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View Full Version : Why is this %^%##!! scraper grabbing????



Arno
4th November 2006, 07:26 PM
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
4th November 2006, 07:46 PM
is it particually tough wood, (buggerwood):D

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
4th November 2006, 07:47 PM
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
4th November 2006, 07:49 PM
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
4th November 2006, 08:43 PM
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
4th November 2006, 09:07 PM
I could be the nut on the end of the handle:D :D :D


sorry couldn't resist.:D

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
5th November 2006, 12:24 PM
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
6th November 2006, 11:48 AM
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
6th November 2006, 12:30 PM
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
6th November 2006, 10:57 PM
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
7th November 2006, 01:54 AM
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
7th November 2006, 02:40 AM
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
7th November 2006, 03:58 AM
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:p ......

soundman
7th November 2006, 10:56 AM
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:D

cheers

baxter
7th November 2006, 11:21 AM
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.

OGYT
7th November 2006, 11:54 AM
Arno, Hickory is right. I must disagree with Soundman on this. (Sorry old bloke) :o
When I have my toolrest too low, I get catches. Now lets all look at this problem from a different perspective. Think about this deal from the standpoint of the center of the bowl (don't try to stand there), and as if the wood is going around in a circle (Well, it is!), and as if you are going to do all your work in the upper left quadrant... from 9 to 12 o'clock. :o
Now, draw an imaginary verticle line through the center of the bowl, top to bottom. As the wood circles over the top of the arc, it's moving away from the verticle position, and arcing downward. When it passes the 9'oclock position, it heads back toward the verticle position. Make sense?
If you will turn the flat on your scraper to a 45 degree angle... like this slant mark /, and take very light cuts using the lowest edge of the cutter (like cutting on the lower edge of a skew), cutting only in the upper left quadrant, the wood is moving away from the cutter, which will make the cutter "shave" the wood.
If you have the cutter below center (anywhere between 6 and 9'oclock), the wood is moving toward the cutter, which will make the cutter dig into the wood (catch).
When you go "around the corner" (transition from bottom of bowl to side of bowl) you want to rotate the scraper just slightly back toward a horizontal position (slightly less than the 45 degree slant / ), and keep cutting on the lower edge of the slant as you move up toward the rim of the bowl, but remove the scraper from the wood, before you get to the rim.
Now this all makes sense to me, cause I just did it today. But, judging from my inability to explain the theory of relativity and nuclear fission, I'll bet it's confusing as heck to the rest of you. zatso?
Well, anyhow, it works for me. :o)

TimberNut
7th November 2006, 12:56 PM
Arno,

I sourced some green Blue Gum ;) a couple of years ago and discovered some of the following. Hope it helps.
1. Wet (or green) Blue Gum will scrape well and with a burr on your scraper you can remove a LOT of bulk quickly.
2. Don't try that when it dries out though. Any sap inclusions in the timber will nail you every time. The sap goes like granite and the timber gets so damn hard. If the Blue Gum is dry, sharpen your scraper, then use a diamond stone to flatten the top and remove the burr or you'll wear the handle in your chin almost every time.
3. If there are sap inclusions, put the scraper away and use a gouge.
4. Shear scraping will help if you have room to roll the scraper on its side.
5. Dry Blue Gum will dull the cutting edge REAL quick, so make sure you don't try too many cuts between sharpening.

Best of luck.

Christopha
7th November 2006, 01:03 PM
Arno, see PM

soundman
7th November 2006, 07:21 PM
AHHHH but if you drop the rest too low, you will need to raise the cutting tip and again you will end up cutting above centre just the center has rotated and is in a different place.
I'm no great expert turner but the physics is obvious to me.
If the cutting edge of the scraper is presented higher than a line between the rest and the spinning center a catch is almost certain.
If the cutting edge of the scraper is presented below that line a catch is almost imposible.
Of course this all gets more complex inside a vessel but i maintain the phisics is the same.
cheers

baxter
8th November 2006, 10:44 AM
The following is an extract from Turning Bowls by Richard Raffan.

"Scrapers inside Bowls
Scrapers are wonderful finishing tools if handles properly, but they can also be involved in some of the worst catches.

I try never to use scrapers near the rim of a thin-walled open bowl, because catching the edge and severely torn grain is a near certainty... [text goes on to mention distortion of thin timber]

Scrapers can be used flat, or they can be tilted on edge to shear-scrape. On open bowls, the nearer you work to the base and center, the less the bowl wall distorts and the more useful scrapers become. The broader the edge you present to the wood, the bigger and nastier the catch you risk, especially near the rim where flexing and vibration occur most.....an edge tilted to shear-scrape presents virtually no metal in the horizontal plane, in contrast with the scraper kept flat on the rest.

Scrapers Near the Rim
Never use a scraper flat on the upper half of a bowl unless the wall thickness is more than one-fifth of the bowl's diameter, and even then proceed with caution...[refers to lowering speed down to 300rpm for a thinner bowl].

Scrapers in the Lower Half
In the lower half of a bowl, there is substanially less vibration than up toward the rim, and you can use the scrapers flat with less likelihood of a catch. You can remove a lot of wood within the diameter supported by the chuck by pushing the tool straight in in a series of steps...

To blend the gouge-cut surface into the bottom, move the scraper in a series of arcs so the edge just brushes the surface to produce a very think curly shaving...A scraper with as broad a curve as possible for the curve you want to cut is much easier to use than a tight-radius round-nose scaper, which does not relate so well to the curve being cut. Nevertheless, do not use more than 1/2" (13mm) of the edge at one time. For a finer surface, use the same sweeping cuts more gently so you remove only dust...Always have the tool pitched slightly down, especially when cutting at center or on a flat surface. Away from center on an internal curve, work slightly above centrer so that in the event of a catch, the tool carries into space.

Because the fibers lie lengthwise across the bottom of a bowl, scraping tends to leave a rougher surface than a gouge but a more flowing, less undulating curve. Bumps and dips are difficult to get rid of, so I go for the overall shape even though it might not be as well cut. Sanding soon makes the surface smooth.

A cylindrical bowl with a flat bottom is best cut using scapers because of the difficulties shearing with a gouge....Across most of the bottom, I use a square-end scraper, which in fact has a very slight radius to keep it from catchg on a flat surface. I use a skewed scraper into the corner..."


I think that this probably gives a good explanation concerning the use of scrapers inside bowls. I have omitted references to photos and sketches in the text and I trust that I have not breached copyright. If so I apologise and trust that the Mods will delete.

The use of shear-scrapers on the outside of a bowl is also described in that book.

joe greiner
8th November 2006, 03:41 PM
Soundman, you're holding my initial confusion. The thread actually started with a problem inside the bowl.

To summarize (in the most basic terms), scraping outside the bowl, cut below the equator (of the bowl); scraping inside the bowl, cut above the equator. This way, the wood moves away from the edge and catches are reduced. In any event, slicing is almost always preferred. Works best with tomatoes, too.:D

JG

OGYT
8th November 2006, 03:57 PM
Yeah, maybe I should have stated that what I tried to ramble on about was meant for the inside of a bowl.
My apologies, Soundman, I should have known you were talking 'bout the outside.
Thanks, Baxter and JG...
I also never have turned any Blue Gum... dry or not.....
Maybe I shoulda just skipped this thread. :o

soundman
8th November 2006, 06:57 PM
Understanding the action of a catch outside is easy.
Inside it all becomes more complicated because it depends on where in the inside you are.

The behaviour will be very different depending on where on the inside surface you are and the nasty points are where you are at a point where the behaviour changes.... like transfering between the bottom and the side.

I notice from raffins video that he seems to avoid this transition... he either works on the side or the bottom and when there hops right into it, but is very ginger arround the transition point.

But one thing is sure you are in much greater risk of a catch inside than out.

cheers

powderpost
8th November 2006, 11:01 PM
On the outside of a bowl, the cutting edge of a scraper needs to be on or slightly below a horizontal line through the centre. Inside, the cutting edge needs to be on or slightly above that horizontal centre line. Raise or lower the tool rest as required. To finish the bottom inside, raise the handle higher so that the cutting edge is on centre. In both cases the handle of the tool should be slightly higher than the cutting edge, and the tool rest as close as is practical to the job, but far enough away so that the bottom edge of the bevel doesn't want to 'ride up' on the tool rest. For the 'transition' between the bottom and side, use a small (12mm) scraper shaped like a cathedral window top. Shear scraping is a finishing cut , very effective in experienced hands, but can produce disasterous results.
Jim

Arno
9th November 2006, 08:14 PM
Righto!

I did have another go last Sunday but did not make it back to the Forum....

I did two things I (and guess what, you have covered all of this...):


Put the tool rest above the centre.
Did not us my toolrest inside the bowl - instead put it at the mouth.
Tossed the offending piece aside for a bit and turned something that was not as deep (but used the same timber).


All right that's three.... :rolleyes:

Result: nice shavings and no catches. Having the toolrest (both the straight and the curved one) inside the bowl results in the scraper being presented at an angle from true and yes I think it can twist and cause problems. In the original piece, because it is so deep and narrow, it is also difficult to place the rest above the centre line inside.

The comment about the gum inclusions in this type of timber is also quite true. You know, I have turned it green but after a month or so I called the piece "Scotty" (from StarTrek - Mr Scott, warp 4!) because it became oval and twisted quite a lot. Now that I have let it dry properly I don't get that problem any more but have to sharpen tools quite regularly.

Anyway, thank you all!! I'll have a go at the offending piece again on the weekend.... :eek:

Cheers,

Arno

baxter
9th November 2006, 08:19 PM
ARno
Good to hear and good for it!

Hickory
11th November 2006, 01:42 AM
Great Arno, Soundman and I were speaking the same thoughts but in a different manner. The key is the placement of the toolrest and the angle that you hold your tool (don't go there). Almost all problems can be traced back to the tool presentation and turning speed. When you begin to have problems with catches, rough turnings, lack of progress, etc. Make adjustments in the tool rest and/or your tool presentation OR play with the turning speed. These are the variables that are most subjective to your own personal style and performance. Each of us develope a technique, we can't always discribe what it is, but it produces for us. Just have to keep playing around until you develop your own.

soundman
11th November 2006, 11:40 PM
One thing I firmly believe is that an understanding of how the tool cuts and reacts with the spinning work is the beginning of getting the damn thing to do what you want.

One thing that several people recomend is turning the lathe off and presenting the tool to a stationary job, looking, thinking and trying to make the tool cut by hand turning the job or pushing the tool.

cheers

tashammer
12th November 2006, 01:45 AM
that is a very sane idea Soundman!

soundman
12th November 2006, 11:02 PM
It works for me.

It the technician in me.... If a machine isn't working properly... you turn the bugger of and turn it over by hand so you can see whats wrong.

cheers