Help - Mitres opening up & underpinning big carved obech

[Bill Henry]

I’m going to take a slightly contrarian approach to this “out of square” problem.

On the rare occasion that I need to check/adjust the fences on the Morsø, I use a ten foot length of metal moulding and extend it the full length of the machine. I believe that since wood is so prone to warping that an aluminum rail of moulding will give you a straighter, truer guide to set your fences. This way you can guarantee that both right and left fences are perfectly aligned.

But, I would be very reluctant to adjust the left fence to compensate for “untrue” miters. In the unlikely event that the both right and left fences are not perfectly perpendicular to the blades, it shouldn’t matter!

Since the blades are factory set at perfect right angles to each other, even if they are not perpendicular to the fences the right and left cuts would have to equal 90°. (If, say, the right cut were only 44°, the left would have to be 46°, so the corners, once joined would be 90°). This is, of course, assuming that the Morsø wasn’t dropped off the loading dock at one point.

I might suggest that your less than perfect joins may be due to the underpinner rather than your chopper. It sounds to me like the corners are separating slightly either because your “hold down” clamp is not tight enough or the vise grips are not holding well enough. Also, you might check to see if you using the right type of V-nails. My underpinner makes V-nails for both “hard” and “soft” woods. (It is not a Cassese)

But one more thing to check. It is possible that some grit managed to find its way behind the blades of the Morsø and the knife block and bumped them ever so slightly out of square. You might try removing the blades and lightly clean both the blades and the knife block with fine steel wool.

[deejay]

I agree it's more likely to be the underpinner than the Morso. Incidentally, that bit about stuff lurking on the cutting bed and causing the cut to be out of true, I actually make use of this effect if the moulding is warped. A small strip of card or other material that balances the warp can often result in an out of true cut that enables an almost perfectly flat frame if the joints are then carefully aligned for pinning.

[Not your average framer]

Although we have not yet any answers to the questions I asked, I take the liberty of going through a few points and maybe I hit on some key matters along the way.


First point.

We are told that the mitres were good before the frame was placed on radiator. Well even if there was no glue in the mitres at all, as long as the mitres were adequately pinned, the mitres opening up so easily would come as a bit of a suprise.

So my first point is that large mouldings should be adequately pinned, with sufficient stacked points to fully secure the deeper parts of the moulding and also additional pinning as appropiate to obtain good secure tight mitres.


Second point.

Obeche is an absorbent wood, so it needs enough glue to ensure that sufficient glue remains in the mitre to ensure a well bonded joint. Use a genuine woodworking grade PVA or other appropiate wood glue. Sometimes a second application of glue may be advisable.

Builders or general purpose PVA is much thinner as it has been watered down. More water, means less PVA resin, means less of the stuff which make the bond in the joint. It's not rocket science, use enough of the woodworking stuff and it will be strong.

Make sure that the whole of both surfaces in the mitre are glued, not just a drop in the middle!


Third point.

Understand how the underpinning wedges (or V-nails) work. Normal wedges are ground much more on the inside edges than on the outside edges, so that they pull the joint together as they go in. Unfortunately under some conditions, normal wedges can easily do this too much in hardwoods and come out of the side of the frame. Also normal wedges don't stack quite so well in hardwoods, but can slide out from under the wedge above, instead of penetrating the the full depth intended.

This is why hardwood wedges are provided, these are ground closer to an equal amount on the inside and the outside edges. They work better in hardwoods and are designed to aviod the problems mentioned above.


Conclusion.

Large frames and large mouldings require extra wedges and more than enough good quality glue to ensure strong mitres.

[blaisehall]

...always check after putting on new blades (check there is know burr on the blades first)

Thanks Mitreman, for that very detailed advice. I'd forgotten - they are new blades. I'm going to have a fiddle with them tomorrow. I spent the whole of today moving all my mouldings into a warm, dry place to stop them absorbing damp. I will do as you suggest about the blades and test on some bits of wood.
What makes me think it isn't the blades is that I cut larger mouldings too - and the mitres are pretty perfect front and back. The large obeche is a sod though. Also, it looks good before I pin it - when I just hold it together.
Blaise

[blaisehall]

Get a thin cork floor tile and cover one side. Good when you have to pin a moulding with a sharp top.
A chrevron made from approx 3/4" flat moulding (also cork lined) is handy for clamping odd mouldings where there is a steep slope and the pressure pad can't reach without marking the side of the slope.

This is a really good-sounding scheme, I'm going to try it. Thanks Prospero.
Blaise

[blaisehall]

I cut a triangle from 1/8 alumium drilled and counter sunk a screw into the rubber bung (which i bought a spare for) and covered the alumium trangle base over the screw hole with Upholstery rubber glued on with evo-stick.

That sound like a good solution to this problem. I'm getting the feeling that it's ok to be a bit creative with engineering solutions as long as they work! I've got more energy to solve this pinning problem now that I'm inspired.

[Not your average framer]

The large obeche is a sod though. Also, it looks good before I pin it - when I just hold it together.

Ah! it's a difficult moulding! Some large mouldings can need some extra help when underpinning them.

Often tall mouldings can open at the front as the wedges are going in and sometimes benefit from a band clamp around the top.

Another trick which can sometimes help is to glue the mitres together first using a band clamp and underpin them later when the glue is fully set.

[blaisehall]

I'm not sure if we have established what's going on here ... exactly where are the mitres opening?

The mitre is lovely all along the back of the frame (where it's pinned) but open a crack all over the front.

...do I understand that this is only a problem on this particular moulding? If so is the moulding much bigger that ones that you normally use? Where are you placing your wedges? How much of the thickness of the moulding is fully secured by wedge penetration? How much glue are you using in each mitre?

• Yes, just a problem with the really big carved obeche
• I place the wedges a cm from the edge and 2cm from the middle.
• Wedge penetration is a good point - that could be an answer - maybe I need bigger wedges for this one.
• Not much glue

Where I was trained I was told to always underpin the thickest part of the frame first. I've not heard it elsewhere, but it's a good tip to follow with difficult mouldings.

Thanks, good tip.

[blaisehall]

we need to see the exact problem ... why not post some photo's...
MITREMAN

I've dismantled the offending article so can't photograph it I'm afraid. I'm doing lots of tester frames to test out some of the tips. I've got so many ideas on how to fix the problem from you lot I'm sure I can get rid of that crack.

[Not your average framer]

The mitre is lovely all along the back of the frame (where it's pinned) but open a crack all over the front.

It still could be a Morso problem too! Some mouldings can try and push apart as you make the final cut. You control this by securely gripping the moulding against the two fences on the Morso. Sorry, but you need a strong grip, (or a couple of G-clamps) and by taking very thin finishing cuts, which also helps.

[blaisehall]

blaisehall. :) Meant to ask......What glue are you using?

Wood glue, but only a tiny bit. Perhaps I should be adding more.

[blaisehall]

The other factor that can cause problems is the dreaded snot wood - you generally notice this when cutting as the back of the moulding often just breaks away - sometimes the whole moulding just wont cut clean - when you get this soft sappy wood your not going to get a joint that holds.

I don't think it's snot wood.

[Not your average framer]

Wood glue, but only a tiny bit. Perhaps I should be adding more.

Hi Blaise,

Have you seen my earlier posting where I explain about glue. It's a big moulding, so it needs strong corners to cope with any forces in the mouldings trying to spring them apart.

Strong glue, enough wedges and in the right places are the essentials with big moulding or big frames.

[blaisehall]

It sounds to me like the corners are separating slightly either because your “hold down” clamp is not tight enough or the vise grips are not holding well enough.

I think it may be this too. I'm going to try out a few different ways of holding it tomorrow.

But one more thing to check. It is possible that some grit managed to find its way behind the blades of the Morsø and the knife block and bumped them ever so slightly out of square. You might try removing the blades and lightly clean both the blades and the knife block with fine steel wool.

[/quote]
I think the blades are ok because I cut bigger mouldings than this ok (but they're flat, not carved. I think it must be the pinning I'm going wrong on.
Blaise

[blaisehall]

We are told that the mitres were good before the frame was placed on radiator. Well even if there was no glue in the mitres at all, as long as the mitres were adequately pinned, the mitres opening up so easily would come as a bit of a suprise.

I think they opened up easily because the contrast was too great between the two environments. Wales is damp (no big surprise) and storage heaters are hot. The unheated barn was a bad place to store them. I rang my brother who bought one of these off me to see if his were alright and they were, (thank goodness), so I think it was the extremely different environments that caused the dramatic opening up. The mouldings are now safely tucked up in the nice warm (but not too warm!) studio.

So my first point is that large mouldings should be adequately pinned, with sufficient stacked points to fully secure the deeper parts of the moulding and also additional pinning as appropiate to obtain good secure tight mitres.

I do stack the pins.

Make sure that the whole of both surfaces in the mitre are glued, not just a drop in the middle!

Yes, I don't think I've been using enough glue.

Understand how the underpinning wedges (or V-nails) work. Normal wedges are ground much more on the inside edges than on the outside edges, so that they pull the joint together as they go in. Unfortunately under some conditions, normal wedges can easily do this too much in hardwoods and come out of the side of the frame. Also normal wedges don't stack quite so well in hardwoods, but can slide out from under the wedge above, instead of penetrating the the full depth intended.

This is why hardwood wedges are provided, these are ground closer to an equal amount on the inside and the outside edges. They work better in hardwoods and are designed to aviod the problems mentioned above.

This is very interesting. Does that mean that you must never use a hardwood wedge in a softwood or does it not matter much?

[prospero]

When joining the frame, the method I use is to pin from the outside to the inside. If only one wedge is required, put it nearer the outside than the inside.
When you come to join the last corner, the faces should be just a little apart. If they touch, then the morso left fence probably needs tweaking. (I'm talking minute amounts here, if you overcook it you start to gets gaps on the outside). When you close the final gap the inside of the other three corners get a final squeeze and you get a nice rigid frame. It doesn't work the same way if out pin inside first. It's a question of leverage. Hard to explain, but think about it. When you have pinned the frame it should feel like it is one single pice of wood. If it feels the slightest bit wobbly, then there is a bad joint somewhere.

Try this little demo that will illustrate the point:

Get 4 scrap bits and mitre them. Pin near the inside of the moulding (no glue). Stand the frame up on the bench with the frame facing you, hold it by the top rail and try rocking it sideways. It will deform easily and the backs of the corners will open. Now try it again, but pin near the outside......

[osgood]

I've stayed out of this topic so far, but am putting in my two cents worth now.

Sorry, Bill, but I have to disagree with one aspect of your response. That is changing the angle of the left fence.
My left fence gets changed several times during the course of trimming a batch of various frames. (I cut the pieces on a double mitre saw, then skim many of them on the chopper to give a crisper joint.)

I have found that there are several things that will change the angle of the cut on different mouldings. Different profiles, widths, hardnesses of wood all contribute to very small angle differences. Those differences will result in some joints being open on the inside, some open on the outside and some will be perfect if the left fence remains locked in place.

This effect can be tested easily by cutting 4inch x 4inch frames in a number of different profiles, widths and hardnesses of wood and place each of them together, dry on a flat bench top.

[John]

My left fence gets changed several times during the course of trimming a batch of various frames.

Let's be clear about what is happening here though. The geometry of the machine is constant so you would think that, once set, it should give identical results regardless of the profile or hardness of the wood.

However in practice, when cutting the larger profiles and harder woods, as the blade slices down, slight movement occurs either in the moulding to the left of the blade moving to the left, or the cutting head moving back and away from the fences, or both.

To compensate for this, many experienced framers, after years of practice, develop a knack of knowing just how far the left fence needs to be misaligned to correct for this movement for any given profile.

I really can't be bothered with all the fiddling about with the fence and prefer to ensure that no movement occurs either in the head or in the moulding, specially during that final cut. The down side with this method is that cutting the larger hard woods requires either a second pair of hands or a clamping system to stop movement in the moulding to the left of the head.

[prospero]

I've stayed out of this topic so far, but am putting in my two cents worth now.........

Wise words and worth more than two cents.

There are may variables to consider. As soon as you start with a freshly sharpened set of blades they will start to dull. In rountine useage the front of the blades will tend wear more than the middle, so when you come to cut a wide moulding, the sharpness is not consistent along the cut. If regularly cut a 100mm scoop and it is a swine to get the angles perfect. Because of the shape, the middle (sharpest) of the blade starts to cut and is a good way though when the front (dullest) of the blade contacts the high back of the profile. This can cause the cut to deviate.
Wood varies from length to length, even along the length. Some profiles will jiggle about when cut, particually those that are higher than the width.

[Not your average framer]

This is very interesting. Does that mean that you must never use a hardwood wedge in a softwood or does it not matter much?

It's not a hard and fast thing, you can with experience make the decision according to the wood and the moulding. You will notice I mentioned understanding how the wdges work, that's the key. Eventually these things become instinctive.

If you are not sure how a moulding will cut and join, order enough to do a couple of trial mitres. Also you can often turn your left over bits of moulding into ready made frames and get some extra practice and income as you do so.

Some suppliers do discounted clearance mouldings, which can be a cheap way of practicing and hopefully making some extra sales. I like to do framed mirrors, using 3 ft x 4 ft 4mm thick mirror glass, which I cut up to make several mirrors. It can be a profitable thing to do.

You can't beat practice and experience. As a new framer, get as much as you can using your left-over scrap. Practice and experience are what will give you the confidence to do the difficult stuff.