Kaliko Systems Limited

Make a Low-Power Media Server

Machining ABS

ABS can be cut either conventionally using wood or metal tools, or by a thermal cutter. I believe that thermal cutting with a heated knife must lead to significant “lips” on the ABS surface that I am keen to avoid, so I have not tried thermal cutting.

 

The ABS case bottom is shown below immediately after machining.

Back of machined box

When cutting ABS, swarf (cutting waste) can build-up around the cutter once the temperature of the cutter rises. This can give rise to over-cutting and “jolting”. Jolting occurs as the swarf melts, re-solidifies as a brittle lump attached to the case and then breaks off under pressure. This jolting can cause the cutter to wander and give an uneven finish. The types of holes required for the relay board case are:


1. Round holes for LED status indicators and DC power sockets


Drilling round holes requires a sharp drill to keep the temperature down. Blunt drills will melt their way through the ABS, leading to oversized holes and ugly “lips” on the ABS top and bottom surfaces. Because ABS is soft, a sharp twist drill will often catch in its hole and try and lift the ABS box, so the box must be secured by clamps. When starting the hole a sharp drill can tear the ABS surface just beyond the hole leaving a lighter colored crescent. Surprisingly, this only happens when starting the hole, so I recommend that you start your hole from the inside of the case. As with all drilling, creating a small pilot hole first can improve accuracy and should reduce the chance of melting.

2. Rectangular holes for IEC plugs and sockets

There have been some interesting articles on the Web describing different techniques for cutting rectangular holes in ABS panels, but I find the quickest and cleanest method for cutting rectangular holes in ABS is to use a woodworking router. I have a Trend T4 router (costs about $100) that is light to hold, has variable speed adjustment and low vibration. To cut the set of rectangular holes required in the case you should first make a template. Trying to cut rectangular holes without a template requires using the router with a guide fence and improvised end stops for each cut, it is very slow and error prone (with at least 4 cuts for each hole). As the number of socket holes I needed was 8, I carefully made a template for one hole then used that to make a template with 8 holes (I had already added the single plug hole to this second template). I used MDF board (10mm thick) for my templates, as many people recommend this material for low usage templates. Of course, to make the templates you should use a guide fence and improvised end stops. I used clamps to both hold the template to a work bench and act as temporary end stops. The bottom of the template, where the ABS box fits, is shown below.

Template for cutting case bottom.

Routing Issues

  1. Traditionally, router collars or bearing guided router bits are used with templates. Router collars cannot be used as their required clearances would make the gaps between sockets far too large. For a bearing guided router bit with a template above the ABS box a top bearing is required between the bit’s cutter and its shank. However, as a 6.4mm bit is required for the corner radius of many plugs and sockets, and a top bearing is only available on larger diameter bits, so a top bearing bit cannot be used. Instead, use a bearingless 6.4mm bit that has a 6.4mm (1/4 inch) shank, so that the shank can simply rub against the top part of the template. A bit with a long shank is preferred, as you will need to have the shank extend well below the router base, as shown below.Diagram showing router, jig and ABS box.
  2. Melting of ABS is always a potential problem, but you can smell it pretty quickly. My non-professional understanding is that the fumes should be avoided as much as possible and reasonable ventilation should be provided. The router should be used at minimum rotation speed. Surprisingly, melting often occurs even with sharp bits when your cutting feed rate is too slow. So if melting occurs, withdraw the router, let the bit cool down, clear out the flutes and try pushing the router a little faster next time. Trend sell a special 6.4mm bit for machining ABS, this is expensive (about $20), does an excellent job and should be considered if you intend to route ABS, PVC, etc frequently.
  3. With first cuts there is a tendency for the side of the cut that is in the forward rotation direction to be less smooth, so it is best to cut each hole in a clockwise direction and to plunge the router in the middle of the hole.
  4. With first cuts the rotation direction of the routing bit produces additional sideways forces in the corners that can result in overcut. A standard technique to remove first cut errors is to follow the first cut by a shallower final cut of only one or two millimetres, but this requires making separate templates for both the first cut and for the final cut.
  5. When attempting to cut the rectangular holes be prepared to scrap you first and perhaps second ABS box  (good job these boxes are low cost!).
Making rectangular holes using a hand-held router is usually good enough for the minor imperfections to be hidden by the bezels on the sockets and plug. However, the holes usually are not to factory quality, so this is now my excuse to make a computer controlled 3 axis routing machine.


With the case bottom machined then the mounting, wiring and labelling of the relay board and case will complete the case.

 

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