Do you need to clamp glue joints in MDF?

Have you ever wondered whether clamping up glue joints actually does anything? How much difference does it make if you don’t bother to use clamps? It’s not always practical to use clamps on joints in all situations so today we’re finding out, are clamps for wood pointless in MDF? Welcome back to Test Tuesday!

You can watch the full video here:

Does clamping glue joints make ANY difference? Test Tuesday!
Watch this video on YouTube.

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Tools & products used

  • DeWalt DW682K Biscuit Jointer (UK / US)
  • Festool Quick Lever Clamps (UK / US)
  • No. 20 Biscuits (UK / US)
  • Titebond II Wood Glue (UK / US)

Please note that for this test I used biscuits by UJK Technology.

A bit of background

Last week we had a look at biscuit joints in MDF to see whether biscuits actually make any different to the strength of a joint. They results were surprisingly close but the biscuit joints were nominally stronger and certainly less susceptible to catastrophic failure. So I figured it would be interesting to find out how much difference it make clamping up the joint while the glue dries. After all, it’s not always possible to use clamps depending on the customer project and the size of the item. Sometimes you just need to crack on and get the job done! For practical reasons and in the real world, cabinet carcasses will often just be biscuit jointed and brad nailed together with zero clamping pressure.

The test specifications

The test was exactly the same as the test we did on clamped biscuit joints but with one important change. This time the glue was allowed to dry with ZERO clamping pressure – only gravity held the joint together for 24 hours while the glue dried.

  • MDF used: Trupan 18mm non-MR
  • Biscuits used: UJK solid beech No. 20 biscuits
  • Glue used: Titebond II (NOT clamped)
  • Size of back test piece: 142mm x 190mm
  • Size of front test load piece: 142mm x 50mm
  • Distance of base to start of joint: 50mm
  • Metal L-bracket location: Central to underside of test load piece
  • Biscuit location: Biscuits are located with 2mm gap to edge and approx. 2mm gap in middle

The test involves pulling the T-joint MDF (end-grain to face-grain joint) at 90 degrees to the backing panel. This is pretty much worst case scenario for a joint like this and would simulate, for example, a cabinet being significantly forced out of square.

Are clamps for wood pointless? The results!

The loading was applied as evenly as possible directly above the pull-point on the test piece of MDF using my very unscientific load measuring jig. Here are the new results in red compared to our results from the week before (peak loads):


As you can see there was very little difference in the strength of the biscuited joint as a result of using clamps.  The clamps only added 2.6kg of force to the overall strength of the joint making it around 6% stronger. On a joint that would never experience anywhere near these sort of loadings this is really a negligible difference.

The big difference however was on the non-biscuited joint and this dramatically illustrated the importance of clamping the joint. The clamped joint was some 53% stronger than the non-clamped joint when biscuits weren’t used.

Have we learned anything from this at all?

Of course we have! Firstly, even with zero clamping pressure and zero biscuits a glued MDF joint is still incredibly strong. For a joint to withstand 29kg of leverage force before failing, that’s WAY beyond the forces you’d expect in a static cabinet as part of, say, an alcove unit or even a kitchen cabinet (less likely to be MDF). SO if all you can do is brad nail and glue your butt joints, they’ll probably be absolutely fine. Also bear in mind most cabinets are built with a backing panel that would offer additional strength to prevent it being pulled out of square.

Secondly, clamping DOES make the glue joint significantly (53%!) stronger if no other jointing methods are used. So clamping certainly doesn’t do any harm and if you’re building MDF furniture that may need to be moved around, like a stand-alone bookcase, it’s probably wise to clamp it while the glue dries… if possible. But…

Thirdly, when the joint is biscuited there’s very little difference in the strength of the joint due to the clamps (only 6%). The biscuits also offer significant protection to the joint to prevent catastrophic failure. When the non-biscuited joints fail they’re toast – whereas the biscuited joints are still relatively stable (although loose).

To summarise, biscuit joints are good. Clamped joints are good. If all you can do is biscuit, glue and brad you’ll probably be absolutely fine. Obviously bear in mind this is a totally unscientific test and just a bit of fun. Feel free to do your own tests if you want different results. Cya next time!

Last updated: 21 Aug 2018
Originally published: 21 Aug 2018


Andy Mac
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