Sunday, April 20, 2014

Circular Logic, Part II

Project X has been an utter monster in recent weeks. I hate calling it that. 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. But 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. It's resulted in a lot of sleepless nights, staring at the ceiling in the dark, 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 all of that come together in the physical world. In theory, this piece will go into production, so I have to get it all figured out. It's forgivable to fudge a few details on a one-off piece, but once you get into multiples, the time spent correcting for errors gets magnified.

I can't wait to do a real write up on the project. I've been wrestling with some really interesting stuff, and came up with some cool solutions. But I hate reading... and writing... about abstract solutions without a meaningful context. I've had to re-invent the wheel on radius cutting, which has resulted in this series on dealing with cutting arcs and circles. 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.

There's also going to be some work on veneered panels, possibly with marquetry in the future. There's going to be a bent lamination... and possibly a bent tapered lamination in future iterations. I'm still working out how I'm going to cleanly mount a piece of curved glass with radiused corners. And all of it has to be streamlined...

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...


So, this is where we were last time:

Sliding center point base for the band saw, and for the router table. Dog-leg scissors jig with pins to 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 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 just what I could learn from it, and how much it would do that the regular screw-through-the-center jig wouldn't.

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 I have two holes with identical spacing from the ends and edge.

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

Initially, I'd used a smooth pin to hold the center. I switched to a threaded bolt that ended in a smooth pin to hold the pivot point/center because there was slop 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. But I wouldn't have learned about just how sloppy the pin was if I hadn't gone this way. 

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.


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.

Monday, April 7, 2014

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.

Friday, April 4, 2014

Circular logic, Part I

Things have been busy. It would have been great to get this series of posts out last month... preferably on Pi day (3/14/14) or just during last month, which was Pi month. (3rd month, of '14) Alas... c'est la guerre.

The top of my current project is curved. 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, it's a curved structural member, involving two different radii, one convex, one concave, and with two different centers. And that starts to get sticky. Added to that, I had this hair-brained idea of making a cut on the band saw, and a finish pass on the router table, to make for a smooth finished part. Lastly, I'm working with 12/4 stock, and the notion of just drilling a center through a beam that's wide enough to also contain the center, was just wasteful to the point of being dopey. So, I set out to untangle radiused cuts (inside and outside radii) on pieces that aren't wide enough to contain the pivot point.

The picture below is the starting point for the jig. As I said, most of the radius cutting that Ive seen involves driving a screw through the center of the curve, into a precisely placed hole 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... so I simplified things. 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 can set up whatever jig I'm going to use, and because the centers aren't drilled or driven into anything, I should be able to just move directly from the band saw to the router table.

Part of the problem 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 removed the part being cut, from the center that was your reference point. I thought about using a simple jig that mounted to the center point, and to mount the blanks to that, but again, it seemed awkward. Eventually 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 1/4" holes in the hooked end, and into 1/4" holes in the blank. In this way, the pin can be removed to allow the blade to get in between the arms, and re-inserted before cutting. And the hooks are there to give clearance for the blade to exit the material without having to cut into the arms of the jig. And, the pin being used for the pivot point slides into the brass bushing in either the band saw, or the router table.

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

To be continued.

Wednesday, March 26, 2014

BFFI to the rescue

Accuracy is still an issue. I have a long-term solution, but it hasn't been implemented yet. And, to borrow an unfortunate, albeit accurate quote, "You don't go to war with the army you want. You go with the army you have."

Bombs away...

Apropos, then, that I borrow from that great military tradition of Brute Force and Ignorance. If at first you don't succeed, get a bigger hammer. Above, I'm using a 3 lb hammer and a center punch to make the Kreg miter bar fit my miter slot in an ugly, if workable way.

Tuesday, March 25, 2014

Oldie but goodie shop tip: cheapo glue bottle

I put this up a few years back, and after a few years (!) of using my last one, I started refilling a small titebond bottle. But it's been a few months, the pull-open tip is very much the worse for wear after many clean-outs, and so it's come back to this. 

Glue bottles at the fancy store will run $2-3 at the minimum. You could go the restaurant supply route and pay the same for a refillable ketchup bottle. And in either case, they'll still clog up, need to be cleaned out with a drywall screw, and not last too long.

This one will only cost you a nickel, it comes with a drink of your choice, and for some chemical reason that's beyond my pay grade, the glue doesn't stick too well to the top. When it dries, it pops off pretty easily. 

No, it doesn't have a roller, and it won't get into crevices, but that's what glue brushes and scraps are for.

(S)Crap! Another use for scraps! I'll put that in the next roundup...

Wednesday, March 19, 2014

Scrap Roundup

I once knew a guy who ran a shop making cabinets... He was very fanatical about getting rid of scraps. He wouldn't keep anything smaller than half a sheet of plywood. As far as he was concerned, anything he threw out had been paid for by the client, so he wasn't throwing away any of his own money. It was someone else's trash, and it was taking up valuable space in his (admittedly, small) shop. He encouraged people to take what they wanted from his dumpster. It meant less waste that he'd only have to pay to dispose of anyway, so it worked out for everyone. I got some good hunks of bamboo plywood that way.

I'm not that vigilant yet, but I do at least try to keep the amount of scrap to a reasonable level. I have a designated scrap bin, and anything that doesn't fit in there, goes out. That said, I do find myself looking for productive uses for scrap that is otherwise dumpster-bound, as do other folks that I know, so here's a rundown of some of the good uses that I've found, to date.

---- Push Sticks ----

Anything slightly larger than a full sheet of paper is push stick material, and it gets stored in a milk crate for that purpose. I laid out the design for my push sticks on a full size sheet, and so the blanks I use for making new push sticks are that size. I get two push sticks out of each blank. The design I came up with hooks onto the side of the fence, so it's always at hand when I need it. The top edge is parallel to the bottom edge. That means that when the bottom has gotten chewed up beyond safe use, I can reference the top edge against the fence, rip the chewed up section away, and cut a new notch. This way I get a few uses out of each push stick. The way the back is angled, pushing forward will lever the front end down, to help keep the board from popping up if it hangs on the back edge of the blade.   Once in a while I'll make a batch that fills up about 2/3 of a milk crate. Last time I did that, it lasted me roughly 5 years.

Looking down into the milk crate above, the push stick on the right is 1/2" ply, faced with quartersawn oak veneer. Some folks might think that's pretty fancy for a push stick. I think it keeps me from hanging on to a scrap that I would very likely never actually use, but that I'd save, simply because it was 'fancy.'

---- Thin/ Narrow Push sticks ----

When you get down to stock that's less than 1/4" thick, a regular push stick just feels unsafe. Especially when you're working with 1/8" or thinner... it gets tickled upwards by the back edge of the blade, and... things happen. Usually, not good things, either. So, I'll use a length of scrap like this, to hold the material firmly down against a zero clearance insert. It keeps the full length of a thin, and narrow (in this case, 1/4" wide) strip under control, while it's being firmly escorted past the blade.

Basic criteria here, the scrap should protrude above the fence by 1/2"-1". Run the blade at least an inch or two higher than the material... The higher up the blade is, the more the leading edge exerts a down-force, instead of pushing back, and is less likely to splinter thin stock.

I've also used a similar tactic to rip really thin strips from thicker stock, in this case 1/16". Having a riving knife really helps, but having direct pressure on top of the strip to keep it pressed against a good zero clearance insert is also a good way to keep things stable. The fact that it's a fresh push stick with clean, square edges helps. And the fact that it's just a hunk of scrap means that pretty much every time I do this, I'll be using a fresh, clean push stick.


---- Short Cross-cut setup blocks ----

I have a scrap of mahogany that's cut to 6" long, and a hunk of ash that's 12" long. I use them for a lot of things. In this case, they're great for helping with short cross-cuts of narrow material. Anything less than 2-3" is probably too short to controllably cut with the fence on a miter gauge. So, I'll set the rip fence to 6" over the length that I need, place the 6" block against the fence, and bump the material up against the block. (In this case, I'm cutting 1" pieces, so the fence is set to 7".) After that, I'll hold the material against the miter gauge fence by hand, and make the cut while leaving the positioning block behind.

---- Router table setup blocks ----

This is a by-product of the fact that my router table is hooked directly onto my table saw, but either way, if you're running grooves that need to be a specific dimension from an edge, it's ridiculously easy to cut short chunks of whatever to use to help set your fence: Insert block between bit and fence, check to make sure that the bit just barely grazes the end of the block, adjust fence accordingly. I'll label them if they're a reasonably common size, but because scraps are everywhere, and the short cross-cut setup block makes it so easy to cut accurate short lengths, it's almost easier sometimes to just cut a new one than it is to find one that's pre-cut.

I've tried to think up faster ways to set up the fence... For instance, I could add a L-R adhesive scale to the fence rail, to show distance from the fence to the center of the router collet, but then I remember that I use plywood bits a lot, so the math gets hazy. ("Okay, center of the bit is here, diameter is 31/64, half of that for the radius is 31/128", that's the location of the edge of the bit... wait, I can't even see that small... Who thought this was a good idea?") There is a scale for setting up the fence to the left of the blade, and it's good enough to help me move the fence in predictable increments, relative to the bit, which you can't do with most router table fences. So... the setup block routine is pretty quick.

I love my router table.

---- Drill Press fast blocks ----

I mentioned these recently, but they do get a lot of use. It's just so much faster to drop in a couple of chunks of plywood to elevate the material when it doesn't need to be a super- accurate hole, or when there's a lot of bit-changing going on. It's also proven to be very useful as a secondary surface when I'm drilling aluminum or steel, so the oil and swarf doesn't trash out my drill press table... or anything else that will someday get put on that table.

Monday, March 17, 2014

MFT: quick tip, checking miter angles before cutting

I'm set up to make a mitered cut at a particular angle, that's not a simple 45 degree angle. I could try to check the angle at the guide rail, but because the rail is squared up to the holes in the top, I can actually check the angle farther out, using Qwas dogs. Additionally, I get a longer hunk of board to reference from on this side of the guide rail. That extra length generally spells better accuracy when you're checking an angle. And, having the bevel gauge on a solid surface, rather than out in thin air helps hold it steady during the procedure.

In this picture, I have the board set up, the guide rail lowered, and I'm checking to make sure that the board is actually set up at the proper angle before making the cut. In this case, I'm using a 10" Shinwa bevel gauge.

The board being cut will be used for a shooting board, so having an accurate cut is really important.