Blast Furnace Gallery

Clamping of transfer port whilst glue dries

Gluing transfer port

You can't have too many clamps. Eight clamps were required to hold the two layers of the transfer port for gluing. Cling wrap comes in handy to stop glue sticking to unwanted surfaces


Preliminary assembly

Preliminary assembly

19mm MDF was used for this speaker and is much nicer to work than the chipboard of earlier models, although the router dust is apparently quite toxic meaning a respirator is needed

An internal frame was made from strips of MDF which allowed everything to be screwed together without glue at this stage. Once all the panels are finished, they are glued and screwed one at a time. After the glue dries, all internal seams are sealed with gap sealant to prevent any possible air leaks. The external seams and screw holes are filled with putty and then 3 coats of polyeurathane is applied


Bottom view of power head

Bottom view of power head

The transfer port will couple the two chambers together and is not glued in place at this stage


Lower chamber with bottom removed

Lower chamber

Visible in this view are the rebated port openings which allow the glue more surface area to work on. The cutout at the back is for the amp and the one at the front is for the controls


Shelf brace - powerhead

Shelf brace

Guess who just bought a router!
A central shelf brace is used to make the powerhead more rigid. Because the ports exit from the centre of the sides, cutouts had to be made for airflow. All internal edges were cleaned up with the rollover bit so there are no sharp edges to cause any whistling


Bottom view of powerhead

Bottom view

The shelf brace is rebated into front and back panels and has cutouts for airflow into the side ports. Also, just visible are the screws for the top which were screwed from the inside so there were no visible holes to be filled


Top view of transfer port

transfer port

This view of the powerhead with the sides removed, shows the top edge of the transfer port which, along with the bottom edge, was smoothed with the rollover bit. The port is still removable at this stage


Concept session

tubes bottle drawing

Using the Sidewinder as a sample power head, and some large pipes to hold everything in place, various ideas were tried out. With creative assistance supplied courtesy of Uncle Jim, the Blast Furnace was born


Rebates and edge bracing

Power head

The front panel was made out of double thickness of MDF glued together. The larger radius was cut first and then the smaller cut through


Closeup - transfer port

transfer port


Circle cutting jig

Circle cutting jig

To cut circles, a homemade jig was constructed using the slides that came with the router, and took about 4 hours to make. The pivot point is just visible under router. The angle piece was rivited to the baseplate, which was made out of some thick aluminium sheet that had been saved for something special.

Another view of circle jig


I recently had to cut a hole for an 18 inch driver and found that my jig was too small. This is what I used as an emergency single use jig. With care, it worked fine!

temporary jig


Lower chamber

Lower chamber

These braces were cut with the router and given a rollover edge, then glued into position


View through port opening

port opening

The shelf brace detail and location can be seen here. On the far side, you can see where the inside face of the port opening is rebated to increase the surface area for the glue


Nearly There..

Bare box

Three coats of Polyeurathane, before the ill-advised flirtation with Scandanavian Oil




Here it is in all it's glory! The background has been removed and the shadow added, but this is the photo taken on the back porch




The sub was recently sold but the new owner wanted to mount it behind a wall (shock horror!). In addition, the opening had to match the existing in-wall sonance main speakers. This adapter shroud was made up.


Port section


The output from the ports needed to be channelled inwards by this section. The air is compressed horizontally, but has room to expand vertically. Adapter will be sealed to sub using some foam tape and a ratchet tie-down. I had to learn some new router tricks to cut those angles!


Nice Face!


The new owner hopes to purchase a front panel from Sonance to match the mains. Meanwhile this grille is the same size. To facilitate airflow, it is as narrow as possible, meaning some cross braces were needed to stop the tension in the grille cloth from distorting the shape. These were rebated to keep them clear of the cloth, thus avoiding any "grille slap".

Grille clips


The grill is mounted on standoff blocks to fit through the drywall. The other half of the clips is mounted in blocks that can be unscrewed and discarded when the Sonance panel arrives.


/bl-gal/blast-gallery.htm last modified: 31 December 2018
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