D bit Geometry Question

[phil burman]

What is the theory behind D bit design. Specifically why do some of the D bits in the photo at the bottom of this web page have such long half sections. Will this not destroy or at least minimize the self-aligning property that is so important? Or am I missing something?
What is the theory behind D bit design. Specifically why do some of the D bits in this web page:

http://easyweb.easynet.co.uk/~chrish/dbit1.jpg

have such long half (D) sections. Will this not destroy or at least minimize the self-aligning property that is so important? Or am I missing something?

Regards
Phil Burman

[aboard_epsilon]

SAYS IT ALL HERE PHIL...and more

http://www.antaresinc.net/EngravingFAQ.htm

all the best.mark

[phil burman]

Sorry Mark I was not clear enough in my question. I'm talking about D bits as in reaming deep holes. In my case that's 25 mm diameter by 110 mm deep sing a 10 x 18" center lathe.

Regards
Phil

[phil burman]

Sorry Mark I was not clear enough in my question. I'm talking about D bits as in reaming deep holes. In my case that's 25 mm diameter by 110 mm deep sing a 10 x 18" center lathe.

Regards
Phil

[aboard_epsilon]

ok right .thats a question for the gunsmiths to answer not engraving people like i first thought.
all the best.mark

[Timleech]

I need to do a batch of holes precisely 1/8" dia, 1 1/4" deep with a 40 deg included angle at the end. I'm planning to drill slighly undersize with an ordinary twist bit & then finish with a D-bit ground to the 40 deg angle. Any comments on how successful this might be? Or on better ways to do it?

Thanks
Tim

[John Lawson]

I have used many D bits for finishing chambers in gunsmithing operations. The reamers used are designed to finish the chambers only, and all have a slight to very slight amount of taper, so the reamer is cutting full length from the first to the last cut.
The only "secret" of a successful D bit is an extremely well fitting and properly lubricated guide bushing ahead of the cutting edge.
It is very important to use a tool steel that can be kept from warping during hardening to make an effective and accurate reamer.
I heat with an acetylene torch and plunge out, guide end down in a tall can of salt water having a half inch of motor oil on top to "soften" the quench. Then draw to specification and finish sharpen with a fine stone, finishing to razor sharpness with an Arkansas black stone.
These D bits always cut better if there is a slight relief ahead of the cutting edge, stoned on after sharpening. This takes patience, a good magnifier and a good sharp, well lubed 1/2" square stone.
The Brits, especially "Meticulous George" seem to make much more of the D bit than just the bare essentials. It is just a flat reamer with a sidewall support.
When properly used by hand or when the workpiece is driven by the lathe there is no chatter, as can occur when multi-fluted reamers are used.
When reaming, make haste slowly and all will go well. Retract, clean and re-lube often. And, keep an eye out for over-reaming, which is fatally easy to do with such a sharp reamer.

[Norman Atkinson]

Phil- greetings!

Nice job for a Quorn- ahem!


The best(?) reference is Ian Bradley's The Grinding Machine. A MAP publication.

The bit is ground to a flat of D/2+.005"
The front clearance is given as 10 degrees and the front relief is the same.

As originally mentioned- regular cleaning is necessary to avoid wandering.

With such a tool, I have bored African Blackwood to a depth of up to 13" and only 7/64ths. In case, you ask- these were for Northumbrian bagpipes. A bit like a Hardanger fiddle, a musical instrument with limited appeal.

The bits were 13" silver steel rods- and hardened. I think the US name is drill rod.

Does this help?

Tim,
This should be a doddle- compared to Phil's.

Cheers

Norman

[Mcgyver]

Norman, your description sounds the same as those of home shop classics like ME and the amateurs workshop, etc, and my preparation and use is in accordance. try as I might though, I’ve never had huge success with D bits. The problem seems to be the bit getting stuck or binding in the hole. The close tolerance and large contact area between bit and hole is a problem. Even the slightest heat build up (expansion) in the bit and things get jammed up. obviously the binding isn't an issue in wood as its got some give to it....but still i would like to master it!

I've tied all the tricks and can't detect any warping of the bit, I've also relieved some of the shaft after the first inch or so, although too much of that and you might as well drill




[This message has been edited by Mcgyver (edited 08-17-2005).]

[Norman Atkinson]

Mcgyver,

I can agree wholeheartedly with the heat problem which you mention.

The blackwood and lignum vitae both charred and the rod got - very(?) hot making it difficult to withdraw.

Two answers are worth noting.

The first is to waist the rod- after the halfing. Difficult to explain in words- but fingers crossed.

The second is to work very slowly- and poke the rod in and out- sort of pecking.
When the old boys made bag pipes, they held the loose end in a fixed steady- and removed the poppet from the tailstock- substituting a piece of drilled round to guide the bit. That end of the bit was secured with a small tap holder.

I didn't want to do a bit on pipe making- but perhaps the principles can be applied elsewhere.

Cheers

Norman

[phil burman]

Hello Norman,

You answer partly helps but I'm trying to understand as per my original question as follows:

"Specifically why do some of the D bits .... have such long half sections."

Best Regards
Phil

[darryl]

I've made and used D bits many times. I don't make a very long D simply because it takes time to grind it, and it's the leading edge that does the cutting anyway. I do have to withdraw the bit often because there isn't room for a large chipload on the flat portion of the bit. My first impression of why the D might be made longer is to leave more chiproom. Secondly I think it may lessen the tendency to jam, and thirdly it may be done in the case where the reamed hole in not deeper than the D part of the bit. In that case it's conceivable that the reaming is done in one go, without removing and reinserting the D bit to finish the hole.
As far as the self-aligning property of the bit, the half round part of the bit is forced towards the hole by the cutting pressure, so even if the D were very short, only that 'back' side of the bit would be in contact with the hole anyway. I don't think any control would be lost by using a long D instead of a short one. My opinions only. It could well be that a long D would skim a little more material out of the hole so that the remaining shank wouldn't seize in the hole.

[This message has been edited by darryl (edited 08-18-2005).]

[Norman Atkinson]

Hi Phil!

I think that the best answer is to suggest that you try a dummy run on something which doesn't matter.

I think that you will discover just how little swarf( material) which can be accomodated in the little flat of the D bit.
As you will note, I sorta said this earlier.
It soon gets bunged up- as the art mistress said to the gardener!

Moving away- slightly, a comparison with a twist drill explains that apart from anything else, the room for swarf in the spiral flute is considerably greater- as it has somewhere to go. With a D bit, it simply hasn't.

Could I flog the old nag a bit further?
Traditional drills were simply flat pieces of metal, hammered from the round to a flat on each side. Then two cutting edges were formed. The swarf went down the flats.
Here, moving onto a different level is still the classic clock and watchmakers' drills.

Whilst continuing to Philibuster- wicked me,I wondered whether you had considered making or sharpening conventional twist drills on your Quorn. Yep, this guy has made one. No, slouch is our Phil.
If you go along the Four Facet system twist drills don't need a centre drill to start.
It's all in Chaddock- folks.
As a tip- I have a teeny three jaw scroll chuck mounted on a 1" arbour to hold the drills.

I dread talking down to people which old age
gives. Hopefully, Phil(and Tim), this diatribe will have exercised the grey matter.

Cheers

Norman

[Norman Atkinson]

I am sorry Darryl!
Obviously, you beat me to pressing the send button.

The expert agrees with this old fart!

Norman

[Swarf&Sparks]

It has crossed my mind, but I've yet to try. What about a single straight (ish) flute? What I mean is something like a silver steel rod with a quarter or less, cut away. Seems like the remaing 3/4 circ would keep it fairly true and a longer single straight flute would give more chip room. Naturally, I'm talking about final cut operation.
Anyone tried this direction.
Rgds, Lin