Underpinner Issue

[RobM]

Hi All,

A quick amateurish question that I'm sure many of you will have an easy answer to...

In putting together a frame, I'm installing two wedges in each corner, as I would expect to be normal.

In the attached pictures, the wedge which is supposed to be on the "inside" side (albeit it is more in the centre) is inserting fine. However, on every corner, the wedge on the "outside" is not inserting fully and is sitting proud.

What causes this?

[NTG999]

The way the underpinner works is to press down on the top before pushing up the wedge. If the clamp bung is less dense than the pressure required to push in the wedge then the moulding will lift. I would look at the operation, test without a bung - using a block of wood instead. You want to be using as dense a bung as possible that doesn't mark the top of the moulding. When I used a foot operated underpinner I used a scrap of MDF instead of a bung when joining hardwoods.
Separately I would place the wedges nearer the inside of the frame

[Justintime]

To expand on NTG999's comment, place the inside wedge as close to the inside edge as you can and the back wedge, try placing it half way between the two in the picture. Placing the back wedge as far back as you have often causes the back of the moulding to flare open.

[JKX]

Is it a flat moulding?

If so then maybe the back v nail is so close to the machine’s fences, that the pressure pad is hitting them.

If not then maybe the pressure pad needs to be lower.

I agree the back v nail is far too close to the back of the moulding.

It seems the v nails are not perfectly central to the join too.

[prospero]

The main thing when underpinning is to get the top clamp pressure directly over the wedge insertion point.
I use a wooden triangle of wood faced with cork and wrapped in clingfilm. To prevent hardened glue building
up on the cork I wrap it in clingfilm which can be easily changed.

It's an extra operation to place the triangle but it does allow you to get pressure on just the right point. You don't
always need it, it depends on the underpinner and the moulding. Very good for deep scoops.

* I always like to pin all the outer wedge positions first and the (changing the top clamping if necessary) and then
do the inside. There is a good reason for this from an engineering standpoint but I won't go into it just now...

[vintage frames]

* I always like to pin all the outer wedge positions first and the (changing the top clamping if necessary) and then
do the inside. There is a good reason for this from an engineering standpoint but I won't go into it just now...

That's interesting.
I always used to do it the other way, ie inside pin first and then the outer.
Can you tell us the reasoning?

[Justintime]

Yeah I'd love to know too, as I do outside in from trial and error, leading me to believe that the framing gods insist it is done this way or catastrophic failure of some description will inevitably happen. I'd love to know a less superstitious reason.

[JKX]

With some underpinners the pressure pad and v nail block move together.

With the CS200 wot I had, there was also automatic pressure sensors, don’t you know The pressure pad would not return until the v nail was in.

[Mark Thornton]

An incorrect or badly set top clamp pad results in the moulding lifting whilst the wedge is being inserted.

Mark

[pramsay13]

I put the wedges in from the inside to the outside, but I'm not sure why.

[RobM]

I will try to improvise with some alternatives, as you recommend. I've got to admit, I'm entirely sure I could discern between a dense bung and a non-dense one!

You've made another good point, which I hadn't mentioned; yes, the bung did mark the front of the frame, which was a bit alarming.

Noted about the wedge positions.

Thanks a lot.

[RobM]

I'll be trying this from now on, thanks Justin.

Looking at some of the outer wedges I put in, I can see how they have damaged the outer lining of frame.

[WannabeFramer]

You pin from outside in? Ooh, I’ve always gone inside out so will try swapping. I’m all topsy turvy now!

[Justintime]

Until Prospero gives us the engineering sense behind it all, I reckon do what works for you!

[NTG999]

My Alfamacchine U300p inserts wedges from outside to inside so I'll stick with that

[prospero]

It's a fine point but I'll try to elucidate....

If you have cut the angles spot-on when you come to the last corner there should be a small gap between the
faces. When you push them together it 'cinches up' the whole frame. The three other v-nails act a fulcrum points
and the joints will all be super tight. Desirable for a good glue bond. That's if the v-nails are near the outside.
But... if they are toward the inside then the tendency is to pull the other joints apart at the back, the fulcrum points
being in a different place.

Imagine this... Pin a frame with no glue with the wedge positions on the inside. Stand it on end and try to deform
it sideways. You can. But pin on the outside and you can't.

As I say, a fine point but a good principle.

[vintage frames]

Ah-ah, I see your logic.

But ..

It depends what sort of an underpinner you're working off.

The one I used was a simple but thuggish beast, (Brevetti Aut82) which didn't have any clamps. You just pushed the two mitres together into the fence and fired - inner pin inserted - then you pulled back and fired in the back pin.
To follow your method would require more gymnastics than a 1-2-3 cycle and you're done.
But that's with the one I used - still have it sitting brooding in the corner - never gets used, - must sell..

[RobM]

will try to improvise with some alternatives, as you recommend. I've got to admit, I'm entirely sure I could discern between a dense bung and a non-dense one!

You've made another good point, which I hadn't mentioned; yes, the bung did mark the front of the frame, which was a bit alarming.

Noted about the wedge positions.

Thanks a lot.

[RobM]

I'll be trying this from now on, thanks Justin.

Looking at some of the outer wedges I put in, I can see how they have damaged the outer lining of frame.

[RobM]

That particular moulding was flat, yes. I also tried with a curved moulding, where the wedges seems to fix in better.

I'm getting the overriding impression that the outer wedge was just too far back, so I'll be moving them further inside from now on.

The idea of the bung behind too high is a good point. I hadn't actually tried lowering it previously, so will adjust that and see if it makes any difference.

Thanks John.

Is it a flat moulding?

If so then maybe the back v nail is so close to the machine’s fences, that the pressure pad is hitting them.

If not then maybe the pressure pad needs to be lower.

I agree the back v nail is far too close to the back of the moulding.

It seems the v nails are not perfectly central to the join too.