For anyone contemplating their own acoustic horn factory........

Horn Mill

Here is the azura machine – a 1” shaft mounted on bearings with a 1m round table driven by a 3000 rpm bench grinder and skateboard wheel.  An auxiliary wood lathe / belt system provides lower gearing.  Low gear is used for the grinder passes.  High gear is used for hand fairing.

Eureka! # 1 - Cutting Tool

	Jig up a 100mm angle grinder on a plate like this

The angle grinder jig has a template follower (1/2” bearing) concentric with the cutting wheel – this means the cut is not effected by the angle the grinder is held relative to the work. This has the enormous advantage of enabling the curve to be ground exactly to the template – not hand finished to match it.  There’s a big difference.    

Eureka! #2 - Template

The Template is made from 6mm MDF.  Mount steel ruler as shown and ensure the set square runs smoothly up and down.  Plot the curve from x,y coordinates.

The cutting tool fulcrum must however follow a curve equidistant from the desired curve.  This curve is achieved by the simple expedient of describing multiple arcs back from the desired curve.  The equation defeated me – but that’s what geometry is for. Offset is half the grinder wheel diameter - PLUS half the bearing diameter.

Method & Materials

The plug mould blank is made from polyurethane foam – built up in layers from sheet, or 2-pack poured into a rough mould.  The initial shape is turned from this blank with the grinder, a few mm short of the final curve.  Next, a fibreglass layer is built up on this, and ground to just shy of the final curve.  The last layer is gel coat, or an easy sanding primer, ground to the finished profile.  Remove the grinder marks by light hand fairing, and polish. The trick is to sand back the template in stages until achieving the final curve for the last pass. The finished plug mould is only used once.  The first female is taken off and further finishing is done on this. From this female the production mould is made.

Finishing

Cutting throats

Casting the flange

Marking up and drilling driver mounting holes.

Surface finishing

Making your own Flanges

There are two basic alternatives: 1) Cast the flange as above, or 2) Turn or otherwise fabricate the flange and bond it to the horn.

1) Cast flange

Ensure the horn is concentric with the mould by positioning over the concentric circles marked on the mould base.  For narrow neck compression drivers a lcator spigot can be bolted to the mould base.

If the horn neck is sufficiently flat there will be little leakage of the casting resin – however the inside of the horn neck should be well waxed to prevent any intrusion from sticking.  The outside of the horn neck should be well keyed with coarse grit and some light hacksaw cuts.The inside of the mould should be well waxed.Special Casting resin should be used because it is low exothermic.  Mix with calcium carbonate powder and milled fibre until thick, and add catalyst. Pour into the mould.  Slight shrinkage makes the mould release easily once hardened.  There are a number of alternatives to calcium carbonate that a fibreglass supplier may recommend eg Talc.  Avoid Q-cell as it may crack.After release the mounting holes can be marked using the tracing paper method.  The holes should be drilled with a drill mounted in a fixture. See shed pictures for ideas. Hand held drills will not easily make vertical holes.  Use a very small pilot hole to begin with, and work up, to ensure no drill bit wander.Mounting holes should be drilled large enough to allow some adjustment of position of the driver – flange holes are not intended to be locators.

2) Bonded on Flange

For Compression Driver horns, a very attractive flange can be turned from hardwood. The bolt hole circle can be marked whilst on the lathe.  The bore may be tapered to fit the horn neck. The holes may be drilled accurately on a pedestal drill prior to mounting.Mounting holes should be drilled large enough to allow some adjustment of position of the driver – flange holes are not intended to be locators.The Flange should be mounted by placing it face down on paper and then inserting the horn from above.  This will keep the face / neck interface flush and normal to axis.  The way to ensure the flange is concentric with the horn axis is to mark the paper with concentric circles; one for the horn neck ID, and one for the flange OD. If a locator spigot can be made, this is even better. There will likely be a certain amount of fitting to do prior to applying the glue – possibly removing material from the horn wall where it is irregular. Finally apply the bonding agent - 2-pack epoxy glue is recommended.There is no difference in principle for an 8” driver.

A flange can be cut from Ply sheet, using a router jigged up as a hole cutter. The holes should be marked and drilled after the flange is bonded on – using the tracing paper method shown in the photos.Horns for 8” drivers that are intended to be back loaded by a closed or ported box may be bonded directly into the front baffle of the box.