Wednesday, June 29, 2011

Small Subwoofers Project

I picked up a couple of cheap 8 inch subs to help fill out the bottom end in my RX-8. Not sure yet what the install will be, hoping they will fit between the back seats in an IB configuration.

I remembered seeing an image of a driver installed in this location on the forums, but couldn't remember whether that install had used one or two drivers. With a little googling, I was able to locate the thread about the installation, which included this image provided by the guys who did the install (MotorMusic, Inc, who appear to be based in Oakland, CA).

The stock configuration looks like this.

And with the pass-through cover removed.

When I picked up the speakers I had intended to install both in this location, because I thought I remembered seeing an install like that. While the space is physically large enough to fit both drivers there (particularly if mounted basket-out), the layout of the plastic would make it a pretty tight fit. It would be possible to strip it out that stuff for a fully custom install, but I'm not going to get that deep into it.

From this it appears that if I want the driver facing into the cabin here without a lot of work, it'll need to be a single driver. There are a couple of other installation options to consider though.

A 4th order bandpass with the speakers in the trunk with a shared vented chamber ported through the passthrough would allow me to use both speakers, but would require a lot more space. This isn't necessarily a problem, since I'll probably have to remove the spare in any case and will likely have more trunk space even with a bandpass box.

Stacking the drivers in an isobaric configuration would reduce the sealed box size from 0.5 cubic feet down to about 0.25 cu.ft. I'm not all that interested in this configuration for one and only one reason: power. Isobaric configuration trades a driver for box size, and I hate to trade half of the possible output power for a quarter of a cubic foot.

A more typical configuration for this car would be to place the drivers at the sides of the trunk in sealed boxes.
I'm a little hesitant to hide relatively low powered drivers in the trunk though. Might have to stuff the spare 12" box in there and see how it sounds.

To test fit the driver at the side of the rear of the trunk, to verify that i would have enough volume to meet the recommended 0.5 cubic feet spec, I cut a cardboard panel to fit where the front baffle would be.
This defines the space and lets me see what the speakers will do to my available trunk spack. To estimate the volume I filled the cavity behind the cardboard with packing peanuts.
I then transfered the peanuts to a rectangular container to calculate the volume.
In this case it worked out to 0.8 cubic feet. That is a good start. The front baffle will be 3/4 inch MDF, and so ends up taking up about 183 cubic inches. Combined with the 10 cubic inch displacement of the driver, this reduces the space to 0.69 cubic feet. This will be reduced somewhat by the fiberglass shell and bracing, but it looks like it will work out quite well. I will still need to mount the amps somewhere. I think the best place for these is going to be on the backs of the seats. Thay will be way back out of the way that way. This configuration will let me keep my spare tire.

Automatic cat feeder project

Here is a short video of the slide portion of my automatic cat food dispenser project.

Ruben's tube

We put together a little Ruben's tube at the OMG meeting today. There has been some discussion about building a large tube for a week or so, but no one has yet come up with a length of tubing to use. We were standing around talking after the meeting and someone observed that the remnants of a small telescope that was sitting on the table could probably be used to make a small Ruben's tube. So we scrounged up some additional parts from the parts bins (pc speaker, vinyl hose, bread bag, aluminum sheet, duct tape) and whipped up this little guy.

One of the guys got some good video of it running, which I expect will show up on the Omaha Maker Group website or mailing list before too long.

Sunday, June 26, 2011

Cat feeder coupler

Tonight I built a really crappy coupler to connect the motor on the cat feeder mechanism to the drive screw on the slide. I was having some difficulty getting anything to work and started on a new slide with a scotch yoke design, but, not surprisingly, I had trouble connecting to the motor shaft for that too. So I poked around a bit and found a grub screw in the junk box that I figured would work to secure an adapter to the motor shaft. To make the adapter I cut a short length of aluminum rod which I then needed to bore out to the diameters of the two shafts.

Since I don't have the equipment to properly bore a shaft, I did it improperly, and with really poor quality. To do this I mounted the rod in the drill press chuck, put a drill bit into the vise, and made an attempt to set the bit vertical and on center, then drilled away. This technique can be used to put a hole exactly in the center of the rotating shaft, give or take a few inches. Flip and repeat with the smaller bit for the motor shaft.

I tapped the larger hole for the screw, then mounted it up and marked where the motor shaft flat was so I could drill a hole for the grub screw. I didn't have a tap small enough for the grub screw, but since the screw is steel and the shaft is aluminum, it can self-tap. I ground the outside of the shaft a bit to remove some of the worst of the imbalance from the crappy, off-center boring job, dripped some loctight in it, and mounted it all up. I'm too impatient to wait for loctight to dry though, so I crushed the threaded portion to secure it to the shaft.

It's working pretty well. I'm not sure how much torque this motor generates, but at stall it generates a reasonable amount of linear force through the screw, a pound or two. At stall the motor draws 2.5A, so I'll need to scrounge up some appropriate transistors for the h-bridge.

Friday, June 24, 2011

Evening Project: Automatic Cat Food Dispenser

I spent the evening working on the mechanicals for an automatic dry cat food dispenser. It consists of a screw-driven slide with a 2 inch hole. The assembly will be placed under a large cat food bucket with a hole in the bottom, with a chute directing dispensed cat food into a bowl.

At the moment the screw is driving the slide simply by threads cut directly into the wood. If necessary I will embed a nut in the slide, but I'm hoping the forces will be low enough that the wood will be sufficient. I've got the motor coupled directly to the screw. This means I'm using a far larger motor than is really necessary for this job. Unfortuantely I didn't have any gear drives that were really suitable. Also unfortunately I didn't have any reasonable way to couple the motor to the screw, so I just bored the end of the screw out to the diameter of the motor shaft and put some Loctight in it. This obviously won't last long, but it was a quick fix. I'll need to come up with something interesting to couple the shafts. Maybe it would be interesting to cast a three-part coupler for this.

The next part of the project will be the electronics for driving the motor. This will consist mostly of some end switches for the slide and an h-bridge driver for the motor. I'll probably put an AVR into it, just because they're cheap and I've got a bunch lying around looking for work. On the other hand, since it just needs to cycle once per input trigger, it might be fun to do it with something more rudimentary with some basic logic.
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Saturday, June 18, 2011

Spectralight mill update

We spent the afternoon tearing down the mill, cleaning and oiling the ways, and getting the tabletop painted. Its running pretty good now.

Monday, June 6, 2011

Zinc Casting

I picked up some casting sand from a local aluminum foundry this weekend and spent Sunday putting together a small test casting. Rather than using aluminum, which is a challenge to melt in a soup can, I used a few rolls of pennies as a source for some zinc.

It took a few tries to get the cope rammed up properly. I had trouble getting the sand inside the pattern rammed hard enough and strongly connected to the rest of the sand in the cope. It did work out nicely though. I took some pictures of the casting process and the resulting object.