Parabolic drills, where is the parabala?

I have a few parabolic drills that work well and have read they are wonderful and better then the standard twist drill. I have looked and I can not find any explanation of what is parabolic about them. Can a regular twist drill be reground and made into a parabolic drill or is it something about the flute shape?
Are they harder to make or resharpen. If not why not only make those type.
Bill D

This look like yours?

Bill D said:

I have a few parabolic drills that work well and have read they are wonderful and better then the standard twist drill. I have looked and I can not find any explanation of what is parabolic about them. Can a regular twist drill be reground and made into a parabolic drill or is it something about the flute shape?
Are they harder to make or resharpen. If not why not only make those type.
Bill D

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Parabolic drills are for deep holes, they evacuate chips better than normal drills. The down side is they are a bit fragile.

Here's a excerpt from an article on advanced cutting tools.

PARABOLIC FLUTE DRILLS

These drills are designed with open fluting, heavy parallel web, and high helix (Figure 16). They usually have split points to reduce the thrust required by the thick web.

Figure 16 parabolic flute drill

Because of their heavy web construction, parabolic flute drills can be used under heavy feed conditions in most materials, in shallow holes, as well as deep holes. They are very popular for deep hole drilling because they work best under heavy feed conditions, with minimal reductions in feed required even in holes 12 or more diameters deep, and usually require one-third or less of the number of retractions required by conventionally fluted drills.

The whole article is available here:

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I don't know why the software changed the URL to a bunch of question marks, but it does work.

The article explains the differences between 27 different types of drills.

They are a time and tool saver when drilling deep holes.

thermite said:

NO flatted "split" tip", IOW, just a smooth curve. Same as we learnt to hand-grond, "back in the day".

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Nothing ventured, nothing gained ... sounds like the Spiropoint that Cincinnati drill grinders used to create ?

I was told that the parabola is the shape of the flute gullet if the drill was sectioned through its axis, this was to better carry waste up the drill, it seems a rational explanation though looking at one makes me think hyperbola as the curve is skewed but I’ve never sectioned a drill!, snapped a few though
Mark

I do agree, but snapping is a radial section, splitting right down the middle is an axial section, or at least that’s what I understood of what I was told, seems plausible
Mark

I’ve split one down the middle once, my jaw dropped, it must have been some heat treatment fault, I wish I kept it but the rep from the supplier took it mainly because he’d never seen anything like it, it was a spiral fracture that did breach about 2 1/2” up the 1.5” drill, done on an old Elliot 4e drill, amazing sight to be honest, mind the old Elliot court shear a 2” drill like a carrot, not something to mess with, I used to cringe at people holding work to drill, quite a few got injured when it came back around, surprising people don’t think of drills as dangerous machines, the big ones are ruthless
Mark

Bill D said:

Can a regular twist drill be reground and made into a parabolic drill or is it something about the flute shape?
Are they harder to make or resharpen. If not why not only make those type.
Bill D

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I worked at a screw machine shop making aircraft fittings in the mid 70s. We used to grind regular and fast twist jobber drills on the heel are on Gordon Heaton's drawing the full length of the drill. This sort of approximated the profile of a parabolic drill. It certainly made more room for the chips to get out and for the cutting oil to get in. Not sure why the boss did not buy parabolic drills back then as they were available under a number of different descriptions. EZ Torque or something like that was one.
In theory they should be more fragile as their is less material, in practice they can be fed harder and deeper since there is more room for the chips to get out. This is definitely true on screw machines and CNCs were you can flood the heck out of them and control the feed probably less so on manual machines.

thermite said:

Not quite like the mis-use of the term "billet", but you get the drift?

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I'm still confused why anybody cares about the mis-use of the word billet. I make Billet parts for the local Harley shop. 'cos that's what they want, that's what their customers want etc etc. They can order as many Billet parts as they want as long as long as they pay in $'s for their parts.

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As for parabolic drills, the first time I used one I looked at it for 2-3 seconds, wondered why it was called a parabolic drill, the reason wasn't obvious, haven't cared since as to why it's called a parabolic drill. All I care about is that it drills deep holes effectively.