Spotting Drills?

Martin

I've always known them as slow spiral drills.

Available here (and many other places I'm sure).

http://www.drill-service.co.uk/Tools.asp?Tool=020240000000

The angle on the end is greater than 118 degrees, rather than less, looks like a stub drill meant for sheet metal to me.

Neil

Definitely slow spiral; intended for hard materials, as the cutting edges are stronger than standard helix drills, or for materials that tend to 'snatch', like brass. Presumably if it is self-centring it is 4-facet ground, although that is not clear from the picture.

Andrew

Talking of drills I was looking through the box of stuff that came with my lathe for a 12mm drill came across a Dormer drill still in it's red sleeve it was a 21/32 don't know why but opened it up and was surprised to find it contained a 3 flute drill never seen one before what would be it use /

H

Sounds like a core drill that's designed to follow a pilot hole and produce a nice true and size hole without the web snatching that would probably occur with a twin flute.

I remember buy some years ago for drilling brass.

It used to be possible to buy straight flute jobbers drills specifically for brass.

Never mind pedantry, if we're being mathematically correct a helix is simply a special case of a three dimensional spiral. There's nothing that says a spiral has to be two dimensional, or indeed limited to Euclidian geometry.

Andrew

I suppose that if you stretch a point a helix is a spiral on the surface of a cylinder.

Isn't there a version of a spiral that links the two 'poles' of a sphere?

Yes

Neil

Hi Gents

Surely a two dimentional spiral / helix is merely a zig-zag line, unless of course its dimentions where away from you then it could apear to be a stright line.

sorry to pedantic

A Spiral starts at a centre point and flows away from said point in a circular path with a progressively increasing radius on a flat plane so is two dimentional.

A helix starts at a point on a circumference and rotates around the circumference at the same radius, if it is a parallel helix, moving forward with a given pitch eg. a screw thread. It can also move out with a progressive radius as on a cone. With the cone, when viewed from the tip, you can see a spiral in two dimentions but you won't see the helix as the always have effectively some form of forward motion.

Also, just in case someone notes, a helix can also have a variable pitch but I don't know why you would want one.

Spiral staircases are so called because of how they look from the plan view where the tread appears to rotate out around a given centre point ie the column. which geometrically is actually a cylinder with a very very deep thread with an equally deep pitch wrapped around it therefore a helix.

Edited By Bodgit Fixit and Run on 31/01/2015 15:48:43