How does a glass scraper actually work?

[Jamie Buxton]

Dan Barr's recent thread wondering how a handplane actually works brings to mind another similar question. You can make a scraper from a piece of glass. You break the glass, and the broken edge can serve as a scraper. You can pull shavings off a piece of wood. However, unlike our usual steel scrapers, there is no burr on the edge. You can't turn a burr on glass. That means there is no cutting edge. How can the thing cut without a cutting edge?

[Brian Ashton]

It doesn't have a hook like what can be created on a steel scraper but the two surfaces, where the glass is cut or broken, meet at an extremely sharp edge. When held at the right angle, the sharp edge can easily penetrate the wood enough to allow you to peal off light shavings and leave a clean surface. As the edge dulls (glass or steel) you have to push harder for the edge to penetrate the surface and as a result you actually compress the fibers of the wood more than remove them leaving a poor finish.

The glass is actually much harder than the steel scraper and if treated well (which means don't press hard or run it over knots...) will last a fairly long time but is far less durable when you get a bit rough with them.

[James Taglienti]

My grandfather used to strip antique furniture with a piece of broken glass. It works well- the cut is gentler, and there is no burr to clog with old finish.

[Jamie Buxton]

What I don't understand is the cutting action. With other tools -- think of a chisel -- there is a sharp edge shoving between the body of the wood and the shaving that is getting peeled off. With the glass scraper, that doesn't exist. How does the shaving actually get separated from the body of the wood?

[Dave Anderson NH]

Jamie, think of it this way. A sharp edge is defined as the line where two geometric planes meet. It has no thickness at all, or put another way, it is infinitely thin. Conversely, when the line becomes rounded over, or develops a radius that is .0005" across, the edge is now dull. Since you are angling the scraper when you use it, you are presenting an edge that is as sharp as the burr that is sometimes used on a scraper. I say sometimes, because depending on the situation you can often eliminate rolling a burr on a scraper.

[Kent A Bathurst]

I saw that LN offered a "90* scraper blade" for the LA jack - that had me scratching my head - couldn't quite "get it", until an email conversation with Deneb. Same principle as explained by Dave. Had me confounded, I'll admit.

[mike holden]

Jamie,
A glass (or metal) scraper without a burr has a "negative rake" angle. If you have a RAS or a chopsaw take a close look at the teeth, the tip actually trails as it hits the wood.
This is the negative rake. Your glass scraper has the same thing. I used metal scrapers quite a bit on metal (patternmaker) on my job, didnt know about burrs till I got involved in woodworking.
If you want to get a surface flat, you lay the scraper flat on the surface and push it across. (be careful when using a piece of broken glass though)
BTW, didnt know about negative rake back then either, that was just how scrapers were sharpened and used.
Mike

[Phil Warnement]

I have done stained glass for about forty years and just got into woodworking recently. One day when my scraper was dull, the thought of using a piece of scrap glass crossed my mind. I tried it and it works.

I don't know all the angles and I really don't care. I just know the wood comes off where I want it to come off.

Also, the glass I used was a little thicker than window glass is. That made me feel more comfortable using it the first time. Don't try to put a bow in the glass. As you know, glass does not bend. Gloves may be an appropriate wear.
Last thing. The glass will wear down, but all you have to do is score the glass and break off a new edge and two new edges appear.

[Ron Hock]

A glass scraper is a true scraper -- sharp edge, zero to negative rake. A steel scraper with a hook is actually acting like a very short plane blade with a severe chip breaker -- it's shearing fibers but limiting tear out by smashing them against the face of the blade before they can lift ahead of the edge.

A true scraper simply presses against the wood fibers, compressing them until they fail and are scraped away. Because there is no lifting action, true scrapers can flatten and smooth just about any wood surface. But the surface left by true scraping won't tend to be the shiny, satiny surface obtained by the shearing cut of a hooked scraper or a plane. The wood fibers being compressed ahead of the true scraper rebound slightly as the edge passes. This rebound is the reason for the matte surface (it's also why all blades need to have a relief or clearance angle behind the edge -- that fiber rebound can lift the edge from the cut if there is no clearance to accommodate it.)

Hoadley's book Understanding Wood explains in great detail the action of wood machining and I highly recommend it to any woodworker. I referred to it frequently during my research for the chapter in my book, The Perfect Edge (http://www.perfectedgebook.com), "How Wood is Cut".

(I worked that shill in pretty well there, didn't I?)

[george wilson]

On the other hand,I have watched guys scrape wood with a piece of glass that had an irregularly broken edge,scoring the wood into a mess,and they thought they were doing good scraping. Perhaps you could do some scraping on rounded objects where there was a single point of contact with the glass. I just don't prefer it.

Another thing,you had better be careful,or you can cut the devil out of yourself if you have the glass suddenly break when scraping with it. Personally,I'll stick to steel scrapers.