Fixing Drill Bit Misalignment in the DBS-22 Drill Bit Holder

[RickKrung]

While I was working on grinding secondary facets on drill points recently (https://forum.tormek.com/index.php?topic=3601.0), I ran into a problem getting the large diameter drill bits (3/4") to align properly in the Drill Bit Holder. I had held somewhat similar drills (1/2", 19/32" and 5/8" in the holder and successfully sharpened them without misalignment issues.  I wasn't sure at first what was happening, but on looking closer, it was possible to see the misalignment.



Placing the misaligned drill on the jig platform and holding it near the grindstone made it more obvious that uneven grinding would occur on the facets.





Although the drill bit holder has several fingers on each side of the drill that should hold the bits evenly and straight, that was not happening for this drill bit.  The best I could figure out is that there weren't enough contact points of the flutes with the fingers. 



I could not get it aligned such that it would remain in place during grinding, so I devised a method of holding it, essentially a long, thin-walled spring collet.  It consisted of a length of stainless steel tubing, 3/4" ID and 7/8" OD, which I put in a horizontal metal cutting bandsaw and cut slits along its length.  One slit went all the way through one side of the tubing.  Several more went most of the way through, but not all the way. 







This allowed the tubing to flex enough to securely hold the drill in place by gripping the drill bit flutes all the way around each flute over the full length at all four locations at which the drill bit holder made contact.  Viewed on end. 



I was fortunate to have the appropriate size tubing on hand to make the slit spring collet.  Most of you will likely not have the right material.  I have not tried it, but I think it is likely that PVC tubing could be used instead.  It probably would require drilling out some pipe that isn't the exact right size and then cutting the slits.  This could work for several drill sizes close to the ID of the pipe.  The OD doesn't have to be exact, as the pipe could expand a fair bit, with a wide gap at the full depth slit, as long as that gap didn't fall right on one of the four clamping surfaces of the holder. 

It would be nice to understand better why this misalignment occurred and be able to correct it without resorting to additional gadgets or tooling.  In lieu of that, I found a fix that worked for my situation.

Rick

[Grizz]

nice simple fix for a complicated problem ! good job !

[Ken S]

Well done, Rick.
Ken

[aquataur]

Rick,

I ran into the same problem recently. It was a brand new 12mm drill (close to a half inch drill), and I just sharpened it without checking.
When I changed to the second side I noticed that I would not reach the stone at all, which is a clear sign of misalignment. I thought I just turn the knob a bit to compensate, but this resulted not only in unequal lip lengths despite all fumbling with the caliper, but also in wrong point angles.

I read here that drills are made for a specific point angle, in so far as the helix angle correlates to the point angle. Grinding a different angle will invariably make the cutting lips convex or concave - which is what I experienced. The resulting angle was considerably smaller than the desired 118°.

The drill was unusuable this way - nothing but flutter.

I remembered your article:

The best I could figure out is that there weren't enough contact points of the flutes with the fingers. 

but contrary to your assumption I believe that the fingers of the V-vise start to approach in size to the valleys between the flute contact points. In other words, they become too small to make a secure contact on all of them. Some of them grip into the nothing. This may be subject to manufacturer´s peculiarities but is prone to happen with increasing diameter and/or increasing flute angle.

This is my explanation. That said, it does not matter.
Your collet solution is brilliant, but restricts you to a single diameter, plus most of us won´t be able to make one.

Since the drill only contacts the V-vise´s sides on the flat parts, you may as well use something flat - like strips of scrap PCB epoxy board. I chopped down four strips á 10mm, and with some slightly broader strips, maybe three different widths you cover all sizes up to the max. For small drills the flute gap is small compared to the vise´s fingers, so this is not needed.
This way you may not be able to sharpen the maximum size drills as per the jig´s spec´s, but then a drill that size will a) not often be seen with hobbyists, b) overload typical hobbyist´s drill press devices anyway besides being dangerous, and c) may as well be sharpened with something like the simple jig forum member Jan devised.

(For those calibers we might rather dwell on some web thinning, but this for another time.)

Since the poor drill was mutilated beyond recognition I followed the often uttered suggestion to straighten the bit out on a bench grinder beforehand to do the motherload of work.

This suggestion has to be taken with a grain of salt, because it comes from people who do a lot of tool sharpening. I have never seen one of them not using a wheel with white aluminium oxide which is said to run much cooler than anything else. I have tried this action on my cheap bench grinder equipped with a normal corundum wheel and the drill´s point turned blue immediately.

So I continued with the wet grinder. Needless to say, this time the drill bit sharpens flawlessly.
(I´ve also put SPA´s on it, but again, more on this shortly)

[RichColvin]

I´ve also put SPA´s on it, but again, more on this shortly

Please elaborate on the SPAs ... would love to hear more about how you added them.

[RickKrung]

Nice solution.  You are quite right, most would not be able to make the sleeve collet type device and it is single-sized.  Flat inserts, in whatever form, should be quite doable by most. 

The article linked on drill point angle leads to a series of articles on drill design and use.  Should be very informative reading. 

WHAT are SPSs?

Rick

[aquataur]

Here


(Source: https://gadgetbuilder.com/DrillSharp.html)

[RickKrung]

Of course. Know what they are and use them on some of my drills, just didn't recognize the acronym.

Rick

[aquataur]

(...) just didn't recognize the acronym.

It was you who was the first to use this acronym during your mission 

[RickKrung]

Yes, I'm sure that is true, but I haven't been paying attention on this topic for a long time and it is one of the factoids that didn't survive...    

Rick

[tgbto]

The best I could figure out is that there weren't enough contact points of the flutes with the fingers. 
<snip>
It would be nice to understand better why this misalignment occurred and be able to correct it without resorting to additional gadgets or tooling.

A possible explanation might also be that the spatial frequency of the flutes along the z axis of the drill is close to that of the grooves on the drill bit. So each finger could fall on a sightly recessed part of the bit on only one side, and hold it askew.

[aquataur]

This is exactly what I was driving at. At some pitch dependent on the drill type will grip into the nothing. This will become increasingly probable with drill size.
Anything rigid like a strip of metal or, as I used it, a strip of PCB board, will simulate a infinite contact area. The pins only contact throughout their centerline axially.

Using a strip of PCB board turns out to be a happy coincidence, because even thin ones are pretty rigid because of their glass-fibre reinforcement. They are always straight, don´t bend, don´t produce burrs on cutting edges and are easily cut.