning issue

Post by jonbon » Fri 04 Sep, 2020 4:03 pm


I'm having an issue with my underpinner, and I wonder if anyone might be able to explain what might be going on.

The front of the mouldings look great but at the back, there is a gap in the top corner. My main concern, is that aesthetically on the outside edge of the join, there is a little gap which doesn't look great. If anyone has any ideas as to why and how i might be able to avoid it, it would be much appreciated.

Thank you

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Re: Underpinning issue

Post by Orde02 » Fri 04 Sep, 2020 5:14 pm

I don't know for sure but isn't this more to do with how the mitres are being cut rather than the underpinning? I'm sure someone who knows more will offer more help than I can.


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Re: Underpinning issue

Post by JFeig » Fri 04 Sep, 2020 5:59 pm

The inner V nail appears to be ok while the second v nail has split the wood.

My first guess is that the wood is punky (bad) and gave way.

Second, you could always use one nail vs two. Place the nail close to the middle of the moulding's width. Also note that the second nail was set between growth rings and that might have caused the split.

Third are you using "softwood" or "hardwood" nails? That can make a difference as well. My preference was to use only "hardwood' nails.
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Re: Underpinning issue

Post by StevenG » Fri 04 Sep, 2020 9:35 pm

All good advice above & I'd also check on the vertical fences with your underpinner

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Re: Underpinning issue

Post by prospero » Sat 05 Sep, 2020 9:33 am

Is that Oak? If it is, that's par for the course.

Oak (and Ash) need a different approach, particularly on tall, narrow mouldings.

I would put a shallow (7mm?) v-nail near to the inside and secure the top with two crossed panel pins. The v-nail
will hold the join in alignment and the pins will complete the join. OK, you have holes to fill. Before the advent
of underpinners all frames where joined with pins. It's an 'old school' technique, but one worth learning. Same with
hole filling.

If you do a lot of this type of moulding, a Hoffman router might be in order. But like underpinners they have their
drawbacks. They reduce the glue area and this is the main strength on a join.

Strap clamps and the good old mitre vices have their place for 'difficult' mouldings. :D
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Re: Underpinning issue

Post by prospero » Sat 05 Sep, 2020 9:39 am


You could rescue those joins by forcing glue into the gap. Non-Solvented cartridge glue will stick and fill.
Brummer Stopping in the right colour is very good for filling oak.
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Re: Underpinning issue

Post by Not your average framer » Sat 05 Sep, 2020 12:33 pm

This can be quite a difficult issue with some wood types and in particular I would mention Oak. I have a good stock of Oak and it's a big favorite in my shop, but it can be somewhat problematic as well. Not all underpinners do the best job of joining hard woods like Oak, even some underpinners with rebate clamps are not necessarily up to the job and not all rebate clamps are as effective as each other.

Also not all makes of wedges are as good as each other. I use a Cassesse CS-88, but I reload the cartridges with the Alphamacchine powertwist universal wedges as these work really well with hard woods such as Oak. I also have an "L" shaped wooden insert which I fit between the fences and the frame mouldings being joined, This insert is linning on the inside with fine abrasive paper, which prevents the wood of the moulding from sliding side ways away form the centre of the joint.

The CS-88 is an underpinner which I like, but the rebate clamp is pretty much much useless on woods like Oak and the problem is a lack of enough grip, when more pressure is needed from the wedged to penetrate the wood and this is where the abrasive paper provides the added grip. Don't get me wrong, Ii am not displeased with my CS-88, but sometimes it needs a little extra help. The rebate clamp is used to help maintain the presure between the moulding and the abrasive paper lined "L" shaped insert, I also supplement this force by using my hands to provide some extra clamping force too.
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Re: Underpinning issue

Post by Justintime » Mon 07 Sep, 2020 11:33 am

This and far worse is why I bit the bullet and bought a second hand Hoffman Dovetail joiner...
And believe me, I had tried all of the tips above. Brummer "Natural Oak" filler I find the best for Oak and Ronseal Multi Purpose Wood Filler "Natural" for Ash.
Before the Hoffmann I used to cut lots of tester pieces and keep testing until it came right, fractional differences, including where you put your hand pressure (on a foot operated) can all make the difference. Unfortunately one piece of Oak can be very different to another.
Have you tried band/strap clamping these corners? (FrameCo steel band clamp)

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