Official Luthiers Forum!

Owned and operated by Lance Kragenbrink
It is currently Fri Mar 29, 2024 2:27 am


All times are UTC - 5 hours


Forum rules


Be nice, no cussin and enjoy!




Post new topic Reply to topic  [ 9 posts ] 
Author Message
PostPosted: Tue Mar 14, 2017 6:51 am 
Offline
Contributing Member
Contributing Member
User avatar

Joined: Fri Jul 10, 2009 4:44 am
Posts: 5398
First name: colin
Last Name: north
Country: Scotland.
Focus: Build
Status: Semi-pro
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?

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.


Top
 Profile  
 
PostPosted: Tue Mar 14, 2017 7:31 am 
Offline
Contributing Member
Contributing Member
User avatar

Joined: Wed Oct 08, 2008 11:36 am
Posts: 7241
Location: Southeast US
City: Lenoir City
State: TN
Zip/Postal Code: 37772
Country: US
Focus: Repair
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.

_________________
Steve Smith
"Music is what feelings sound like"


Top
 Profile  
 
PostPosted: Tue Mar 14, 2017 8:24 am 
Offline
Brazilian Rosewood
Brazilian Rosewood

Joined: Sun Mar 30, 2008 8:20 am
Posts: 5924
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.



These users thanked the author Clay S. for the post: Tim Mullin (Tue Mar 14, 2017 3:57 pm)
Top
 Profile  
 
PostPosted: Tue Mar 14, 2017 8:37 am 
Offline
Koa
Koa
User avatar

Joined: Mon Sep 05, 2011 10:45 pm
Posts: 1470
First name: Trevor
Last Name: Gore
City: Sydney
Country: Australia
Focus: Build
Status: Professional
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

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au



These users thanked the author Trevor Gore for the post: david farmer (Wed Mar 15, 2017 5:43 am)
Top
 Profile  
 
PostPosted: Wed Mar 15, 2017 1:46 pm 
Offline
Brazilian Rosewood
Brazilian Rosewood

Joined: Sun Mar 30, 2008 8:20 am
Posts: 5924
"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?


Top
 Profile  
 
PostPosted: Wed Mar 15, 2017 2:26 pm 
Offline
Contributing Member
Contributing Member
User avatar

Joined: Fri Jul 10, 2009 4:44 am
Posts: 5398
First name: colin
Last Name: north
Country: Scotland.
Focus: Build
Status: Semi-pro
[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.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.


Top
 Profile  
 
PostPosted: Wed Mar 15, 2017 3:50 pm 
Offline
Brazilian Rosewood
Brazilian Rosewood

Joined: Thu Feb 12, 2009 10:27 pm
Posts: 2082
Location: South Carolina
First name: John
Last Name: Cox
Focus: Build
Status: Amateur
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....


Top
 Profile  
 
PostPosted: Wed Mar 15, 2017 5:26 pm 
Offline
Cocobolo
Cocobolo

Joined: Sun Jun 21, 2009 6:14 pm
Posts: 439
First name: Mike
Last Name: Imbler
City: Wichita
State: KS
Zip/Postal Code: 67204
Country: usa
Focus: Build
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


Top
 Profile  
 
PostPosted: Thu Mar 16, 2017 12:10 am 
Offline
Contributing Member
Contributing Member
User avatar

Joined: Fri Jul 10, 2009 4:44 am
Posts: 5398
First name: colin
Last Name: north
Country: Scotland.
Focus: Build
Status: Semi-pro
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.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC - 5 hours


Who is online

Users browsing this forum: No registered users and 41 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group
phpBB customization services by 2by2host.com