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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