>I'm going to attempt my first square bowl tonight and I'm sure there are some helpful hints to make it go smoother. I haven't done anything like this yet, but have done lots of bowls, natural edged, hollow, and goblets. I saw today a couple square bowls and thought, why not give my new lathe a try at one of those.
What speed does one use, how does one attack this thing, any warnings, what about how to finish the bottom after hollowing out the bowl...jam chuck? Ahh...so many questions...just need some insight...
>I have turned a couple square bowl;s and can offer some advice. The following is taken from an article that I wrote for the AAW Journal several years ago. It may be helpful.
Turning the wings involves an interrupted cut that is nearly invisible at their tips � the "ghost". There are a few rules to remember when turning the "ghost" on a square bowl.
SAFETY RULES --- Again!
Safety Rules are published and promoted by the AAW to make the lathe a safer place for all turning activities. All of these rules are especially important when turning a large "square". From personal experience, the following four are critical:
1. Be alert at all times. Know what you are doing and where you are going with every movement. Know where the "ghost", tool, hands, arms, face, and body are, and their relative locations at all times.
2. Always rotate the work by hand BEFORE running the lathe, and NEVER move the tool rest while the lathe is running. No further explanations are needed.
3. Always wear a full face shield. It is far better to hit the mask than your nose when we forget Rule #1. Also, sharp splinters can be broken away from the wood at the corners.
4. Relocate the START/STOP switch so that it can be reached without passing either hand or arm near the �ghost�. The switch on most lathes is not in a safe location. If you don�t want to move it, or install a remote STOP, you have been warned!! I have had two encounters with the "ghost", the first while reaching for the OFF button, the second the ON button. Then I moved the switch.
The Tools
The corners of the "square" are flexible and easily deflected by any cutting force that is applied perpendicular to their surface. However, they can be turned successfully with a tool that makes a shearing type cut whose major force component is parallel to the wood surface, and the perpendicular component of the cutting force is kept to a minimum. The best tool for this (in my opinion) is a side-ground bowl gouge with a small tip radius and a very sharp cutting edge. There may be other tools that will give similar performance, but I haven�t found one yet. I use the 5/8" diameter Mastercut bowl gouge from Oneway�. Its "V-shaped" flute allows the cutting tip of the tool to be ground to a very small radius. For an especially fragile �ghost� on long and thin corners, I use the 3/8" diameter gouge from the same source. The cutting forces are even lighter with the smaller gouge, and I have turned corners that were over 5" long to less than 1/8" thickness.
Vibration and sound will tell you when it is time to sharpen the tool. Lightly honing the inside of the flute with a fine slip-stone will make it even sharper.
Turning square corners will improve the quality of other turning forms because we will learn to keep our tools sharper.
A shear cutting scraper can be used on stiff corners, but the cuts will have to be extremely light. The scraper must be held at an angle of 45� or greater to ease its entry into the corners and to prevent any large deflection of the wood. A scraper cannot be used flat because the load across the wide cutting area is nearly perpendicular to the wood surface, and it is applied abruptly at the leading edge of the corner. At the least, the vibration can make cutting impossible, but heavy vibration can pull the tool into the open spaces between the corners. If I want to remove large tool ridges, I rotate the corners across the face of a scraper by hand.
The Lighting
Visibility is as important as a sharp tool and safe working habits for turning on the �ghost�. A 100-watt clear incandescent bulb with a reflector is ideal. More light than that and the background becomes illuminated through the �ghost�. Many turners have advocated placing sighting boards behind and opposite the turning to enhance the �ghost�. I have found them to be a poor substitute for a bright light aimed at the surface being cut. The sighting board only provides a uniform background, while the bright light accents the surface of the �ghost�. Both might be necessary in a brightly lit shop (I don�t have that problem).
Back To Turning The Inside Of The Bowl
Now that we know how to work with the �ghost�, we can complete the turning of the wings. Always turn the �ghost� first, leaving as much of the solid area in the center as possible to provide rigidity and vibration damping. Follow the outside curve with the inside cut to develop a uniform thickness. As the wings become thinner, it is usually easier to start the cut in the solid wood at their base, and move into the "ghost" at their tips. If the opposite is true and the cut wants to be started at the tips of the "ghost", don�t be alarmed, be careful. Some wood will cut better in the wrong direction. The wings can be very flexible, and I always let their final thickness be determined by their stability. With care, they can usually be turned to less than 3/16" thickness. It will be difficult to get a smooth surface, so concentrate on a uniform thickness. Tool grooves are easily removed when the bowl is sanded.
After the wings are completed, pick up the inside surface at the bottom of the "ghost", and chase it into the center of the square in two or three steps. Establish the tool bevel on the finished surface, and ride it into the unfinished area to prevent gouging the surface at the transitions. Be aware that it is very easy to cut a shallow band at the transition between the "ghost" and the solid wood.
On lathe speed, it must always be slow enough to prevent fluttering of the wings. For a 12" bowl, I would never spin it faster than 500RPM. Depending on the wood, its flexibility, and the thickness of the wings, 200-300RPM may be a good speed to start and slowly work up to the higher RPM.
I know that there are those who will sand the corners of a square while the piece is spinning in the lathe. Do this only if you wish to live dangerously, it isn't necessary to do it this way.
Power sanding while the work on the lathe is stationary is required on the corners (my opinion). Besides being the only safe way to do it, it is faster, and it is easier to get sharp edges on the square than with everything moving. It will take practice to become proficient, but it is a skill worth learning. Use the free hand to support the thin corners if they need it.
>This is one of my "square" bowls. It is 12" diameter and the wood is Magnolia. Please pardon the tacky background. It was taken with fluorescent shop lighting, no flash, and a dropcloth to cover the workbench.
>I'm going to go down and give it a shot. My cherry blank is about 7x7x5" thick. I think I will try to put some type of sweep on the wings outside and make the actual bowl fairly deep. I assume I should use the tailstock up until I cut the tenon for the stronghold.
Thanks again and I'll post a pic...good, bad or otherwise.
>Russ That was very well written. Having turned maybe 100 square mirrors and several square bowls I agree with you on all but one point. I turn at high speed. This is just my opinion but I find that there is less time for the tool to drop into the void when spinning at higher speed.
I teach people to "glide" the bevel arcross the wood. The gouge will sink into the wood very rapidly at the tip of the square because you are only touching 4 very small areas out of 360 degrees. Any pressure on the bevel at this point will make it cut more aggressively. Excessive pressure also makes much more noise. Listen to the turning. The quieter you keep the cut the less pressure you'll have on the bevel and your cut will be more controlled.
In all fairness Russ and I may use our tools differently and this can easily make the difference in our approach to the same turning.
I strongly urge you to put a scrap piece on the lathe and try different cutting angle and speeds. There is a learning curve to square turnings. Have fun and follow the rules Russ gave about lighting and keeping your hands away from the edges.
>Russ- what an unusual piece of wood? I'm trying to visualize how bark grew nearly into the pith- was the tree damaged and then healed over the split? S'plain please!
>I tore a 4" long gash in my are on one of the corners. Couldn't have done that on a round piece of wood. The sharp points on the corners can be a hazard whether the piece is spinning or standing still.
>I'll second the stationary sanding. it is incredibly easy to round over the outer edges when power sanding. Not only is it dangerous it can tear up your sanding pad$$$. I do it on my hand mirrors but the outer wings only protrude about an inch. I wouldn't dream of doing it on larger pieces. Well OK I have tried but no matter how carefull you are you thin out the outer edge and you get swirl marks. simply not worth the hassle.
I see Russ has given you some good information, as well as John. There is another approach if you are using a regular piece of lumber for your bowl blank. Some people (I have not done this) will glue pieces of scrap wood onto a square bowl to make it round, and turn it as if round. Once it is done, the scrap pieces are removed and the edges sanded.
I haven't even really turned a square bowl, but I have done some unusual shapes. I agree with John that high speed is an aid to smooth cuts in the areas where there is interrupted wood surface. Below is an example of what I have done in the past.
This is a three winged bowl from a maple crotch. It was exciting to say the least, and I was able to get two of this type from one piece of log.
>Kurt, I'm attaching a picture of a piece I did with the tchniques of square bowl turning, but a little easier.
For now forget about the Mahogany parts and just visualize the Maple bowl. It was turned as a rectangle. (Something like 10x6x3) What makes it easier is that the legs are cut out after the turning is done. (I used a jigsaw) The technique is the same as a square bowl, but the legs have more support because they are wider. You are only turning about 50% air instead of 98% at the tips of a square bowl.
Once you are comfortable then turn a square bowl. The process is the same. Or you can glue on an offset square piece of Mahogany and turn a square and a rectangle.
The piece sits on the three legs, with the bowl about and 1/8 inch of the table.
>Stayed up til past 11:00 last night turning the outside of the bowl and wings. it has really turned out quite nice and I'm happy with the cut. Russ...you're right about keeping that bowl gouge sharp, I stopped and sharpened several times.
I hope to finish the bowl tonight and will post a pic tomorrow.
>These things always get a lot of attention because of the "how'dya do that" factor. It's real bark that I saved from the rest of the log section. And it was glued onto the ends of the square with CA glue. Gaps and seams were filled with coffee grounds and CA glue.
Marnolia bark is easy to work with because of its texture, and it is quite pliable when it is wet.
