Pentagonal Hole

[DaleCooper]

Are there any toolmakers or similar on the forum who can explain this? I wanted a 12mm hole in some thin (~1mm) rigid plastic. I started with a 3mm drill then a 7mm drill but decided any bigger would be too aggressive on the thin material so I would open it up with my tapered hand reamer which has four spiral flutes. When it was nearly to size I took the reamer out to look at the hole, to my surprise it was pentagonal with radiused corners. Any ideas how that happened?

 

[ComUtoR]

Any ideas how that happened?

Have you tried drawing it out on a piece of paper with a compass ?

Sounds perfectly logical to me but ye olde pen and paper may better reveal what you did.

 

[Dr Hoo]

Hmm... I have had a similar thing happen in the past and couldn't work it out either. I am all agog for the answer from one the forum's technical experts.

 

[Cowley]

That's what you get for trying to make coffins out plastic.

 

[DaleCooper]

That's what you get for trying to make coffins out plastic.

"Well you can't get the wood, you see", as Henry Crun was so fond of telling us.

 

[Railsigns]

This page shows how you can use a triangular drill bit to cut a square hole with radiused corners. It mentions that tools are available to cut pentagonal holes too.

 

[Cowley]

"Well you can't get the wood, you see", as Henry Crun was so fond of telling us.

I'll send you a photo of a hexagonal hole. You can use that Min.

 

[DaleCooper]

This page shows how you can use a triangular drill bit to cut a square hole with radiused corners. It mentions that tools are available to cut pentagonal holes too.

I was thinking it might be connected with the square hole drill, that has three flutes and cuts a square so maybe four flutes is capable of cutting a pentagon with a bit of wiggling.

 

[DaleCooper]

I'll send you a photo of a hexagonal hole. You can use that Min.

There's a problem with hexagonal holes, bees try to lay eggs in them.

 

[ComUtoR]

Just spent the best part of the last hour looking into 'broaching' Hexagonal holes galore !

 

[AndyPJG]

Are there any toolmakers or similar on the forum who can explain this? I wanted a 12mm hole in some thin (~1mm) rigid plastic. I started with a 3mm drill then a 7mm drill but decided any bigger would be too aggressive on the thin material so I would open it up with my tapered hand reamer which has four spiral flutes. When it was nearly to size I took the reamer out to look at the hole, to my surprise it was pentagonal with radiused corners. Any ideas how that happened?

I read somewhere the trick to drilling out a pilot hole larger in thin metal is to place a small piece of folded cloth between the drill tip and the sheet metal.

See youtube old machinist trick