Dowels in MDF

[Carlneedshelp]

Hello again

I'm hoping to pick your brains. I know that you all recommend biscuit joints as the best way of joining MDF at right angles but I don't have the space or tools to do this as I am working in the bedroom and don't want the dust. I also don't have the confidence.

So, I have been using 8x30mm dowels and screws. Width of wood is 53cm and I have used 6 dowels and 4 spax screws per side. However, it turns out that I am not the best at drilling my holes straight so when I insert my dowels they don't line up straight. So I've been thinking whether it is an option to join the wood with the screws and then drill through the sides and insert longer dowels (8x40mm) so that you can see them from the outside of the box but guarantee that they line up. I can then fill and sand the sides so that they are not seen.

Would this be okay or would it impact on the strength of the MDF side panels as there would only be 4-5mm space from the bottom of the carcass? I'm using 18mm MDF at currently drilling 9mm into the side panels and the rest into the ajoining bottom.

Thanks

Carl

 

[Penny]

If you have an issue drilling at 90 degrees - look at this idea: https://www.youtube.com/watch?v=vZ58uj6RwSQ

 

[Mike.S]

What are you building and what weight might the joints have to carry? Is it to be permanently fixed together or take-apart e.g. no glue?

 

[JakeS]

If you're worried about the structural integrity of the boards, you could always get a length of 6mm hardwood dowelling from any DIY shop and drill a narrower hole. It's not great-quality stuff usually but it'll do fine for reinforcing a joint. And yes, if you're only concerned with reinforcement, there's nothing wrong with drilling after the joint is screwed together.


That said, leaving aside the joint itself:

MDF
...
working in the bedroom

I wouldn't cut or drill MDF in my bedroom. It's near-impossible to catch all the dust and it's the kind you really don't want to breath in. Drilling possibly isn't as bad as biscuit-jointing, but it still comes out in hangs-in-the-air-and-bad-for-lungs format.

If you have to, work with a couple of windows open and vacuum thoroughly afterwards.

 

[Dee J]

Given the low strength of mdf, and its propensity to split / delaminate... do dowels add any thing useful to a glued and spax screwed join?

Dee

 

[Carlneedshelp]

Thanks people.

Jakes, noted on the MDF and drilling. I hadn't noticed any dust as it comes out in worms. I have the hoover running as I drill to catch anything I cannot see.

Dee, I would be interested as to whether it adds anything to the joint. I'm using them as I was advised that 4 screws, glue and six dowels are what someone else on here recommended. They do feel sturdy and secure.

It is all for a couple of built in wardrobes and access in and out of the house isn't great and I will have to move the carcasses into place which are quite heavy. Wife is pregnant so she can't even help support any weight.

I'm about to start painting the first couple of sections so that I have more room on the floor to work with. Between coats I intend to do a little sanding, is this dust just as bad as I think it will just be paint dust rather than much MDF.

Thanks

 

[petermillard]

Dowels/biscuits/dominos in MDF are usually to help with alignment when putting the carcass together; in my experience most mechanical strength comes from the carcass screws (and probably as much again from having a back fitted to it) and as you say these can easily be countersunk, filled and painted if they show - or there's alway pocket-hole screws if you prefer to keep the surface unbroken. Also, avoid the temptation to over-engineer, it just makes work for yourself and will frequently add nothing to the carcass, especially if fitted.

If you're struggling to keep everything aligned whilst drilling and driving screws, there are some cheap & cheerful sprung corner clamps that can help, btw.

Re. finishing; painted MDF shouldn't need much sanding, just a very light de-nib between coats.

HTH Pete

 

[MikeH]

I have found pocket hole screws great for joining MDF. Reinforced with glue or not. Also the better jigs have built in dust extraction when drilling.

 

[Giff]

Screws are the most secure, counterbore to 10mm and use a plug. Spax do an MDF screw, but I still pre drill them. Geoff