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Dennis J

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Everything posted by Dennis J

  1. The width of the central convex part of an arching along its length defines that arching. If it is narrow, particularly at the widest part of the upper and lower bout, the overall shape will be quite different than a wide, full cross arch in those places. The same applies to the front and back long arches. So controlling overall arching shape, especially without a specifically designed long arch template, is problematic. My guess is that a full, not necessarily high, wide convexity at the upper and lower bouts (inflection point about as near as practical to the edge) is the best option. The difference in curvature between the two extremes there (narrow or wide convex component) is only about 2 mm, and achieving that outcome is not easy. Not so hard with properly designed cross-arch templates. With such an approach applying to the upper and lower bouts the inflection point (where the convex component changes into the concave scoop) can be well defined at edge height, where it should be. Narrow arches at the upper and lower bouts have wide scoops which have to flatten out to achieve that result.
  2. I've got a large collection of gouges and chisels and these are the most suitable for roughing out, not too heavy. They all are forged cast steel from way back. Ward, Robert Sorby, Thomas Ibbotson, 2 Henry Taylor, Mathieson, and a 2 inch US made. Can't remember the maker stamp too faint. Replacing the handle on this one with a longer one would make it an ideal roughing gouge.
  3. I use the worn diamond plates that I have as indicator plates to test for flatness. I have only two that still work reasonably well. One is a Japanese one and the other an Eze Lap. They will flatten smaller, about 6 mm or so chisels, but not larger ones. In the past when I have bought new diamond plates to flatten larger old chisels they removed material reasonably well but left irregular deep scratches which were hard to remove with finer grades. The only way I've been able to flatten larger chisels is to very, very slightly hollow grind the surface area behind the cutting edge with a soft aluminium oxide wheel and finish on a glass plate charged with silicon carbide grit, repeatedly. Very time consuming. I believe Japanese chisels are hollow ground behind the cutting edge so that they can be kept flat with finer grit stones. I have lapped my Japanese chisels on a glass plate with silicon carbide grit. It only needs to be done once.
  4. As I see it the most important thing is that the flat side of chisel and plane blades is just that, flat. They should not be polished, just flat with a fine matte finish, like Veritas blades out of the box. I've resurrected a lot of plane, chisel and gouge blades and arriving at that level of flatness without deep scratches is time consuming. Old Stanley type plane blades are often buckled, but it is usually possible to get the last few millimetres of the blade flat, and that is all that is necessary. I like Veritas tools but honing guides that run on a stone or diamond plate, especially when sharpening chisels, is messy and unnecessary. Hand sharpening is just as effective, although I use Veritas steel plates and diamond paste and kerosene (no rust) with wooden guides that run on the saw table when sharpening plane blades. It takes just about the same time to lay out the three plates (6, 3 and 1 micron paste) as it does to sharpen because the various blades I use are ground at the right angle and I keep the final bevel no wider than about 3mm. After sharpening I buff off any residual burr with honing compound, never touch the flat side with a stone or anything. In the case of Veritas plane blades the backs are perfectly flat and shouldn't be touched with anything, apart with a light buffing, or they are ruined.
  5. There are plenty of old cast steel British and American gouges and chisels out there. If they are forged chisels marked cast steel they are probably OK. I believe most were made from about 1890 to 1930. I wouldn't bother with anything else. The main problem is finding ones which can be sharpened properly. Older Japanese ones are also worth having.
  6. I would say that the scoop around the edges is probably not deeper than usual but very narrow all the way around making it look so. The sub-edge depth of the scoop should vary from wider at the corner locations to narrower elsewhere so that it blends in with differing cross-arch recurve shapes. Simply put the pictured violin is ugly.
  7. In what direction the cuts are made is really not the issue. How the gouge is ground and sharpened is the most important factor. A proper carving gouge with a curved cutting edge is needed. David Sora's videos couldn't be more clear as to what is necessary.
  8. OK, I'll admit I'm a bit short on the practical stuff. To tell the truth I've said just about enough on the subject. I'm ready to move on.
  9. Yeah. Fifteen pages of mostly waffle and argument.
  10. Well James I've posted quite a lot of arching geometry stuff which explains how I set the vertical (height above edge level) and horizontal (distance from centre/edge) position of inflection points above edge level for the 4 central cross arches. The inflection point for the upper and lower bout cross arches is at edge level. All heights are taken from a long arch template. On the blanks you can see the edge height line, the termination point of the arch, the vertical and horizontal location of the inflection point and the apex of the arch. I use french curves to complete the upper convex and lower concave (including the scoop below edge level) parts of the arch profile. If the inflection points are in the right spot it is easily drawn as you can see in my posted pic. The height and horizontal position of the inflection point is crucial. That is why the calculation of that is important.
  11. As far as I know arching heights are usually measured from the bottom of the plate to the apex of the arch. When planning arching templates I use a default edge height of 4.3 mm marked from the bottom of the aluminium blank with the actual arching profile on top of that. So, to be specific the highest arch is marked at 14.9 and that is made up by adding the thickness of the edge (4.3 mm) to the actual arch height above edge level at that position. When I cut the profile out I leave a tab at each end which fits over the edges.
  12. I've marked up a set of cross-arch templates based on the long arch design posted here. I've shown the front arch here. The back arch is .5 mm lower so calculations will be much the same. All calculations are based on an edge thickness/height of 4.3 mm. And inflection point positions are as per my arching layout based on an arc. But one thing that has worried me about inflection point height calculations in the past is that the two central cross arches (middle and bridge) are a little high to comfortably complete the arching profile. So I've decided to average the four central cross-arch inflection point heights as calculated and make the lower corner arch height the highest, the upper corner the lowest and the middle two a little lower (about 1 mm). It is not of much significance in the scheme of things but is in making templates. It works much better that way. You might be able to see how I've taken the curve below the horizontal and vertical lines at the waist. So the arching point heights calculated at a fairly high 7 degrees are 3.4 mm lower corner, 2.7 mm the two central, and 2.4 mm upper corner. Measured from the bottom of the plate that works out at 7.7 7.0 7.0 and 6.7 mm. I realise that probably not many members are getting into all of this but It is just a matter of dotting the I's and crossing the T's.
  13. I was referring only to the old photograph. To my eye it is a lot closer to circular than most early instruments. Can't see any sudden downturns there. The newer photograph is quite different, hard to believe it is the same instrument.
  14. Looking at Ho Go's del Gesu pic, at least the older one. The front arch apart from being close to circular is quite low. The neck end seems to dip well short of the upper block suggesting a fairly long recurve there. The back arch is low as well and definitely off centre so that the lower section is more full than the upper section which shows a very gradual slope to the upper block. The recurve there can't be seen but must be there and it would be quite long. And those features of the back arch are something I've noticed in some instruments before. Not dissimilar to the one in my posted drawing. I don't think there is anything simple about arching. There are a lot of variations and subtleties involved and it's something that requires a bit of careful analysis.
  15. I take note of what everybody posting says. I'm thinking increasing C-bout rib thickness from say .9 mm, which is what seems to me to be what some makers end up with, to 1.3 mm would result in a significant increase in strength. Now that's not increasing weight much but is a relatively large increase in thickness and presumably strength. So I'm not about suggesting anything radical. I can't plead guilty to violin making heresy.
  16. I've been thinking about violin structure on and off for about 12 years. I've spent a lot of time formulating what I think and I'm quite confident about where I'm at now. Of all the posting I've done I can't recall anyone pointing out where my arching or arching method is wrong or even differs from Cremona work. Just vague comments about it's not how they did it. And nobody knows that. An arch is an arch, it's just a matter of how well you can handle its creation. Making plate edges thicker in the C-bouts is not just something I've thought about on the spur of the moment, it's a feature of some instruments. If nothing else it enhances the look of an instrument. I don't see anything about what I've posted as inconsistent with traditional violin making.
  17. The idea of strengthening the mid section of the violin body makes a lot of sense to me. Apart from thicker C-bout ribs edge thickness should be increased between the corners.
  18. The way I see it the front full arch is or can be of equal thickness end to end with the high point in the middle. That means that the outside and inner arch curves are or can be just about concentric. That requires a small adjustment to the inside arch near the end blocks. The back arch is complicated by the taper from a thick centre to a thinner one which seems to me to be best handled by placing the high point behind centre. And the inner arch is more central terminating at the back edge of the upper and lower blocks. If I wanted higher arches front or back I would use the same termination points for the curves that I have shown.
  19. That is my impression of the front arch but not the back. I've pushed back the high point of the back arch as far as is practical based on what seems apparent to me. Whether that is a feature of some Cremona instruments or otherwise, I don't know. To my eye it looks right.
  20. Here is an update on my first posted pic. I've checked how circular my back arch is and it is very close and that doesn't surprise me. The difference between circular and any other sort of curve is not much. I did use a french curve to develop the arch from about 16 mm behind long arch centre. I've gone through this process just to nail down the ins and outs of how to design arches and how the inside profile interacts with the outer. The main aspect of the back as I see it is where to locate the high point. By placing it behind centre, the upper recurve is much longer than the lower one. And that seems to me to be a feature of many early violins. The back arch is at the top.
  21. As mentioned the inflection surface/lines go through the sound holes, just about through the middle as I locate it. I just like the idea of keeping it well defined and narrow assuming that is the best option. I think inevitably there really is a big difference using templates to achieve that and freehand arching.
  22. Yes, I do realise that. I have wondered whether well defined arching profiles with a clear inflection point, achieved with precise arching guides, is desirable or not. I get the impression that a lot of makers relying on arching by eye are creating profiles where the upper convex and lower concave scoop is separated by a flat section. So the possible consequences are it doesn't make any difference or it is positive or negative acoustically.
  23. If this effect holds true with violin arching it might be that confining the sweet spot to a well defined area between convex and concave, not flat, would be an acoustic enhancement.
  24. I'm a bit out of my depth with terminology here. What I'm looking at In the C bout above the arc is a surface which is convex on one axis while being concave on the other axis. Below the arc both axes are concave.
  25. STLs aside here's a pic showing the arcs. They they show an inflection point line from the upper bout to the lower. The height of the inflection point above edge level is marked at each cross-arch location. Everything above the arc is convex and everything below is concave. This is an old pic but close to what my current iteration is. The numbers across the arc are used to calculate the height of each inflection point using a right angle triangle calculation.
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