rib crowning

[Greg Newell]

Ron,
    I didn't mean for it to take this long to reply but, THANKS! This is good
info to ponder for a while. I'm getting much closer now that I have what I
consider to be an adequate approach to jointing. I've never had any luck with
the short bed jointers I've owned and I made a long aluminum channel to use
with the router but that is a little too bulky to use frequently. I've just
purchased a Jointech system for table saw and router and will use that to
joint the Sitka pieces. Once done it'll go in the clamps and then I have to
get really serious about what I wish to do for ribs. I think I will buy a
little extra time though while I play with the new toy. I have to get used to
using it before I do something important with it, right?

Greg

Ron Nossaman wrote:

> >Greetings list members,
> >    I am in the process of learning how to go about pre-crowning ribs
> >for use in a new soundboard installation. I have seen at least one jig
> >which seems to cut the rib with the crown directly in the center of the
> >rib which will not necessarily be under the bridge. As I understand it
> >it makes the most sense, at least to me, to have the thickest part of
> >the rib directly under the bridge for the maximum support. Am I correct
> >in assuming (Ooo there's that word again) that a different degree of
> >taper exists from one side of the bridge to the rim than the other?  Am
> >I correct in noting that the rib thickness at 3 inches from the bridge
> >on either side would not necessarily be the same? If this is the case
> >(on a middle rib for examples sake) then how does one go about laying
> >out an even taper but with different degrees on either side? Does any of
> >this make any sense at all? Am I even on the right track?
> >
> >--
> >Greg Newell
>
> Greg,
> Good observation. Here are a few more for you. With a constant radius
> crown, the high point will always be in the center of the rib because the
> crown is an arc segment. A crown curve based on a parabola or catenary
> curve will let you put the high part of the crown anywhere you want it, if
> you want to go to the trouble. I'm not sure it's necessary. The old
> compression crowned boards had a crown that was whatever the panel
> expansion happened to bend the rib into, and was anything but constant
> radius through the feathered ends of the ribs, or from rib to rib for that
> matter. Some of them sound pretty good. In a rib crowned board, you can
> build the crown and load handling characteristics you want. Looking at a
> rib like a beam under concentrated load(s), you can move the load center
> somewhere other than the center of the beam with the feathering, which
> doesn't have to be symmetrical. The best way to see how different
> feathering configurations and proportions work is to make up some small
> scale model ribs out of scrap pine or something, try different feathering
> approaches under different loading points on the rib, and observe the way
> they bend under load at that point. Go with the approach that produces the
> deflection action you think you want, and remember anything else
> interesting you discover along the way for future prospecting potential.
> The rib scale will be different on every model piano, so there will always
> be a series of decisions to be made in the process.
>
> Ron N

--
Greg Newell
mailto:gnewell@ameritech.net