Canadian Plastics

Keeping second ops simple

R egular readers of this column know that I'm a big fan of "big picture" thinking when it comes to shop floor productivity. It's a concept so overused that it's downright trite, but what I mean by "bi...

April 1, 2008   By Jim Anderton, Technical Editor



Regular readers of this column know that I’m a big fan of “big picture” thinking when it comes to shop floor productivity. It’s a concept so overused that it’s downright trite, but what I mean by “big picture” is a look at the cost of production from a plant floor throughput perspective. Big volumes mean efficiency, but at the cost of lower flexibility and higher up-front tooling costs.

What if you’re a small-run job shop? One “ace in the hole” that a smaller shop has is its ability to trade off processes between primary (injection/extrusion, etc.) and secondary operations like degating, trimming and finishing. This lets the business quote lower up-front tooling costs for the customer, but adds labour cost in the form of “second ops” during production.

Since second ops are generally not directly charged to the customer, an essential part of the bidding process is to calculate them carefully. For short run jobs, it helps to keep the second op system as flexible as possible. How? It depends on the part’s needs. Trimming is the most common, and here there are options as cheap as a razor blade and as expensive as lasers. Sensible low cost options are pneumatic presses with simple trim dies and wood working tools. Woodworking? Yes, it’s not only possible but profitable to adapt woodworking equipment to plastics second op processes in many cases.

Band saws, routers, sanders and drill presses can all be used effectively, but there’s a catch: fixtures. The key to repeatable use of these low-cost processes is a carefully designed jig to hold the part and guide the cutting/boring/ sanding process to eliminate (or at least reduce) the influence of the operator and add safety. You might be surprised to learn that the fixtures can cost more than the machinery, but they’re essential for quick, precise work.

How complex should they be? Ideally, a fixture should hold the part with a minimum of clamping or holding devices, be easy to load and unload, keep the operator’s hands clear of the cutting path and be cheap to build. I’ve seen and built workable devices ranging from nails in a plywood board to CNC-machined aluminium jigs with vacuum clamping.

The trick to keeping costs down here is to use as much of what the machine has to offer as possible. Cast tables and rip fences can be drilled and tapped for mounting bolts, and can generally be tilted with reasonable accuracy. Most tables have machined slots that are a natural when building manual feed jigs that approach the blade or bit.

Blades and cutting/feed speeds are generally within the range of woodworking settings and the equipment can often be mounted at angles radically different from the regular upright stand mounting.

The equipment itself can often be had with three-phase or single-phase motors and requires minimal maintenance.

Simple downstream automation can make a smaller job shop both flexible and profitable with a little creativethinking.

CPL

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Jim’s Tip of the Month

Woodworking tools for plastics

This one is straight out of my well-worn notebook for cutting and drilling commodity resin parts. They’re a great starting point, but your application may need special parameters:

Cutting Rake: 5 degrees Clearance: 20 degrees or more TPI: 100-120 (Band saw/table saw)

Drilling Tip angle: 65-70 degrees Speed: Max until melting or burning hole

In both cases feed speeds are relatively fast, but bits and blades should be sharpened or replaced regularly on a conservative schedule; don’t wait for burned or melted edges to appear. I like keyless chucks for fast drill changeover and consider low -cost sharpening equipment for drills and bulk coils for band saw blades. Making your own band saw blades is surprisingly easy and many professional machines have built in resistance welders to do it quickly.


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