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Musings on Four-Facet grinds, split points and SPA´s

Started by aquataur, August 29, 2023, 10:56:03 AM

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Most hobbyists like me (until recently) don´t view a drill (drill bit as the American call it) as something special. But this is not the case. There is a lot of brainpower behind it.
After looking into this matter deeply, I no longer view drilling as a simple task, but rather as an art form. To be able to quickly apply the optimum grind to a given task is certainly the supreme discipline, but this assumes a firm understanding of the subject. So even the latter alone is empowering.

I write all this down in the hope that it may speed up somebody´s learning curve and that It may serve as a thought provoking impulse for further investigation. 
The following reflects the apex of my insights and investigations on the subject up till now and is by no means claimed to be universally valid or true or free of errors.


Grinding  Four-facet (hereafter: 4F) drill points has the undeniable benefit of being within the realm of the home sharpener since the introduction of Tormek´s drill sharpening jig;  and while this grind may alleviate some problems, it is not the end of all those problems, although the impression is fostered.

With 4F the chisel, the non-productive area at the center where the web resides is reduced by half, but it still exists and it is the cause of wear, heat generation and drill walking (although reduced).

Mazoff is a famous promoter of the 4F grind, and so is (naturally) Tormek.
Both report similar increases in service life, but neither of them reveals the surrounding parameters.

As briefly hinted above, the weak point (literally) of the 4F grind is the region around the web. Like the standard conical point (SCP hereafter) it does not cut – it extrudes, although it is smaller than on a standard conical grind. Indeed I looked at a freshly ground small drill (6mm) with the enlargement glass after a drill session and noticed the cold-weld bead on the point coming from extrusion... This region is naturally marred first.

Both of those grinds´ center regions will run hot eventually. Now this is poison for trying to cut stainless steel. Before you realize it, hard alloy components from your drill metal migrate to the parent metal, and the piece of stainless steel turns out harder than your drill is. It will run blue and even the best and freshly 4F ground drill can fail within seconds, making it a candiate for the bin.

It would seem that a cutting point like a conventional split point (hereafter: CPS) would eliminate that problem, and to an extent it does, but the downside is, that this faces you with yet another grinding challenge.

Honorable members of the scene like ,,Gadgetbuilder" John Moran think, that CSPs or similar measures are only necessary for bigger drills (say from 12 mm upwards), while he thinks that small drills perfectly get along using a 4F point.

But there is a different, equally valid approach that came to my ears:

My brother-in-law works in a factory making balconies and fences out of aluminium and stainless steel (wooden balconies being almost extinct). They use small drills like 3.1mm (for some reason)  for pre-drilling, and those do employ a CSP grind. They swear by them. Those bite into the parent metal more readily and thus reduce the risk of burning. They also have a limited tendency to walk. None of the drills they use hereafter are split. They don´t see the purpose for splitting them.

Moran again, in total contrast, advocates the use of a small drill (like 5mm) ground with 4F for pilot drilling. To be precise, he prefers spotting drills (ground to a very flat angle) over center drills to eliminate chatter and walking. Link.

So what is going on here? The answer is: different environment parameters! The prior clientele mainly uses drill presses and hand drills, the latter speaks of lathes resp. NC machinery (where Moran comes from) that have a very tight grip on both parent material and drill.

Furthermore, Mazoff in his well-known article tells us that his modified split point (hereafter: MSP) yielded much bigger service life than other´s, which will be true beyond doubt, but what did they use it for? Cast iron cylinder heads? Those are notoriously bad to drill even with pilot holes I heard.

We cannot view the performance of a drill (or rather a drill´s point geometry) isolated from neither the parent material nor the machinery.
The logical conclusion to this is that none of the grinds provides the final answer to all cases. Nor can the service life be attached to a certain grind indiscriminately.

Before I came to all those conclusions, I tried to apply my artistry to the Tormek. I modified the jig somewhat and managed to successfully grind something probably akin to Mazoff´s MSP (after many failures...). I can elaborate on this if there is a demand.

This can be applied to 4F (although with the relief facet only ground to where the corner meets the web rather than up to the center) and store-bought SCP´s. They work well into virgin parent metal, but apparently no better than using a pilot hole with a stock SCP.

That all said (and John Moran has come to the same conclusion eventually) you may instead want to equip bigger drills with secondary point angles (SPA, see Mazoff). I find that those much reduce the tendency of drills to block upon break-through on the bottom side of the parent metal.

SPA´s can be realized easily and faithfully on the DBS-22 with a slight modification of the sled (shown elsewhere here). Being cutting edges, they eventually need relief, but not if you keep it decent. John Moran told me this upon a private contact.

So the upshot is, for a hobbyist sharpening with a Tormek (and this unit is no doubt geared towards the advanced hobbyist), particularly if you mainly drill mild steel or stainless by hand (or drill press), you do not desperately need to equip big drills with a CSP (or similar), but you may want to invest into a pack of high quality drills with a split of sorts for piloting (you cannot grind a split point onto small drills with the Tormek). While you are in the shop, you may also get a specimen of a precision center punch.

Alternatively, a 4F small drill (such as 5mm) may serve well for this purpose too.
(Note that Moran grinds smaller drills without relief facet and that very small drills cannot be ground at all on the Tormek without additional gadgetry).
You may want to consider applying SPA´s to bigger drills, such as from 10 mm upwards.

Note that Mazoff goes on at length that that the influence of point angles and relief angles on the result may have a much greater impact on the performance than the point geometry itself. I recommend studying his essay ,,Drill Point Geometry" in depth.

Non-iron material must be investigated separately, and to my knowledge no mention has been made as to the relevance of 4F to those. That said, I have read that SPA´s are known to be beneficial particularly on plastics and thin sheet material.

One thing that has not been touched upon yet is the general aspiration level concerning drills. A company may have as one of their goals that a maximum number of drill operations per time slot are completed. Mazoff obviously comes from that corner, he clearly speaks for large scale manufaction where every second translates into money. So the necessity for pre-drilling may mean a loss of time that cannot be disregarded. On the other hand, a manufacturer of drill bits themselves may skip any web thinning action in favor of shorter (read: cheaper) time-to-market.

Again for the hobbyist (particularly of the model maker type), I guess quality may be the main aspiration, but I know many car tinkerers that could care less about precision and just want a hole done, period.

This is another perspective, from which we may have to view the discussion.

Another facet of the subject (pun intended) might be worth considering: the experiments quoted by Mazoff and Uddeholm are no doubt executed under controlled environment conditions. In practical terms this means for example: a rigid drill press or similar, controlled feed, controlled speed, controlled thrust, controlled cooling and lubrication, a certain type of material and probably more. The kind reader will immediately recognize that none of that can be met by a layman. I leave it up to you to decide how meaningful such tests can be for practise.


Interesting essay, but I need pictures to fully understand it. In addition, names such as 'web', 'split point', 'modified split point' and ''secondary point angles' referring to a part of a drill are completely unknown to me. That is of course also due to the fact that English is not my native language, but also that I am a layman or at most a hobbyist in the craft field. Links to referenced articles might also help.
I hope you will comply, as your knowledge of the subject is apparently of great interest to those members of this forum.


Dutchman, thank you for your kind words. I much appreciate your work too, which is the basis for many programs to follow.

Until recently I did not know any of those terms myself, as I said above, I used to look at a drill as most do: nothing noteworthy.

But when you you set out for something new, you have to dig in deeper, and now I know many of those buzz words...

Quote from: Dutchman on August 30, 2023, 12:02:58 PM(...)your knowledge of the subject is apparently of great interest to those members of this forum.

I didn´t notice ;D

Many of these terms are explained here. You have to flip open the Drill Bit - POINT window.

The vintage book The Use and Care of Twist Drills : Cleveland Twist Drill Co shows on p.21 a nice illustration of a (conventional) split point. Sometimes those vintage illustrations are better than a photograph. (By the way, it is surprising how little has changed in the decades since this book has been printed...)

For SPA´s, you may want to glimpse at an earlier thread I started: More on Secondary Point Angles (SPA). It is all there, links and everything. No reason to re-invent the wheel.

For more links on four&more facets, Gadgetbuilder and Mazoff look at 6-facet SPA style or 6-facet tertiary relief style?

This is the quickest way, since those threads are full to burst of exactly those subjects.
Note that I have added a paragraph in the end, which I had forgotten.
Hope that helped.