Top Edge Thinning - Measurement of Thickness

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Mitch Lees
Beefwood
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Joined: Mon Jan 08, 2018 3:54 am
Location: Findhorn, Scotland

Top Edge Thinning - Measurement of Thickness

Post by Mitch Lees » Sun Sep 01, 2019 1:07 am

I have thinned the outer edges of the tops of my previous guitars, but this has been done by guess-work alone using scrapers. I do have a good metal thickness caliper which is fine for measuring the unattached plates, but it will not go through the sound hole of the completed box.

I am building a Medium SS Falcate as per the book and the plans. The box is closed, frequencies look OK, and neck angle and 2.5mm at bridge all achieved. My top is 2.9mm and I now want to reduce the thickness of the outer 60mm of the lower bout by a further .5mm. How do you guys measure progress? Ultrasonic devices are no good on wood seemingly, and the Stu Mac magic (magnetic) ball device is expensive. I have built a couple of plywood calipers which will go through the sound hole but they are inaccurate and will only measure the top in a couple of small areas which is no good. I did think about doing some thinning before I attached the top, but decided not to because I did not know how close to my target frequency I would be, so working on the principle it is easier to remove wood rather than put it on I left the thinning - unfortunately I did not think things through carefully enough.

Is there a simple technique I am missing, or am I going to have to spend $300 US plus UK tax on the magnetic device? Or - just go back to my 'guess -work' method!

Grateful for any advice.

Mitch Lees
Beefwood
Posts: 26
Joined: Mon Jan 08, 2018 3:54 am
Location: Findhorn, Scotland

Re: Top Edge Thinning - Measurement of Thickness

Post by Mitch Lees » Sun Sep 01, 2019 5:32 pm

Further to my previous post. I was thinking about this (in the middle of the night!) and I have decided not to buy the magnetic device. I will just thin to a target frequency rather then an actual thickness, and use my judgement to ensure I do not thin too much. My 'out of the mould' frequencies are 106, 204 and 224. My final targets are 95, 180 and 226 (already missed the T3 target). If I assume a reasonable amount of top edge thinning, say .5mm or so, will lower T2 by up to 10hz, then if I stop thinning at 198hz I should be OK. The choice of a 6hz reduction being arbitrary, but erring on the 'safe side'.

Does this sound like a reasonable strategy?

Mitch Lees
Beefwood
Posts: 26
Joined: Mon Jan 08, 2018 3:54 am
Location: Findhorn, Scotland

Re: Top Edge Thinning - Measurement of Thickness

Post by Mitch Lees » Mon Sep 02, 2019 12:12 am

Sorry to labour this issue, but others may find it interesting. I have done the thinning, and in doing so I have learned a few things. First, experience does not always help - I have made a couple of arch-top jazz guitars and carving the re-curve at the edge of the soundboard leaves the outside edge at original thickness and the depression created is inboard of that. So this is how I visualised the thinning would proceed on this guitar - that is why I was so concerned about having a caliper to measure the sound board thickness; however, once the process had started I realised that the thinning could go right up to the edge, and, because there are no bindings yet, I could measure the top thickness at the side. I realise this was probably obvious to everyone else.

T2 is now 198 and the top has been reduced around the outer 60mm of the lower bout to 2.2mm from the original 2.9mm, ie .7mm gave me the 6hz reduction I was looking for - I do not think I could have got a 10hz reduction without going thinner than the limit of 2mm Trevor advises.

I also have found that even though I have a 'feel' for how thick the top is as the thinning proceeds, I have no idea at all as to what would be the thickness at which the top response would start to deteriorate - hence that is why I stopped at 2.2mm.

One positive that came out of it was that, because I took a lot of spectra to measure progress, when I averaged out the T3 figures I found that I was now quite near the original target. I don't think it was the thinning that caused the slight increase in the frequency of T3 (?) - just the number of reading gave a more accurate average.

So I now have a T2 of 198hz and I have calculated that my bridge, saddle and pins when fitted will reduce this by a further 11hz. This leaves a further 7hz to get to my target of 180 - will the bindings and BOBO neck delivered that sort of figure?

For those with sufficient patience to follow these ramblings - thank you. I would really welcome your views.

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Trevor Gore
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Re: Top Edge Thinning - Measurement of Thickness

Post by Trevor Gore » Tue Sep 03, 2019 1:59 pm

My procedure for all of this is in Section 12.7 and I haven't changed what I do.

You will likely thin the edges a bit more when you clean up after gluing on the bindings, and maybe a bit more again in preparation for finishing. With a bit of side mass, you should be in reach of your 180Hz target.

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