Circular Logic, Part II

Project X has been an utter monster in recent weeks. I hate calling it Project X. I'd rather come out with the name and details, but it's not my product, so I'm reluctant to say too much until it makes a debut somewhere.

In any event, this piece has required so much problem solving that it's eaten up a lot of time. Every time I solve one problem, another one pops up, if not 2. It's resulted in a lot of sleepless nights, staring at the ceiling in the dark, and trying to wrap my head around changing curves and angles, so I have a better understanding of what's going on. Then I have to figure out how I'm going to make the mental image that I come up with come together in the physical world. In theory, this piece will go into production, so refining the process is important. It's expedient to make a few corrections by hand if you're building a one-off piece, but once you get into multiples, the time spent correcting for errors gets magnified.

I've had to re-invent the wheel on cutting circles, which has resulted in this series of entries. I've made a lot of patterns to shape individual parts. There are some 3-way miters that are square, and some that transition into compound curves, that will be visible from all sides, so there's no room for error. I've had to re-examine accurate miter cutting several times over, as well as calibration of angle measurements. (Hence the review of the Shinwa bevel gauge.) And even once I could cut accurately, the first time I cut a test joint, it didn't come together at all. Normal 3-way miters are 45 degree angles, cut at 90 degrees to the surface. But once you move one of them away from 45 degrees, everything changes, and you get compound angles. And again, because it'll be visible from all sides, everything needs to be perfectly cut. Some of it starts to feel like a mathematical proof sometimes, because the ground work needs to be fully developed before it can be referenced in a larger work, and then there's still problem solving to be done on that higher level. The fundamentals must be solid before higher levels can be achieved... and that starts with accurate work.

As always, the devil is in the details. So much effort goes into making everything look clean, so that the supporting elements can fade quietly into the background. But once you get gaps in the joinery, kinks in your straight lines, tearout, etc, the mistakes all stand out like a squeaky clarinet in an otherwise harmonious symphony. You'll notice nothing else.

Back to cutting curves...


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So, this is where we were last time:

In the photo, we have sliding center point jigs for the band saw, and for the router table. A dog-leg scissors jig with pins will mount to a blank, for cutting inside or outside radii, with a center pin that will transfer neatly from band saw to router table. For my next trick, I wanted to use it to make a pile of precise identical parts to use for making a bending form. I figured that would test the system, to see how robust it was.

I'll confess here to being a little too cerebral. Mark wandered over, asked what I was up to, and pointedly remarked that I was doing things in 'long-hand.' And he was right. There are many ways to skin this particular cat, and almost all of them are much more efficient. (Make one master curve, and pattern-rout the rest from that, would be the fastest.) But I wanted to see this experiment through, and see if the long-hand proof would result in something that would save me time down the road.

Using the jig for cutting radii in either direction (inside or outside) is pretty simple.  Because the jig has so many holes, it's easy to find a setup that will work. But I realized pretty quickly that mounting pin placement was an X-factor. The holes for the pins are drilled at identical distances from the center, but the distance between pins is also relevant. Once the arc is laid out to locate the mounting pins, you can drill anywhere along that arc. But the chord length- the distance between the two pins- will determine how far into the blank that curve gets cut. Two different chord lengths will result in two cuts with identical radii, but different placement of that cut in the blank. It was one of those details that's obvious in hindsight, but still made me scratch my head for a minute. Since the object is to create a bending form, all of the layers must be identical, so pin placement needs to be the same on all of them.

I laid out the first blank, and set up the pin holes to be exactly the same distance from each side, and from the front edge, and drilled them using a fence and a stop block. Drill, flip, drill, and the result is two holes with identical spacing from each end, and the edge.

To the band saw, and then to the router table...

 

Initially, I'd used a smooth pin, loosely installed in a hole to hold the center. I switched to a threaded bolt that ended in a smooth pin, because there was slop in the radius with just the loose pin. It only made for a difference of maybe 1/64"- 1/32" from one blank to the next, but for a bending form, everything has to be exactly the same. This was the part when Mark made the comment about doing things longhand, and flush trimming being faster. Obviously, he's right, at this point. But being able to swap parts from one operation to another without having to attach a pattern for flush trimming will save time in production later on. 

With the slop issue ironed out, the final stack was just about perfect. There were inconsistencies that I could feel, but they were small enough to fix with a plane. It felt a little bit like cheating, since I was trying so hard to make the the jig accurate enough to not need to smooth anything out, but any play in the pivot point makes inconsistency unavoidable. All things considered, it's still a very accurate system. The fact that I can re-adjust the center point and take a second pass, means I can creep up on a very accurate radius for a master pattern, or on a wooden part. And with the router table,  I can make a finished curved surface that's ready for sanding.

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Part 3 will go into a little more theory on dealing with radii. The form is a 2 part form, so it will have a mating piece. But cutting that means taking the convex off-cuts from the concave form, with identical but unknown radii, and finding a way to locate the mounting pins to change the radius.

Tool review: Shinwa Bevel Gauge, and a quick tip.

One of the phrases that I've come across in recent months with regard to layout tools is 'Accuracy has to start somewhere.' Typically, this phrase is used in conjunction with a review to justify a new try square or bevel gauge or some such, as a reliable reference standard. I'm in agreement with the phrase, and I'm using it in a tool review, but not the way I've seen it used.

I think accuracy has to start somewhere, but I think it has to start with an understanding of accuracy, and degrees of accuracy.

This is a picture of two lines, drawn with a sliding bevel gauge.

Please note that a) the lines diverge, and b) the divergence isn't really readily apparent for the first couple of inches. THIS is where accuracy begins: with the understanding that minor and minute errors aren't apparent until magnified or multiplied. And you may not detect them until they can affect a bigger picture. (If this was a picture frame, or something with big miters, and your angles are slightly off, your miters won't close. Period. Yes, you can use putty or wood filler, but the joint will lose strength.) It's easier to detect a minute discrepancy if it's projected out far enough. This is why you need to draw LONG lines to set your bevel gauge to. It's why it helps to have a bevel gauge with a long blade, AND a long beam: You want to make sure that the angle is true, even when projected out. 

I've seen some stores offer little 3" setup blocks milled out of aluminum from companies like Incra or Woodpeckers. I've also seen firsthand that even inside the 3", these little doo-dads weren't actually square. If you can't make a block square enough within 3", that's beyond egregious. If you're shopping at the store, ask to borrow a Starrett combination square to check any other squares or setup blocks, and see for yourself. 

These are my Shinwa bevel gauges, and my Starett gauge:

I bought the Starett years ago. I like it.

I like the Shinwa gauges better.

I like the 9" long beams, the 8" long blades, and the fact that there's room to write on them, to keep track of multiple angles. (Sharpie marker ink wipes clean with denatured alcohol) 

I also like that the handle has a hole, not a slot. The slot in the Starrett can lead to errors, like so:

I've exaggerated the issue for the sake of illustration, but a little bit of beam protrusion can interfere with the ability to set up the blade on a table saw accurately. 

Lastly, I like the screwdriver slot in the nut on the Shinwa. I don't torque down on it too hard, but the blade locks very rigidly in place. I usually set to finger-tight while I fine-tune the setting, and tighten afterwards, to lock it up.

Quick tip, for an even tighter lock-up, pulled from an old book on drafting: Old-school draftsmen would heat up their dividers and melt beeswax onto the pivot point. Unlike paraffin, beeswax is a little sticky. Melted into the milled steel surface, beeswax will add just a bit more 'sticktion,' which is the static friction that must be overcome before an object moves. In motion, beeswax glides beautifully. But it will also help hold a setting a little bit better. I haven't needed it on the bevel gauges that I have, but for those of you who have been fighting with the gauge that you have, it might help... And, it'll help prevent rust.

Auto-regulator: Circular logic, Part I

The top of the Auto-Regulator is curved, and tapers in thickness. Most of the methods I've seen for cutting a radius involve driving a screw through the center, and using that as a pivot point. And that works well enough for many things. But in this case, I'm working not only with two different radii, but with two different centers. And that starts to get sticky. 

Because of all of the curves I have to cut into the blank, I need to start with 12/4 stock. The end result will be an arch with a 7.5" inner radius, and an outer radius of 11.5". Using the screw through the center technique, I'd want some extra material to contain said center hole. To use the simple, drill a screw into it method of curve cutting, I'd need to drill a center hole through a 12/4 beam that would have to be at least 12-13" wide enough to also contain the center. The finished part would be 15" long, and could be contained within a 7" wide board. There's enough waste involved in cutting an arch, without adding 6" of width to a 15" long blank of 12/4 walnut, just for the sake of locating the center. 13" wide 12/4 walnut isn't exactly rare, but it's not exactly lying around, either. And it's certainly not cheap.

A side note: Not only would the above method be incredibly wasteful, but a 6" x 15" blank would also fit squarely into that sweet spot on the Venn diagram of 'I can't throw this out, it's too big,' and 'I'll probably never actually use this.' And that's a recipe for hoarding scraps.

Added to that, I had this hair-brained idea of making a rough cut on the band saw, and then moving directly to the router table with the same jig, for a finish pass.

And so it was that I set out to untangle radiused cuts (inside and outside radii) on pieces that aren't wide enough to contain the pivot point... and a few other things.

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Most of the radius cutting that Ive seen involves driving a screw through the geometrical center, into a precisely placed hole on the jig that's the proper distance from the cutting action. But the screws are usually short, it's almost impossible sometimes to see where you're supposed to be driving it into... and it never really felt like an accurate method. Not only that, it doesn't allow for subsequent fine tuning: I can always take 1/16" off of a straight part on the table saw. But I can't do that with the wood screw technique for curve cutting. Especially not if the relevant dimension is the concave side of the cut, because the process involves cutting away the center of the curve. In that case, you just have to make a new blank, and cut a new piece.

So I simplified things. Or, it could be argued, made them more complicated, to make them more simple.

The base mounts to the band saw table. And on top of that is a sliding dovetailed piece, to adjust for different radii, with two options for pivot centers: a removable pin that sticks up, to be used with a 1/4" hole, and a 1/4" brass shelf pin sleeve that serves as a bushing for a center that will be seen shortly. The pin started out as a 1/4-20 bolt that was only partially threaded. I cut it off, chucked it up in a hand drill, and domed the end with a bench grinder.

Making the cut on the band saw is as simple as I assumed it would be.

As I said, I also wanted to be able to make finishing passes on the router table. The jig below is a sliding mount for two more centers, as on the band saw. The idea is that I should be able to set up whatever jig I'm going to use, and move directly from the band saw to the router table.

Part of the problem I had with cutting an inside radius with the traditional screw-center method is that you only get one shot. After that, if the radius isn't quite big enough, you no longer have a center to work from, because you've just removed the concave part, from the center that was your reference point. I came up with the idea of a scissoring pair of arms to mount to a blank, and the only problem I had then, was that I'd cut right through the arms when I used it. So, I came up with this:

Again, 1/4" bolts cut off into pins. The pins slide through the jig, into 1/4" holes that are laid out and precisely drilled in the blank. The dog-leg shape is there to give clearance for the blade to exit the material without cutting into the arms of the jig. And, the pin being used for the pivot point slides into the brass bushing in the band saw jig, or the router table jig.So the whole pivoting assembly can be simply lifted off of the band saw table, and dropped onto the router table for a finish pass.

Coming up: laying out the mounting holes, using the jig to make a bending form, and a few other things...

To be continued.