Tool tips – making a rectangular hole

John KZ1G notes a recent post by Fred (K3TXW):

“…I have problems making a rectangular hole in a piece of aluminium of exactly the right size, with the edges exactly parallel to the edges of the case. I inevitably make the opening too big or slightly wavy. The result screams “homebrew carelessness” though I’m not careless. Maybe there is a rectangular punch or something to do this right, but I don’t have such a tool; I use a set of files.”

In addition to files for work on front panel fabrication, I recommend an electronics shop include:

1.  A drill press with a chuck that will accept 0.50-in.-diameter bits.  Harbor Freight sells a couple of bench-top units for under $100.  Put a magnet on the base as a place to keep the chuck wrench.  (It’s usually best to clamp work to the drill press table.)  Once you have a drill press you’ll find many other jobs for it.

2.  A good set of sharp drill bits.  Bits with a titanium nitride coating remain sharp for a long time.

3.  A step drill bit.  I use an Irwin Tools Unibit 3/16-Inch to 7/8-Inch Step-Drill Bit with a 3/8-Inch shank.  Great tool when you need to make larger holes for controls or to start a rectangular cutout.

4.  An Adel-brand metal nibbler.  They show up on Ebay.  Or buy a new one at https://www.adelnibbler.com.  I’ve used one since I was a teenager and couldn’t work on chassis or panels without it.

Lay out your hole with masking tape around the outside.  To make a rectangular hole (see attached image) I use a step bit and smaller bits to make round holes that remove a lot of metal.  Just don’t get too close to the rectangle’s edges.  Next I use the nibbler to remove remaining metal close to, but not at, the rectangle edges.  Finally I clamp the panel or chassis in a bench vise so an edge of the hole aligns with the top of the vice jaws.  File away any remaining metal until the edge is parallel with the vice jaws.

Kevin KU8H hides imperfections around a rectangular hole in the front panel with a bezel. A bezel will hide a lot of sins. They are also easier to fabricate with straight, clean edges. Rough edges that might show a little on internal panels of chassis – who cares.

Your use of the bench vice to keep the edge straight during filing is good but will eventually damage the vice. I use a sacrificial piece of steel like a piece of angle iron clamped up along with the workpiece. In woodworking we call that a “fence”.

Kevin VK3DAP / ZL2DAP clamps a short length of angle iron along the line of the opening, and files to the edge of angle iron. This gives a nice straight line. Rectangular chassis punches are available but are costly, and the larger ones require lots of force, and may need an hydraulic press.

Reference

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