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Thread: "4-facet" vs. "split-point" drill grind definition

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    Default "4-facet" vs. "split-point" drill grind definition

    This is in reference to the current HSM cover article. It is titled a "4-facet" drill grinder. I have read it through and would like some sort of confirmation my thinking is correct: the two terms are synonymous, yes?

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    I have not read the article, but I will try to describe the diff. between the two point styles.
    A 4 facet drill point has a flat ground primary cutting edge and a flat ground secondary edge. The primary and secondary edges are parallel to the flute face of the point. The secondary will be behind the primary edge and will bring the back side of the primary releif to the center of the drill point.

    A split point will have an additional grind that will be ahead of the cutting lip of the drill point and at an angle less than that of the chisle edge. The split point drill point is used to reduce the dead spot at the center of a drill to reduce the hp needed to force the tip into the material. So you would basically would have two cutting edges on each lip of the drill.

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    "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

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    It is possible to have a 4 facet drill with a split point.

    The drill point below has 4 facets, secondary point angles (SPA's) and a split point. The split is a variation on web thinning that causes the cutting lip to extend almost to the drill center. Especially on drills with thick webs this reduces the thrust required for drilling.

    You can never be too thin or have too many facets

    John


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    I must admit, when getting into the nuances of cutting geometry with drills, I am pretty green. I find the article linked to explains the benefits of 4-point drill grinding much the same as I have understood the benefits of split-point drills. For example, "...it produces a self-centering point, eliminating center punching and pilot holes. It won't walk". Can they be considered, then, as two means to the same end? What are the practical differences?
    [EDIT] Re-reading again.. So maybe the split-point affects horsepower needed for the chisel edge to advance into the work. The four-facet affects the cutting action of the flutes. As I understand it, the chisel edge does not actually "cut". It is more of a "plow" Am I on the right track here?
    Last edited by Arthur.Marks; 01-09-2012 at 11:18 PM.

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    The how's and why's of what drill configuration is needed depends on the drill material and the job requirements, the capacity of the drilling machine as well as the means (off-hand included) of sharping the drill/s.





    There are not any of the various drill sharpening configurations that cannot be done by hand (ie "off-hand") - it just takes practice - same as sharpening lathe or fly-cutter tools etc.

    A final "lick" to the cutting edges with ahnd-held diamond hone improves edge sharpness and longevity quite a bit.

    Sometimes a relatively minor drill re-sharpening only needs another lick with the hand-held hone.

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    Quote Originally Posted by Arthur.Marks
    Can they be considered, then, as two means to the same end? What are the practical differences?
    I suppose they are the same means to an end; holes! But they're not the same. While a split point can have four facets, a four faceted twist drill does not have to a split point. Split points are a little trickier to grind (imo), and also are freer cutting. A four faceted twist drill otoh still has a chisel and the drill is more likely to wander without a centre punch mark. btw, still do centre punch if using in the drill press, if not to avoid wander then to pull the drill start to where you want it.

    to compare, here's a commercial split point and facet drills i ground. To distinguish, the spit point has the secondary clearance ground on two surfaces - both where you'd expect it, and second perpendicular surface that extents the flute to the point - that's the spitting of the point. The faceted ones do not split the point, the and essentially is the same as the traditional conical grind in performance but let's a twist drill be done on a T&C grinder....which means they drill very accurately because the lips are are dead on symmetrical



    Last edited by Mcgyver; 01-10-2012 at 08:44 AM.
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    Quote Originally Posted by Arthur.Marks
    I must admit, when getting into the nuances of cutting geometry with drills, I am pretty green. I find the article linked to explains the benefits of 4-point drill grinding much the same as I have understood the benefits of split-point drills. For example, "...it produces a self-centering point, eliminating center punching and pilot holes. It won't walk". Can they be considered, then, as two means to the same end? What are the practical differences?
    [EDIT] Re-reading again.. So maybe the split-point affects horsepower needed for the chisel edge to advance into the work. The four-facet affects the cutting action of the flutes. As I understand it, the chisel edge does not actually "cut". It is more of a "plow" Am I on the right track here?
    Both 4 facet and split point drills have much reduced tendency to walk compared to drills with a conical grind with a chisel point. The chisel is flat so if the end of that flat touches as you start drilling, the drill can pivot about that end, forcing the drill off the desired position. And this can happen even if the desired position has a center punch.

    4 facet and split point drills have a point rather than a flat chisel so given a center punch mark they typically locate into it. Even without a punch mark the narrow point has little tendency to walk because the initial contact is at the center rather than slightly off center as with the chisel point.

    4 facet points have negative rake in the web area, similar to chisel points, so this area extrudes the material rather than cutting it. That is, the material ahead of the web is "plowed" outward and then cut by the drill lip. However the negative rake is slightly steeper on a 4 facet drill so they extrude the material in that area slightly better than conical point drills. A good fraction of the thrust needed for drilling is used to extrude this center material.

    The point can be split on 4 facet or conical drills. This is a variation of web thinning where the web just ahead of the cutting lip is ground away, leaving an edge with zero or positive rake between the lip and the center. The resulting drill has a central point, reducing the tendency to walk at start of drilling. Further, the web area now cuts rather than extrudes material so thrust required to drill is reduced, especially obvious for drills with thick webs. And power required for drilling is reduced because cutting is much easier than extruding material.

    It's easy to verify some of this by comparing the divot produced in mild steel by a 3/8" or larger chisel point drill to the divot from a similar size split point drill. The chisel point divot will have a fairly large flat area at the center while the split point's central flat is tiny in comparison. Often, the material extruded by the chisel point appears smeared in the central area.

    If you take a freshly sharpened drill and aggressively drill through 1/2" of mild steel, examination of the drill point's web area will generally show some blue in the negative rake area, evidence of how hard the drill works to extrude the central material.

    John

    Picture repeated for easy reference. The split shown has a slight positive rake.

    Last edited by GadgetBuilder; 01-10-2012 at 09:47 AM.

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    Gadget builder, I apologize for not being more astute. I decided to look through the article once again last night. First- it is your article! A very interesting project; Thank you for authoring it. Secondly, the actual title inside the magazine mentions an optional split-pointing tool. I missed that at first because it was not touched upon in this issue (part one). I get the impression that the image you show above will be the ultimate functionality of the grinder. I look forward to reading the following installment(s). Considering your reply above, I was just struck at how a 4-facet, split point grind might approach something I know of as an "Oliver self-centering point". While those grinding machines use complex, eccentric grinding profile mechanisms, the one in HSM uses various facets. The analogy I am proposing is akin to "analog" vs. "digital". Feel free to refute all this speculation on my part . As mentioned, drill point geometries have always left me off-kilter. I'm still trying to wrap my mind around what actually goes on. I have trouble keeping track of all the various aspects of the point and how they interact in the final grind.
    Last edited by Arthur.Marks; 01-10-2012 at 10:28 AM.

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    Arthur,

    The Oliver point you linked to is closest to a plain 4 facet drill point, i.e. the Oliver will extrude the center material with its rounded chisel. A split could be added to the Oliver point, likely to good effect.

    Sharpening is mostly a matter of holding the work solidly and moving it on a predictable and accurate path past the grinding wheel, then indexing and repeating for the other side. Curved paths, per the Oliver, are more complicated to analyze and produce in a home shop.

    Faceted drill points are easy to produce in the home shop because the work path for sharpening is a simple straight line. A two facet drill can work well for many things. Performance of two facet drill points can be improved by adding secondary relief facets. This can be further improved for some tasks by adding SPA's. Split points are essentially another facet in that they're produced with a straight line motion. Each facet added may improve some aspect of drill performance, depending on the task involved.

    With this sharpener there's a little judgement needed from the operator to get the 4 facets to meet precisely at the center - generally an additional quick pass on the secondary facets or the primary facets, depending on alignment and wheel wear. As illustrated in my video showing the sharpener in use.

    I think of point splitting as an art form because it's open to more variation than sharpening -- although most any reasonable looking variation will work. The point splitter part of my sharpener allows varying several parameters affecting the split so each user may settle on a slightly different looking split. The key is that the jig makes the path predictable once you set the parameters.

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