Why is this %^%##!! scraper grabbing????

[Arno]

I have been turning of and on for a while and I am no expert but have made a few decent pieces. The last few bowls I have been turning I have had problems finishing the bottom with a scraper. Make that two different scrapers! I have put the tool rest as far as possible into the bowl (I have a rounded one so it goes in a fair way) have very little overhang. Gently gently (after it almost ripped the bowl out of the chuck - not that I think I was being too aggressive) but no, it grabs and tears a chuck out of the bottom :mad: :mad: . Mostly happens towards the sides. Have reduced rpm but still having problems. Re-sharpened the scraper (70 deg) but still not joy...

Any hints??

Thanks, Arno

[WoodNerd]

is it particually tough wood, (buggerwood)

Try not putting the scraper flat on the tool rest. that is, angle it so it is sheerscraping. It a bit hard when doing details on the bottom, though. i aloso find that if you can get the little carbide burnishers that create a burr on the end of your scraper it will not cut as much and reduce catching.

[Caveman]

Hey Arno - no expert myself, but what angle are you holding the scraper at and at what hieght is the tool rest. From what I know, the tool rest should be above centre hieght and the the scraper should be held with the cutting edge to the bowl slightly downwards.
Hope that makes sense - I know what I meant:confused: .

I'm sure the guy's here will come up with a solution to your problem.

[Don Nethercott]

Hi Arno,
Could be any of a hundred things, but here are a couple of ideas.
Are the corners of the scraper sharp or rounded. A sharp corner can catch and cause this kind of problem.
Is the end ot the scraper straight or slightly curved. A curved scraper can be a lot less troublesome.
Are you cutting croos grain or end grain. A scraper works better on end grain than cross grain. Had a similar problem with jacaranda today, so used a big bowl gouge for most of the work and just touched up a few ridges with the scraper.
If it is happening towards the sides would say you are catching the corner.

Also makes a difference if you change the angle of the scraper - try it angled up (handle down) or the other way round. One way is scraping the other is shear cutting (forget which is which). Find out which works best for you on the particular wood you are using.

Could also be too slow - If you are close to the centre line the cutting speed is quite low, so you could try increasing speed. I was scraping a bowl bottom today at over 1200rpm.

Is your scraper really sharp? When you sharpen it do you see the glow along the edge?

Hope this is of some use.

Good luck
Don

[Arno]

Thanks Woodnerd, Don and Andy...

Let's see...

I am turning blue gum - dry and hard... Small bowl, maybe 12cm diameter at the top, and 8 cm deep.

The toolrest is a little below the centre of the piece and the scraper is angled down, maybe 10 degrees, not much. When it grabs the angle becomes much steeper of course!

Both scrapers I used are curved.

Cutting cross grain - and I don't think I am catching the corner, took particular care to avoid that one! Ran it first at 1800 rpm than at 600 - had the worst catches at 1800.

I will play around a bit with angles (up - haven't tried that) and tool rest heights tomorrow.

Don, I don't really understand the "glow" in relation to sharpening???

Thanks again,
Arno

[soundman]

I could be the nut on the end of the handle


sorry couldn't resist.

interesting to note that even richard raffin gets a catch near the edge of an item he is turning on his video. So don't feel bad.

cheers

[Don Nethercott]

Arno,
When you are sharpening the scraper on the grinding wheel, you should make a couple of passes to get the angle correct and to get the finish as neat as possinle (ie no ridges, etc). Then for the final pass lift the handle a little to hone the very edge. As the edge runs across the wheel you should see the very edge glow as little. Too much and you will blacken the steel, which you don't want. This glow tells you that you are getting the edge itself sharpened. The final product should feel very sharp with a burr on the top edge. Leave that burr there, it helps with the cutting.

You should have the tool rest above centre when using a scraper.

When cutting try and hold the handle as close to the steel as possible with the right hand and let the length of the handle rest under your forearm. This will help hold the scraper if it tries to catch and you won't be hit in the chin, or wherever.

If I were doing a bowl that shape I would definately be using a bowl gouge for the sides and onto the bottom, and finish off the bottom with the scraper.

Keep trying,
Don

[Hickory]

Tool rest too low. When the tool rest is too low, the edge of the scraper meets the wood as it begins to turn back toward the operator, this creates a more aggressive cut. More agressive than expected and grabs the edge, you hold on and next thing is a gouge or grab. Wood wins.

Tool rest higher the wood is moving away from the operator and lessens the attack angle of the scraper meeting the wood, Cut is lessened but the operator win. Ideally the scraper and wood meet perpendicular to the piece.

When I was teaching young fellows to use the lathe I would have them move the tool rest up a bit to ease their problems. Serves the same as riding the bevel on the outside of the bowl.

All this of course is relavant to the operator's stance and the height of machine compared to the height of the operator. There is no set height adjustment just as there is no set operator height.

But play with the toolrest height (raise a bit) and see if it doesn't help.

[PAH1]

Another alternative is self feeding into the wood. This has happened to me before and only happens occasionally. Knock the burr off the top edge of the scraper with a hone/sandpaper and see if that changes things.

[joe greiner]

Hickory has it nailed. Didn't make sense at first; thought maybe he had translated his remarks for Southern hemisphere :confused: Then I realized we're talking about the inside of the bowl. But I'd say perpendicular or less. Works the same way when scraping flat work. I get best results with the scraper leaning a little bit toward the direction of scraping.

JG

[Hickory]

Thanks Joe (I think) I almost agree with PAH1's comment about the burr. (Sort of) Old school has always said to hone the edge of the chisels and scrapers, this removes the false edge or burr. (Wet turners adore the burr as it gobbles up wood and makes lots of noodles and curls, but dry wood is different) Removing the burr will lessen the agressiveness of the cut, Honed edge will make a clean cut as well, although not as quick in the material removal.

Part of the problem comes from modern society's trend at quick results, many of us "want it now" and can't wait for the tool to do the job in its slow pace. We hurry up and then we screw up. Same holds true with the scraper, It does a fine job if you let it do what it does in the way it does best (Cypher that for a while) So My suggestion is to be agressive till you are near the finished depth, and then adjust the cut (resharpen and hone) to shear off small layers till you reach the finished size.

Arno, you need to adjust the angle of your dangle and get the tool rest in position so that you can hold the scraper level and greet the turning (Inside the bowl) at a level plane with the tangent of the curvature. Slight adjustments in elevation will find the cut you are looking for. Although not as agressive and fast material removal, but far better results. (And as a finished cut, sharpen and hone a keen edge on your scraper to prevent tearing the fibers. )

[OGYT]

I agree. With all. Scrapers require a gentle touch for the final shave, and getting the tool above center will help to keep the catches down.

[bikerboy]

I concure'th with thee Mr. ogyt....one other point,when you bring the tool across the bottom of the bowl,it's all going well?then when you come round the side from the bottom,this part of the wood might be thicker as you curve your tool round the edge, so you might be trying to take to much wood off as the tool enters this part of the bowl,you might think that you are shaving the same small amount,but as you bring the tool round and into the curve of the bowl the wood might be thicker?in that area????..just an idear,some thing to think about,I just hope it makes sence ......

[soundman]

So summarising

The top edge of the scraper should be at or below centre otherwise it may grab... when it grabs, it will self feed because the edge will encounter the curve of the wood comming closer to the rest... result.@#$&%$:mad:

So the current wisdom is to either lower the rest or present the scraper in a downward fashion.
Inside a vessel ther is no option the rest must be lowered.

Some are saying a burr is good but the hone off the burr for final passes particularly on dry hard wood.

Of course this requires adjustment of the nut on the end of the handle

cheers

[baxter]

Arno in your first post you mentioned that you are using a curved bowl toolrest inside the bowl. This could cause the scraper to twist as you move it across the bottom of the bowl.

Try using a flat toolrest across the outside of the mouth of the bowl.