The design...

You can download the Sketchup drawing here which is pretty close to the finished project - only a few minor changes around the metal components, depending on what I had on hand.

Click here to download the drawing.

Well over 30 years since I studied Technical maths so working out the 3 points of the crank to give the same clamping pressure on all sides of the box took some serious nutting out!

I haven't got around to making the 'spacers' (in red) yet as they are only required for finger-jointed and dovetailed boxes which I don't make so often . . . one day!

Making the clamp...

I made the clamp out of melamine covered MDF simply because I've collected a lot of off-cuts just for this type of job and I thought it might be easier to remove any glue squeeze-out in use . . . until I did some glue testing on the stuff before I started. I found that once you clean the surface with acetone, PVA will hold the melamine better than the melamine sticks to the MDF, and actually rips the melamine off the MDF when you break it. I've added some screws to many of the connections just to make sure it has a good physical connection as well.

Right: MDF parts completed.

The side stops are just a right-angled MDF bracket with a 'pressure-pad' that can be pushed toward the box 3 or 4mm. The key to the design was to have the attachment point of the crank on the side fence which not only avoided ripping the side-stop away from the fence, but also gave me room to fit the crank that pushes on the pressure-pad closer to the corner of the box.

The corner block uses a sliding-door wheel for the tensioner - you can get them at any hardware shop or window makers. I cut a shallow groove into the nylon tyre to stop the chain from riding up. The wheel is bolted to a piece of 5mm angle iron that was cut down to suit but you could probably get away with 3mm. The block under the angle iron stops it from twisting when the M6 wing-nut is turned and keeps the assembly sitting in just the right spot. The plate the wingnut runs against is an offcut from a piece of 25mm RHS - strong enough but lighter than solid flat-bar (I don't throw much out!).

I found that as I applied more pressure (than neccesary) to the clamp in use, the first undesirable thing that happened was for the side-block to slip outwards along the fence . . . unless I used a bolt tightened with a spanner instead of the thumbscrews. I have since ordered some 'clamping levers' which will give me more leverage to tighten the blocks, but they are also much taller and will interfere with the act of attaching the chain so I have modified the attachment point of the chain now to extend it out from the side-block further. Having the hook out where it is now makes it very easy to attach the chain.

Using it...

In use, the clamp can accommodate a box from 100mm x 100mm up to 410mm x 410mm which is bigger than most boxes I make. I still have the slow type clamps for anything bigger.

As the side fences are 120mm high, when clamping shallow boxes or trays, I would expect some 'racking' of the push-blocks so I will be putting spacer blocks under the box sides to lift them to roughly centre of the fence to avoid any issues.

To prove to myself that the clamp was applying enough pressure, I cut some strips of 2.5mm EVA foam to go in each mitred corner of a dummy box. I then clamped it up using my 4-way quick clamp and applied more pressure than I normally would for a box and measured the amount the EVA had compressed - down to 1.5mm in each corner.

I then put the dummy box in my new clamp and tightened it to a 'comfortable' tension where the side-blocks were not yet slipping - about the same result at 1.5mm. Perfect! I'm only pushing the corners together, not trying to crush it!

Box clamped and squared in under 20 seconds! - you'd better believe it!