Tuesday, November 13, 2012

Cutouts for LCD/OLED Mounting--Hot Knife vs Razor Saw (Updated)

Update: This note originally covered only the popular hot-knife method for hand-making display cutouts, but we recently tried again with an 8-dollar Zona Saw. Won't keep you in suspense: The razor-sharp saw cuts plastic faster and better than the hot knife. Details here.

Hot Knife Method

The cheapest, simplest One tool for making a display cutout in plastic is a hot knife, like the Weller SP23LHPK soldering kit, which comes with an adapter to attach Xacto-type blades.

Using this tool to cut openings in ABS speedy boxes (like this Wisher H2851 we used) requires some technique to overcome the limitations of the crude tool. We experimented, cutting dozens of holes in several boxes, in order to fine-tune the procedure. Here's what we found:

We attached the blade to the soldering iron and let it heat up thoroughly, then made some experimental passes on ABS plastic. We found that the tip of the blade lacks the thermal mass to really melt into the plastic, so we snapped it off with pliers and tried again with the stump of the blade. Much better.

After more experimentation, we arrived at a three-step technique that worked well.

  1. Score a guideline into the surface of the plastic. 
  2. Cut through the full thickness of the plastic. 
  3. Carefully snap/pry out the plastic slug.

For the scoring step, dragging the flat edge of the blade produced the straightest lines. For cutting, a more upright angle did the trick. Neither of these was particularly fast; even with practice, making a complete cutout required about 5 minutes.

Since the soldering iron is unregulated and gets very hot, and since the fumes of burning plastic are noxious if not toxic, it's important to do this work in a well-ventilated area, and to work quickly. If you pause between steps, the temperature of the blade will increase to the smoke point. Use a piece of cardboard to scrape the molten plastic off the blade to minimize fumes. We experimented with using a lamp dimmer as a crude temperature controller, but found that full power was required for effective cutting.

Demo: Mounting a GLO-416Y with BEZ-416G

We used our hot-knife procedure to mount a GLO-416Y serial OLED into the cover of one of the ABS boxes using the BEZ-416G mounting kit. The kit includes a self-adhesive window that conceals the edges of the display cutout, a real plus given the ragged edges left by the hot knife. Here are pics from our project:
Practice cutouts. 

Chopped hot knife.

BEZ-416G template.
(Masking tape used to protect the plastic mounting surface)

Drilling mounting holes using a cordless screwdriver
with hex-shank drill bit.
Removing masking from cutout area.

Scoring the outline of the cutout.
Second pass: Cutting.

Slug removed from cutout.
Cleaning up bead of waste plastic. Mounting surface must be flat.
Holes must be countersunk. We used a hand tool.
OLED bolted in place for test fit.
Window mounts with industrial peel-n-stick adhesive.
Finished ABS Speedy Box with GLO-416Y Serial OLED display.
Plenty of room in the bottom half of the enclosure for an Arduino (or two or three!).

Update: Zona Razor Saw

Zona razor saws are well-known and loved by woodworkers, who use them to make fine cuts for dovetail joints and the like. We use them around the shop to cut down chip rails for small-quantity shipments. When we recently stumbled upon their sabre saw, it seemed like the perfect tool for hacking ABS plastic. And is it ever! Below are some pics of the same cutout process as above executed with the Zona. Two minor differences in setup:
  • Additional 3/32" starter holes were drilled at the corners of the display cutout area. 
  • Three layers of corrugated cardboard cut from shipping cartons were spray-glued together, making a thick spoil-board to support the plastic panel during cutting. The cut went right through the plastic and the cardboard. The extra support prevented the thin saw blade from bending. 

Zona 35-40 Saw with small, push-cut blade.
In addition to mounting holes,
starter holes were drilled at the corners of the cutout

The plastic panel was secured to the cardboard backer
and clamped over the edge of a worktable. Here the cut
is started by poking the saw into a starter hole.

Cut went completely through the plastic and cardboard backer.
(That's a wastebasket underneath to catch crumbs.)

Finished cut wandered a bit, but the edges were smooth
and required no additional work. Since the BEZ faceplate covers
minor cutting mistakes, this panel is ready to accept a mount.
Elapsed time: about a minute.

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