D Bit Confusion

Allan

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G'day.
I am wanting to make a largish D Bit- .567"- but am baffled by the geometry. There are many versions:

1. some versions- looking from the top- have the cutting edge cut as a single angle cut across the nose and then relieved underneath to allow it cut.
2. some versions are shaped as a hemisphere at the nose and then relieved underneath and the trailing edge relieved a bit so as to not drag and prevent the bit from advancing.
3. Some versions have multiple angles on the nose and usually have two different angles across the cutting edge. Usually the apex is not at the centre line of the bit. It is usually off set so the shorter and steeper angle is on the cutting edge and the the shallower but longer edge is on trailing position.

I hope that makes sense.

This last one would seem to me to leave portion of the metal not being cut very well. It would seem to have the apex of the two angles cutting a "valley" off centre when looking from the top. Why would one want to have a bit cut this way? What function would be served by this design. It appears that the big bucks gun drills are made this way. It would seem to me to put weird stresses on the bit that would make it tend to wander. ButwaddoIknow?

I have only made one a few years ago to use in wood (with a single angle across the cutting face) and it wandered something terrible. I would like to NOT repeat that in metal.
 

rgray

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Kinda funny how that works. You want a straight accurate hole and you think you would need the stiffest hardest drill bit ever, but a gun drill is very flexible.
It works by being self guided. Can't start it's own hole but once in a hole it drills very straight.
Copying that geometry for your bit would be a good idea, but most just do the half bit style as it's easier to make.
Depends on what your using it for also. Drilling new hole? enlarging existing hole?
Put your #1 and #3 together and you have a winner. Start the hole with something else so your Gun drill is in a hole when in use.
 

Allan

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Thanks, rgray. I am enlarging an existing hole. Trying to make a sizing die for my muzzle loader out if a grade 8 bolt. I drilled it and then tried to bore it to size but a boring bar small enough to fit the hole is quite flexible. The finish is rough. So I want to use the D bit to enlarge it the last .005 so it will be .001 undersize and then lap it.
 

rgray

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Not sure a grade 8 bolt will be hard enough with out surface hardening it. I have made a few from O-1 steel and even that after hardening is not great. With the small amount your cutting it may work just fine.
Here's a couple of pics of one of my follow/hole D bits. Underside shows shank relief that isn't obvious in top pic.
It's made from O-1 and hardened.


20191127_085645.jpg 20191127_085655.jpg
 

pontiac428

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I do route #1 with mine. It is a good design for chasing holes because it reams as it drills, giving you a very straight result.
 

Allan

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So, rgray, it looks like you have a pilot on the nose to guide it? As for hardness I would think that it would be hard enough for pure lead. If it wears out after 500 bullets I will have to use O-1 for the next one ; )

Thanks pontiac. That was how I made the one for drilling a 3" long hole in end grain wood that didn't do well. It was 1/4" off center. But I'm sure the wood grain does weird things. I shall give it a try again. I'm thinking of using a 90 degree groove down the length to make for a stiffer bit. More like a gun drill.

I'm sure it would be easier ( and probably cheaper) to just buy one but I look at it as paying tuition. Thanks for being my instructors.
 

Latinrascalrg1

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Im pretty sure the purpose for the groove running the length of the bit has everything to do with clearing chips and keeping the part cool and has nothing to do with stiffness......but I've been wrong before!
 

rgray

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I would think that it would be hard enough for pure lead.
Wasn't familiar with it's use. defiantly will cut lead. I'm gonna vote no on the V groove adding stiffness. Gun drill shafts are very flexible and they drill straight so stiffness is not really required.
 

Latinrascalrg1

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Agreed. That is its purpose but it would make it stiffer as well, I'm sure. Two birds. One stone.
I could see that being possible, infact maybe even likely to be the case if the groove was forged into the rod but Not if the plan was to cut/grind/mill away the material! Please don't take my curiosity as me stating this as fact or confrontational in any way shape or form...its simply for my knowledge base and well maybe those whom may also be interested. Although I Don't see how cutting away the material would increase anything besides deflection away from the cut I Am Open to learning the Physics/science that supports your belief on the matter if you would be so kind as to share that knowledge with us please? Maybe even link a website with the info I could do the research myself....ive looked but my Google-fu skills are lacking and i dont even know what to type to look for in order to begin the search!
 

Allan

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In stating that the groove would make it stiffer I mean that it would be stiffer in comparison to just milling a flat across the end of the bit which is what most shop made ones are like. I would imagine it would add some stiffness where the cutting takes place and would have tendency to guide it more accurately as well. More material can't make it less rigid. There must be a reason that guns drills are made that way. This is only a guess on my part. But it couldn't make it worse at any rate.
 

mmcmdl

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I do route #1 with mine. It is a good design for chasing holes because it reams as it drills, giving you a very straight result.
Is this what ou consider a dreamer ? I have a lot of these down the basement . Drills and reamers combined .
 

Latinrascalrg1

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In stating that the groove would make it stiffer I mean that it would be stiffer in comparison to just milling a flat across the end of the bit which is what most shop made ones are like. I would imagine it would add some stiffness where the cutting takes place and would have tendency to guide it more accurately as well. More material can't make it less rigid. There must be a reason that guns drills are made that way. This is only a guess on my part. But it couldn't make it worse at any rate.
I guess I just Dont Understand what it is you are trying to describe, Not the first time thats happed to me!!! Thanks for posting that link......
I actually quickly read through that site before you posted it but I fail to see any information that supports your theory, oh well, not a big deal for me at the moment but will definitely look more into this before I proceed down that path.
Thanks anyway.
 

Allan

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pontiac, that looks nice. I guess you could call it that. As in most things of old people call them by different names. My understanding is that there are D Drills and then there are D Reamers. The former is for making a hole where there was not one before and the latter is for enlarging an existing hole. There seems to be lots of overlap in terms and uses. "Dreamer". I like that.

Can you fill us in on angles and tell us how well it works?
 

Allan

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pontiac, in looking at your example it looks like the apex of the angles is in the centre of the bit. Is that correct?
 

pontiac428

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Yes, it's centered. This one was meant to work as a countersink, but it drills too. I wouldn't change the tip location, but for a dedicated drill I would add 5 degrees of back relief and grind it further up the shank for better chip exits.

Edit: The point can be any angle. The back cut can be 0 or 5 degrees, but I haven't tested enough to say one is better. The facet covers 15 degrees because that's the step my indexing workhead is divided in.
 

Latinrascalrg1

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Well if you mill half of a bit away it will not be as rigid as only milling 1/4 of it away. Lets see if this posts:
Ahh i think I see where the confusion is coming into play! You seem to be speaking of the cutting portion of the bit itself where I thought you where speaking of the "single flute" yhat runs the entire length of some of that style drill bit.
 

Allan

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but for a dedicated drill I would add 5 degrees of back relief and grind it further up the shank for better chip exits.
-----------
Would you be speaking of adding a rake angle to the top of the side edge parallel to the length the rod? (everyone has their own terms and it gets confusing)
 

mmcmdl

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Can you fill us in on angles and tell us how well it works?
I will post some pics of my dreamers tomorrow . It is a combined drill and reamer .
 

Allan

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OK. so today I took delivery of thin book that answered my initial question- why the funky geometry on D Drills in general and gun drills in specific. The book is one of the old Lindsay publication called "Making Rifle Barrels". IT is a reprint of an old article showing how the Enfield Armoury made barrels for the No1 Mk 3 in 1916.

One sub article is on gun drills and how they are made and function. It is stated that the reason the front cutting edge is ground off centre is so that it forms a circular valley and ridge to the inside next to the centre line of the bore. This gives the bit added support to keep it drilling straight. It also says that those barrels were drilled at 2000 RPM!!! With a .003" feed per revolution!!! Yikes! I think I shall slow mine down a bit given that I don't have 500 PSI oil flushing the chips out.

I looked at the bit I had made previously that didn't cut true and I see that I had the geometry out of whack, It is now in whack but I don't have there pressing need to drill a long 3/8" hole. If I ever get caught up I may drill one just for funzies.

It also showed that the D Bit they used was a short cutting end with all the geometry and clearances and it was soldered onto a tube that supplied the oil. The business end also had a small oil supply hole drilled into it before being soldered on. So it takes a gun drill to make a gun drill.

P.S. The book was purchased from Your Old Time Bookstore along with a number of others. Great prices on these old gems.
 
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