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Measuring wall thickness (Read 3,562 times)
 
Keith_Bundy
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Measuring wall thickness
Jul 19th, 2005 at 8:47am
 
Alright, I've had it. Time to swallow my pride and ask for HELP!!!  THe hollow forms I have done in the passed have had fairly large openings or have been vase like forms so checking wall thickness has been relatively easy. The forms I have been trying of late are shorter and wider with smaller openings and vase like forms with small openings. I use the Stewart system and some home made tools that are 3/8" steel bar with high speed steel cutters. My problem is that no matter how or what I measure with it is not accurate. I think I'm at the thickness I want to be and the piece blows up. Looking at the wall afterwards I see that I was thinner than I thought. Chris, Paul, and any other holow form turners here, howdo you measure wall thickness and with what? I have double ended calipers, calipers with one straight leg and one curved leg, all kinds. I even tried the bent wire caliper that Ellsworth uses. Nothing seems to work correctly. Can you give me some help even if it's just to say that I'm an idiot?
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« Last Edit: Jul 19th, 2005 at 8:49am by Keith_Bundy »  
 
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Chris Wright
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Re: Measuring wall thickness
Reply #1 - Jul 19th, 2005 at 3:01pm
 
Hey Keith,

This happened to me alot in the beginning too.  Part of it is a matter of reading the calipers.  I found that you want to measure the inside of the two points, not the points themselves or the outside as many of the instructions say.  This will give you at least 1/32-1/16 more wood than the other way will.  As for Ellsworth's technique, that has never worked for me, the wire always seems to flex just enough that at 1/8" wall thickness I think I am at 1/4" or more and go through the side.  When I measure the wall on a hollow form, I turn using the inside of the points as my measurement and when I measure for posting or sales facts I use the tips of the points as this is where most others will read them.

My true test though is the sound of the piece as the tool is cutting.  When hollowing I turn everything else off, radio, respirator, etc so I can listen for the distinct sounds of the wood thinning.  The pitch changes and becomes noticably higher when it gets thin.  When it gets right there or too thin is almost sounds like the tool is scraping cardboard.  I also use a small gooseneck LED probe to check how much light passes through the sidewall as well as checking for rough or high spots on the inside I might not be able to feel.  Even dry wood will let light through as it gets thin enough.

On thing that turners can forget about turning small opening, thin wall pieces is that once the wall thickness is thin (I use 1/8" as my 'control' measurement) then you can't go back behind the current cut.  There isn't alot of material to support a cut in the thin sections so if you forget or discover a small ridge a 1/2" behind your current cut, you're best to leave it and do better on the next piece.  Otherwise you seriously risk tearing the piece apart between the high spot and the current cut below.  This is especially common with scraper type tools like Ellsworth and Hosaluk use.  The shearing and tearing action of the scraper, even on what you believe is a light cut can be enough pressure to blow it up.

Hope this helps,
Chris
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E. Bud Gillaspie
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Re: Measuring wall thickness
Reply #2 - Jul 19th, 2005 at 3:09pm
 
Are we supposed to "measure" (as in caliper, ruler, etc.)wall thickness? Shocked  I must have missed that part of the "lesson." If I can't reach it w/my thumb and middle finger it doesn't need to be "measured." I either determine thickness by feel or, if I want to go really thin which I hardly ever do, I'll shine a very bright light inside the piece and when I "see" light from the outside I STOP.  Grin

The sound of the wood is also a great indicator. Chris is right on there.
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« Last Edit: Jul 19th, 2005 at 3:11pm by E. Bud Gillaspie »  

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Keith_Bundy
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Re: Measuring wall thickness
Reply #3 - Jul 19th, 2005 at 3:25pm
 
Chris  - I understand what your saying. Part of the problem though seems to be a false reading from the calipers. I realize that if the calipers are not aligned correctly that they give a false demension. It seems that no matter what I use there are always areas where calipers can't be aligned right. Now this is not to say that this is the sole reason for my problems, but, it would be a big help to have an accurate way of measuring until I get used to working through smaller openings and develop "sense" of how things are going. I'll have to try the sound thing. I usually work with an air helmut on as dust is a problem for me. I guess when I do the hollowing I can switch to my Dustbegone maks.
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Keith_Bundy
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Re: Measuring wall thickness
Reply #4 - Jul 19th, 2005 at 3:29pm
 
Bud -  I'm trying to get down to an 1/8" or slightly less. It seems I always have an areathat seems thick and when I take that light cut to thin it I find in reality it was already thin and cut through the wall.
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E. Bud Gillaspie
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Re: Measuring wall thickness
Reply #5 - Jul 19th, 2005 at 3:39pm
 
I thought I said enough on the subject of wall thick/thinness but I was wrong.

A nice, thin-walled (3mm or less) gives ya braggin' rights, "Look how thin I turned the walls on this hollow. They're thinner'n yours." If you're comparing yours to mine then 9 times out of 10 you'd be right. On the other hand, mine don't break easily and they don't warp if put in a sunny place.

True story: I sold a very lovely, very thin-walled (less than 2mm), piece to a collector in CA. He'd had the piece less than a month when a small earthquake hit his area, the piece fell 3' on to the carpet, and broke in half. What are ya gonna tell him, "To bad, so sad, you bought it, it's your problem." I can't do that. I made another one with 4mm walls and sent it to him. I've sold 5 more pieces to him over the years. All I was out was 5 hours of my time and chuck of myrtle.

No more really thin walls for Buddy. Grin
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Re: Measuring wall thickness
Reply #6 - Jul 19th, 2005 at 3:43pm
 
The biggest problem I had with calipers was angular placement. If they are not angled properly, I would be measuring a diagonal instead of straight through, and a diagonal is longer ""thicker"" than straight through. So it was possible to blow up the piece and still be reading wood remaining with a angular measurement. I am OK now, but only because I am very careful to measure at 90 deg to a wall.
Light penetration does help a lot, and the sound of the tool cutting is a good measure of thickness too (after you are used to sound vs. thickness.
I may change my mind later, but I feel that thin walls are best cut with wet wood.
Maybe the laser is a good way to get accurate thickness readings?? I do not have one , but they seem to work good in that arena.
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« Last Edit: Jul 19th, 2005 at 3:46pm by N/A »  
 
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Keith_Bundy
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Re: Measuring wall thickness
Reply #7 - Jul 19th, 2005 at 3:57pm
 
I'm not meaning to be thick headed about this though I know sometimes I can be. I have never had a class of any kind when it comes to woodturning. I've been to symposiums, watched DVD's and read books. It seems everyone is measuring in some way. At least those I know about. I'm just trying to find the best way to go about it. I'm not looking for bragging rights, just trying to lighten up my pieces and improve my skills.
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E. Bud Gillaspie
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Re: Measuring wall thickness
Reply #8 - Jul 19th, 2005 at 4:01pm
 
Keith, I don't know how big a piece you're tryin' to make but is there somethin' magic about 1/8"? When your piece begins to "sing" to you ya, STOP. You'll know the sound when you hear it.

I'd almost bet you go through the side somewhere near the middle of the piece. The next most common is to go through the bottom. How to avoid these? Work the entire wall not just the middle or bottom? Maintain a wall thickness that doesn't create a problem? The latter is what I try to do.
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Keith_Bundy
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Re: Measuring wall thickness
Reply #9 - Jul 19th, 2005 at 8:21pm
 
Bud - I seem to do okay down to about 1/4", though to be honest, it is probably not a consistant 1/4". I don't want to turn thinner just to say I can do it. I like the lightness of a thin hollow form and I do have an idea that I want to try out and I think the pieces will have to have at least 1/8" wall thickness. The pieces I've been working with lately are about 4" to 6" in diameter and 3" to 4" deep. I may eventually want to go bigger. Your right I do through about the middle. It seems that as I work down I get thinner even though the calipers tell me different. Again I think this is due to not being able to get the calipers at the right angle to get a good measurement. You mention working the whole wall. I normally turn the whole piece to about 1/2" wall thickness and then starting at the top I work my way down an inch at time to finish wall thickness until I reach the bottom.
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Chris Wright
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Re: Measuring wall thickness
Reply #10 - Jul 19th, 2005 at 9:30pm
 
Keith,

That is how I do it also.  I rough them out of green wood to 1/2-3/4" depending on the expected movement.  The when I finish turn them I turn an inch at a time and work from opening to bottom this way.  I admit that I started with pieces that had 1 1/2" openings until I got the hang of the "sound" of the wood, it is one of my most reliable indicators.
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E. Bud Gillaspie
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Re: Measuring wall thickness
Reply #11 - Jul 20th, 2005 at 11:19am
 
Keith, as Chris and I have said, the "sound" is critical. What is that "sound?" I can't explain it, perhaps Chris can, but it's almost "musical" in nature. That's why I said, 'When the pieces sings to you STOP.' It's not a "whine" and it's not a "whistle," maybe a "hollow whine" is the best description. The faster you turn the piece the higher the pitch.

I may be a bit strange but the faster I turn a hollow form, especially the smaller ones, the less likely it is that I'll "go through" a piece. I'm comfortable turning a 4" x 4" hollow form @ 2,000 rpm, perhaps as high as 2,200rpm.

I assume I turn @ high rpm because I learned on a lathe that had only 4 speeds--1,100, 1,800, 2,300, & 2,800. So 1,800-2,300 is my "comfort zone."

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Keith_Bundy
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Re: Measuring wall thickness
Reply #12 - Jul 20th, 2005 at 1:02pm
 
I appreciate the help the guys. We shall see what the future has in store.
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Re: Measuring wall thickness
Reply #13 - Jul 20th, 2005 at 1:33pm
 
I use a lazer to measure the wall thickness on some of my turnings mostly larger hollow vessels.  This helps speed up the cutting process but mostly the sound tells me as much or more than the lazer due to vibration.  It does get me to 1/4 to 3/16 with comfort.  Then the rest us up to me.  I have cut down to paper thin but usually stay around 1/8 to 1/4 depending on the size and what I will be using the vessel for.  Good luck and happy hollowing if nothing else it sure is a challenge.  That is what makes it so fun.  Even when you blow one up and it goes into the junk pile.  They make good firewood for winter.  Nothing like throwing a few bowls and vessels in the wood stove and watch them burn.
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E. Bud Gillaspie
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Re: Measuring wall thickness
Reply #14 - Jul 20th, 2005 at 3:05pm
 
Keith, you'll do well. This turning thing isn't science, it's art. One can't quantify art, unless it's digital art, so wait for the piece to sing. Think of it as removing a tumor in very close proximity to the carotid artery--slowly w/a very light touch.
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