Back joint prep

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cajunkev
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Back joint prep

Post by cajunkev » Tue Jul 30, 2019 3:26 pm

I am building a flat back/front mandolin and am having great difficulty getting the back and soundboard edges perfect.
Can anybody help please?
The books say use a shooting board and sand to finish. Spent many hours doing this and running out of wood.
Using 20mm thick board which I plan to run through a thicknesser to drop down to 2-3 mm. Have access to a very good one and have dropped mahogany strips down to 2mm ok. Tried dropping in a back strip but this only compounds my problem.
I built a shooting board with an adjustable fence, which I adjust to get square and true. Then I tried the Stanley No.6 plane, but never perfect!
Next I added a long strip of fine sandpaper and ran the board along the fence to sand the edge. This got me square but a banana shaped board? Light at one or other end of join. Shimmed the middle to flatten the banana but only gets worse!! Reversed the boards to offset but still not perfect.
Tried a router and straight edge but not perfect.
May try clamping to a bit of mdf and running the joint through a table saw or router. Will parallel edges?
Any other ideas?
Using Matai a native timber used a lot by local guitar builders for backs and kahikatia another local native white pine also used by local guitar makers. Possibly a mahogany joint strip.
Appreciate any suggestions. :x

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rocket
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Re: Back joint prep

Post by rocket » Tue Jul 30, 2019 4:19 pm

Most people take their stock down to about 4mm and then join them with the help of a shooting board , with 20mm stock I would chuck it in a vice and plane it that way, sounds to me your technique might need some attention ie,,, be sure to have the sole of the plane hard against the stock at all times especially the start and finish.

Rod
Like I said before the crash, " Hit the bloody thing, it won't hit ya back

www.octiganguitars.com

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kiwigeo
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Re: Back joint prep

Post by kiwigeo » Tue Jul 30, 2019 4:59 pm

I use a jointing plane and a shooting board. If I'm having trouble getting a good joint I'll cheat by flossing the edges with sandpaper covered builders level.

Are you shooting both halves of the side at the same time and with the sides face to face? When running the plane along the edge be aware of pressure on the plane...I but my weight slightly more on the front of the plane at the start and by the end of the stroke I have more weight on the back of the plane.
Martin

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56nortondomy
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Re: Back joint prep

Post by 56nortondomy » Tue Jul 30, 2019 5:32 pm

If I can't get it quite right on the shooting board I have a 40mm thick piece of MDF 100mm wide by a metre long with 80 grit glued onto it, I then run the plates back and forth to get rid of any bumps, as long as you hold it straight it works well.
Wayne

yakka
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Re: Back joint prep

Post by yakka » Tue Jul 30, 2019 7:13 pm

https://www.youtube.com/watch?v=HRB8EypxVPU
this works very well and if you have a hollow in middle the sandpaper on a level works really well to flatten out the edge till flat
regards
chris

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rocket
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Re: Back joint prep

Post by rocket » Tue Jul 30, 2019 7:24 pm

Just keep in mind that you don’t want to glue up a sanded edge,,, needs to be clean, planed, scraped or cleaned of any dust

Rod.
Like I said before the crash, " Hit the bloody thing, it won't hit ya back

www.octiganguitars.com

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Trevor Gore
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Re: Back joint prep

Post by Trevor Gore » Tue Jul 30, 2019 10:55 pm

The usual problem when trying to shoot panel edges and having trouble is that the plane sole is not flat enough. The usual out-of-flatness is that the sole is concave and I would say that ~90% of Stanley/Bailey pattern planes have this problem. A secondary problem is that even if you flatten the sole perfectly, there is no guarantee it will stay that way due to residual stresses in the casting continuing to pull it out of flat. For example, I have a Clifton #7 (bed-rock style) that I have flattened at least 3 times over the last 14 years. The only bench plane I have that was flat from new and has stayed flat is my Veritas bevel-up jack. With that plane I can shoot perfect edges with 10 or so plane strokes and demonstrated that at a recent "Hand planes for luthiers" class I held. With a concave sole plane set very fine, you can only get a continuous shaving when the shape of the edge closely matches the shape of the sole, so the edge has to be convex. Bring two such edges together and they touch in the middle but have gaps at the ends. This situation is very hard to remedy with a concave sole plane. If the plane sole is flattened to perfectly flat (and your technique is good), you get no gaps, of course. If the plane has a convex sole, you will naturally get gaps in the middle, but with a convex sole plane you can do something about it by differentially planing the ends, and you can get a fit that way.

Getting a plane sole flat is a whole game in itself that I don't want to get into here, but that's what you need for consistently good results achieved quickly. But to check for flatness you need a seriously good straight edge. A typical steel rule is nowhere near good enough. If you can't get the sole dead flat (which is not easy to do) settle for slightly convex, which at least makes the plane usable.

If you factor in the cost of your time in flattening a plane (and likely having to re-flatten it sometime down the track) it begins to make sense to buy one of the expensive brands that starts out flat and hopefully stays flat. A workable alternative is to use a short plane (e.g. a block plane) and differentially take off the high spots until you have a fit. It takes much longer than with a longer flat plane, but it is possible, which is not the case with a concave sole long plane.

Dave M
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Re: Back joint prep

Post by Dave M » Wed Jul 31, 2019 2:47 am

I would just add that you are not alone! Edge jointing of boards is a skill that many people struggle with. Most of us have gone through that litany of sanding, routing etc at the start.

However the reasonably long plane (I use a number 5) and a shooting board is by far the best setup to use.

Trevor's point about a flat soled plane is the starting point, and Martin's point about where to apply pressure is also right. As was said you must do the two leaves together, facing each other. That way if there is a slant to the cut it will be compensated on the other board. Working at 20 mm will make this quite difficult. Even with the lightest of cuts planing a 40 mm width on a shooting board would certainly be beyond me.

The good thing is that once you get this technique it is very quick and satisfying to get a tight fitting joint
------------------
Dave

cajunkev
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Re: Back joint prep

Post by cajunkev » Wed Jul 31, 2019 8:55 am

Rod,

Good points. I have over 50 years DIY but this is my first instrument. Will keep trying!

cajunkev
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Re: Back joint prep

Post by cajunkev » Wed Jul 31, 2019 8:57 am

kiwigeo wrote:
Tue Jul 30, 2019 4:59 pm
I use a jointing plane and a shooting board. If I'm having trouble getting a good joint I'll cheat by flossing the edges with sandpaper covered builders level.
Will try. Thanks for your comments.

cajunkev
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Re: Back joint prep

Post by cajunkev » Wed Jul 31, 2019 8:58 am

yakka wrote:
Tue Jul 30, 2019 7:13 pm
https://www.youtube.com/watch?v=HRB8EypxVPU
this works very well and if you have a hollow in middle the sandpaper on a level works really well to flatten out the edge till flat
regards
chris
Chris, I thought I had tried that but will have another go. Thanks for reply.

cajunkev
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Re: Back joint prep

Post by cajunkev » Wed Jul 31, 2019 9:00 am

Dave M wrote:
Wed Jul 31, 2019 2:47 am
I would just add that you are not alone! Edge jointing of boards is a skill that many people struggle with. Most of us have gone through that litany of sanding, routing etc at the start.

However the reasonably long plane (I use a number 5) and a shooting board is by far the best setup to use.

Trevor's point about a flat soled plane is the starting point, and Martin's point about where to apply pressure is also right. As was said you must do the two leaves together, facing each other. That way if there is a slant to the cut it will be compensated on the other board. Working at 20 mm will make this quite difficult. Even with the lightest of cuts planing a 40 mm width on a shooting board would certainly be beyond me.

The good thing is that once you get this technique it is very quick and satisfying to get a tight fitting joint
Trevor, interesting insights. Not something I would ever have thought of? Thanks for the reply. I will ruminate and act. Cheers.

cajunkev
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Re: Back joint prep

Post by cajunkev » Wed Jul 31, 2019 9:01 am

rocket wrote:
Tue Jul 30, 2019 7:24 pm
Just keep in mind that you don’t want to glue up a sanded edge,,, needs to be clean, planed, scraped or cleaned of any dust

Rod.
Good tips...thanks

cajunkev
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Re: Back joint prep

Post by cajunkev » Wed Jul 31, 2019 9:04 am

Trevor Gore wrote:
Tue Jul 30, 2019 10:55 pm
The usual problem when trying to shoot panel edges and having trouble is that the plane sole is not flat enough.
Great ideas Trevor....appreciate it.

Jim
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Re: Back joint prep

Post by Jim » Wed Jul 31, 2019 2:47 pm

Robbie O'Brien has a good video on it.....

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Steve.Toscano
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Re: Back joint prep

Post by Steve.Toscano » Fri Aug 02, 2019 1:01 pm

Like Trevor said, a straight sole plane is important.
And as others mentioned, don't join at 20mm, get your stock down to ~4mm.

Here's my edge shooting setup:
edge joining 2.jpg
edge joining 2.jpg (263.42 KiB) Viewed 5277 times
The plane is a veritas Low angle jack.
I tried many methods to get to this point, mostly:
- I started out doing Strato's method like in the Video Yakka linked to above which works well even with a slightly concave or convex non flat sole plane but takes a good 20-30minutes to get straight.
- Then the sandpaper on a level
- Then a stanley no7 going straight across end to end - but got sick of flattening the bloody thing.

With this setup and the veritas i can typically shoot the edges of spruce to bang on straight in under 10 strokes.

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