Finishing off the Minibass

Just about there now. I found a couple of old “top hat” knobs in my parts bin. In the spirit of using stuff I already had, these seemed to fit the theme.

As I was wiring it up and testing it out one of the biggest problems I found was that the string anchor holes were too close to the bridge which gave a very steep break angle. This had the effect of canting the bridge saddle forward, so that the main weight of the strings was actually pushing against the back edge of the saddle slot. This meant that there was very little downward pressure onto the piezo pickup. For now I’ve implemented a bodger’s solution of a rounded bar which forces a much shallower break angle over the strings. I may just live with it and call it the Sustain-o-Barâ„¢.

Finishing with Danish oil

I’ve spent a lot of time experimenting with the best way of applying Danish oil and I think I’ve just about arrived at the perfect approach for me.

Day One

The first phase is all about getting the first coats to soak in as deeply as possible to give a good foundation to the top coats. I don’t know for certain but I’m hoping this deep soak, means that it will be a more durable and longer lasting finish too. I get the oil and piece to be finished good and warm as a start. Using a brush I apply a liberal coat, let that soak in for 15 mins and repeat. You can see the areas where it has reached saturation point because the surface retains the satin sheen. Keep going until the whole piece has got that satin sheen. Give it a wipe down with a lint free rag and give it an hour or two to dry. After the drying period, if there are spots that appear “dry” I do it again. Once you’re happy let it dry over-night.

Day Two

The second day is all about applying the thinner top coats to give it a deeper finish. For a long while I used 400 or 600 grit wet & dry paper to apply these coats. This helps flatten out any imperfections as you apply. It worked well but because the paper doesn’t hold any oil, you are continually dipping the paper back into the oil; inconvenient, messy and unnecessarily wasteful. The breakthrough came when I tried an old, well used, green nylon scouring pad. It went through the washing machine first to ensure that it was clean and grease free. This works brilliantly. It is gently abrasive and it’ll hold a decent amount of oil so that you’re not continually stopping to reload. Once the thinner coat is on, any excess gets wiped away with the lint-free cloth and allowed to dry. Throughout the second day I do this every 60 to 90 mins. I leave it to dry a minimum of 24 hours (48 is better) and then top it off with a good wax polish.

Here are some of the progress pictures from day two of applying oil to the minibass.

Minibass: Cavity cover and prep for Danish oil

Before I got started I took a picture of the minibass being held by my son, to give an idea of just how small this thing is.

So putting yesterday’s theory into practice, after a number of test runs on MDF I worked out a slightly different approach. I use the same technique but, rather than cutting the shapes into the final timber I cut them into MDF and then used those as templates for routing with the top bearing cutter. Here’s the main cavity.

I drilled the hole for the barrel style jack socket.

I laid paper over the cavity and using the old schoolboy brass-rubbing technique, traced the cavity shape onto the paper, so that I could draw the shape of the cover itself.

The paper shape was transferred to MDF.

This MDF master was then used to create an oversize template.

Using the different sized collars, I then used the oversized template to cut a male and female template.

Using the female template I routed the lip into the body.

And with the male routed the shape into an offcut of sapele.

Perfect? Not quite, but better than my previous attempts by a large margin.

So after giving the whole thing a good sand, and cleaning up my work area, it was time to apply the Danish oil.

Tomorrow I’ll continue applying light coats of Danish oil and start work on the changes to the preamp. Currently it has the volume and tone surface mounted to the PCB. I need to desolder these and run wire from PCB to pots, so that I can fit everything into the cavity and have the volume and tone where I want them.

Cutting cavity and cover to match

When I’ve previously made control cavity covers, I routed the shape into the body and then, as best as I could, have cut the cover to shape. Getting an exact match is really tough and whilst my efforts have been adequate, they’re far from perfect. Look closely at this picture and you’ll see the ~0.75mm gap between body and cover.

It always struck me that there must be a way of using a single template and router to cut both the cavity (female) and cover (male). I finally stumbled across this page which shows a technique to do it. In essence it involves using an oversized template and different guide bushings (also known as collars).

I hit my first problem – all of the wide choice of router collars only seem to be available for Porter-Cable type routers, whereas I have a Bosch POF 1400 ACE, which has a different fitting in the router plate. I spent many fruitless hours searching for the parts I needed – mainly looking for what is called an “inlay kit” which combines two collars with a matching cutter

The way the geometry of this approach works (see picture below) is that the difference in collar diameter needs to be twice the diameter of the cutter. I currently have a 17mm collar, that came with my router. I finally managed to navigate to the correct page on Bosch’s UK website to see what collars were available that would fit. Put that part number in at Amazon, preceded by “bosch” and I found a supplier of the part I needed. You can find this at bosch.co.uk. Of course as soon as Bosch refresh their website this link may stop working, so I will reproduce the collar sizes and part numbers, should it be useful for you.

Diameter (mm) Part Number
13 2 609 200 138
16 2 608 000 471
17 2 609 200 139
24 2 609 200 140
27 2 609 200 141
30 2 609 200 142
40 2 609 200 312

So going back to that equation of the difference between collar sizes needing to be twice the diameter of the cutter, and given that I already have a 17mm collar, the only viable option for me was a 27mm collar with a 5mm cutter.

So how does this magical combo of two collars and one cutter allow you to make the male and female cut from the same template?

This diagram attempts to show how this works. The template needs to be 11mm larger than the final pieces. For cutting the cover we use the small collar, pictured at the top. Here you can see how the left edge of the cutter will trim the piece, to follow the shape of the template.

To cut the ledge for the cover to sit on we switch to the larger collar and here you can see it is the right hand edge of the cutter that replicates that same shape, albeit in female.

There’s more to this than initially meets the eye. Using the same principles you can make an exact sized cover for any cavity or make an exact sized cavity for any cover. For example, I have considered rebating a Telecaster’s control plate so that it sits flush with the surface of the body. I could mount the control plate onto MDF, use the small collar to create the 11mm oversized template, and then use the template and the large collar to cut the recess into the guitar body.

There is one limitation of this approach is that it is not possible to cut a corner smaller than the radius of the router bit. This means that the template corners must have a minimum radius of 22mm. It could cut a square corner on the male part, but it would mean that you’d need to use a chisel to square up the corners of the female.

And that’s the theory. When my 27mm collar arrives in the next couple of days I’ll report back with my experiences.

Minibass: Carving the neck and stringing it up

Carving the neck, first with a rasp…

…and then with a spokeshave.

Last night I strung it up (E and G strings) and adjusted the bridge until the intonation was correct. Here are the marks.

I glued the bridge into position and then glued up and clamped in the neck too.

With the spindle sander in the drill I shaped the heel and upper reaches of the neck.

I used the radiused sanding block to shape the top edge of the bridge saddle and nut. First I covered the edge with pencil and then sanded until it was gone.

Then I strung it up to check everything aligned correctly, marked the position of the strings on the nut and saddle and filed the slots for them.

The last piece of woodworking required is to cut the control cavity and cover. I’ve been investigating ways of using a single template to cut both, to guarantee that they match exactly. I finally tracked down the approach, which involves using the 5mm cutter, a 17mm collar and a 27mm collar. I had everything except the 27mm collar so while I’m waiting for that to arrive there’s not much else to do. I’ll write up the details of how I propose to use the two collars and one cutter when I get the chance.

Minibass: cutting the neck pocket and drilling the string anchors

We’ve had intermittent heavy showers today so I haven’t made as much progress as I had hoped. First job was to cut the neck pocket, so I made myself a routing template. I clamp the neck to the MDF and draw lines along each side, extending out onto the board so that I can draw on a centre line. This means that when I match it up to the centre line of the body everything matches up.

I then sticking a couple of guide pieces either side. At this point I unclamp the neck and tap it down into those two guides to make sure it is really snug. I then stick down the end guide.

A few passes with the top bearing cutter to follow those guides, cutting into the MDF sheet below and I’ve got a template that matches the neck dimensions.

The neck pocket needs to get routed in at an angle, so I set up a pair of guide rails, and chock up the back egdes to get the angle I want (a rise of 30mm over a distance of 600mm). The neck pocket template gets secured to the rails and the body flat to the bench below. Rout the pocket to 13mm deep at the front edge and we’re done.

While it was still off the body I trimmed the back of the neck to start the job of carving the profile. The real work on this won’t start until it is glued to the body, so I can shape the neck heel at the same time.

Last job of the day was to drill the holes for the string anchors. I drilled 2mm holes all the way through the body in the positions where I wanted them.

From the back I drilled 10mm holes to for the ball-ends to sit in. These are much bigger and longer than a traditional ball end so these holes are 18mm deep.

Back to the front and I drilled holes for the strings to pass through – these are matched to the individual sting gauges. Sizes are 5mm, 4.5mm, 4mm and 3.5mm.

And this final shot shows the yellow ball-ends seated in their “pockets”.

Next job is to rout the control cavity and then glue the neck on. Once that has been done I can do an initial assembly and rough setup to correctly intonate the bridge. I’ll disassemble, glue the bridge into position and then it is onto shaping the neck, sanding and application of finish.