Negative Rake Scrapers

[wizer]

I want to turn a round nose scraper into a square end neg rake scraper.


I take it I just grind using the tool rest, quenching often?

Can anyone advise of the angles involved?

 

[cornucopia]

you dont need to bother with angles tom just grind a secondary bevel onto the top of the scraper.

 

[duncanh]

If it's HSS don't quench it - it can tolerate the heat and quenching may put stresses and micro fractures into it.

If it's carbon steel then just don't press it onto the grinder so much that it needs quenching.

If you have an angle grinder then I'd be tempted to remove the round part with a thin cut off disc - much faster and kinder to both the tool and grinding wheel.

 

[wizer]

Thanks Chaps. I'll have a play next time I'm in the shop. I did find a whole site dedicated to the Negative Rake Scraper, which I find slightly odd

http://negativerake.com

 

[cornucopia]

I first saw stuart batty use them at a awgb seminar at the time he was setting up a site- it might be his? but i find no mention of it being accredited to anyone

 

[wizer]

it's registered in the US

http://www.whois-search.com/whois/negativerake.com

 

[TEP]

Looking at that site, the information that is given could just as easily be about conventional scrapers. Which is where most go wrong. ALL scrapers work on the burr which never lasts any more than a few seconds on the job. I'm talking about finishing here, not tearing the wood away. All scrapers need to be sharpened as soon as they lose the burr other wise you will end up tearing the wood.

Some talk about rubbing the burr off on a oil stone for some hard woods, but I've never had much joy without a burr on any wood.

 

[mikec]

Being a tool-a-holic I bought the Veritas Scraper Burnisher at the Yandles show. Leaving aside the arguments about cost I found it to be great for giving a consistent controllable burr. The burr is easily adjusted from fine to coarse by the amount of pressure applied. I have it next to the lathe and it takes seconds to re-establish the burr.

More on-topic I have never been convinced by the argument for negative rake scrapers. If you are using the burr and the bevel is not rubbing, surely a few degrees either way on the angle of the 2 faces doesn't make any difference. The angle of presentation is easily changed by lifting the handle.

Can anyone with more relevant experience explain the advantages etc ?

Regards,

Mike C

 

[wizer]

I had a go at this last night. Ground down the scraper to square and then applied a back bevel. I then flipped the tool over and set the rest to match the angle of the primary bevel, turned the grinder on, let it come up to speed and then turned it off. As the wheels where spinning down I touched the tool on them to raise the burr.

Results where mixed. It seemed like the edge lasted for barely half a traverse and then went back to tearing. It was a rushed attempt, so I'll try again.

I've also been looking at the Veritas Burnisher which looks ideal to me.

 

[TobyB]

I got the veritas burnisher earlier this year, and seemed to make a significant difference to how sharp the scrapers seemed to cut - definitely producing more shavings and less dust. Doesn't completely stop tear-out - some spalted beech I had was dreadful - but suspect that is technique rather than the tool. In my hands the burnisher makes a very useful difference.

 

[TEP]

I had a go at this last night. Ground down the scraper to square and then applied a back bevel. I then flipped the tool over and set the rest to match the angle of the primary bevel, turned the grinder on, let it come up to speed and then turned it off. As the wheels where spinning down I touched the tool on them to raise the burr.

Results where mixed. It seemed like the edge lasted for barely half a traverse and then went back to tearing. It was a rushed attempt, so I'll try again.

I've also been looking at the Veritas Burnisher which looks ideal to me.

Don't understand the reason for switching the grinder off? Surely you want it on so as to raise a constant burr around the complete edge.

mikec - I agree whole heartedly with you, about the so called 'Negative Rake' the only real advantage I can see is if you are working deep inside a vessel and haven't got the room to angle the tool for presentation. With a negative rake you can have the tool horizontal on the tool rest while the edge is at the correct angle to present for the cut. That's my take on it, and I don't profess to be an expert by any means.

Whether you use a burnisher, or just off the grinder, your burr will only last seconds. I would suggest the Veritas MAY give a more consistent sized burr, but again is it worth it if only for those few seconds. It's quicker off the grinder IMO. With a burnisher you have to rub the top face flat on a oil stone each time you need to renew the burr, then to the burnisher. No contest in my view. Don't listen to anyone who says you don't need to use the oil stone before burnishing the edge either, if you don't all you are doing is raising an already worn away edge.

The folks you should really be talking to on this are the 'Flat folk'. They no doubt use Cabinet scrapers a lot of the time, and the burr don't last on them very long. How long can you imagine the burr lasting on a lathe with the work whistling round. As I said seconds.

Practice, practice, practice, with the tools. The better you get the less you will need to use a scraper.

 

[cornucopia]

Can anyone with more relevant experience explain the advantages etc ?

Regards,

Mike C

the main advantage of neg rake is that it wont snatch at the tool like some normal scrapers can- i can happily neg rake the bottom of a box or goblet 3-4 " over the rest or neg scrape a 3mm thin bowl all the way up the inside- you wouldint dare do that with anormal scraper- i hated scraping before i saw neg rake now i use it quite allot.

 

[robo hippy]

A scraper is my primary roughing tool for bowls, and also a good finish cut tool. I have tried burnished burrs, and never liked them much, as they didn't seem to be any better than the one off the grinder (a fine, hard 80 grit wheel). I do take the top surface to a diamond stone first to remove the old burr (it never goes away, it just gets dull), and a pass or 2 on the grinder, actually pushing it into the wheel rather than lightly going across the face, then on to turning.

You can do 2 cuts with a scraper. One is a scraping cut where the tool is flat on the tool rest. This is great for fast stock removal, but not so good for finish cuts as it tends to pull the wood fibers more (more tear out). The other is a shear cut where the scraper is at a 45 degree angle on the tool rest. This is more of a slicing cut, and can give as fine of a finish as a gouge can. It is the same steel, and same edge, just a different tool.

I found negative rake scrapers to be better suited to really hard woods, and not as good on softer ones. The burr on it seems to be really delicate, and just not vert effective for my turning style. I do use a skew, which Stuart Batty origionally used as a negative rake scraper, for trimming up the shoulders on my threaded boxes. It takes off so little, that it is easy to micro adjust so that you can line up the grain. Other than that, not very useful to me.

robo hippy

 

[wizer]

Has anyone tried Eli Avisera's finishing tool from Hamlet Tools?

 

[duncanh]

Whether you use a burnisher, or just off the grinder, your burr will only last seconds. I would suggest the Veritas MAY give a more consistent sized burr, but again is it worth it if only for those few seconds. It's quicker off the grinder IMO. With a burnisher you have to rub the top face flat on a oil stone each time you need to renew the burr, then to the burnisher. No contest in my view. Don't listen to anyone who says you don't need to use the oil stone before burnishing the edge either, if you don't all you are doing is raising an already worn away edge.

One advantage of using the Veritas burnisher (after a diamond hone in my case) is that the tool will last longer than if you grind it every time (even allowing for the fact that you're only at the grinder for a few seconds).
I actually find it faster to use than the grinder as well

Duncan