Acoustic archtop non-traditional design and build

[Nick Kitchener]

So I've now made the two plate holding chassis:

12mm depth - the centre is 6mm thickness

Can you see where the router bit dropped?

15mm depth and the centre is 3mm thickness

The idea of doing these this way (rather than outline+backing board) is to teach me about which techniques work best - in the end I tried the dremmel press but the speed burns the bits (even at lowest speed), the large pillar drill is too imprecise, and so it was down to drilling a hole manually and then routing with a palm router.

The reason for having two chassis is simple - my outline is not symmetrical hence I can't use one chassis for both plates in sequence..

There's mistakes in these but it doesn't matter - it's taught me alot, and there's less errors on the second one.

I think tomrrow/tuesday will see the start on the actual side plates.

[Alan Carruth]

Many people make cradles with a thin plywood rim for the outline and corrugated cardboard for the steps, with a plywood backer. The cardboard can crush a bit around the edges to accommodate variations, and still give decent support.

[Nick Kitchener]

Many people make cradles with a thin plywood rim for the outline and corrugated cardboard for the steps, with a plywood backer. The cardboard can crush a bit around the edges to accommodate variations, and still give decent support.

That would be the faster route. Although this has given me a more time and practice with the router.

I'm going to modify the program to create a higher resolution image, so when it's blown up to real size the lines for the contours are smaller. I'll then print and stick the paper to the wood. I think the contours I have are still a little too jagged and the pixel width scaled up is too large which creates an inaccuracy. Note that it will be massively accurate - it will be a variable I can minimise. After all it will be carved during tuning etc.

I've not returned to the original hand drawn plans since before starting the application etc. With the plate measurements, the bridge point sits higher than the original hand drawn plan (I had 12mm from the rib and not the engineering line). As a result the bracing, the spar and neck can shift up. I can then start finalising the spar, neck etc which will give me the final downforce. I can test in a range but it's nice to have a centre point to base it on.

[Nick Kitchener]

So I'm progressing..

Originally I tried routing but after two layers the depth guage dropped 0.5mm. So I've switched back to the ye-olde drill depths. I'd routed with some additional space so that will clear up at a later date.

I'm adopting this drill plan:

I setup the drill gauge on the brad boint tip from the base. It's now got measurements of the tip off the base plate. Tested that, then drilled a number of different drill heights cut them open and checked, finally re-testing the measurements.

So.. it was time to get holey..

I think whilst the top has a stable base, I'll drill depth the underside. At least the deeper areas. Then, once that's done I'll carve, scrape and sand down the upper surface using a gauge to check the actual final height. If the gage falls on a drill hole in the base I'll use a scraper (1mm thick - handy!). Then after that's prepared I'll switch to carving/scraping/sanding.

I'm aiming for about 8-9mm thickness after the drilling, then scraping/sanding back to about 6mm uniformly before I finally remove some more in the required areas to tune. The final could be around 6-4mm depending where on the top.

[Nick Kitchener]

A little update.

The front is coming along after the router mishap. I still need to finalise this but:

It's looking like that router mess is salvageable (just!)

Today was rear plate day - depth marked that both sides, and then took to it with a power plane to remove the excess:

I knew about the growth (dark marks) and that wasn't a problem as this sounded the best, however on removing alot of excess it seems that it has quite a bit of flame too which was unexpected.

I designed the rear plate to have a centre dome rather than mirror the top plate bridge position. The concept is it reduces the number of duplicate standing wave distances.

I still have a distance to go with both but happy with the progress today!

[Alan Carruth]

Was it the depth gauge that dropped, or the bit? I've had lots of problems with router bits pulling out of the collet over the years, particularly on heavy cuts. I've never had, or heard of anyone having, a problem of that sort with the drill press planer, nor have I had a quill drop out. That's why I've used the drill press planer for this for the past 40+ years.

I have gotten nicked by the DPP a couple of times, and some of my students can't seem to avoid the thing somehow. I've also had some memorable run-ins with routers. Again, there is no 'safe' power tool, with the possible exception of the 'Saw Stop', and even then...

[Nick Kitchener]

Yup the depth gauge.

However *blows trumpet*

The router mess cleaned up. Where the router mess was now has 12mm of thickness so I have a spare

Next up will be to verify the contours and top heights again after that, and then finalise that before moving to the thickness.

[Nick Kitchener]

A little more progress, the front is almost there - I've marked out brad pin holes remaining, the underside is the same. The rear plate is looking better after giving it a 40-grit roughing again but stopped for today as last night's partying is starting to kick in.

I've been using an inspection light to give incidental light to get a better idea of the curve smoothness and identify remaining depth marks.

[Alan Carruth]

When sun light isn't available I like to use a single element LED spotlight, as it gives a reasonably sharp shadow.

[Nick Kitchener]

The rear plate is coming along - I missed posting this yesterday but it's progressing.

I still have about 5mm of thicknesss to remove, so you can see form the brad points that there's 2-3mm from this slide:

There's some interesting growth patterns which I quite like, it makes it look more organic.

I may have some time today to get onto that just to get both front and back closer together in preparation. I can then clean up the garage and finally get back to get the next steps going.

Now I feel happier that the top will sit more or less as planned, I can finally start having a look at the neck/spar joint but I don't want dust around when attempting to join those.