• May 2025 Turning Challenge: Long Neck Hollowform! (click here for details)
  • Congratulations to Phil Hamel winner of the April 2025 Turning Challenge (click here for details)
  • Congratulations to Paul Hedman and Donna Banfield for "Fire and Ice" being selected as Turning of the Week for 5 May, 2025 (click here for details)
  • Welcome new registering member. Your username must be your real First and Last name (for example: John Doe). "Screen names" and "handles" are not allowed and your registration will be deleted if you don't use your real name. Also, do not use all caps nor all lower case.

negative rake scraper

I use smaller ones. This tool is for 'whisper' cuts, so very light, and not a heavy roughing tool. The 'more mass = less vibration/more stability' thing does not apply. 1 is a repurposed skew, another a Raffen spear point tool which is the same as the skew 1 inch by 1/4 inch, and the last is a small scraper, 3/4 by 1/4 inch. Some people have heavy ones, but I don't know why...

robo hippy
 
Well I have both. I repurposed a thick scraper and a thin one. I don't notice much difference. I use both for the whisper shavings. The biggest dissadvantage of the thick scraper is that it takes too long to grind. The thin one just takes a second.
 
One of my NRSs was changed from 35 to 25 degree bevels. I took it to the 40 grit belt and took it down. The belt bevel is over 1/2 inch wide, the grinder bevel is about 1/8 inch wide. I do relieve the heels on all of my standard scrapers as well. On most grinding situations, it doesn't do much, but going from 180 to 600 or 1000 grit wheels, it can save time, even if the platforms are set to an 'almost' perfect height for angles from one grinder to the other...

robo hippy
 
Dave Fritz said:
Generally speaking would you want a thick heavy scraper or a thinner, lighter one to grind as a negative rake scraper.
Click to expand...
From the experience I've had so far, it really depends. I have two, a 1/2" and 3/4" that are ground specifically for box making -- the bevels are asymmetrical. They are both 1/4" thick, and they feel just right on the tool rest. I have used a 1" that was beveled all the way around the tip, and is often used with either side laid long-ways on the tool rest. Not sure how thick it was, but certainly no less that 1/4". NRS's are mostly a finishing tool, not a hogging out tool (except for Reed?:D)

I can attest that one can make an NRS that's tooooo light -- a 1/4" round-nose that I ground with symmetrical bevels (22.5 degrees both sides). It's about 5/32" ;thick, and requires a much steadier hand than I have. Might have to "weight it up.":p
 
Jamie Straw said:
Wouldn't that mostly apply to the first grinding?
Click to expand...

It would apply every time that you sharpen it because the bevel is much longer on a thick tool so more metal to remove. If the bevel is twice as long then twice as much metal to remove. I have some miniature tools that are about ⅛" thick. I repurposed the skew to be a tiny NRS. As you said, we're only taking away whispers of wood (except for Reed of course) so no need for a big slab of steel.
 
Bill nailed it. I grind one side, use that until the burr is gone then flip the tool and grind the other side to create a new burr. That's how Stewart does it and it works wonderfully. The thick metal doesn't really take much longer once you have actually established the grind but the thin tool raises raises that burr with just a touch of the wheel. Repurposing an old skew is an excellent way to get a good negative rake scraper.
 
Bill Boehme said:
It would apply every time that you sharpen it because the bevel is much longer on a thick tool so more metal to remove. If the bevel is twice as long then twice as much metal to remove. I have some miniature tools that are about ⅛" thick. I repurposed the skew to be a tiny NRS. As you said, we're only taking away whispers of wood (except for Reed of course) so no need for a big slab of steel.
Click to expand...
When sharpening the NRS during turning, which has to be done frequently, it's only to re-raise the burr. Swipe-swipe, takes perhaps 5- 10 seconds on a platform that's ready and waiting. The two turners I've taken workshops from often just leave the grinder running.:cool:
The top bevel needs grinding only periodically, the bottom frequently during turnig.
 
Eric Lofstrom uses a carbide tipped rod to burnish a burr on his NRSs. This burr does out last anything from the grinder, and can be used for more heavy cuts. The burrs from CBN wheels way out last the ones from standard grinding wheels, but still go dull fairly quickly.

robo hippy
 
I hope I'm not hijacking this thread but I'm going to little off topic based on what you mentioned a moment ago Jamie.

I've often thought about just leaving the grinder running and if I didn't have a cheap grinder I might do that. I'm hesitant to just flip it on and let it rip for an hour at a time.

I'm using a half horse Rikon end it does take some time to come up to speed so does anyone else live their grinder running?
 
Mark Hepburn said:
I hope I'm not hijacking this thread but I'm going to little off topic based on what you mentioned a moment ago Jamie.

I've often thought about just leaving the grinder running and if I didn't have a cheap grinder I might do that. I'm hesitant to just flip it on and let it rip for an hour at a time.

I'm using a half horse Rikon end it does take some time to come up to speed so does anyone else live their grinder running?
Click to expand...

Here's something to mull over. A typical 4-pole (in other words, 1725 RPM slow speed grinder) half-horse single phase AC induction motor running on 120 volts draws a full load current (FLC) in the range of 8 to 10 Amps. For typical turning tool grinding, it doesn't put out anywhere close to full load power and in fact isn't needing much more than no-load current. BTW, no load current consists of current needed to overcome losses such as copper loss, iron loss, magnetizing current, bearing friction, and windage friction loss. All of these are unrecoverable losses.

For a grinder, you can add a slight windage loss for the grinding wheels. I mention all of the above because they add up to a significant power loss. A good engineering rule of thumb for a motor of this size is that the no load current is approximately a third of the FLC. Using this rule of thumb and a best case motor that draws a FLC of 8 Amps, we have a NLC (no load current) of 2.66 Amps for our half-horse motor. If we bother to calculate how much power is being lost while the grinder is idly running, but not actually being used it comes out to 320 watts! That's not exactly a trivial number if you are going to let the grinder run for long periods.

Before you rush for the OFF button there's more to consider. When you first turn the grinder on there is a very large start-up current that can have an initial peak as great as six times the FLC. The initial peak magnitude of the current before the rotor begins to move is called the locked rotor current (LRC). Because the rotor isn't actually locked, the current will quickly decrease to nominal running value as the motor spins up to its running speed. All this typically takes place in less than a half second. However, I wanted to mention this because many people have said that the smaller grinders like the Rikon with CBN wheels installed take much longer to come up to speed. How much longer, I don't know because nobody has given a number. Is it two seconds or twenty seconds? If we're talking many seconds then maybe that needs to be considered as a reason to let the grinder continue running. If you're touching up a NRS every few seconds and it takes X number of seconds for the grinder to come up to speed, then there would be a reason to let the grinder run under those circumstances.

One other consideration is that the start winding often uses lighter gauge wire than the run windings because of its normally short and infrequent duty cycle. When the grinder is used outside of the design duty cycle and with CBN wheels that presents a far greater than normal inertial load and we are left with more unanswerable questions. This also puts a greater load on the start capacitor. Imported motors often come with limited technical information, usually nothing beyond what's on the nameplate.

So, now is everything is perfectly clear about turning the grinder off or letting it run?
 
Yes. Very clear. I just ordered two Tradesman grinders and a photovoltaic array.

Seriously, it seems that with a cheap grinder like mine what becomes more important to consider is its duty cycle and ability to withstand frequent starts. Mine takes 14.83 to come up to speed. It has 2 D-Way 1.5" wheels. That can seem like a maddening wait.

If I'm using my gouges I don't have to go back and forth because I have three with the same grind. But intuitively, it seems like it might be better for this grinder to let it run if I'm doing an operation that requires frequent trips back to the grinder.

That is if I'm not considering energy consumption. Am I even close here?
 
I'm in a real bind here Bill. I'm one of those rare people in south Louisiana who doesn't own a boat.

That means I could buy more tools though
 
I have seen 2 variations on the NRS. On the ones that have the same grind on both sides, they seem to be between 20 to 30 degrees on each side. I think Glenn Lucas uses 33 degrees or some thing like that. I don't know that I can feel any difference with how they cut, but that could be because I don't use them that much. The other variation is a 70/20 grind, which for me is my standard scraper with a relieved top bevel. I use them on the inside of smaller hollow forms and end grain forms.

robo hippy
 
Mark Hepburn said:
I'm in a real bind here Bill. I'm one of those rare people in south Louisiana who doesn't own a boat.

That means I could buy more tools though
Click to expand...

Sorry, I should have said whatever floats your pirogue. :D

Mon ami, you no got a pirogue? Dem gators, how you gonna catch? Should I start a "Go Fund Me" to help you through this rough spot in the swamp?
 
From a safety standpoint, I very much disagree. Even though I work in my shop alone, I follow certain safety rules religiously. It takes only one slip up to cause a long lasting injury.
 
Bill Boehme said:
Sorry, I should have said whatever floats your pirogue. :D

Mon ami, you no got a pirogue? Dem gators, how you gonna catch? Should I start a "Go Fund Me" to help you through this rough spot in the swamp?
Click to expand...

No, mes ami, I'm on high ground me. Dis here land is tree feet above de sea level. I can see dem gator comin and choot em. :cool:
 
I have mine at 75 degrees with the top bevel the lowest position on the Roborest which I think is 25 degrees. I tried the 70 degrees but went back to 75 as it seemed to work a little better. I do mostly NE bowls.
 
Bill Boehme said:
. If the bevel is twice as long then twice as much metal to remove.
Click to expand...
Oh, methinks it's not a noticeable difference. With a CBN wheel, and the upside-down approach for re-establishing the burr, it takes no time at all to swipe the tool. It takes me longer to turn, take two steps to the grinder, and turn it on than it does to swipe the tool once or, occasionally, twice. Perhaps I'll shoot an 8-second video.:D The whole bevel is on the wheel, tool pressed against the platform, index finger pressing it down and serving as a pivot point. Taking off a very small amount of metal.
 
Mark Hepburn said:
I hope I'm not hijacking this thread but I'm going to little off topic based on what you mentioned a moment ago Jamie.
I've often thought about just leaving the grinder running and if I didn't have a cheap grinder I might do that. I'm hesitant to just flip it on and let it rip for an hour at a time.
I'm using a half horse Rikon end it does take some time to come up to speed so does anyone else live their grinder running?
Click to expand...
I find myself more concerned about the stress of the grinder powering up with a CBN wheel than I do about it running for a period of time (not an hour, I'm too ADD for that). I actually spin the CBN wheel by hand before I throw the switch for start-up to give it a little boost. You're grinder has a leg up, because it's labelled. Mine is a no-name from Woodcraft.:p Back in the day, they didn't reveal who made their grinders.
 
Fred Belknap said:
I have mine at 75 degrees with the top bevel the lowest position on the Roborest which I think is 25 degrees. I tried the 70 degrees but went back to 75 as it seemed to work a little better. I do mostly NE bowls.
Click to expand...
That's very interesting, Fred. The only so-called rule of thumb about NRS's seems to be that the total of the 2 angles should be no more than 90 degrees. Sounds like that may not be a bullet-proof "rule."
 
Coming up to full speed time with the D Way steel wheels is about 3 seconds with both my Baldor and my 1 hp Rikon. The Rikon appears to have an electronic brake. Can't remember how long it took with the old blue no name grinder from Woodcraft, but much longer. I never ran the heavy wheels on it other than to see what happened, and sold it years ago.

I have no clue about why a more acute angle is 'supposed' to work better. Don't think I have ever heard it explained either. I haven't played with them enough to notice any real difference, and both seem to work fine.

robo hippy
 
That's about 1/5 the time of my grinder. I knew when I bought the 1/2 hp units that I was going cheap but it's a low priority as it provides similar utility for the next year or two and I can move up in quality over time. As long as I don't burn these up, which I won't because, as a hobbyist, I don't get anywhere near the max duty cycle of these. Still, I do plan on going with a better grinder. I like the one that Leo posted - a really old monster, military surplus unit that will outlast me.
 
If the angle is more acute then it might have a larger bur. When I first started using a NRS about a decade ago, I ground it with an included angle that was greater than 90 degrees. The bur was smaller and probably didn't last as long, but it left a nice smooth finish. The advantage of the blunt nose is that it couldn't damage the wood if pushed too hard. The disadvantage is the short lived bur.
 
robo hippy said:
Coming up to full speed time with the D Way steel wheels is about 3 seconds with both my Baldor and my 1 hp Rikon.
Click to expand...

With 2 D-way steel CBNs on a Jet grinder, one reason folks leave the running is how long it takes them to stop spinning.:D They will often still be rolling along when it's time to touch up the burr, seems silly to shut 'em down. Wish I had that problem, but a Jet grinder is way down the road for moi.
 
Jamie Straw said:
Ahhhh, but one should not "push" an NRS.
Click to expand...

And you're not supposed to do a lot of things that people do anyway. And, yes, I've heard the aphorism about the futility of trying to make things "idiot proof".

But, also consider that a finer bur takes finer shavings ... although there's probably a fine line somewhere between getting a smooth finish and parsing molecules. :rolleyes:
 
I will have to go out and measure my negative rake scrapers. I use the Robo rest to sharpen them but can't remember whether I have it set at 35 or 40 degree setting for those tools. I'll try to remember tomorrow when I go out.
 
Just measured my negative rake scrapers. 68 degrees. I use the Robo rest at 40 degrees so I don't know exactly why it comes out to 68. Anyway, the actual angle doesn't matter as long as it's less than 90. More than 90 raises a lousy burr if at all. Anything less than 90 raises a good burr on all scrapers whether they are negative rake or not. By flipping the tool for each grind to raise the burr you remove less metal. Ideally when you raise a burr you grind or hone off the original burr and then grind or use a diamond hone to raise a new bur. Flipping the tool eliminates this step because it grinds off the old burr and forms a new on one the opposite side at the same time.
 
john lucas said:
Just measured my negative rake scrapers. 68 degrees. I use the Robo rest at 40 degrees so I don't know exactly why it comes out to 68....
Click to expand...

If your scraper is meeting the grinding wheel above or below the grinding wheel centerline then you also need to take that into account when determining the angle of the grind.

I don't have a Robo rest, but I seem to recall that it has an angle scale. The scale would need to be relative to where it meets the grinding wheel in order to give an accurate indication of the grind angle.
 
You must log in or register to reply here.
Back
Top