PDA

View Full Version : A brief history of the burr



NeilS
7th Feb 2011, 12:25 AM
Brendan Stemp prompted me to take a closer look with my digital microscope at what happens to the burr if left on a scraper.

There is an excellent article (starting on Pg 2 here (http://www.woodturninglearn.net/articles/newpdfs/Scrapers.pdf)) by Alan Lacer and Jerry Wright that examines the effectiveness of the different methods of scraper preparation. One of their key findings was that the level of polish on the face of the scraper is a key determinant of the finish off the tool. However, they didn't cover the durability of the burr if left on a scraper.

Brendan thinking is that the burr off a grinding wheel is too fragile to last last long enough to be useful in cutting wood cleanly, but thought it worthwhile taking a closer look at the burr after a short periods of use to confirm this.

The scraper I used was one of these (http://www.cws.au.com/shop/item/robert-sorby-rs213-large-tear-drop-cutter). It had the advantage of being detachable so it could be easily put under the digital microscope. The grind angle was about 80deg.

The wood was native cherry, a dense hardwood, running at 1500rpm. A shear angle was used of approx 45 to 60 deg.

The face of the scraper was polished to a level where no scratch marks could be detected under magnification.

The following images are at 200 X magnification and show an area approx 1mm wide.


160652 Burr from #120 diamond wheel before use.


160653 Burr from #120 diamond wheel after 30 secs use.


160654 Burr from #120 diamond wheel after 60 secs use

The burr off the #120 diamond wheel could be felt but not seen with the (my) naked eye. The scraper continued to readily produce fine gossamer shavings for about 30 secs when I stopped and took an image. As can be seen, the burr appears reasonably intact, but could only just be detected by touch. The serrations were starting to lose some of their sharpness.

By 60 secs there was a noticeable change in performance. Another image was taken that shows the burr in effect gone.

Conclusion: Over to you?
.

rsser
7th Feb 2011, 06:09 AM
Interesting. Clearly not that strong - but how many metres of scraping would the edge have done at the 60 sec mark?

I've not found that Sorby steel all that strong and have read forum posts that early versions used stronger alloys.

Equally, a burr on a thick scraper used flat for end grain hollowing wouldn't give me much better than 60 secs either.

By contrast, the original Farrance study of the burr on a spindle gouge found that dry grinding produced a long lasting burr, so let's say that's a function of the grinding angle. There's some suggestion that such a burr might be compacted with abrasive particles off the wheel.

NeilS
7th Feb 2011, 10:40 AM
....but how many metres of scraping would the edge have done at the 60 sec mark?

I've not found that Sorby steel all that strong and have read forum posts that early versions used stronger alloys.



Ern, not sure if my scraper tip is made from an earlier alloy or not. I bought my Sorby RS200KT when they first came out in Australia, so perhaps it is. I'm still using the original tip and find it as good as any final cut steel that I have. If I get the time, I might run some other steels through the same exercise to see if there is a difference.

Should have said my test was on the outside of a 9" cross grain bowl, so more end grain than face grain scraping.

As for the 'scraped meters' travelled, that's a bit tricky. Lots of variables. Having thrown my engineering tables away a few years ago and not wanting to fry the old grey matter, let's say the average diameter scraped was 7" at 1500rpm, at a circumference of approx 22" or 0.56m = 840 scraped meters. Not surprising what happens to a burr after that.

I did also have a look at the burr raised on an #80 alox wheel and by hand with a 45 micron diamond plate and will post those images when I have processed them.
.

artme
7th Feb 2011, 01:01 PM
Hmmm..... I would think that a truly sharp edge has no burr and that such an edge provides true cutting.

Does the burr then, actually cut or tear the fibres?

For me, leaving the burr intact means leaving a rounded edge.

From what I have read on the subject no one can properly explain this apparent conundrum.


I was talking with a well known lady turner who thinks that Sorby tools have been incosistent with regards to metal quality for some period. Lost their edge, so to speak.:rolleyes:

rsser
7th Feb 2011, 01:30 PM
Another contribution to Neil's instructive post ...

I did a test using the Veritas (http://www.veritastools.com/Products/Page.aspx?p=122)scraper burnisher.

This bends a 'hook' on the cutting edge.

The process was this: the bevel was ground on the #120 diamond wheel, the scraper top was lapped to remove the grinder burr with a fine diamond bench stone, and then the edge was levered against an angled carbide rod to turn the hook. Comparisons using a microscope in the past compared with a grinder burr (Alox wheel, #60 or #80) show the hook to be more even in height and across the width of the edge. The pic is of the burnished edge from that past comparo. Lapping scratches on the top are prob. from a fine diamond stone (25 micron). That doesn't look that brilliant but compared to the dry ground edges it's clearly superior.

Veritas describe this as a cutting tool. The bevel rubs; the hook cuts.

The timber was a Euro Ash bowl outside. 28cm diameter run at 1000 rpm. The scraper was a 1" wide Record with a slightly radiused edge. It was used flat on the rest and at the bowl equator as per instructions. Most of the edge was engaged throughout the test.

After 6 mins the tool was still producing shavings.

The end grain was cleaner than what the gouge produced when I trued the piece.

By fingertip assessment of the hook, there was no noticeable diff over the 6 minutes.

So is this a good result? I think so. Qualifications are that Neil and I don't know whether our steel alloys are the same; Euro ash is not as dense or as full of silica as some Aussie timbers; this scraper prob. had a greater length of cutting edge available than the Sorby tear-drop.

Hope this is of use.

rsser
7th Feb 2011, 01:49 PM
Arthur, this is my 2 bob's worth.

Off a dry grinder, Alox wheel, the edge in close up looks like a hand saw edge that's hit a few nails.

The 'scope pics I have of such show a ragged edge, not at all turned back like Neil's pics.

Len Smith of the Woodsmith once said to me that's fine with medium density timbers, but not high or low.

I assumed that what would be happening was that the ragged edge would chip out the wood in high density and drag fibres out of low density.

I haven't commented on the finish left by the burnished edge on the Euro Ash bowl.

This species has distinct growth rings and distinct ring porosity so is prob not a good test. But 'with the grain' finish, you wouldn't need to sand at all. End grain however has clearly been raised.

NeilS
7th Feb 2011, 04:44 PM
Thanks, Ern, for that contribution. I'm not surprised at the result, there being some agreement amongst the experts that a burnished hook is stronger and will last longer than a burr raised on a grinding wheel. The time taken to eliminate and raise the hook each time is another factor that needs to be put into the equation.

BTW, what magnification have you used on that image of the burnished hook? I'm thinking about whether I can visually compare the size of your hook with the burr off the #120 diamond wheel.

Artme, if you hone back the burr on your gouge flute every time you sharpen it and then hone the bevel to remove the micro burr created by honing the flute... :oo: ... I guess that would be close to sharp. e.g here is the scraper edge before I raised the burr. It was honed to about #12000, I guess you could call that sharp! But, not something I would bother with on my gouges, or normally on my scrapers or skews.

160674 Scraper face honed to #12000 - before burr - at 200 X


And, for what it is worth here are the burrs raised on an #80 alox wheel and by hand with a 45 micron (approx #320) diamond plate. All images are at 200 X magnification and show an area approx 1mm wide.


160675 Burr from #80 alox wheel before use.


160682 Burr from #80 alox wheel after 30 secs use.


160683 Burr from #80 alox wheel after 60 secs use

This burr off the #80 alox wheel (not surprisingly) produced more dust than fine shavings. By the end of the 60 secs the scraper was still producing dust but, as can be seen from the 60sec image, parts of the burr are giving way and what remains is of little value, if it ever was.

160678 Burr from 45 micron diamond plate before use


160679 Burr from 45 micron diamond plate after 30 secs


160680 Burr from 45 micron diamond plate after 60 secs


160681 Burr from 45 micron diamond plate after 90 secs

The cut off this burr was finer than that off the #120 diamond wheel and was still intact and could still be felt after 90 secs, but some areas of the burr are starting to go. Had to go home for tea so didn't test out how much longer it would cut before the quality of the cut deteriorated, but expect it would go for a bit longer. Will test that out when I next go to the workshop.

For comparison sake I guess I should also have a closer look a de-burred and honed edge...:rolleyes:
.

rsser
7th Feb 2011, 05:33 PM
Yep Neil, nothing's settled till you've polished the bevel to #12000, and the top to the same :hooray:

What mag was my pic? Good question. It was the max. Think they claimed 400x but that was only digital mag'n. Any case, it's a USB 'scope and not far from a POS.

NeilS
7th Feb 2011, 10:31 PM
Yep Neil, nothing's settled till you've polished the bevel to #12000, and the top to the same :hooray:

What a tough task master you are, Ern... :bns:

What mag was my pic? Good question. It was the max. Think they claimed 400x but that was only digital mag'n. Any case, it's a USB 'scope and not far from a POS.

OK, that hook is quite fine then. It sure does a lot of work for such a small piece of sharp metal.

I've now used the scraper tip with the burr from the 45 micron hone until it is was no longer cutting off fine fluffy shavings. All up, 10 minutes of run time. I was quite surprised about that, not such a brief history after all.

Still sharp enough to do some heavier horizontal cuts but not delicate shear cuts.

Here is what the burr looks like now, all gone!


160734
.

rsser
8th Feb 2011, 07:59 AM
That's a good result. The hone was a paddle style medium grit?

Re the magnification on my pic, overnight I started to recall that the unit did 200x max. Whatever, in my case to feel it properly needed a finger tip pushed towards it, not a thumb dragged casually across it. By eye you can see the small bright band left on the bevel side by the carbide rod.

NeilS
8th Feb 2011, 10:09 PM
The hone was a paddle style medium grit?

.

I'm currently using a #325 credit card hone, but have used finer. I find I can apply more pressure with a credit card than a paddle to burnish the edge.

Have now done a test run on the scraper tip with the burr honed off, just down to #1000...(you win, Ern). The bevel angle is about 80deg.

This first image is looking in at the edge before use. The edge is just a few microns wide.

160765

This edge was not as aggressive as those with a burr. It did remove very fine short shavings, but very slowly, and did not leave quite as good a surface as the finer burrs. However, it did keep cutting in its fashion for longer, 15 minutes all up.

Here is what the edge looked like after the 15 mins. The width of the edge now about 15 microns.

160770

Don't reckon I'll be adopting this method of scraper edge preparation any time soon...:D
.

rsser
9th Feb 2011, 07:04 AM
Still on the native Cherry Neil?

NeilS
9th Feb 2011, 11:55 PM
Still on the native Cherry Neil?

Yep, but a bit smaller now. Thought I should stay with the same blank to eliminate at least one variable.

For comparison sake decided to finally add a burnished hook to the Sorby scraper tip and run it over the same blank. Ern's test run with the scraper hook confirmed what is generally agreed by many turners to be its superior scraping performance. So, to complete the comparisons, here are the results.

Used a TC burnishing rod after honing the scraper face and bevel to #1000. It cut long fine shavings for 13 mins before it stopped cutting.

Then re-honed to #4000 (it was the only fine stone I had in the workshop at the time) and gave it another run. This time it cut for 21.5 mins, a significant increase in run time. Only variable was honing with the higher grit stone. Interesting, perhaps there is more to Lacer and Wright's observations about the polish on the face of scrapers than just an improvement in the quality of the cut, which I confirmed for myself with the best finished surface of all methods used

The before and after micro images that I was able to capture don't tell much. In the after use image the base of the hook is still there but rounded over and no longer effective.

160869 Hook before use


160870 Hook after use

I did also go back and time how long the burr off the #120 diamond wheel would cut before packing it in, something I didn't follow through in the earlier test run. All up 13 mins. Not bad at all, although the surface finish was not as good as either the #325 credit card hone or burnished hook.

So, after all that, what do I reckon. The hook is the best, duration and finish wise, but requires more preparation time. Whether that extra prep time is worth it over a quick hand hone with the diamond credit card (the next best option IMO) may depend on whether finish or time is more important. But whatever method, I'll definitely be polishing the top of the scraper to at least #4000 in future.
.

brendan stemp
10th Feb 2011, 08:32 AM
Wow, there is almost enough material for a book! Very thorough experiment Neil. Thanks for doing that.:2tsup:

Not many surprises for me except for how long the off-the-bench grinder burrs lasted; I would've given them 5 secs not 30.

OK, you can go back to your normal turning now while you give me a bit of time to think of some more homework projects:D

rsser
10th Feb 2011, 09:47 AM
heheh .... wonder if the honing improvements are arithmetic .... think how long #12000 would last :devilred:

You'd be waist deep in fluff.

NeilS
22nd Sep 2018, 02:35 PM
Without the original images this thread is useless (the images disappeared at the time of great image vanishing episode). Having referred to it in another thread, here (http://www.woodworkforums.com/f8/sharpening-trouble-223688#post2107667), I thought I should attempt to reconstruct at least part of it with the original images that I have on file.

Here we go....


There is an excellent article (now here (https://www.mnwoodturners.com/cms-files/sharpeningturningtoolsbookdo.pdf) on page 67) by Alan Lacer and Jerry Wright that examines the effectiveness of the different methods of scraper preparation. One of their key findings was that the level of polish on the face of the scraper is a key determinant of the finish off the tool. However, they didn't cover the durability of the burr if left on a scraper.

Brendan thinking is that the burr off a grinding wheel is too fragile to last last long enough to be useful in cutting wood cleanly, but thought it worthwhile taking a closer look at the burr after a short periods of use to confirm this.

The scraper I used was one of these (https://www.cwsonline.com.au/shop/item/robert-sorby-rs213-large-tear-drop-cutter). It had the advantage of being detachable so it could easily be put under the digital microscope. The grind angle was about 80deg.

The wood was native cherry, a dense hardwood, running at 1500rpm. A shear angle was used of approx 45 to 60 deg.

The face of the scraper was polished to a level where no scratch marks could be detected under magnification.

The following images are at 200 X magnification and show an area approx 1mm wide.

442799
Burr from #120 diamond wheel before use.

442800
Burr from #120 diamond wheel after 30 secs use.

442801
Burr from #120 diamond wheel after 60 secs use

The burr off the #120 diamond wheel could be felt but not seen with the (my) naked eye. The scraper continued to readily produce fine gossamer shavings for about 30 secs when I stopped and took an image. As can be seen, the burr appears reasonably intact, but could only just be detected by touch. The serrations were starting to lose some of their sharpness.

By 60 secs there was a noticeable change in performance. Another image was taken that shows the burr in effect has gone.

rsser
22nd Sep 2018, 03:01 PM
Thanks for the rehabbed thread Neil.

I'd forgotten how well the hooked scraper went.

And there was a piece of Rock Maple I tried scraping with no burr or hook at all that produced a cleaner result than a burr.

BTW The V burnisher is FS if anyone's interested. Was pretty much only used for this test. Make me an offer.
Veritas Tools - Sharpening - Scraper Burnisher for Turners (http://www.veritastools.com/Products/Page.aspx?p=122)

Paul39
23rd Sep 2018, 02:39 AM
Thanks for bringing this back.