leveling the ribs

[ctregan]

Is this a good idea?
When leveling the ribs of a bass , should one take 2-3 ml more material off the ribs at the top and bottom blocks, leaving the c bout ribs higher than the upper and lower ribs of the bass. This, (in theory), would help compensate for the pulling up on the bottom block from the string tension and decrease compression on the bass top plate.

[Don Harris]

Never thought about that. I made a sanding board of MDF and made the top and back of the ribs dead flat (other than the break). Made for easier fitting. Interesting idea though.

[ctregan]

With my limited experience, I will probably make the ribs as flat as possible and not deviate from the norm. I did measure a cello and a bass though, and, on both of these the instruments the bottom blocks had been pulled up higher than the surrounding ribs. Also, the cello had cracks running from the bottom block up the belly, "possibly" from the string tension pulling on the lower region of the instrument? My mind will wonder wile I am building this bass, in some ways, it makes sense to make the ribs a bit lower at the bottom blocks, feel free to tell me if this is wrong thinking.

[Matthew Tucker]

Sometimes having ribs wider at C bouts makes the instrument look better proportioned. I believe cellos are often done like this. A bit like the "swell" in stone pillar design. Basses are sufficiently large, perhaps, not to need this, although many are made with the lower ribs wider than the upper ones. This could be because of the volume of the lower bout, but knowing the little i do about design proportions I tend to think it is probably equally so that the instrument looks "right" from above!

I think, make the ribs fit the plates well. Doesn't really matter if they aren't completely flat, but they should fit well. Flat is usually easier.

[ctregan]

I was thinking to ease compression on the top plate, more than aesthetics, but looks are important too.

[Ken McKay]

According to some sources, building in "tension" is a no-no. I don't know myself but would just make them flat.

On my cornerless bass I am building, the ribs have a little slope (slight radius) in them from the mid point of the upper bout to the block.

[Darren Molnar]

I think the cracks that you mentioned seeing, around the bottom blocks, don't have much to do with the normal tension instruments live under. It's more a case of the top shrinking over time. The saddle doesn't shrink with the top, so, something has to give. Same with the ribs.They won't shrink with the top, either.

On a bass, in my workshop, a 2-3 mm tolerance is called "dead flat "

[ctregan]

Originally Posted by Ken McKay

According to some sources, building in "tension" is a no-no.

It seems like a "slippery slope", it would easy to loose your points of reference by building this way.

[Ken McKay]

Originally Posted by ctregan

It seems like a "slippery slope", it would easy to loose your points of reference by building this way.

I doesn't change anything but the overstand, understand?

Something important that I learned from Ken Smith's comments over the years. The ribs are meant to crack if necessary rather than the top or back.

[Darren Molnar]

The glue should act as a sort of releif valve on instruments, that is, the glue should give out, before any cracks should appear in any wooden part. That is one of the reasons you glue the top on less strongly than other areas. If glue seams let go, be happy, because it did it's job of preventing a crack.

But if you ignore open seams or glue joins, then that little bit of planned engineering can cause other problems. If you own a fine instruent, you have the resposibility to be on top of this stuff.

[Darren Molnar]

I didn't post that to say that Ken Mckay's point on ribs was wrong. I posted it to make that info available, in case someone reading the thread didn't know this point. It seemed that the thread was going in that direction.