Lathe-less turning

[wtnhighlander]

Someone recently asked to see how I produce plates & dishes without a lathe. First let me warn everyone, I am experimenting with different jig designs, and what you see here may not be the safest way to do things. Please bear with me...

I like working with tree rounds from a local timber mill. The end grain looks pretty nice. Maple is my favorite so far. Here is a log round, broken up to make 2 round dishes about 8" diameter each, and an oblong chunk for carving.

I tried to round off a blank with my pivot-point circle jig, but the thick maple and smallish diameter made it feel squirrelly. 

So I split away most of the waste with a mallet and chisel, instead.

With it sort of round, I went back to the circle jig, and just raised the blade enough to mill a circular rabbet around the edge. With that as a reference, I could chisel it a good deal closer to round.

The pipe-clamp vise on my bench is great for holding odd shapes.

The blank was a bit wedge-shaped in thickness, so I use a crude 'tenoning jig' and a stop block, cutting a guide line around the side, evenly spaced from the arbitrarily chosen 'bottom'. 

I switch to a handsaw to slice off the uneven top. 

With a clean, smooth face, I could now attach a faceplate. My shop-made faceplate has a locating point to help align it to the center mark. 

I added the point by spinning the carriage bolt in my drill press, and lowering it onto a stationary drill bit held in a vise. The point is the end of a finishing nail epoxied into the hole.

The next photo needs some explanation.

The bearing block for the faceplate is mounted in a pivoting jig that rides the fence. I needed the blank to be rigidly suspended above the saw table, so I used clamps and scraps to make a outrigger. With the blank suspend just over the surface and spun by the drill, I slowly advanced it across the saw blade, milling away the roughness of the opposing face. With two smootg and parallel faces, machining operation #1 is complete.

Next, I need to true up the circle, so another temporary jig is needed. A crosscut sled, some scrap blocks, and some clamps, and we are good to go.

Again, advancing the jig slowly forward over the saw, and spinning it with the drill, soon produced a nice round blank.

Next post will go into more saw machining operations. Might be a few days...

 

[Mark J]

@wtnhighlander, you must have a lot of necessity because that is the mother of invention.    I am very impressed with your ingenuity.  

[Dave H]

Izzy's the man! I've thought about trying this method just for fun, as I own a lathe I use it but some day..... It looks like you've got this method down to an art, I'm looking forward to your next post.

[wtnhighlander]

Izzy Swan is a good reference. Most of these ideas came from watching him, and a few others. Some are simply adapted from the world of metal machining. 

[Immortan D]

Very impressive Ross, I wish I had your patience and commitment!

[Chestnut]

This is awesome and is definitely something different. I'm excited to see the rest of the process.

[pkinneb]

Thanks for taking the time to share this is great!

[wtnhighlander]

3 hours ago, Immortan D said:

Very impressive Ross, I wish I had your patience and commitment!

Patience and commitment are required where money and proper equipment are not to be found! 

[wtnhighlander]

Since I won't get into the shop before the weekend, I thought a post to discuss the logistics of milling hollows with a tablesaw would be in order. Without diving into a pool of mind-bending math, let's consider the geometry of the machine in relation to the work piece.  I may come back later and drop in some illustrations, but for now, your imagination will have to suffice.

Let's look at this as a 3-axis machine, with the saw table surface defining the X and Y axes, with blade elevation being Z. X is axial to the saw plate (left - right) and Y is radial to the saw plate, 90* to the X, so Y is fwd - rev for the operator. Zero for X & Y is where the blade attaches to the arbor. Let's position the top edge of the blade flush with the table surface, and call that Z=0.

First, compare the limits of blade travel against the constraints of the work. Elevate the blade until the apex above the table equals the maximum desired cavity depth. This puts the maximum depth at around 3.5" for most 10" saws, and nothing short of a larger blade will increase it.
However, there is a second constraint. The minimum diameter of the cavity opening is  equal to the cord length formed where the plane of the table intersects the blade. If your work requires a smaller opening diameter, a smaller blade is needed. Circular saw blades or 6" dado blades can help excavate a deeper cavity if the opening needs to be small. 

Starting with the work centered at 0,0,0, we can rotate the work by hand or motor, and slowly elevate the spinning blade to excavate a spherical cavity. For hand-rotation, a V-shaped fence, clamped to the table and oriented so that the point aligns with the blade, can hold the work from being moved by the saw. This first operation is the hardest on the saw, as all the exposed teeth will engage the work. Be patient.

Once the cavity is to depth, and if the work is large enough, the rotating work can be fed straight into the blade (trnaslating +Y direction) to enlarge the opening diameter. The "walls" of the cavity will retain a curve matching the radius of the blade.

But what if you want a steeper radius? Once the opening diameter is achieved, reset Y to 0, and translate X+ until the points where the blade intersects the plane of the table meet the rim of the cavity. This changes the angle of attack between the cutter and the work, reducing the effective radius without reducing the height of the apex, just like using different feed angles to produce different cove profiles. When the opening diameter is large, the angle can become severe enough to cut a near-vertical wall, with a small radius fillet at the transition to the floor. Again, the smaller the cutter is in relation to the opening, the steeper the angle possible.

Now for a really cool trick...we can utilize the blade tilt as a fourth axis, and actually achieve a bit of undercut. To do this, stop short of the full opening diameter in the previous steps. After performing the X translation mentioned above (at the reduced opening), tilt the blade so that it "leans in" toward the cavity wall. Now slowly feed in the X axis until the blade starts to cut, still spinning the work. Adjust the blade tilt along with the X and Y positions of the work, to achieve the desired wall curvature. There are limits, of course, but I plan to determine them experimentally. Math seems like too much work, but if you dig that sort of thing, Matthias Wandel has some articles on calculating coves on his web site.

The outside is less complicated, although the same principles apply. Fewer constraints, too.

For deeper vessels, a staged approach with segmented rings allows you to mill the inner walls with the blade protuding out the top, to achieve vertical cuts. Then build the vessel from the rim down by gluing the rings together after milling. The outside can be milled after assembly.
The girth of such a vessel is limited only by what your saw table + outfeed can support. 
Really big.
 

[wtnhighlander]

If a picture is worth a thousand words, I'm writing a novel.

So, step one is to determine how much blade I can expose without cutting through my blank. Note the sharpie lines transferred around the edge are to facilitate centering the blank over the blade later.  With just under 2" of thickness, I want to max the blade exposure at about 1 5/8" for hollowing the bowl, because I want a small "foot" on the bottom. Let's do that first.

The foot needs a recessed center, so the bowl won't wobble. I think 3/16" is deep enough, and that exposes a 'cord' of 1 7/8" along the blade. Works for me.

Centering the blank as best I can, using guidelines drawn on the saw table. It is important to note that the arbor of a trunion-elevation style saw moves a good bit in the "Y" axis as it elevates, making a true center problematic. This is just close.

Spinning the blank as I raise the blade (counting turns of the wheel to mark my elevation), a shallow recess is formed. Just enough so that it won't touch the surface beneath. Now to determine the distance from the edge of the blank to where I want the outer rim of the foot.  About here will do...

Now transferring that distance to the position of the blank relative to the cut line.

With the blade still at the (approx) 3/16" height, I spin the blank and feed it in the Y direction across the blade to produce the foot.

 

While the blank is still attached from the top, I experimented with various blade elevations and approach angles to mill away at it until a rough curve was achieved.  This is tedious, and not particularly comfortable with the crude jig I am using.

 

Time to switch to the other side of the blank. A quick & dirty method for locating the center is to spin the blank while still attached, and drag a pencil across the unmarked face. The line will end, or fold back on itself, as it crosses center. My less than perfect alignment while cutting the foot recess left a bump that interfered with the pencil, so I broke out the center finder head for a combo square and made it all better.

I didn't include a pic, but a handy way to make such marks at 90* across a circle is to mark the first on, then adjust the head so that the rule lines fall on an even unit when you place the head approximately 90* to the first line. Aligning the unit marks on each side of the rule to the first line get's you pretty darn close to square.

Checking max depth again. Better stick to no more than 1-1/2", looks like I lost a bit while forming the foot!

With the blade at 1-1/2" elevation, I get a strong 5-1/2" of cord. Since the blank is almost 8" across, we're good.

 

So, lining up to cent as previously shown, I start the slow process of milling out the center. A bit more than ha'f-way to maximum cut depth, I ran into problems. I think the irregularity of the face as it turned against the saw table, or perhaps a slightly off-center attachment of my pivot, allowed the back of the blade to catch as it rose into the tight opening. Since my crude jig isn't 100% rigid, this allowed the blank to bounce and some nicks were cut around the periphery.  Hhmm... time to stop going deeper, and start going wider.

Being loath to move the work while cutting, because of the lack of rigidity in the jig, I lowered the blade a bit, re-positioned the jig & blank, and again raised the blade into the spinning blank. This removed most of the gouges, but left some ridges where  my cut depth adjustment was a little off.

By this time, I was getting a cramp in my back from reaching above the table to runt he drill while reaching below the table to mange the blade elevation at the same time. Rather than delay any more, I positioned the jig above the saw throat, so I could access it, and take advantage of air flow to the dust collector. Then I broke out the side grinder...

Locking the drill at full speed, I used a series of carbide grinding disks and flap wheels to smooth out the interior of the bowl.

Obviously, this is a messy proposition!

I quit for the day, after dropping some epoxy into a couple of small cracks. Mixed in a dab of gold pigment powder for bling.

I wouldn't call this exercise a complete failure, but it does clearly illustrate the extra effort required when trying to use one tool to do another tool's job.

Some points I can identify as problematic include:

1. The size of the cutter in relation to the blank. A larger blank does better, according to my previous experiences with this jig.

2. Construction of the jig - this is a crude and quick experiment, I think it would work much better with more rigid construction.

3. Using the saw table as a support point - this increased stability and safety of the operation, but really needs a smooth, flat face on the blank. Any irregularity causes issues.

4. Accuracy of centering on the pivot / drive center. Obviously, this causes issues similar to it would with a lathe. The hollow of my bowl is slightly off-center from the outside shape.

Thanks for following along. I may do some more experiments, but for now, I am going to move away from trying to make small bowls with this method.

 

 

[RichardA]

I'm looking at this as a Mother of Invention type thing, so far I'm impressed.  Well done Ross.

[wtnhighlander]

1 hour ago, RichardA said:

I'm looking at this as a Mother of Invention type thing, so far I'm impressed.  Well done Ross.

I dunno about 'necessity', but it was interesting to try. Certainly, if you have access to a lathe that is large enough for the job, use that. By ALL means, use that.

But if you have something of greater diameter than your lathe can swing, this type of jig seems like a viable option. To use it 'safely' with the table saw really requires that the jig be built like a Bridgeport mill, big and beefy.  And it would probably work better if all the movements could be controlled from above the table, reaching under to elevate the blade was a real pain.  But by that time, the jig would essentially be the gantry system for a CNC table.

One of these days, I'm going to turn it around to use on the wing-mounted router in my saw table, and try to shape some large segmented rings to stack up into a big vase, just for giggles.

Don't hold your breath waiting for a journal, though...

[Mark J]

Ross, some of the pictures you posted look like you have two blades on your saw at one time.  Am I seeing that correctly?

[wtnhighlander]

41 minutes ago, Mark J said:

Ross, some of the pictures you posted look like you have two blades on your saw at one time.  Am I seeing that correctly?

Yes, two 7-1/4" circular saw blades. Smallest diameter blades I had on hand, and they are kind of flimsy, so I stacked two for stiffness.  A dado stack is even better, but mine is 8", and a little large for this job.  A six-inch stack would have been great.

[Gary Beasley]

You ever get south to my area I have a 6” set for you?

[wtnhighlander]

@Gary Beasley, how far South from Spanky's place? I don't get much call to go that way, but when corona settles down a bit, I'm considering a road trip. I have a cousin in Chattanooga that might like a visit, and another in Atlanta.

[Gary Beasley]

Im in a northern suburb of Atlanta. Hoping the covid gets settled soon, the vaccines are rolling out now. Wife and I have had our first shots, she gets her second in a few days. Wife says covid cases here are just starting to drop a bit.

This is the set. I dont need it as I got a 10” set and my saw handles it quite well.