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| Sanding grit vs scratch depth? http://luthiersforum.com/forum/viewtopic.php?f=10101&t=49131 |
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| Author: | Colin North [ Tue Mar 14, 2017 6:51 am ] |
| Post subject: | Sanding grit vs scratch depth? |
Recent post on planing tops has me wondering what the scratch depth of sanded wood is compared to the grit, or micron size, of the abrasive is. Basically this is to assess how much "dead" or "inactive" wood is left on the surface after sanding. Anyone know a way to work this out? |
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| Author: | SteveSmith [ Tue Mar 14, 2017 7:31 am ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
If you Google "sandpaper grit size chart" you can find the size of the grit used for the various grades which I would think should correlate reasonably well to the scratch depth. For example, standard 80 grit uses 0.00749" grit size so I would expect to lose about 6 to 8 thousandths of an inch by the time I have it sanded smooth. Interesting question, I hadn't thought much about it before. There are some good charts out there and I'll be printing one out for the shop. |
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| Author: | Clay S. [ Tue Mar 14, 2017 8:24 am ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
Different types of sandpaper will give different results. "Grit" type sandpapers (CAMI and P grades) have variation in the size of the grains on the paper, where micron papers tend to have more uniformly sized grains. The pressure applied during sanding might also determine how deep the scratches are. The amount of use the paper has had might also affect scratch depth (some grits fracture into finer particles or become less sharp with use). Another interesting question might be the affect the application of a finish that "fills in" those scratches has on the sound. Does it reactivate these supposed "dead weight" areas? The old timers that used a scraped and unfinished soundboard may have been the purist of the pure. |
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| Author: | Trevor Gore [ Tue Mar 14, 2017 8:37 am ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
Colin North wrote: Recent post on planing tops has me wondering what the scratch depth of sanded wood is compared to the grit, or micron size, of the abrasive is. Basically this is to assess how much "dead" or "inactive" wood is left on the surface after sanding. Anyone know a way to work this out? If you google "SURFACE AND SUBSURFACE CHARACTERISTICS RELATED TO ABRASIVE-PLANING CONDITIONS" you should eventually get to a downloadable pdf, which will provide some background. The depth of the furrows is related to the distribution of grit size (rather than just the average; better quality abrasives having less dispersion) and the stiffness of the backing (rubber roll, steel roll, etc.) Here are a few ways of estimating the depth of the "mush": 1) Estimate it from the distribution of grit sizes. e.g. 40 grit (average) might give you up to 0.5mm of "mush" when the grit size distribution is taken into account 2) Abrasive plane a surface, then go at it with a very finely set, very sharp plane and see how much you take off before you make the grit marks disappear (use a magnifier to inspect the surface) 3) Tap test an abrasive planed surface to get a Young's modulus figure, then plane the grit marks off and get a new Young's modulus figure, then reverse the equations to get an equivalent thickness |
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| Author: | Clay S. [ Wed Mar 15, 2017 1:46 pm ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
"Basically this is to assess how much "dead" or "inactive" wood is left on the surface after sanding" Honest question - Why do you assume the "furrowed" surface is dead or inactive? |
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| Author: | Colin North [ Wed Mar 15, 2017 2:26 pm ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
[quote="Clay S."]"Basically this is to assess how much "dead" or "inactive" wood is left on the surface after sanding" Honest question - Why do you assume the "furrowed" surface is dead or inactive?[/quote Honest answer - what others more knowledgeable than I have said. Posts from Al Carruth, Trevor Gore (and The Book) and the article mentioned above which I have read previously. |
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| Author: | truckjohn [ Wed Mar 15, 2017 3:50 pm ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
Those sandpaper charts are interesting. One trick I have used when I am having trouble getting a top or back joint planed out right is to sand on the surface plate till it candles - then take one or two passes with a well set plane. I noticed that this sanded surface planes like it's not there at all.. While the underlying good wood planes normally. While I didn't connect the dots - it makes sense. Say you joint with 180 grit.. You could have 0.002" of mush under the sanded surface.... And one or two good licks with a plane takes that off.... |
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| Author: | Imbler [ Wed Mar 15, 2017 5:26 pm ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
Clay S. wrote: "Basically this is to assess how much "dead" or "inactive" wood is left on the surface after sanding" Honest question - Why do you assume the "furrowed" surface is dead or inactive? The way I visualize this (rightly or wrongly) is that the sanded surface is like kerfed lining with the sandpaper grooves being the grooves in the kerfed lining. Therefore, that surface is essentially limp like a noodle, and adds no stiffness, just mass, Mike |
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| Author: | Colin North [ Thu Mar 16, 2017 12:10 am ] |
| Post subject: | Re: Sanding grit vs scratch depth? |
Both good points. Here's a chart I came across, admittedly for belt sanding, but interesting anyway. Average Depth of Scratch Grit Size Depth of Scratch 36 .028” - .030” 60 .025” - .022” 80 .015” - .018” 100 .010” - .012” 120 .008” - .010” 150 .005” - .006” 180 .004” - .005” 220 .003” - .004” With 180 grit, I really don't fancy a glue line of up to 8 to 10 thou wide of glued "mush" - which is why I joint with a plane. Probably hand sanding is less damaging of course. |
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