June 2011 archives
June 30, 2011
cutting rods to size
by sven at 7:00 am
In making armatures, the smaller you work, the more you have to focus on precision. By the same token, when you start doing batches of more than 100 parts at a time, you start looking really carefully at each step of your process.
Rods are the simplest part of an armature. But during the past year, I've come to realize that they're as deserving of attention as any other component. Today I'm going to walk you through the process I've developed for making good rods… In excruciating detail.
Here are five guiding principles:
- Don't assume your rod is actually round.
Like in cooking, start with the best ingredients possible. I've moved to using type 303 precision ground rods, which are pretty close — but even then, while lathing you might discover a rod is .0005"-.0010" off on one side.
- You can't get an accurate measure of length if the ends aren't flat.
Which means that before you can even check the length of a rod, both ends need to be lathed.
- Protect your lathe's collet by chamfering rod ends.
You may not be able to see burrs on the end of your rod, but you'll feel them scraping the inside of the collet when you try to shove it in. Collets are precise tools; take care of them.
- You can't put a rod in a hole of the same diameter.
If your rod is precisely 1/16" wide, and the hole it's supposed to go into is precisely 1/16" wide, the rod won't go in. You have to narrow the rod just slightly, so there'll be a little bit of play.
- Don't keep numbers in your head.
When machining big tedious batches of things, trying to remember numbers will inevitably lead to error. Write things down instead.
OK… On to the step-by-step instructions. When you get good at it, this should take about 7-9 minutes per part.
Step 1: Rough cut your rod into segments
Start by marking the rod you're going to cut with a fine-point sharpie marker. Cut it into segments using a 1/16"-thick abrasive wheel on a mini cut-off saw. These things only cost ~$25 at Harbor Freight, as opposed to ~$60 at Micro-Mark. Great tool. Wear a tight-fitting face mask — you definitely don't want stainless steel dust in your lungs.
Step 2: Chamfer one end of the rod
One end of your rod segment will be fairly smooth, the other will have a big burr hanging off it. Prep the smooth side to go into the lathe by filing the edges. I find it useful to put the rod into the chuck of hand-held electric drill, and simply run it across a stationary file.
Step 3: Flatten one end of the rod
With one end of the rod chamfered, insert it into the lathe collet. If you're trying to work precisely, don't use a jawed chuck — it's just not as accurate. Lathe that big ugly burr right off.
Step 4: Chamfer and mark the flattened end
The end of the rod is flat, but there are still microscopic burrs on the edges. While the segment is still in the lathe, use a tiny diamond file to very slightly soften that edge. While you're at it, use a normal-sized sharpie marker to blacken this end of the part. It's helpful as you go through the process to be able to tell one end of the part from the other.
Step 5: Flatten the other end
Take the segment out of the collet and turn it around. Repeat step 3 and flatten the other end of the rod.
Since you've blackened one end of the rod, you don't have to bother chamfering this side. Just make sure that when you do further work on the rod, it's always this side that you're cutting.
Step 6: Measure the part
Using digital calipers, measure your part to see how long it is. Hopefully you've left yourself ~.0100"-.0800" of extra length, since we're still going to need to lathe the part to its final size.
Step 7: Write down the measurement
However long the part is, write that number down on a little scrap of paper. If you're doing a big batch of parts, I recommend getting yourself a cheap jewelry organizer (~$3) to use as a palette.
I've got a 3/4" square paper punch that came from a craft supply store — very useful for making the tiny bits of paper. Card stock works a bit better than typing paper, because the paper doesn't bend when you lean it against the walls of the jewelry organizer.
Step 8: Calculate how much to shorten the rod
For the rod we're looking at below, I wanted the final length to be .1880". Since the rod at this stage was .2300" long, I just needed to subtract to figure out that I needed to remove .0420".
Step 9: Write down how much the rod needs to be shortened
On the same tiny square of paper, write down how much the rod needs to be shortened. I find using a red pen here helps keep the new number visually distinct. Anything that helps prevent errors is good!
Step 10: Locate the end of the rod
Put the chamfered end of the rod segment back into the lathe. Blacken the flat end of the protruding side with a normal sized sharpie marker. Bring your cutting tool in until it just touches the end. If you're careful, you should be able to remove less than .0005" of material while doing this. Listen for a change of sound when the tool touches the spinning rod, and look for whether or not the sharpie ink is being rubbed away.
Step 11: Cut the rod to its final length
Zero out the DRO (Digital Read Out). Use the lathe to cut off the amount of material that you wrote down in red. For 1/16" rods, I found that .0050" cuts worked well. You can take off more at a time safely — but the stress is more likely to cause the rod to move in the collet.
Step 12: Check that you hit the correct length
Sometimes things go wrong. Like when a rod moves in the collet, and you don't actually cut off as much material as you thought. Check your work with the digital calipers. In the photo below I was off by .0005" from my desired length of .1880". That was a perfectly acceptable level of error for this project.
The part is now at it's correct length.
Step 13: Position the rod for trimming its diameter
If this end of the rod is going to go into a ball, then you're going to need to trim its diameter. I find lathing about .0010" off all around gives just enough play to get the rod into the ball's hole while still keeping a close fit.
For this project, the ball holes were 5/64" deep (.0780"). I decided the diameter trim should be .0850" long. I wanted the rod to stick out .1000" from the surface of the lathe collet, so there'd be a margin of safety between the cutting tool and the collet. To accomplish this, I created a little tool that allows me to push the rod into the collet to a (reasonably) precise depth. Using the .1000" tool, position the rod in the collet.
Step 14: Zero out the DRO where you want the diameter trim to end
As in step 10, blacken the end of the rod, and carefully bring the cutter in until it just touches. Zero out the DRO. Without performing a cut, move the cutter along the Z-axis to .0850". Zero out the DRO again.
Now, when you're trimming the rod ends, all you have to do is travel to zero each time. It saves a lot of brain power. (If you're doing a big batch of parts, you're only going to have to do this step once.)
Step 15: Locate the side of the rod
Blacken the side of the rod. Move the cutter on the X-axis and watch for when it makes a mark in the ink. Figure out where the edge of the rod is and zero the DRO. Move the cutter in to X=-.0010. You're now ready to make cuts. (For a batch, you're only going to have to do this once.)
Step 16: Trim the end of the rod
With the cutter set at X=-.0010, move it to Z=0, and then back off from the part on the Z axis. Leave the X alone. Test the trim by placing a ball on the end of the rod. If it fits nicely, then you're done — or ready to trim the other end of the rod.
There, that wasn't so hard, was it?
Oh… I guess it was. Well, at least you've got a really great part to work with now. ;-)
June 29, 2011
evolution of the ball-drilling jig
by sven at 7:00 am
Drilling balls is a fundamental of armature building. It's a tougher problem than you'd think. At this point, I believe I've gone through seven methods of drilling balls — and I'm still not entirely satisfied.
Today I'd like to do a retrospective of the methods I've tried and discuss their various shortcomings.
Fig.1: The classic method for drilling holes in balls is to put the ball in a lathe chuck. This is what I did for the Man of Steel back in 2006.
The advantage is that your hole should be perfectly centered. The big problem is that the drill bit wants to push the ball back into the chuck, ruining your control of hole depth. You can try to crank down on the chuck and make it really tight — but the stainless steel ball is harder than the jaws of the chuck and will leave a ball-shaped deformation. Oops.
Fig.2: If you put the ball into a milling vise, supported by a riser, you can be sure that your depth will be accurate. However, now you're going to have to worry about getting the hole perfectly centered.
My milling vise has a nice built-in groove for holding the ball in place. But if you use it, where do you measure from to find the center of the ball? You can't use the fixed jaw because you don't know how deep the ball sits in the groove*. You can't use the moving jaw because milling vises pull that jaw down at an angle. If the jaw isn't vertical, there's no way to tell exactly where the ball is making contact.
(*Technically, you could figure it out with geometry — if there was a way to get really precise measurement of the groove's dimensions.)
Fig.3: I tried moving the ball over in the vise, so I could use the fixed jaw as a vertical surface to measure from. But with only two points of contact, the ball wants to spin on the Y-axis when the drill makes contact.
To give the vise a better grip, I tried making a grooved insert that attaches to the moving jaw. As I recall, it did an OK job of holding the ball in place… But then I discovered that the ball was deforming the vise jaws. Again, the stainless steel ball is harder than the things holding it. No good.
Fig.4: Thinking about sandwich plate joints, I created this jig that would hold two balls from the sides. Several problems… It's fussy to get balls in and out. You have to take the versatile milling vise off the XY table and put a bunch of effort into getting the jig parallel with the milling column. There's a lot of room for compounding errors if the jig itself wasn't milled perfectly straight. It's an overly complicated and irritating solution.
Fig.5: This jig was inspired by step-block joints, and worked pretty well most of the time. It involved a few superfluous steps, though. After clamping a ball in place, I'd fill the hole above it with oil. (The hole held the oil in place quite nicely.) I'd use a cylindrical diamond burr to grind a flat spot on top of the ball. I'd use a center drill to start a hole. Then I'd use a drill to make the actual hole. I've since found a method that doesn't require flattening the ball or using a center drill.
Fig.6: My most recent attempts take their inspiration from this jig by Jeremy Spake. The top holes guide the drill directly to the balls' centers, making center drilling unnecessary. This design is intended for use with a drill press. Unfortunately, I don't have a drill press with depth control that I really trust, so I've had to adapt it for the mill.
Fig.7: Using three balls guarantees that the top plate will be parallel with the bottom. The hole you make in the ball will be centered, the drill bit won't go into the ball at a funny angle, and you've got good depth control. Excellent!
My version for the milling machine theoretically gives me depth control down to .0005" accuracy. You do have to pull the drill out promptly, though, or else Z-axis backlash might allow the drill to keep going down after you've stopped turning the handwheel.
With Jeremy's design you can do a couple of balls per minute. With mine, you have to re-load the jig every time you want to drill a new ball. Slower… But I'm willing to give up some time for the sake of getting accuracy.
Fig.8: Much to my frustration, I've run into problems with this jig design becoming deformed. If your speed and feed for the drill aren't right, the ball will work harden and the bit can't cut into it anymore. After that point, the drill is simply smashing the ball downward into its seat.
Fig.9: The previous jig was made out of 1018 steel — just about the softest steel you can buy. For my most recent jig I switched to type O1 tool steel, which is harder than 302 stainless. It too has deformed with use, but it survived a much longer time. Progress.
I think the next improvement would be to mill ball cups into the jig… And to really control speed and feed. Unfortunately, I think the only way to achieve that is to purchase a CNC machine. A computer would stepper motors to precisely control the descent of the drill bit.
In the meantime, let's take a closer look at using the current jig design.
Fig.10: A blog post for another day would discuss the ordeals I've gone through trying to learn how to make the ends of rectangular stock truly flat. Recently I discovered a very simple solution to my woes: four-jawed lathe chucks.
I've been prejudiced against the lathe ever since I couldn't get it to drill balls correctly. Now I discover it's the solution to a problem that's given me countless headaches. Ironic? Anyway, this jig is my first time using the independent-jaws version of the four-jawed chuck.
Fig.11: When you put the smallest balls into this jig, they can fall through the larger holes if you accidentally knock them. It's a good idea to put tape or stickers over the larger holes while you work.
Fig.12: You can see here how parallel the jig's top and bottom plates are. When one of the holes starts deforming, the plates become less parallel. To my surprise, the drilled balls coming out of the damaged jig are useable until the parallelism is strongly compromised.
Fig.13: It's very useful to use a ball-nosed hex screwdriver when initially tightening the jig. It allows you to get in at an angle.
Fig.14: The screwdriver doesn't have enough torque to get the jig really tight. For the final quarter turn, you need to use a regular hex key.
Fig.15: One of my best discoveries for milling has been that you can use a syringe to apply cutting fluid where you need it very precisely. Trying to put oil where you want it with the short spigot of the tin can is messy if not impossible here.
Fig.16: Especially with 1/8" balls, it's difficult to use your big meaty fingers. Tweezers are necessary. Metal tweezers tend to become slightly magnetic, which causes lots of trouble. Using a pair of plastic tweezers works better. Normally I use these for cleaning parts in acid baths… Yet another small detail that saves grief.
June 27, 2011
by sven at 11:55 pm
Immediately after the Northwest Animation Festival ended, I got hired to do some armature work for a feature film. I was asked to fabricate 160 puppet hand paddles.
I made parts in two sizes, using 10mm and 8mm jump rings.
The 10mm rings are solid brass. The brazing process draws copper to the surface of the metal, making it reddish in color.
The 8mm ring appeared to be made of brass — but that burned away during brazing, revealing a steel core. I was initially worried that plating would interfere with getting a good bond. It didn't turn out to be a problem.
The balls are 1/8" diameter, the rods 1/16". We're talking SMALL. I constantly had to use needle-nosed electronics tweezers to pick things up.
Doing just about anything 160 times gets tedious. At least the tedium gets punctuated with panic when things go wrong!
Armatures are a love/hate thing. There's a very special misery when things aren't working out in the machine shop… But when things do work, perfect shiny metal is all kinds of sexy.
A couple weeks of living night and day with this project. Then? All those neatly laid out parts get dumped higglety pigglety into bags and I take them off to FedEx, never to be seen again. Good luck in your new life little hand paddles!