More on Secondary Point Angles (SPA)

[aquataur]

As a follow-up to my thread on 4+facets and SPA´s and fuelled by Rick´s Mission I´d like to speak about my findings regarding SPA´s.

I suggest reading Mazoff´s document (which has been quoted countless times here) if any of the following terms sounds non-familiar.

After making the mod Rick suggested to the jig´s baseplate, which worked perfectly (thanks man), those questions arose immediately:

• do I need SPA´s at all?
• how much SPA do I grind?
• at what point angle?
• at what relief?

Gadgetbuilder mentions in his great building document ZIP file containing the sharpener plans

(...)claimed by Joseph Mazoff in his "Drill Point Geometry"[1] document. Adding Secondary Point Angles (SPAs) improves the drill's self centering tendency, improves hole finish, reduces drill wear, and reduces the exit burr when through drilling.

So let´s make it clear before we start, this statement of Mazoff is what everything goes back to. Apart from seeing SPA´s appearing on some drills, we have no evidence other than his claimed experience. I don´t think we have reason to doubt, but let´s agree that this is momentarily a claim.

That settled, Gadgetbuilder´s experience seems to support the claim:

Both split points and SPAs help to reduce wander when drilling deep holes.

and

These extra facets cause the drill to have a stronger self centering action and the sharp outer point on the cutting lip is eliminated so the drill remains sharp longer. The chip is weakened by the difference in cutting action between the primary and SPA facets so long spirals are less common because the chips are prone to break. (...) When through drilling the SPA reduces or eliminates the exit burr on most materials.

but now comes the caveat:

Secondary relief can be added to the SPA but isn't necessary for small SPAs.

Photo 13 shows an 8 facet drill with large SPAs, so large that secondary relief was needed for the SPA -- and even that wasn't enough. This drill was tested by drilling several holes in 1/2" mild steel, which produced heat discoloration in the heel area from rubbing. Hand grinding this area could eliminate this contact but instead I use smaller SPAs since they increase drill life without the hassle of hand tweaking the heel.

SPAs can improve drill performance or, as noted above, can degrade performance if too large so work your way up in SPA size.

Gadgetbuilder´s resumé on the claims was:

The 1/4" drill I experimented on cuts steel nicely but it is impossible to evaluate whether all the claimed benefits are actually realized without conducting a controlled experiment. However, it is easy to do and works well so I commonly add SPA facets to drill bits - mostly bits larger than #25 since smaller bits usually cut well even without an SPA.

Drill # 25 is IIRC something not far away from the smallest drill the jig can sharpen, so this practically concerns all of our drill sharpening tasks.

So my momentary answers to the above questions are:

• maybe not exactly need, but helpful on larger drills
• better too little than too much to start with. Time will tell. Unfortunately no cook-book recipe for this.
• the point angle was never mentioned, but since it is an extension of the primary cutting lip, I presume: the angle chosen for the task (setting "P" on the jig).
• none for small SPA´s, secondary relief for bigger ones. Angle assumed equal to setting "S" on the jig.

For starters, I would leave it away. It is unclear, if any assymetry would impair the drilling action resp. how tolerant the drill is towards imperfect SPA grinds.

Experience welcome.

[aquataur]

More evidence for using SPA´s:

I happened to stumble across a vintage book from 1934 which Elden has linked to to. On page 35 it says:

If the novice finds it almost impossible to grind a drill to the prpoer angle on lip and pitch, he can make use of the trick of first drilling a hole, smaller in diameter than the required hole is to be. (...) The (...) drill of the size of the hole selected should then have the outer end of the cutting edge slightly ground away at an angle of about 45° so a to show a flat of about 1/16" [rem. 1.6mm]. This drill is then used as a reamer (...)

(see attached picture; source: The Drill Press, Walker-Turner Co., 1934)

This sounds suspiciously like an SPA.
In the subsequent chapter they speak about Reduced Rake for drilling Lead that seemingly does a similar thing.

In a different vintage book The Use and Care of Twist Drills : Cleveland Twist Drill Co it says on p.53

If a drill is sharpened with a large included point-angle,as in Fig. 47, the zone of abrasion is comparatively small,as at M. Dulling will, therefore, be proportionately hastened. By sharpening with a small included angle, as in Fig. 48, the area of the zone of abrasion N is greatly increased and with it the life per grind. To further increase this abrasive area, and with it the grinding life, it is sometimes advisable to grind a secondary angle at the corners of the drill, as shown in Fig. 49.

(see attached picture; source: as above, p.53)

Note that with such a large secondary point angle cutting lip length, nearly as long as the main (primary) cutting lip, it is obvious that some secondary relief has to be provided. Again, no details are given here, but this supports what Mazoff has said and lets it appear to be not only mere claims.

If you look at Mazoff, figs. E-H (Mazoff, Drill Point Geometry), in fig. H you can clearly see that right behind the SPA area the secondary relief (of the main grinding lip) starts, which clearly eliminates the need of a separate relief for the SPA facet.

With terminology, we have to be ultra careful from now on. Each facet may have its own relief plane. The primary (grinding) facet (which is in the absence of a secondary  grinding facet=SPA probably not termed as such) has a primary and secondary (and maybe tertiary) relief facet, but so has the secondary grinding facet...

[aquataur]

Rick, if you monitor this, I would like to know what the long-time resumé of your SPA mission was.
I have mixed results momentary, but this can be down to the drill which seems to have a precession.

[RickKrung]

Unfortunately, I cannot update or contribute much as I have done almost no drill sharpening for years.  I used to think/say that was because I didn't abuse my drills...  But, I now have quite a few that need attention but have been so busy with other projects, I just haven't gotten a round tuit. 

Kinda ironic and sad, as drill bit sharpening was the larger of the two primary reasons I got a Tormek in 2017. I do recall becoming stalled on drill bit sharpening after I got diamond wheels and found that they were not perfectly flat and drill bits didn't touch or grind uniformly across the face of one or more of my diamond wheels. That was really frustrating and dismaying as I wasn't happy with even the SB wheel for grinding HSS. 

I've since acquired two CBN wheels, an 80 grit for the 8" slow speed grinder and a 180 grit, 10" for the T8, but I just haven't been motivated to get back into it.  I haven't even tried the two CBN wheels to see how flat they are. 

But, I have used the DBS-22 sliding platform and even the drill clamp for a couple of other projects. 

Rick

[aquataur]

Thanks Rick.
It astounds me that those wheels are less than perfect. I thought this was the very reason of their existence...
Well a conventional wheel can at least be trued, and even this only lasts until you change wheels...
Imho mounting it the next time you would have to true it again. As if they grew unbalanced hanging on the stand in the same position because of the water they retain...

[RickKrung]

I'm referring to not being flat across the outer surface, not out of round relative to the shaft.  The bits would touch near the sides but not in the middle.  Probably not a functional issue, more of a psychological annoyance.  It may in truth be quite minimal and within specs, but it was noticeable to me. 

Rick