Twist drills, a study based on openSCAD

[homebrewed]

This is also going to depend on the "shape" of the flutes. (I know the flutes are nothing and the shape is what remains of the drill, but bear with me...)

I'm not sure from the pictures, but it "looks" like you've modeled those as round? If so, that'll make a difference. The final "shape" of the flute is the result of a round (let's say grinding wheel) plowing through the flute at some angle that may or may not be on centerline of the drill, and may or may not be parallel to the helix angle, while the drill is rotating on it's fixturing, giving some 'shape" that's not there, and a "radius like" grind on the outer circumference of the wheel that may be imparting it's own shape, or some angled cross section of that shape... It's a weird shape, and it's not uniform among all drills, although most "not specialized drills" are very similar. But universally, there will be "some" angle at which the side of the flute contains a straight line. Typically that will coincide with the drill point angle that the drill was born with

If one were to change the point angle of a given drill bit, it WILL have some "shape" to it. Typically not enough to notice unless you're looking for it, although sometimes it can be significant. I've never seen it make a "good" or "bad" drill bit by doing so, but that is a real thing you're stumbling on.

Incicentally... I think that's 85 percent of the trouble with a Drill Doctor. That's my weapon, I'm bought into the system since long before other options were affordable. :-( It indexes off of the flutes. The helix angle is not a given, although there's a more common one. Any other drill will throw off the drill doctor's indexing. The helix shape is not a given, which is not a deal breaker EXCEPT, that if the side of the flute where the cutting lip is were taken as a given (because it kind of is), the OPPOSITE side of the flute does not necessarily have any given shape or distance. That throws off the drill doctor's indexing. And some drill bits have a relief on most of the lands, (the outside diameter between the flutes), which again messes with the drill doctor's indexing. Which means you've got to teach yourself to recognize a bad grind, and know how to "eyeball" a fix by clocking ahead or behind of where the built in indexers landed.

I'm very curious, how does the Vevor indexing work? Does it (as I believe) index from the cutting lip? How doe the "dial" interact with that indexing? I'd love to know, but it's hard to cough up a new drill grinder when I have one that works perfectly well*, and yet I'm still more prone to hand grinding anyhow, when I'm home... Is that something that's "practical" to get pictures of, or is that a little peephole that's gonna be meanigless to me anyhow?

*The drill doctor does work perfectly well AFTER the learning curve. I'm 25 years in, and in the beginning, it was a steep learning curve, during which I learned to hand grind. If I needed a new grinder tomorrow, I'd probably get another drill doctor. If I needed a drill grinder 25 years ago, AND there was another affordable option?........ Not really sure, because now I've got a bias....

I thought about the possibility of the flutes being something other than cylindrical, but, in terms of ease of manufacturing, decided that cylindrical would likely be the preferred form. If I was unable to reproduce the shape of the drill lips with cylindrical helices I'd try something else; but the shape was close enough to satisfy my curiosity.

The Vevor indexes off the existing lip of the drill you're sharpening, not the flutes. But it doesn't seem to be well-designed (or I got one with some sort of manufacturing defect). It uses a triangle-shaped landing pad for the lip. According to the owner's manual the height of that landing pad is adjusted to accommodate the size of the drill. I'm guessing that the triangle shape is meant to provide the proper clearance for different drill diameters. But on mine any drill larger than about 5mm in diameter is progressively further "off" in terms of the lip alignment.

I examined the situation using a hand lens and I found that the larger drills were hitting the landing pad too close to the tip of the drill. Now, this could (and likely does) depend on the actual drill geometry. If so, the Vevor approach seems to be flawed to me.

The other troublesome thing about the alignment procedure is the stated requirement to adjust the height of the landing pad depending on the drill size. The lip SHOULD end up parallel to the flats on the collet holder body, regardless of the drill size -- so WHY does the landing pad height need to be adjusted at all??? One could argue that perhaps the web thickness varies with the drill size, but not near as much as what the adjustment procedure calls for. Plus, the web thickness doesn't appear to be standardized. So.....I'm still left with questions regarding the Vevor adjuster.

 

[m1kemex]

Drill bits have a lot of hidden details. Another thing I discovered is that the web of many drill bits is conical and not cylindrical:


Also, the outer diameter of a properly designed drill bit is also conical, with the greater diameter at the tip, to provide axial clearance.

A way to deal with the differential speed and clearance angles problem is to split the point. This removes material from the flute in the area in which clearance angles get so steep that the cutting action becomes negative.

This is a feature shared with center cutting end mills.

P. S. One could argue that, at that point, it's not really a single cutting tool, but two (each edge is "floating" since web thickness becomes zero).

 

[Jake M]

I thought about the possibility of the flutes being something other than cylindrical, but, in terms of ease of manufacturing, decided that cylindrical would likely be the preferred form. If I was unable to reproduce the shape of the drill lips with cylindrical helices I'd try something else; but the shape was close enough to satisfy my curiosity.

Gotcha. Plenty close enough for this issue.

The Vevor indexes off the existing lip of the drill you're sharpening, not the flutes. But it doesn't seem to be well-designed (or I got one with some sort of manufacturing defect). It uses a triangle-shaped landing pad for the lip. According to the owner's manual the height of that landing pad is adjusted to accommodate the size of the drill. I'm guessing that the triangle shape is meant to provide the proper clearance for different drill diameters. But on mine any drill larger than about 5mm in diameter is progressively further "off" in terms of the lip alignment.

Gotcha. Hmmmm...

I examined the situation using a hand lens and I found that the larger drills were hitting the landing pad too close to the tip of the drill. Now, this could (and likely does) depend on the actual drill geometry. If so, the Vevor approach seems to be flawed to me.

It makes sense that they'd have to do that, otherwise small drills would miss it completely.

The other troublesome thing about the alignment procedure is the stated requirement to adjust the height of the landing pad depending on the drill size. The lip SHOULD end up parallel to the flats on the collet holder body, regardless of the drill size -- so WHY does the landing pad height need to be adjusted at all???

This is starting to make sense to me. (For what it's worth, reminder/disclosure, I've never laid a hand on this sharpener). The landing pad touches the cutting lip, but that is not on center. The cutting lips (when viewed straight on from the tip end) are parallel, but offset from center, by half of the thickness of the web. The one you're indexing off is above center by a (for now) unknown amount (half of the effective web thickness) So it makes sense to me that there would need to be that adjustment.


I think you're using drills with a thicker web than Vevor anticipated. I think that makes sense as to why the reviews are so split between "it works' and "just line it up by eye". Vevor is banking on some common, or some average for the web thickness.

One could argue that perhaps the web thickness varies with the drill size, but not near as much as what the adjustment procedure calls for. Plus, the web thickness doesn't appear to be standardized. So.....I'm still left with questions regarding the Vevor adjuster.

So, if you had that pad set low (following their numbers, but with a larger than anticipated web thickness), then because the diameter at which the measurement is taken, the larger the drill bit, the more exaggerated the error is going to be when the drill bit "over rotates" during setup. It's going to follow a sine function. Sine, cosine, whatever way you do the math, the relationship won't be linear, but related to the angle that forms between the center point (fixed), the landing pad (fixed until you adjust it), and the point on the centerline that corresponds to the diameter at which the landing pad indexes, and the cutting lip. Have a look at m1kemex's first pictures, the first set, with an over exaggerated web,viewed straight on. So I wouldn't expect linear relationships at all...

And don't forget the disclaimer, I'm open to correction- I'm doing this on the spot. And I'm doing this without having a similar machine to look at to check my work.....