Don't swear at me...

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lawrieh
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Don't swear at me...

Post by lawrieh »

I know most of the problems with MDF but I need to underpin some MDF moulding, its around 38mm thick/deep and 50mm wide.

I have just ordered some better glue (Titebond Ultimate III) after reading some posts on FF.

I dont want to screw up moulding through trial and error as I dont have much spare, my first idea is to Titebond and 2 rows of stacked 15mm wedges if I can get them in.

I am wondering if soft or hard v-nails would be better or would a different method altogether be wise? I am considering trying Hoffmann wedges or glue and screw (although would be difficult to keep aligned as all the angles are not 90 degrees) or other ideas.

If anyone could give me any ideas on underpinning or joining I would be very grateful.
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prospero
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Re: Don't swear at me...

Post by prospero »

There is a limit to just how strong a butt-joint on MDF can be. I've underpinned bid slabs of the stuff quite
successfully, but only 19mm thick but it was reinforced with a biscuit slot and a steel L plate.

If you can lay your hands on a biscuit cutter then 50mm wide is plenty to get a biscuit into. You need to cut the slot
so it's only partly in the moulding - the slot can continue past the inner side of the moulding. The actual biscuits can
be trimmed so they don't protrude into the rebate. PVA glue will do the job. Put one v-nail in outside (leave enough space
when cutting the slot to allow for pinning without hitting the biscuit). The v-nail serves mainly to locate the joint, it will add
little strength but will 'clamp' the joint and make a better glue bond. If it's a big frame some thin plywood triangles glued over
the mitre will be beneficial.
Unfortunately, MDF is very dense and a big frame will be quite heavy which tends to work against it. It also has a tendency to bend.
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Re: Don't swear at me...

Post by Not your average framer »

A lot of the structural strength in normal wood comes from the grain structure, unfortunately there is no such structure in MDF, so getting the same strength and durability when joining MDF is not a realistic proposition. Also incorporating biscuits into what is only really a particle board means that you are only bonding the biscuits to the layer of particles in contact with the biscuits and the available strength will depend upon the strength of bonding between the various particles within the board.

If the MDF moulding is a paper wrapped one, an unfortunate result tends to be that over the years the MDF absorbs moisture from the air , which slowly leads to some expansion of the MDF and as the self adhesive on the paper wrap can slowly creap, eventually the paper wrap creaps away from the mitres, which does not look nice. If you are cutting the MDF using a Morso, then you should expect to be resharpening your Morso blades with alarming regularity and also find that the same blades very quickly become even more useless on normal mouldings.

In general MDF mouldings are produced for the volume market and the usually method of cutting these mouldings in this situation is with double bladed mitre saw, which then causes the need from considerable dust extraction facilities to be required, together with legally required regular testing of the output dust residue about a specified level of usage. Unless you are already producing very large volumes of mass market ready made frames, I think you may find that the apparent cost advantages may turn out to be somewhat illusionary.

Do not any additional measures , such as using biscuits tend to defeat any cost advantages of using MDF in the first place? Normally volume producers of ready made frames who are using MDF are not so particular about the quality of the joints, also as I look at the offerings that I see it large chain stores, I have noticed that most suppliers seem to have switched to polymers mouldings some years ago. I don't think that they would have made the switch, if the was not some very persuasive reasons for doing this. Perhaps polymer mouldings might be worth looking into as well.
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lawrieh
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Re: Don't swear at me...

Post by lawrieh »

Thanks for taking the time to reply, all good points and ideas.

The moulding I made myself, it's quite complicated, it took significant time for the maths and labour and Ive had to make a special table router to do it. The frame has 10 sides in total, a mixture of straight and curved pieces, there isn’t a right angle in it, every piece is a different length and corner angles. I will have to underpin without the fence at the back and just lineup by eye/feel , risky?

I wish I hadn’t started but I have and I will finish.

I have considered using something to harden the MDF. I have used rustins MDF sealer before, it mainly seems to be water but it does impart some additional strength. Other candidates are ronseal Wet rot hardener or a cellulose based sealer. They stink and I am not sure the cellulose stuff is too viscous to be absorbed into the structure of the MDF like the Ronseal or Rustins stuff. What do you think about using it to add strength at the corners?

I have a biscuit cutter so that is a definitely an option.

It's not a paper wrap, so paper creep is not a problem. I also cut with a chop saw as I sold my morso so blades are not a concern.
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Steve N
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Re: Don't swear at me...

Post by Steve N »

just a thought , can you make the exact shape of the whole frame out of say 6mm MDF , the width of it just the back of the moulding, no rebate, then glue and screw each part of the frame to the shaped MDF, so in effect the depth of your frame would be 6mm deeper
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lawrieh
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Re: Don't swear at me...

Post by lawrieh »

I was thinking of maybe having to add a sheet of ply at the back after the frame had been made, perhaps 1cm in from the edge of the outside edge and screwing the ply onto the frame more to add rigidity if the frame flexed considerably (in the x y directions - if that make sense) but I never through of using it to build a frame on. I forget to mention that this piece is a shadow box design (about a 15mm gap) and glazed so I don't think I will be able to build from back to front, but it's something I will remember for other projects.

Has anyone ever had success hardening MDF?

Any tips about underpinning weird angles like 136 deg without a back support? ( I imagine it should be straightforward but the very fact that I think so makes me worry that I have not thought it through)
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Steve N
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Re: Don't swear at me...

Post by Steve N »

When I said about the 6mm MDF, I meant with a hole in the middle the same shape, so it does not cover the rebate, you will still be able to put the glass etc in, then fit the back into the hole
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Re: Don't swear at me...

Post by Not your average framer »

Cutting a ten sided frame out of MDF does halve it's merits, if you are cutting it out of one piece of MDF. It would be tempting to make by following a reasonable amount of what Steve has already suggested, but personally I would be strongly persuaded to make up the rebate by gluing and pining some sections of pine timber onto the reverse of the MDF and screwing these pine sections together using a pocket hole jig, which will make the whole frame much stronger.

I must admit that pocket hole joinery is not exactly the pinnacle of craftsmanship, but it is quite effective none the less and is relatively quick and easy, which probably helps to keep a project like this viable and profitable. This is not just some odd ball way of doing multi-sided frames, but it's a pretty much standard approach and although the market for such frames may be limited, it's not easy to find framer prepared to undertake jobs like this.

As a result you can charge a good price for doing something, when not many other framers are willing to do this sort of work. Effectively it's a specialist job and if you the only guy doing this sort of thing, then you are the specialist. As you no doubt know from my other posts, I like to do jobs that are a bit different and keep plenty busy by doing so and I don't have to worry about competitors undercutting me, which is also helpful. Perhaps the niche market thing is something that will suit you too.
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Re: Don't swear at me...

Post by prospero »

A man not afraid of a challenge! :clap: :ninja:

With things like this there is always a certain amount of Research & Development. :lol:


** Hardening the MDF may work up to a point, but you are basically shifting the weakness to another point.

** V-nails don't add strength. They are just a means of holding the joint tight while the glue sets. There is the
advantage that on MDF you wouldn't be inserting the nails partially across the grain.

*** If you are going to finish the thing after joining (preferable), the you could drill though the joint and bang dowels
in (after pinning). That way the dowels would be in dead register and stiffen the whole issue up considerably. The protruding
dowel ends would be easy enough trim off and make good.

**** I'm going for my tea now. :P
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lawrieh
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Re: Don't swear at me...

Post by lawrieh »

I like the idea of a pocket hole jig although not having used one I dont know how hard it would be drill pockets for the angles I want, some angles are think it will be difficult as some pieces join at 140 degrees. I have always wanted one just for making stuff around the house so I may invest and test on scraps of other moulding.

I will be hand finishing so dowels is another option, again not something I have done before so would be keen to try, on scraps at least. I suspect it may be easier than pocket holes.

when you say "Cutting a ten sided frame out of MDF does half it's merits" I laughed, it is indeed half... half merit = half strength, half the ease and probably half the quality of finished job.

I am a fool, as I have made a 25mm thick back piece already, I cut it from 10 back pieces, not only do I have to join, them they wont have the same strength, why or why didnt I cut it in one piece? dohhhhhhh.

Of course the 6mm back piece will have a hole cut of it it.... double dohhh, I need a holiday.

Thanks for all your input, it really has helped clarify a few things...
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Re: Don't swear at me...

Post by cleaver »

[quote="lawrieh"]I like the idea of a pocket hole jig.......

A very popular dance in the 1920s.....until replaced by the Charleston. :)
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Re: Don't swear at me...

Post by Not your average framer »

lawrieh wrote:when you say "Cutting a ten sided frame out of MDF does half it's merits" I laughed, it is indeed half... half merit = half strength, half the ease and probably half the quality of finished job.
My brain play funny tricks on me since my stroke, but my laptop has a damaged screen as well. I don't know which one caused the typo, but at least it was worth a laugh.
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Not your average framer
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Re: Don't swear at me...

Post by Not your average framer »

lawrieh wrote:I am a fool, as I have made a 25mm thick back piece already, I cut it from 10 back pieces, not only do I have to join, them they wont have the same strength, why or why didnt I cut it in one piece? dohhhhhhh.
No, why do you say that your a fool. We are just discussing different ways of doing this. If these ten pieces of MDF had ten more pieces of pine, or some other wood behind to create a rebate behind and these extra piece were of adequate size and strength, joined strength of the MDF would be of much less importance, because of the added strength of the pine frames.

The is no right, or wrong way of doing this as long as the finished result is fit for it's purpose, durable and of an acceptable level of quality. It may be tempting to look upon some of us more experienced framers as being very special people, but it's good to realise that we don't all use the same methods and in time you will have your own preferred way of doing things too.

Don't write off the experience, that's how we learn and that includes all of us.
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prospero
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Re: Don't swear at me...

Post by prospero »

:idea: How about this....

Make the frame from pieces of Pine the same width as the back - that is, without the rebate.
Don't bother underpinning, glue and strap clamp it in one fell swoop.
Then cut the 'face' from a single piece of 2.5 MDF and glue it on top.
Cutting the face part might involve a bit of fettling and sanding, but careful marking an cutting will minimise this.
**A Japanese pull saw will make a clean cut.

This will eliminate the visible joins and strengthen the whole thing. 8)
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lawrieh
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Re: Don't swear at me...

Post by lawrieh »

Ive been away, sorry for late response.

I'm a fool for not thinking it through properly before I started. I could have used a one piece back piece made of ply, it would have been easier and also more importantly a stronger support for the MDF front. Ahh well I will know if I make a next one. It's good to talk things through, its one of the major benefits of this website.

Unfortunately i don't think it will take a strap clamp due to the different angle corners and side lengths. I may make the investment in the saw for neater corners though.

Many thanks again for all our input...
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