Tolerance in making flatness needed for rib

[Dennis J]

5 minutes ago, chiaroscuro_violins said:

Why is there friction with sandpaper but not with pumice?  It seems there would be if it is abrading anything.  

There is, but it is very slight. The rib assembly just skates across the slab. I concentrate on the end blocks first, one at a time making sure they are not higher than the corner blocks. I then concentrate on those using both hands to apply pressure.

 

[Dennis J]

Here's a pic of my current work back-down on the glass. I took the time to make sure the block ends were square and glued to the form vertically on this glass slab. I used spacers under the form to get the right height up the blocks.

It does rock a bit but there's no point worrying about that until the ribs are glued on. So the back will need only a couple of minutes on the granite slab after the ribs/linings are planed down to the level of the blocks. The top will need a lot more work to establish the taper.

 

 

 

[HoGo]

12 hours ago, Muswell said:

Keep your eye on skips/dumpsters and find a piece of 40mm kitchen worktop, or see if you can get an offcut from a kitchen installer.

I've had an old kitchen cabinet door for at least 15 years for this. It's got HPL layer on both sides so it's very stable and flat. I glued large sheet of Norton belt sandpaper (I believe 80 grit) which is nearly indestructible by simple wood hand sanding and very agressive. Very light pressure, or almost none at all since I use thick plywood outside mould and rim is held by several clamps to it so it has some weight which keeps things flat and the paper removes wood quickly. I mostly rotate the assembly 30-45 degrees back and forth. If I need to remove more in one spot I movethat spot only for few turns and check progress often.

I don't plane ribs as I'm building mandolins and kerfed lining likes to split off when planed.

[Bodacious Cowboy]

15 hours ago, Muswell said:

Keep your eye on skips/dumpsters and find a piece of 40mm kitchen worktop, or see if you can get an offcut from a kitchen installer.

Just what I was about to post.

Flattening ribs/blocks/linings is a completely trivial task. The sort of thing I'm happy to let my 11 year old do when she asks to help out in the shop.

The tendency of Maestronet to make mountains out of molehills is really something, sometimes. 

No disrespect intended. 

 

[David Burgess]

On 11/12/2021 at 8:25 AM, Bodacious Cowboy said:

Just what I was about to post.

Flattening ribs/blocks/linings is a completely trivial task. The sort of thing I'm happy to let my 11 year old do when she asks to help out in the shop.

The tendency of Maestronet to make mountains out of molehills is really something, sometimes. 

No disrespect intended. 

 

I don't see how some people won't fabricate disrespect, but I agree with you. Over-"mind-phucking" things is one of the most serious hazards to anyone who wants to be a serious and productive maker.

[Nick Allen]

On 11/9/2021 at 6:49 PM, Wood Butcher said:

I guess this is where the skill of a professional maker shows.

Not necessarily. I know many professional makers who don't bother with a plane on this step. It's what gets fiddles made faster that matters, and that's different for each maker. 

[Wood Butcher]

27 minutes ago, Nick Allen said:

Not necessarily. I know many professional makers who don't bother with a plane on this step. It's what gets fiddles made faster that matters, and that's different for each maker. 

My post, which you have quoted, was in response to someone saying it wasn’t possible to make a rib garland flat using a plane.
Clearly this is total nonsense, as a skilled maker, I have no doubt can plane a set of ribs flat. After all, violins were made for at least 350 years before glass paper arrived.

[Dennis J]

Yesterday I finished tapering and levelling blocks on the front shown in the pic I posted previously. I had to take off about 3 mm on the upper and 2.5 on the lower. The lower corner blocks were about .5 mm below the rest. I decided to get them close to finished length before gluing the ribs.

I couldn't use a plane without the ribs attached of course so I pared some off the highest blocks along with using sanding sheet on the saw table. When I got them levelled I tested for gaps. One thing quite noticeable was how the apex of the corner blocks were rounded over as were the lower and upper blocks. I expected that because the sanding sheet wasn't glued down. However I've tried glueing sanding sheets to a board in the past with that same sort of problem, especially with the corner blocks.

It took about 10 min. on the granite slab to get all the blocks level and flat because the end block was close to 1 mm higher than the others. It tests absolutely dead flat with no gaps on the glass slab. The blocks are now about .4 mm too long so when the ribs are glued on I'll plane them close to level with the blocks and then take any excess off on the granite slab.

So it doesn't matter how you reduce or flatten the block/rib assembly it's how you finish that counts. Before I came up with this granite slab and pumice idea I could never get a perfectly flat gap free result.

 

 

[Nick Allen]

3 hours ago, Dennis J said:

Yesterday I finished tapering and levelling blocks on the front shown in the pic I posted previously. I had to take off about 3 mm on the upper and 2.5 on the lower. The lower corner blocks were about .5 mm below the rest. I decided to get them close to finished length before gluing the ribs.

I couldn't use a plane without the ribs attached of course so I pared some off the highest blocks along with using sanding sheet on the saw table. When I got them levelled I tested for gaps. One thing quite noticeable was how the apex of the corner blocks were rounded over as were the lower and upper blocks. I expected that because the sanding sheet wasn't glued down. However I've tried glueing sanding sheets to a board in the past with that same sort of problem, especially with the corner blocks.

It took about 10 min. on the granite slab to get all the blocks level and flat because the end block was close to 1 mm higher than the others. It tests absolutely dead flat with no gaps on the glass slab. The blocks are now about .4 mm too long so when the ribs are glued on I'll plane them close to level with the blocks and then take any excess off on the granite slab.

So it doesn't matter how you reduce or flatten the block/rib assembly it's how you finish that counts. Before I came up with this granite slab and pumice idea I could never get a perfectly flat gap free result.

 

 

You can just spray glue some 80 grit to a sheet of melamine and move on with your life after 5 minutes. 

I got 2 rib assemblies flattened and tapered in under ten minutes the other day. 

For the taper on the upper blocks to the neck block, just put a ruler down and ride the Garland on that for a few strokes and voíla.

 

[Dennis J]

Yes, I think having a flat, hard abrasive surface is the key. As I remember gluing a sanding sheet to mdf or whatever it was didn't seem to work that well. Mind you 80 grit seems very coarse to me.

I only finish the blocks for a height taper from the lower block to the upper, 32/30 approx. So I'm faced with a step down arrangement which has to be flattened.

Anyway for preliminary sanding what you suggest is probably worth looking at because loose sheets on the saw table round over the corner blocks' pointy bits very aggressively. Not hard to go too far.

 

 

[uncle duke]

80 gr. seemed a bit too much for me when reading yesterday - I use 120 gr. when needed.

A reason to not tape the paper down is sometimes one hits the taped edge with the rib assembly,enabling dust to enter under or around the tape.  Then the tape holding down paper doesn't work as well. 

[Bill Yacey]

A friend of mine used to do this; he used  piece of kitchen counter top with Formica / Arborite.

He would glue down a couple sheets of sandpaper with 3M contact adhesive in a spray can. It was a low tack product, and the sandpaper could be easily scraped off with a putty knife and a bit of heat from a hot air gun.

I build in an outside mould and cut all my blocks to within 1/2mm of finished height, so the amount of planing on the blocks is minimal. The ribs I cut slightly higher, and they are easy to plane down after the linings are installed, to the finished height.

[HoGo]

9 hours ago, uncle duke said:

80 gr. seemed a bit too much for me when reading yesterday - I use 120 gr. when needed.

There are differences among sandpapers and the Norton 80 grit belt sandpaper I'm using is quite agressive and durable but still leaves very uniform fine scratch pattern unlike most common papers of similar grit. Was quite expensive, too.

[Muswell]

23 minutes ago, HoGo said:

There are differences among sandpapers and the Norton 80 grit belt sandpaper I'm using is quite agressive and durable but still leaves very uniform fine scratch pattern unlike most common papers of similar grit. Was quite expensive, too.

Yes, it's not like the 10m rolls of 100mm abrasive you buy from the builder's merchant.