Am I missing something re: Mitre joints ?

[raph.basan]

Good Afternoon,
Me again, same problems as before, just less....
I seem to have sorted out the problem with my mitre joints opening up when using Celly or Gesso....
BUT, i'm still getting corners opening, and i really want to nail this one on the head.
I've tried different glues, and am sticking to the Titebond III, unless someone can prove to me that there is a better one on the market.... i've tried the polyurethane types as well, but the glue seems to get brittle and cracks, which does not help..... the frames still crack with the polyurethane...
I'm now using a mixture of Wedges, Biscuits and more wedges....
Two wedges as per usual, un-stacked as my Alfa 4 minigraf can't stack very nicely , one or two biscuits, depending on the depth of the frame, and then for laughs a small 2mm wedge under the rebate, this seems to help... well, i've got the opening of the corners down to a minimum, But, when i use a moulding with no rebate i still get cracking mitres...... Have any of you used dowels on frames before ?

Any suggestions/tips would help me right now, i'll try anything once

Raph

[JFeig]

I see nothing wrong with your technique - other than possible using too many v nails.

I looked at your other posts to see what type of miter machine you are using to cut your moulding, a Morso. How often are you sharpening the blades and are you taking big bites or small bites when cutting. Both issues can effect the angles of the cut miters.

[Davie]

I agree, you may be using too many wedges, but more, it may be more that the blades of your morso are dull, try placing a large piece of moulding in your morso cut in the usual manner but slowly bring the blades down on the moulding and watch for the moulding being pushed away from the blades its more notacible on the left hand side....if it does push away then have them hollow ground, another thing is to make sure that when the frame is put into the underpinner, and offered up to each other your not tilting them in order to get what seems to be a tight joint...check a frame you have already made and see if the underside is tight as well......hope you get it sorted.

[MITREMAN]

Mitre a postage stamp size frame in a 3inch moulding, then hold then frame together (do not join it) this will show up any problem in the Morso draft set up , gap at the front then you will need to adjust the left hand fench backwards, gap at the back then adjust the fence forwards.

If the mitres are ok then yes as Davie wrote, it could be that your blades are not hollow ground.

It also could be that the blades are bowed on the face edge, both will force the moulding away from the blade when you cut, if you cut slowly you will feel the moulding move.

The characteristic of this fault is the frame when joined will be open on the tops.

In which case, you could need a regrind, face grind or a even new set of blades.

Send Your blades to Framers Equipment to have them checked.

Check also that there is no movement in the Morso head. If there is this will force the head back when cutting and produced an open mitre.

Take hold of the head either side and try to rock it forwards and backwards if there is movement, then the head or handle location is worn and will require new parts.

Also check that the fenches on your minigraf are square 90degrees vertical and horizontally.

Make sure you are not using hard wood wedges in soft wood moulding this will open mitres.


MITREMAN

[Not your average framer]

i've got the opening of the corners down to a minimum, But, when i use a moulding with no rebate i still get cracking mitres......

Hi Ralph,

I seems to me that you may be describing one problem and getting answers to a different issue all together, (sometimes a picture helps us to understand things better). Can we just check that we are all understanding what you are trying to say, because the answers so far are about open mitres and you seem to be talking about the gesso over the mitres cracking after the frame has been made with what may appear to be good mitres. Are we focussing on the wrong issues?

Gesso cracking over the mitres is not a new problem, as years ago it was common to overlap the mitres with fabric glue onto the frame before covering it with gesso and can be almost impossible to avoid if the frame profile is part of the overall problem.

If you are happy with the accuracy of the cut mitres and how the frame components fit together, but the frame is able to flex after the glue has set so that the gesso is cracking on the face of the moulding, then perhaps you need a deeper moulding, so that in doing so you will have a larger glued area to bear any strains involved.

Dowels, biscuits and glues may not be the real issue at all. How about starting with telling us which wood the moulding is made from, the cross sectional dimensions of the moulding, the size of the frame which you are making, the size and distribution of the wedges and some photos for good measure. If the wood is something soft enough, it may not be the glue which is letting you down, but the wood it's self and it may still appear to be a glue failure, when it's something else.

You've already tried other glues and come back to using the glue you originally prefered, so maybe the problem is nothing to do with the glue at all. It won't do any harm to make a few assumptions and see if we can try and illiminate some other posibilities, before going over the same ground again.

[raph.basan]

hi.
the problem is NOT my mitres. they are all ok, and just to confirm, my Morso blades are hollow ground at cathandon, and my brevetti blades are sharpened by the suppliers, they have also ben callibrated correctly.
My frames are joining fine, no gaps.
my frames that are stained are not opening up, the frames that we gesso, are.
but not all the frames are doing this, mainly the smaller profiles. so when using the smaller profiles we cannot use silk as we end up using more gesso than is required....
he wood used is mainly tulip and obeche, with tulip being more prone to opening than the obeche.

the next time it happens i will post pictures. I promise.


Raph

[prospero]

Hmmmm... Long time since I used any 'proper' gesso. Seems to me it's the gesso that is the problem. Are you putting too much on at once? Too thick? Too Hot?

If the corner joint itself is sound, a dab of polyfilla in the crack should fix it. And hopefully it should not re-occur.

[framemaker]

What a problem!
I don't know if this is something you can control totally, wood is always going to want to move about.
I get frames that start cracking the day after they are gessoed, these I would then fill again before slapping on the bole. Other frames I have made over 5 years ago are fine with no cracks...

One thing I do wonder about is the use of metal wedges being a factor sometimes (I use Alfamachine power twist), over the years I have broken apart frames that I made (only a short time after they were joined), and the wedges have sometimes started to rust really rather quickly. I believe this would certainly have an effect on pushing the mitre apart (no matter what glue was used) a fraction and causing the annoying cracking.

Maybe the only way is to totally over fill the mitre with a rock hard, two part filler? and then gesso.

[Not your average framer]

the problem is NOT my mitres..... My frames are joining fine, no gaps.

Hi Ralph,

O.K., forget the glue, that is not the problem.

not all the frames are doing this, mainly the smaller profiles.....
wood used is mainly tulip and obeche, with tulip being more prone to opening than the obeche.

I suspected as much. It's not an easy one to solve and the solution may be different according to which type of wood the frame is made with.

Fortunately, there's a useful clue in what you say about the difference between tulip and obeche and here's my take on what this probably means:

I think that the obeche being more flexible than tilip is giving enough along the lengths of wood to reduce any strain appearing at the mitres.

At this stage I will put my ex-engineers hat on a point out that the area of the glue carrying surfaces being the multiplication of the width and depth of the moulding, can increase by quite a significant amount with suprisingly small increases to the width, depth, or both width & depth of the moulding which you are using. So it may pay to experiment with different size combinations and see if this solves the problem.

On harder woods such as the tulip wood, I would seriously consider buying a dovetailing jig or machine and dovetailing the corners. This will massively increase the strength and rigidity of the corners and also lead to a massive increase in glue carrying surfaces, (a big advantage). Since you are gessoing the frames there will be no visual impact on the finished product and as you are already biscuit joining your existing frames, I assume that the dovetail operation would not be out of the question.

[prospero]

Ever wondered how corner ornaments camr to be invented? They aren't just there for show. They cover a lot of things to.

I did a big frame recently and had a bit of a corner-cracking prob with that. Not too serious and to be honest, I was half-expecting it to happen. The main part of the frame was a flat panel, made from two 3" bits of 18mm thick pine, glued and screwed togther. The top layer was mitred as per usual, but the bottom layer had pieces fixed accross the corners so that the grain ran at 90degress relative. The whole thing had about a tubeful of nomorenails on it and I don't know how many screws. Them corners wre strong. No question of the joints giving. But I still got hairline cracks across. Definately a case of the wood settling down. (Used textured ripple paint btw.). Fortunately, the panel was a sort of khaki colour and a quick dibble with wax on a ditry duster made the cracks vanish.