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Steam Punk Mitrebox



During my recent build of the kist I was sawing mouldings on a Millers Falls Acme 74A, and realised that it was similar to cracking nuts with a sledgehammer. I would kill for a small version of this mitrebox, but they are rare as rocking horse droppings and twice as expensive. 

I usually build a tool between furniture projects and, consequently, decided this one would be my own version of a small MF .... well, sort of ... 

The mitrebox is Jarrah and brass, just under 17" in length, 4" deep, and 3 1/4" to the top of the fences. It is used with an 11" carcase saw I made 2 years ago. This has a 2 1/4" plate and is filed 14 ppi crosscut. 

 

In order to make the depth of the mitrebox as shallow as possible, the pivot point for the turntable could not be in line with the fences, where the advantage would have been to enable the gap between them to be small. This would have required a large turntable, which would have caused it to extend as much behind the fence as it would in front. If a semi-circle was used instead, it might look neat when set for 90 degrees, however when it was set for a mitre cut, a section would stick far out the back. Not the look I wanted. 

I considered building a bracket that ran under the mitrebox (as with the MF), but I lack the tools to fabricate such a thing accurately and rigidly enough. 

I also considered a holding devise such as the one used by Jeff Miller in his tenon guide, but considered it too large and clumsy for here. 

So the turntable (for the saw) needed to be small, and this led to placing it pivot point in the centre of the bed. To close up the gap in the fence that results from this, the fence contains a subfence that slides back-and-forth ... In the end I accepted the limitation of the fence issue on my design, and just added a sliding sub-fence. It keeps it small and neat.



 

With the fences back, the mitrebox is optimised for mitres ... 

 

Here is the same moulding from the kist .... 

 

A comparison with a MF Acme 74A ... 

 

The heart of the saw-holding mechanism is a plate with rare earth magnets ... 

 


The magnets are set slightly below the surface of the wood, which is also waxed, and the saw floats across them on the wood surface. The weight of the saw alone provides enough downward force for the guides to drop, and the remainder is pretty much as if you are sawing without a guide. Indeed, it feels less effort than the MF 74A with guide bearings and a large saw.


Another shot, as well as the angle locking mechanism ... 

 

The locking mechanism is built on a morticed Jarrah disk ... 

 

The fence is a 1 1/4" x 1 1/4" brass angle section, with Jarrah subfences. At the rear is a slide adjuster ... 

 

You can make out the angle settings. The fence is set up for quick adjustment to 45 degrees either way and 90 degrees. It has a variable range from about 40 degrees. The saw can be used on either side of the fence. 

Here is a close up of the settings ... 

 

 


I added a simple base, to enable the mitrebox to be clamped on a benchtop ..


Building the mitrebox


There were many requests on the forums to show the parts and how the mitrebox was constructed. Dissembling the mitrebox is not too difficult. Fortunately it is just screwed together.


With the exception of the feet, which are modified drawer handles, everything was constructed in the workshop. Amazing what one can do with a Dremel and files! 

The rotating disk is actually two disks, one screwed to the base and the other pivots above it. The lower disk is morticed for the lock, and the upper disk hold the rods for the sliding blade holder. The brace facing strip started life as flat brass, which came under the knife of a Dremel wheel, and then bent around the disk. Tough work as all the brass is about 3mm thick. 

The knurled locking nuts were purchased on eBay (UK) for the router plane I built some months ago. 


Here we go …



Pulling it apart, these are the main pieces.



The mitrebox base. The brass legs are screwed to this, and themselves are tapped for machine screws for the baseboard.


The saw holding mechanism is the heart of the mitrebox. The rotating disk is actually two disks, one screwed to the base and the other pivots above it. The lower disk is morticed for the lock, and the upper disk hold the rods for the sliding blade holder. The brace facing strip started life as flat brass, which came under the knife of a Dremel wheel, and then bent around the disk. Tough work as all the brass is about 3mm thick. 


Here is the lower disk …


The upper disk. The brass rods are epoxied into the Jarrah disk.



Both disks were cut out on a bandsaw, then machined to size on a belt sander. The lower disk is about 3mm narrower, to accommodate the brass strip.


Everything turns around or is aligned by the centre hole. Below the parts are loosely fitted together.



The lower disk is screwed to the base.


Now is the time to connect the disks via the locking mechanism, which is just a brass section drilled for the knurled nut and an upper screw.


Note that the brass strip is morticed into the upper disk to prevent lateral movement.



The complete base …



Add the upper section with split sliding fences. Note that this section is the same thickness as the upper disk.


The split fences are morticed and screwed into the upper section. Everything is coplanar.



Now screw the upper and lower sections together.



Finally the base board can be attached to the legs with the machine screws.



All that is left to do is add the subfences. The slots for the adjusting bolts were made by drilling a hole at each end, then grinding out the waste between with a Dremel, and then finishing with files.





Test drive


The simple way to safeguard the turntable from damage (from sawing), is to lay a sacrificial piece down the length of the box and against the fence. The sacrificial board does not need to be attached, just placed on top. Sections for sawing will be placed on top of it, and the saw will go through them and into the sacrificial board. This method will only lose 1/4" from the depth of cut (which would now be 2"). 


Below is a sacrificial board …



Sawing a test mitre …


Testing for accuracy. Absolutely spot on. The other feature to note is that the saw cuts are so smooth (from the 14 ppi crosscut teeth) that there is no need for shooting a finish.



Good luck building yours!


Regards from Perth


Derek


August 2013