Modelling the plunger2 with vissim

I'm going to take a timeout and talk about a project that I am resurrecting. It is a paintball gun that I designed and built that ultimately landed me a job at dye precision, a leading paintball manufacture. (I am no longer with dye)

I detailed the project about five years ago and stored it here:

http://attila.sdsu.edu/~thorp/Plunger.htm

It is time to dust off the drawing boards, apply what I have learned, and get back on the lathe. I want to do it in a way that involves the community and is transparent but I also want to protect my work because it very well may have commercial value.

Tonight I will talk about one of the tools that I have been able to put under my belt thanks to George Mansfield, mentor and professor. George (and he'd have it no other way) has used Vissim for much of his consulting work and is the one that got me started on it. It is a software program, as per their website "A graphical language for simulation and model-based embedded development"

So what does it do? It solves equations that you setup, steps through 'time' and then re-solves those same equations with the updated values found from the previous iteration. The equations are setup in a graphical interface, connecting variables with operators and wires.

It is a very powerful program in that any situation can be analyzed as long as the governing equations are known. Let's look at how that can be applied to a paintball gun. I've included some screen shots of a model representing the plunger2 from several revisions back.

Geometry:

Kinematics:

Fluids:

Results:

In a nut shell, the forces on the bolt are summed, then divided by the mass of the bolt. That gives us an acceleration which can be integrated to find velocity and then integrated once more to find position. As the position of the bolt changes so does the volume of gas in front of or behind the bolt and also the pressure on either side. A new applied force on the bolt can then be calculated and we are back to square1, only this time it is square2. Step through time in an incy wincy bit, let the computer do all the busy work, and you end up with a nice plot.

The ability to change a parameter- operating pressure or a diameter on the bolt for instance- and then hit 'run' and see how the bolt and plunger position change with time is a very valuable tool. The whole gun can be virtually prototyped, without a single part machined.

It is important to note though, that the simulation is only as good as the equations used to model the behavior. In the model above, I have modeled friction as a result of a friction constant and velocity. Doing so is not accurate- I've ignored static friction completely. For now, it is okay but later I will go in and model friction in a more accurate way.

I will go into what is to be done with the plots in a later post, once that work has been carried out and I can give a better explanation with screen captures.

Cleaning up masthead.

I need to figure out the best way to clean up the masthead on my boat. It's an old boat and I believe the head is aluminum so its been oxidizing for quite some time. . . . Getting the seized sheaves out was a two weekend ordeal in itself.

I think a wire brush and some kind of spray would be a great start. Any recommendations?

Thanks and make it a great day!

Only one way to follow up a post like that.... pictures.

Extruder Designs

Greetings,

I have been working on the design for my first extruder. I've done my homework and shamelessly borrowed some ideas from others. I think that I am off to a good start but still, I wanted to post up my work and get some feedback.

I've provided an animation for the two revisions and I hope they will do better in conveying the design than the typical single cross sectioned view. The 2nd design was a progression from the first but both are aimed to keep the unit modular. The 2nd concept body is a bit more of a complex part to machine but I'm looking forward to it. All of the pins/bolts/gears are mcmaster parts and then their are three machined parts in each assembly. I will try to post up some work in progress pictures if you guys would enjoy them- probably will only be machining the second version and not both.

I sometimes wonder how much of the conveyed design is lost through communication over the internet. Maybe I will annotate a screen shot or two to specify materials and such?

I leave for a two week vacation in a couple days- won't be getting my hands dirty with this project until I return.

Rev1:

Perspective view:

http://a.imageshack.us/img341/4583/extruderunit1persp1280.jpg

Sectioned view:

http://a.imageshack.us/img828/7296/extruderunit1sectioned1.jpg

Animation:

http://www.youtube.com/watch?v=0VG8BbzyEh8

Rev2:

Perspective view:

http://a.imageshack.us/img843/1543/extruderunit2persp1280.jpg

Sectioned view:

http://a.imageshack.us/img97/3059/extruderunit2sectioned1.jpg

Gear view:

http://a.imageshack.us/img827/2375/extruderunit2gears1280.jpg

Animation:

http://www.youtube.com/watch?v=zzOcbAw69gk

I suppose I've been ignoring this blog.

I've been itchin for a low cost/high rpm spindle solution and it sounds like others have had good results from the Proxxon IB/E. High spindle rpms are important for smaller diameter cutters where the cutting edge doesn't move as far per revolution as a larger diameter cutter would. I'll need to use smaller cutters for engraving work and to create isolation circuit boards ...so I took the plunge... and bought my very own.

The Proxxon rotary tool arrived in the mail and I went to machining a mount. The proxxon rotary tool is similar to a dremel in function but the quality feels a bit better- it features 20,000 rpm and the nose is cylindrical which make it convenient for mounting purposes.

And the pcb router bits also arrived about the same time.

My first go ahead- more of a throw it on their and see what I get.

It may be a bit hard to see in the picture but the problem is that the circuitboard was not oriented flat to the spindle. In one area the cutter would be down very deeply in the work and in others it would be grazing the surface. Deep cuts are not nice on our fragile little cutters and after a bit in the deep stuff, the tip snapped off. My first thoughts were that I had bowed the board in clamping it with the vise. That didn't help matters, but after further inspection, I found that the circuit boards are naturally bowed quite a bit in one direction. Holding the circuit board down at the edges against a tooling plate would keep it flat. I didn't have a tooling plate but I had all the ingredients to make one.

And now to delve into the last several days worth of work.

Since I am going to jump in telling you about a specific part of a big project, I will first give you a short description to bring you up to speed.

I am building a 3D printer- a printer that, as one would guess, prints in three dimensions. More information about the project can be found here. Over the last several days I have been working on the electronics- the control system for the printer. The printer is an open source project and much of the groundwork has been laid which gives me an opportunity to be out of my comfort zone (electronics! eeek!) and still have the confidence that I will see the project through. The support from the community is great and I am learning alot!

So.... what I've been doing. I'm using the Arduino Mega board which is another open source project. The arduino is a beefy microcontroller with a bunch of input and output pins setup to function according to a C program that is loaded onto the board as firmware. In a nutshell, the computer tells arduino to move the print head to a given location. The arduino takes that information and tells the motors what to do and how to get there. The arduino can't interact directly with a motor so a stepper motor driver sits between the arduino and the motor. The arduino tells the driver to step or change direction, and the stepper motor driver energizes the motors coils appropriately. The stepper motor drivers that I am using are the Pololu A4983 boards. They run hot and I plan to use a cpu fan to cool them. The reprap page I've been referencing is here.

For a good two days of work I don't have much to show but much of that effort went into learning the required software. I first sat down and tried to decide between fritzing, kicad, and eagle. They've all got their quarks but I eventually found kicad to be the most intuitive and powerful. We will see if I still think the same when I am through.

So what is the software and what am I using it for? Kicad is computer aided design software specific to circuit board design and layout. It allows you to layout a schematic and then design a pcb tailored to the schmatic. I am making a shell for the arduino- a circuit board that fits over the top of the arduino. The shell will house all the components required included the pololu drivers. There are already several people doing very similar things but I want to do it myself for several reasons. First and formost was to learn the software and workflow but also because I will be making the board on my cnc mill and so my requirements are slightly different than the other reprappers. Getting the workflow down- from schematic to a physical board will prove very valuable in the future, whenever I need to make a circuit board

Part of the physical layout:

Schematic:

Pcb:

Several hours later, a whole lot of text and a picture or two.

This may take some practice, but I will get better!

The blog begins.

Every morning I wake with a list of projects and errands that I'd like to tackle for the day. Writing and keeping up with a blog has been on that list for months now but it seems that their is always something else more pressing or interesting that I would like to be doing, so it gets pushed down the list. Today I stand firm- their will be no reorganization- I will start my blog.

And now, just a few words on my intentions for the blog.

Pictures tell a thousand words. I'd rather be working than writing and I think you'd rather see my work more than you'd like me to tell you about it. I intend to fill the page with pictures and keep the text lite.

I'm going to be putting everything in one place. You'll definitely be seeing machined parts but I'm also going to throw in other whatever else has captured my interest at the time.

And without further ado: I'm off to get started chipping away at that list.