rabbet joint for MDF cabinet

[enkjnfkjekjfewb]

hi all, i'm looking to build some cabinets in my home using 18mm mdf. I'm planning on using rabbet joints, glued and screwed but need some help with the rabbets.

I'm a DIYer so don't have access to a table saw with daedo stack. I'm planning on buying a cheap router to help me but wanted to know what to look out for. If I get a plunge router with a built in fence, can I use this with the track from my track saw? is there any other way of getting straight rabbets?

Thanks

 

[Marineboy]

Depends on the make of router and your track, and if they are compatible (eg Festool) you’ll need another bit of gear anyway to connect the router to the track.

Instead, just clamp a straight edge or wood batten across the sheet of MDF and run the router along that.

 

[nev]

Just use your track saw?

 

[colinc]

Hi,

Am assuming here that you are joining top/bottom and shelves to sides in the following comments....

Others will probably disagree with me, but I am not sure that rabbet joints are necessarily helpful in mdf cabinet construction as the material has little residual strength after cutting one out and it doesn't take screws that well either.

Biscuit joints are an economical fast and, if done carefully, accurate way to join mdf boxes. I have made a number of mdf cabinets for things in the workshop and find glue and biscuits are fine. For 'utility' things like that it is useful to use a few screws to pull everything together if you don't have lots of clamps. Bugle head plasterboard screws from Screwfix tend to work well into mdf (they have a Phillips head not Pozi, so use the right screwdriver).

What to buy depends what you are planning doing in the future, a biscuit jointer is not massively expensive to buy, but a router is more versatile.

For cutting rabbets in the end of a board with a router, a clamped on straight edge and a bit of care is all you need. If it's a rabbet within the length of a board I tend to use a jig I made years ago that has served me well. A quick google search will reveal lots of solutions, mines like this one: router jig

You use a circular guide bush in the router base to locate it in the jig and the jig opening's width controls the width of the cut.

Anyway, I may have missed the point about what you want to achieve, so do tell us more...

regards

Colin

 

[TheTiddles]

You can make a jig with some ply or MDF to make very accurate and repeatable slots in sheets with a router and it’s pretty safe too. Cheap router will be fine, but spend a bit on a decent cutter (like an upcut spiral from Wealden) if you want to make things easy for yourself

A

 

[custard]

hi all, i'm looking to build some cabinets in my home using 18mm mdf. I'm planning on using rabbet joints

Will that be any stronger than a straightforward butt joint, especially if the butt joint is biscuited or dowelled?

It's a genuine question, I really don't know, I'm a solid timber cabinet maker so I don't have that much sheet goods experience. However, I get the impression that rebates on MDF or chipboard are more location aids to simplify flat pack assembly rather than adding strength.

Another point, MDF doesn't take glue all that well on a cut surface, it's like end grain in that it sucks up the moisture and leaves you with a dry joint. You could size the joint first, but that's a load of faff.

I'd think harder about your basic design before diving in and buying kit that may not be relevant to your needs.

 

[AndyT]

For some more information on the strength of joints in MDF, have a look at Gosforth Handyman on YouTube.
He makes his living installing made to measure units in MDF. Here, he looks at joints with and without biscuits:

https://youtu.be/dxwFP4WHS4o

 

[custard]

That's astonishing Andy, I live and learn. I expected the biscuits joint to be at least x10 stronger than the glued joint, instead there was no meaningful difference!

 

[AndyT]

That's astonishing Andy, I live and learn. I expected the biscuits joint to be at least x10 stronger than the glued joint, instead there was no meaningful difference!

Yes, he reckons that they only serve as a location aid. Logically, that means that if you don't have a biscuit jointer, you could use any other method and still make a strong piece of work. Rebates and housings are just as redundant.

Possibilities would include temporary battens clamped, tacked or screwed in place, or corner clamps.

I've made the odd cupboard from mdf and just used screws through the sides into the edge of the shelf. No failures so far.

 

[colinc]

Hi,

I took a look at that video and really it wasn't testing anything that the biscuit brings to the integrity of the joint.

The test treated the joint as a cantilever, so the forces at the glue line were max tension at the bottom of the horizontal part to max compression at the top (assuming Young's modulus of elasticity for mdf is equal in compression and tension). The tensile force at the position of the biscuit being equidistant from top and bottom would be zero so clearly the presence or absence of the biscuit makes no practical difference apart from disturbing the glue line. The test was all about the ability of the glue to deal with the tension in the bottom half of the material and the result quite predictable. The only thing the biscuit might have added was to not fail in tension after the glue failed but it isn't going to do that. Interestingly, the slightest increase in depth of the material would have a massive impact on the strength due to reducing the tensile stress in the glue bond, whereas the number of biscuits has no appreciable impact.

To really test what the biscuit does would require an H-shaped test sample with a load applied at the centre of the horizontal. Doing that means that the joint could fail in either bending, i.e. the glue line fails on the tension edge (which edge depends if the load is applied from above or below), or it will fail in shear across the entire glue line.

The relationship between the bending and shear stress at the joint depends upon the width of the cabinet. For any given vertical load, zero width = 1/2 load shear/zero bending, whereas max width = same shear, greater bending. The joint would fail by whichever case is limiting although a failure in bending means a just crack in the glue line until the joint fails in shear, assuming it has biscuits/dowels etc., to resist then shear, then when they fail it finally it falls to bits!

So, what does the biscuit (Domino, dowel, spline etc.) on the neutral axis of the joint do? It adds to the shear strength of the joint, enhancing the strength of the glue in that axis.

Really, my point is that the test is not representative of the real-world application of the biscuit joint. So, if you were surprised that the video didn't support your expectations of what the biscuit did, I suggest that you were probably right in the first place.

Regards,

Colin