Notched straight edge

[murrmac123]

The surface plate I use I had calibrated and certified to toolroom grade after I bought it .

I could have spent a fortune and had it lapped to inspection grade or laboratory grade but decided that that was probably unnecessary.

As it is, it attains a flatness of 7.0 µm, and that by definition is the flatness which it imparts to the workpieces which are flattened upon it.

[charles Tauber]

Originally Posted by murrmac123

As it is, it attains a flatness of 7.0 µm, and that by definition is the flatness which it imparts to the workpieces which are flattened upon it.

Have you measured the final flatness of the actual straight edges you produce, as opposed to the flatness of the surface against which you abrade them?

[mirwa]

Originally Posted by charles Tauber

Rubbing something back and forth over a flat piece of sandpaper guarantees that the object will come out convex. I make use of that characteristic to slightly back-bow a neck/fingerboard prior to fretting.

Agreed.

The forward backward motion works the ends more than the centre, one can manipulate the centre by adding extra pressure to reduce the phenomenon. When one attempts to join two pieces of wood (example a top) vee the sanding method on a glass plate, you end up with a convex shape, compare that to the true straight edge achieved with a plane and a shooting board, that being said I still like tweaking a book match join using sandpaper and glass, even knowing its inaccuracy's.

I personally use a 245kg black granite surface plate 3ft by 3ft by 6 inches thick for doing any general purpose levelling, it is Manufactured to Grade O, Precision DIN00, Flatness under 3.253/μm, It in no way imparts 3.253/μm flatness to any work pieces which are flattened upon it, I would never sand any of my work tools on it such as a straight edge, it just simply does not work that way, it is good for rough levelling an item

Steve

[murrmac123]

I think the problem is that we have differing conceptions of “rubbing back and forth” . I would agree that the smaller the workpiece and the smaller the available abrasive area, then the greater likelihood of achieving an out-of-truth workpiece.

With a sufficiently long ( and truly flat) abrasive surface, along with proper technique, this ceases to be an issue.

There is no “rubbing back and forth” when you lap a straightedge (or at least there isn't when I lap a straightedge.) The workpiece is drawn along the PSA abrasive on the surface plate with constant downward pressure, in one direction, with the side of the workpiece registered against a fence clamped to the surface plate.

At the end of the stroke the workpiece is lifted and the operation is repeated. IMO a five foot long surface plate is a minimum requirement for success (if you are making straightedges 18” or longer). Six feet would be even better, but if you know what you are doing you can get away with five feet long.

As far as accuracy is concerned, my practice is to lap a batch of three at a time, and inspect them by placing the edges together and holding them to the light to see if any light gap can be observed. I test A:B, A:C, and B:C. If I cannot see a light gap at any point, then the straight edge is of marketable quality and is ready to go out to a customer.

I have in the past tested them by laying strips of tinfoil on the surface plate, laying the straight edge on top, and seeing if the strips can be pulled out from underneath. Invariably, they couldn’t.

[Otterhound]

For a moment , I thought some of you were building hermetically sealed chambers intended for outer space usage . If you are good , a usable straight edge in luthery , can be achieved with a hand held plane . Many fine musical instruments over the centuries have been joined and built that way .
Genius is in making the complicated simple , not in making the simple complicated .
Shall we debate the need to measure each and every hair in order to get a decent haircut ?

[John Arnold]

There is no “rubbing back and forth”

It doesn't matter whether it is in one direction or both. Sanding is more aggressive near the ends of a piece. The only way to avoid this is to sand across the length of the straightedge.

[charles Tauber]

Originally Posted by murrmac123

I think the problem is that we have differing conceptions of “rubbing back and forth” . I would agree that the smaller the workpiece and the smaller the available abrasive area, then the greater likelihood of achieving an out-of-truth workpiece.

With a sufficiently long ( and truly flat) abrasive surface, along with proper technique, this ceases to be an issue.

There is no “rubbing back and forth” when you lap a straightedge (or at least there isn't when I lap a straightedge.) The workpiece is drawn along the PSA abrasive on the surface plate with constant downward pressure, in one direction, with the side of the workpiece registered against a fence clamped to the surface plate.

At the end of the stroke the workpiece is lifted and the operation is repeated. IMO a five foot long surface plate is a minimum requirement for success (if you are making straightedges 18” or longer). Six feet would be even better, but if you know what you are doing you can get away with five feet long.

As far as accuracy is concerned, my practice is to lap a batch of three at a time, and inspect them by placing the edges together and holding them to the light to see if any light gap can be observed. I test A:B, A:C, and B:C. If I cannot see a light gap at any point, then the straight edge is of marketable quality and is ready to go out to a customer.

I have in the past tested them by laying strips of tinfoil on the surface plate, laying the straight edge on top, and seeing if the strips can be pulled out from underneath. Invariably, they couldn’t.

I applaud you for being a small manufacturer of specialized tools and bringing them to market. I have little doubt that the straight edges and sanding beams you make are sufficiently flat for guitar work. However, based upon your manufacturing method, I have no doubt that their flatness is not anywhere near the 7.0 µm you claim.

Hence, the issue isn't whether or not your tools are adequate to the task, but a question of a basic understanding of measurement and manufacturing tolerance and truth in advertising - not overstating what you are achieving.

It isn't my intention to criticize your tools or methods - I didn't bring them up, you did - so I'll leave it there. However, I do think that you really ought to do a little reading about metrology, variation in manufacturing and how that variation is measured and characterized/stated, particularly with regards to "flatness".

[redir]

Originally Posted by charles Tauber

I applaud you for being a small manufacturer of specialized tools and bringing them to market. I have little doubt that the straight edges and sanding beams you make are sufficiently flat for guitar work. However, based upon your manufacturing method, I have no doubt that their flatness is not anywhere near the 7.0 µm you claim.

Hence, the issue isn't whether or not your tools are adequate to the task, but a question of a basic understanding of measurement and manufacturing tolerance and truth in advertising - not overstating what you are achieving.

It isn't my intention to criticize your tools or methods - I didn't bring them up, you did - so I'll leave it there. However, I do think that you really ought to do a little reading about metrology, variation in manufacturing and how that variation is measured and characterized/stated, particularly with regards to "flatness".

Are you making the argument that you have to have something that is truly flat in order to test something else for flatness?

[charles Tauber]

Originally Posted by redir

Are you making the argument that you have to have something that is truly flat in order to test something else for flatness?

No.
.

[mirwa]

Originally Posted by redir

Are you making the argument that you have to have something that is truly flat in order to test something else for flatness?

The issue I believe at hand, is one does not know what one does not know, and the topic derailed a little trying to identify that.

When something is used and sold on the commercial stage it comes with designations and tolerances, flat can mean anything, example the ocean is flat today, or that brick wall is nice and flat and or that straight edge is flat, without giving a tolerance to that flatness it’s just a term that is very open

That is not to imply ethics / product mentioned are good or not.

Custom tooling for commercial sale in the past was made by tool makers, many people made their own tools for personal use, but custom tool manufacturing was a branch of the machinist field, here in Australia, to become a machinist, is a three year apprenticeship.

Hence one does not know what one does not know, as they are never taught it and can make assumptions based on little to no knowledge.

Steve

[murrmac123]

Originally Posted by murrmac123

As far as accuracy is concerned, my practice is to lap a batch of three at a time, and inspect them by placing the edges together and holding them to the light to see if any light gap can be observed. I test A:B, A:C, and B:C. If I cannot see a light gap at any point, then the straight edge is of marketable quality and is ready to go out to a customer.

Anybody care to comment on this ?

[bausin]

>> Anybody care to comment on this ?

I thought you'd never ask. :-)

The smallest thing a naked human eye can see is around 1 mil. I don't know about the smallest gap with a light behind it; probably less.

I think a possible improvement to your testing process would be to use A as a permanent reference and check B against A, then reverse the direction of B and recheck.

[Rodger Knox]

Does anybody think a notched straightedge is a useful tool? I thick we may have some agreement that the notches are not that useful, regardless of how precisely the tool is made.

[ChrisN]

Originally Posted by Rodger Knox

Does anybody think a notched straightedge is a useful tool? I thick we may have some agreement that the notches are not that useful, regardless of how precisely the tool is made.

I'm not a luthier, just a "does some of his own work" guy, but I like mine for making sure the fretboard's flat before fret work, and for measuring/adjusting relief. It also takes the frets out of the equation when I'm looking for unevenness over the fretboard, getting rid of a hump, etc. I'm sure more experienced people have other ways of doing these things, but the notched straightedge does these things for me.

[Rodger Knox]

Originally Posted by ChrisN

I'm not a luthier, just a "does some of his own work" guy, but I like mine for making sure the fretboard's flat before fret work, and for measuring/adjusting relief. It also takes the frets out of the equation when I'm looking for unevenness over the fretboard, getting rid of a hump, etc. I'm sure more experienced people have other ways of doing these things, but the notched straightedge does these things for me.

I wouldn't say I'm a luthier, but I have built a few guitars. Before the frets are in, you don't need the notches. After the frets are in, it's the fret tops that are important. If there's a problem with the fret tops, (as you mention) a notched straightedge is useful in determining if the problem is the frets or the board, but there are other ways to do that. My point is you don't need the notches. I use an 18" clear plastic drafting scale that's flat enough. I know that it's flat enough because I get good results using it, I don't know and don't really care how precise that really is.

Whatever works for you is the right way to do it. Can't argue with success.