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| Checking plate glass flatness - no tested straight edge http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10117&t=31041 |
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| Author: | Colin North [ Tue Feb 08, 2011 5:46 pm ] |
| Post subject: | Checking plate glass flatness - no tested straight edge |
Not really a tutorial, more of a cheap tip, but will maybe help someone out in a bind. A few days ago I received delivery of a new LV low angle No 5 bench plane and was anxious to check it out. But, I first wanted to check if it was flat, and without a reliable straight edge, I had a problem. I did have a Float Glass Plate, 24 inches by 10 inches, used originally to tune up an old beater Faithfull no 4 and block plane, but was that really flat? Because after trawling the web, I found that it might well not be. Not wanting to pay half the price of the plane for a certified straight edge (mine was an unwrapped gift 30 years ago), I had a problem. After a few days head scratching, I had a thought. A fine uniform wire under high tension will be as straight as anything I could find. So, I rigged things up as shown in the attached drawing, leaving about 1/32" less in height for the 2 wooden blocks (in the rather poor drawing attached) than the height of the surface of the glass plate so as not to distort the plate itself (it is 10mm thick mounted on a 18 mm ply board with silicone sealant) or distort the wire itself. Shone a light behind the wire, eyeballed across the plate, and the plate looked concave, but how much? – My thinnest (very beat up) feeler gauge is 0.0013”, and the quoted flatness for the plane is 0.001” So, I then measured the thickness of 10 cigarette papers (about 9 thou = 0.0009”each average) With the wire tensioned over the two ends, one thickness of cigarette paper might have been slightly trapped, but it was difficult to be sure. So, first I trapped one cigarette paper under the wire at each end of the glass plate. I then slid half a cigarette paper, cut lengthways with scissors to avoid the crease in the middle, under the wire at the centre of the plate, and using two fingers, slid it back and forth. No reaction from the wire. Then 2 pieces of the paper under the wire. And slid them about. Maybe some movment of the wire, difficult to tell, hmmmm. Had a thought - switched off the overhead fan. Listened as I moved the two papers about - the wire could be faintly heard vibrating with the two thicknesses of paper, but not with one thickness. - Bingo! So it seemed concave, but by less than a thou. I checked along the length of the plate and found it to be a regular concave shape. With the papers out from under the wire at the ends of the plate, one thickness of cigarette paper in the middle was definitely just touching, I could just hear a vibration from the wire, but not see it., So lengthways was OK, one thou or less - on to the width. Same story, but turned out the plate was slightly convex across the width, and using feeler gauges at each side under the wire, I estimated this to be slightly less than a thou and a half over the ten inch width.- Job done. Hope this helps someone else. I suppose for a shorter length of plate, a top E string could be used, but because of the length of the plate, I used a length of picture frame hanging wire, 21 thou diameter, and a router plus a 6” orbital sander combined for the weight. And the LV plane, I hear you ask – it’s within tolerances quoted, and after a quick hone gives 1.5/2 thou shavings across the width and length of a 1 ¾” block 2 feet long. – Guess I’d better flatten the back before I use it and get it down to 1 thou. While I'm at it, I can maybe check and true my old straght edge and save a few bucks! |
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