Making or Buying a Miniature Counterbore

I have a lot of holes in styrene mouldings to counterbore to a depth of 0.070"

The holes are currently a blind 1/16" and need counterboring out to 0.1098" (7/64" and as its a fragile injection moulding that is to be done it needs to be a not very aggressive cut.

If I were working in something a bit larger then a bought counterbore would be the way to go. For these models (and there are a lot to do) a small version that picked up on the pilot hole to ensure the larger hole is concentric would be ideal

Trouble is I have no idea where to buy such an item or how to make one

Suggestions please?

Paul

If you made a cutter in silver steel the pilot could be made the required depth of the hole, so it acted as a stop. Styrene is rather brittle, though, and melts easily, so I hope you have plenty of 'spare' components to practice on, to get the cutting angles right. It might be worth using a pin-vice as a chuck and twiddling it in your fingers, at least until you were confident you had the tool right.

Or, you might try a burr of the Dremel type, again in a pin-vice - start with a conical one until the OD is what you need, and this will help to centre a cylindrical burr. As long as you can find one the right OD.

Cheers, Tim

hi find some one near you with a cutter grinder,and have an ordinary 7/64 drill ground so it has a short pilot and the cutting edges similar to a slot drill I find these work on virtually all matierials its then just a case of findind the correct speed hope this helps ian c

Given the nature of the moulding material I very much doubt that a pilot on the cutter will ensure that the counterbore is concentric, unless it is already perfectly lined up anyway. In which case one might just as well use a 7/64" end mill, or slot drill.

Regards,

Andrew

Take a piece of silver steel the diameter you need. (Or turn a larger piece down) Drill a hole in the end the size of the pilot you want.

Now get a small triangular file and file cutting teeth into the end. Make sure you file them so that they will be cutting when the cutter is rotating in the standard direction. The idea is to not leave a flat on the top of the tooth, but also not to file too much off any one tooth, as ideally you want them all to be the same height so they all cut. Note that you don't need dividing equipment, they don't all have to be exactly the same depth or width, just the height of the cutting edge should match up.

Now if you were going to use this on metal, this is where you would harden and temper the silver steel. For polystyrene it will probably be fine if you leave it as it comes. If you use it a lot the filler in the plastic might eventually blunt it, but then since you didn't harden it a few more strokes with a fine file will let you sharpen it again.

So now make a little pilot, and fit it into the hole in the end with a bit of Loctite or similar. For cutting this sort of plastic probably the cutter could be twiddled in your fingers.

However...you say the holes are currently 1/16 deep and you want to counterbore them to .070". That is deeper than the existing hole. The only way to do that with a piloted cutter would be to deepen the existing hole first.

I have some cutters like this that I made nearly thirty years ago to counterbore Stuart castings where the studs go through...they are still good.

John