Design

Upgrades, a bigger project, and why that machining job might cost more than you expect (Part II).

By |November 4th, 2015|Design, Machining|Comments Off on Upgrades, a bigger project, and why that machining job might cost more than you expect (Part II).

Too long. It’s been too long since I last wrote on this blog, but sometimes regular work just has to been done first. In this past instance it was one longer design project and an extensive series of equipment and tool upgrades. The design project was a piece of lab equipment for Dr. Jinsook Roh, a neuroscientist at Temple University. Here’s a quick look at the whole thing. There are some detail photos later on.

Photoshopped best top view

Calculations. It was a fun project. There were mechanical engineering calculations — beam deflection, disk brake design, slipping and tipping, screw force. Nothing terribly esoteric, but nevertheless very satisfying. I also was able to use a variety of machines and tooling. Just for fun, I decided to make a list of what I used.

Machines. For machines and more involved tooling:

  • manual mill
  • CNC mill
  • drill press
  • lathe
  • horizontal band saw
  • carbide cutoff saw
  • tool grinder
  • universal cutter grinder
  • bench shears
  • bench punch
  • bender
  • boring head
  • auto-reversing tapping head
  • CNC touch probe
  • 3D-taster

Tooling, software, materials, and pieces. This is in addition to numerous milling cutters, drill bits, taps, edge finders, vises, parallels, measuring  and hand tools. And then there’s software — I count eight different pieces of software that I used for CAD, CAM, and engineering calculations.

It seems like a lot, but there were a lot of pieces, and multiple materials — aluminum plate, aluminum extrusion, low-carbon steel plate, low-carbon steel sheet, some hardened steel pieces to be modified, acetal, UHMWPE, and PVC. Here are some of the small pieces:

Parts

Which brings us back, briefly, to a question that I raised last time, “why that machining job might cost more than you expect.” Even for this project, which required only ordinary precision and did not involve esoteric materials, a rather stunning collection of machines, tooling, and software was needed. All these things cost money to purchase and maintain, which is reflected in the cost of a machining job.

Details. Here are a few details that I thought came out well:

Handle

Brake w_ safety pin

Straps

Arm support close up

 

Why that machining job might cost more than you expect — Part 1: Jigs and fixtures

By |June 24th, 2015|Design, Machining|0 Comments

More than you expect. People are often surprised about how much it costs to make something “simple.” One problem is that most of the manufactured items that we purchase in our daily lives are manufactured by the thousands, if not by the millions. For instance, if you need a nut or a bolt, you can go to a hardware store and buy one for pennies. A 3/8-24 “SAE Zinc Grade 5 Finished Hex Nut” costs only 15 cents at Home Depot. However, if you asked me to machine such a nut from scratch — from a hunk of metal — that would be another matter entirely. Hardware store nuts and bolts are made by the millions; but when you go to a machinist with your job, you might want only one part.

Mass manufacturing vs. a “one off.” Mass manufacturing is often carried out with special-purpose machines designed specifically to allow huge quantities to be produced at low cost. Low cost is achieved by spreading the high initial cost of the machine over the large quantities of parts produced. When you hire a machinist to do a small job, however, that machinist typically uses general-purpose machines — for example, drill presses, lathes, and milling machines — to make the part. For many situations, these machines, along with standard cutting tools, and various kinds of commercially available clamps and vises will do the job just fine. But, often enough, these things alone will not suffice.

Jigs and fixtures. A common situation is that there is no easy way to hold the part so that it can be machined, or there is no easy way to orient the part so that it can be machined accurately, or that there is no standard cutting tool that has the right shape to do the job. In these cases, the machinist needs to fabricate a jig, fixture, or cutting tool just so s/he can do your job.  A fixture holds the part down in a fixed position so that a machining operation can be accomplished. A jig, on the other hand, orients and guides a cutting tool (e.g., a drill bit) into the right position. In some sense, fabricating a jig, fixture, or special cutting tool is a way of making the general-purpose machine into the special-purpose machine required for your job. Naturally, fabricating these special items takes time, which translates into increased costs for your machining job. Sometimes making these special pieces will take longer than the rest of the machining job. This is explains why a “five-minute job” could actually take a couple of hours.

An example. Here is an example from a small “simple” job I recently did. I’ll leave some of the details out, just so we concentrate on jigs and fixtures. The job was to drill some holes in a 1–1/2″ thick piece of plywood to mount an apparatus. The apparatus was to have a number of mounting plates with mounting holes through which bolts would be inserted. These bolts would engage flanged nuts on the underneath side of the plywood. To keep the nuts from protrouding, they would be recessed in counterbores (circular “pockets”). This is illustrated in the following figure.

nut and bolt.77

The blue piece represents a mounting plate with a mounting hole. The figure shows both the hardware and what the underside “pocket” would look like. Pretty simple, except that the holes in the plywood, including the pocket, had to be exactly aligned with the mounting hole in the plate.

Two jigs. The solution to the problem of getting exact alignment was to make two jigs, shown in the figure below — the two aluminum pieces on the left. The piece on the right is a simple fixture, which I will talk about a little later.

jigs and fixtures

The way the jigs work is as follows. There is a jig that is used on the top surface, to allow us to drill straight through the center of the plate mounting hole. And there is a second jig that is used on the bottom surface, to allow us to drill a large diameter bolt hole in perfect alignment with the first hole drilled from the top.

Top jig. The top jig is explained by the illustration below. Again, the blue piece is the mounting plate. The aluminum jig fits into the mounting hole in the plate, which then guides the drill bit straight down through the center of the mounting hole, and perpendicular to the top surface of the wood.

top jig assembly.81


Bottom jig. After the hole is drilled through, the plywood is turned over and the drill exit hole is used as the center point for a Forstner bit, which makes the recess. Now the challenge is to drill a large-diameter hole for the bolt.  If one simply takes a  handheld drill with a drill bit of the appropriate diameter for the bolt, misery will soon follow. The drill bit will refuse to stay centered. The drill bit will “wander” in the soft wood, and the bolt hole will end up far from the center. But, another jig solves this problem, as shown below.

bottom jig assembly.83

This jig fits to the recess, or counterbore, to center the drill in exactly the right place and prevent the drill bit from moving off center while drilling.


And a fixture. That’s the two jigs, but I also needed a little fixture for this job. I’ll skip the details here, except to say that rather than using a regular flanged nut, I used a flanged nut that had two small slots cut into the flange, as shown in the figure below.

slotted nut.79

Since I had twenty of these slotted nuts to make, I used a CNC mill to make the slots. This was more or less a straightforward task, with a simple CNC program, except that the nuts were hard steel so that I had to go a little slow when cutting them. But there was also the problem of how to hold the bolts so I could machine them. For this, I made a little fixture to hold the nuts. The fixture looks like this:

fixture assembly.86

It’s an aluminum rod with a neck that fits inside the flanged nut. The neck has a threaded hole that allows the flanged nut to be bolted down tightly onto the fixture. The fixture was held in a collet block, the collet block in the machine vise, and the nut bolted to the fixture. Once everything was set up, I would pop the unmachined bolt into the fixture, start the milling program, and take the machined nut off the milling machine two minutes later. Here’s a little video that shows how everything works:

For this particular job, for which I made 20 holes and machined 20 slotted nuts, I also had to make two jigs and one fixture. But even if I had needed to make only one hole and only one slotted nut, I would still have needed to make the two jigs and one fixture.

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loopboost progress report #2

I’m been working on the preliminaries of the loopboost project. I have a basic concept for loopboost 1.0:

Loopboost-concept

and I’ve working on the hardware and software block diagrams. I don’t have all the parts yet, but I should be able to start coding next week.

Boxes, Cases, Enclosures — Part 1

By |April 26th, 2015|Design, Electronics|0 Comments

Sometimes it takes more time and energy to make the project box (case, enclosure) than it takes to actually design and build the electronics. For a final product or a production version, that might make sense. But sometimes you just want something that will do the job, not take too much time, but still look good. Over the years I have watched hundreds of engineering students use laser cutters and 3D-printers to make their project boxes. That bothers me sometimes, especially the mess of cut acrylic sheets surrounding the laser cutters and the knowledge that most of these projects will find their way to the dumpster soon. It seems wasteful and ecologically unsound. Not to mention, this is an expensive way to make a temporary box. 3D-printing has its own problems, one of them being that it is a really slow way to make box. Taking four to six hours to print out an enclosure, when there are dozens of other students in the queue desperately waiting to print their parts, doesn’t seem consistent with the concept of unleashed productivity that 3D-printing is supposed to bring.

All of this waste and inefficiency is made worse by the fact that the project box might have to be remade a few times because of measurement errors and design changes. However, over the last few months I have been working on a system that uses small 3D-printed pieces, in combination with decorated foamcore panels, to make serviceable and attractive enclosures cheaply, quickly, and easily. Better still, these enclosures are quickly and easily modified if there are measurement errors or design changes. And a further advantage is that the 3D-printed pieces can be made up ahead of time, before the dimensions of the enclosure are known. Then, when the dimensions are set, the foamcore panels can be cut and the case put together in a manner of minutes.

I’ve worked out several different methods of building these boxes, which I’ll describe in upcoming posts. For now, I’ll introduce what I think is the best approach for small boxes. It basically consists of two plastic (3D-printed) endcaps which contain slots into which four foamcore panels fit into. A couple of pictures say it all:

Small-box-exploded

Small box

Of course, the box need not be quite so plain. Here is rendering of something edgier:

Small box with stripe

In my next post on this subject I’ll provide some construction details. I also think I’ll have an OpenSCAD program available that generates the STL files for the endcaps.