Parabolic Drills

[northwest]

I was having a discussion today with @Tinbasherdan . He was commenting on my fetish for drill bits while I was trying to find a 7.6mm drill and into the conversation the subject of Parabolic Drills came up. It has come as a surprise to me to learn that not everybody is as fascinated with drills as I am!

I have been interested in deep hole drilling for some time now and I have quite a number of tools at my disposal in order to drill clean deep holes. I suppose the first thing is to clarify what I mean be "deep holes". Well in the scheme of things a deep hole isn't actually that deep and they are calculated as multiples of diameter a hole 4x diameter for example a 10mm diameter hole is fine up to 40mm so 4 times the diameter so 10 x 4 = 40mm A regular jobber drill is fine for this and actually even up to 8 times the diameter so 80mm deep using a 10mm diameter drill or 2" with a quarter inch drill. But 8X dia. is the absolute max (yes, I know, we have all done more but where exactly did the hole come out and what was the surface finish like? At 8x Dia and more you should really be using Parabolic Drills.

These drills have a much faster twist in the flutes and the web is 25 to 50% bigger than a conventional drill, it clears chips faster and leaves a nicer finish, after 20x diameter you will need to be casting spells and doing a few incantations as this depth is really a black art, some people are good at it and some people not so much. Luck and experience start to play their part.

 

[Erie Fred]

Now your really getting into it, good to see some enthusiasm.
These people are local:

Deep Hole Drilling - American Hollow Boring

The core of AHB’s business is deep hole drilling. Deep hole drilling is the common name for processes that produce holes deeper than five times diameter.

www.amhollow.com

gun drilling is used for many things, around here, to put cooling lines in plastic molds.
As you get deeper, go for coolant thru and rig up a pump & tank.

 

[Keith 66]

Although i am a rank amateur compared to you in the field of drilling holes I have an interest in it as should anyone who is into guns & rifles.
Some years ago our Wildfowling club had a clear out of its library & in there was a book, Barrels & Actions by Harold Hoffman, Published in 1991.
I bought it for a pound.
He appears to have been a largely self taught American gunsmith & wrote in a short & terse style with much info imparted with as few words as possible.
His book is excellent & goes heavily into deep hole drill design & machine set up. Boring deep 30 cal holes at high speeds with high oil pressures to clear the chips gives an insight into how it was done in factories in the old days. Typically single lip D bit gun drills were run at high speeds of several thousand rpm with up to 500 psi oil pressure on the cutting oil coolant, each machine being served by its own 45 gallon drum of coolant, A typical 30 cal rifle barrel blank 30" long being bored end to end in just 4 minutes or less.
It goes into reaming, straightening & rifling including grinding of carbide buttons & the hydraulics required to pull them through a tube.
all highly fascinating!

 

[pressbrake1]

I used to build gun drilling machines for drilling large industrial engine conrods for the oil feed . Simple plc logic control
High pressure coolant which was very similar to honing machine oil

 

[hotponyshoes]

I did some 3mm holes through 75mm 316 stainless recently.
Fobco Star and Lube manually,
Worked but took a while...

 

[northwest]

I did some 3mm holes through 75mm 316 stainless recently.
Fobco Star and Lube manually,
Worked but took a while...

I can imagine that had a high pucker factor. All too easy to snap drills on jobs like this, if you get lucky there is enough drill sticking out you can manually remove it, if you are unlucky you need the spark eroder.

 

[not done it yet]

If I want a deep, precise hole I use a milling cutter.

Slot drills can be used for plunge milling but I prefer a pilot hole followed by a standard milling cutter. Milling cutters are so much more rigid than twist drills.

The pilot does not need to be perfectly straight but serves as clearance for the centre of standard milling cutter, which does not cut.

Depending on the diameter of the required hole, one can use either a solid carbide or cutter holder with replaceable inserts.

Not the fastest route to a simple hole but I don’t need to rush with anything where precision is required.

 

[Rrobson]

They are a nice drill to behold.
I have a couple of very shiny extra long guhring cobalt split point parabolic drills with the 135 degree grind and polished flutes for that extra satisfying use…
They only come out for special occasions, but are are superb in 300 and 400 series stainless

 

[dieselsamurai]

If I want a deep, precise hole I use a milling cutter.

Slot drills can be used for plunge milling but I prefer a pilot hole followed by a standard milling cutter. Milling cutters are so much more rigid than twist drills.

The pilot does not need to be perfectly straight but serves as clearance for the centre of standard milling cutter, which does not cut.

Depending on the diameter of the required hole, one can use either a solid carbide or cutter holder with replaceable inserts.

Not the fastest route to a simple hole but I don’t need to rush with anything where precision is required.

I do this too

 

[graffian]

I like a nice drill.
Picked an 8.73mm Kennametal TF drill out of the
carbide bin in a local scrap yard. Drills fantastic holes for 11/32” dowels in aluminium, plunge that pretty thing in as fast as possible and get a perfect hole every time. chipped one cutting edge, still drills nice holes!

 

[pressbrake1]

If I want a deep, precise hole I use a milling cutter.

Slot drills can be used for plunge milling but I prefer a pilot hole followed by a standard milling cutter. Milling cutters are so much more rigid than twist drills.

The pilot does not need to be perfectly straight but serves as clearance for the centre of standard milling cutter, which does not cut.

Depending on the diameter of the required hole, one can use either a solid carbide or cutter holder with replaceable inserts.

Not the fastest route to a simple hole but I don’t need to rush with anything where precision is required.

I don’t think your perception of deep hole drilling matches the ops

 

[Johnnybravo]

14mm diameter holes 1100mm deep through a carbon steel manifold for a plastic injection moulding tool is the deepest I’ve been. Did it eight times in the same manifold to prove it wasn’t a fluke….Done with no breakages…

 

[Rrobson]

A wonder you managed to keep the hole straight!

 

[Johnnybravo]

A wonder you managed to keep the hole straight!

As long as your drill is ground correctly, they generally don’t wander. When one cutting edge is 1mm longer than the other and the angles are not the same, you can drill around corners….

 

[northwest]

I have just drilled a hole, okay piloted to a degree, 1.2 metres deep, 7.8mm dia.

 

[northwest]

Poo! got it wrong, 7.6mm diameter.

 

[Maker]

Deep holes in nice isotropic steel is easy peasy, you wanna try 24x diameter/depth ratio in compression moulded GRP.

The glass blunts an HSS drill in no time (carbide doesn't exist off the shelf with a long enough flute), you can't use coolant/lube because it will contaminate the cut fibres, and there's always a chance of a fibre rich region pushing the drill wonky anyway.

 

[Erie Fred]

How about coolant thru and use compressed air or dry nitrogen ?

 

[Keith 66]

Drilling deep holes in GRP isnt easy, one problem is friction on parralel drill shanks causes overheating & outgassing of entrapped styrene. It can ignite in some circumstances.
It is better to design holes in with cores that can be knocked or drilled out easily!

 

[Erie Fred]

Entrapped styrene ?