I've got a line on a decent set of cobalt drills, but they are 135 degree 'heavy-duty' drills. Not 'production grade' 118 degree. I joined up at tech school a semester late, after they'd taught drill grinding, and am sitting here realizing I don't have a good handle on the difference. Plus, I don't think we ever used anything other than normal drills at school anyway....
I know I've never sharpened anything exotic. Is a 135 split-point cobalt something that can be touched up on a regular grinder? Or do you need a fancy drill grinder with a proper split-point setup?
Mainly though, would 135 degree split point cobalt drills make a good all-purpose drill in a home-shop envirnment? I'm completely in the dark about 118 vs. 135, performance and longevity wise.
Many thanks for y'all's time.
As I understand it, the 118 is better for more abrasive material. The 135 works better on harder material and has a longer life to the edge it seems. (Correct me if I am wrong people). I like the 135 for steel and hardwoods. But it does have a tendency to bite more at the exit point than the 118 when you are doing a through hole.
Drill Doctor 400 and above can sharpen the split points on the 135s and 118s, and trying to do a proper split point on the grinder is a paine unless you have a precise jig. (Unless you are one of the older gentlemen on this board who have been doing this since creation. )
In industry, 118 degree are considered "all-purpose", whereas 135 degree are generally preferred for tougher materials. How much difference it makes to the bottom line at the end of the day, I haven't a clue.
To answer your question directly, a 135 degree point will work great in a general shop environment. Split points greatly help in starting the drill and keeping it on center.
Essentially, there is an optimum grind for every type of material. It can make a HUGE difference in how well the drill cuts, hole size and hole finish. See any of the drill maker sites for suggested drill angles on various materials.
I always grind the tip rake for brass, but unless I'm doing a lot of holes in one kind of material, I'll generally go with the general 118 or 135 degrees and not grind the optimum angle for that material. I always split the points when I'm resharpening but I have a drill grinder.
if you use them at home in a hand drill. I would go with the 135's. I like them a lot especially in cobalt.....most of the time you wont drill a small hole and then go larger....especially if its under .5" you just garb a drill and go...well the web thickness can hurt you, because it doesnt cut as easy if its not split.....and since your using hand pressure instead of machine feed...life would be easier with the 135 splits. But you wont be sharpening them at home by hand!... they cant besharpened very well with out the split point.. the web is just too thick.....if you have a drill sharpener, then you ar good to go......I think in a large drill with a 118 point... it will heaat up more...because the is more surface are cutting on the drill... couple with the fact it isnt a center cutting drill( in first pass holes)...so the material in the center is just getting RUBBED to death...but a split point cuts all the way to the very center................good luck...bob
The 118 degree points are used for general purpose drilling in mild steel and other soft material. The 135 degree points are usually used in harder high alloy materials. The cobalt drills will take more heat before the cutting edge breaks down but they will chip easier if you force them. As started in other posts there is an optimal point angle for materials along with speed and feed. Split points will help guide the drill in straight and aid in cutting at the tip. Depends on who you talk about sharping drills by hand and spliting points without a sharpener. I have a Darex sharpener that does an very good job if it is set up and used properly.
The web of a drill bit gets thicker the further you get away from the tip. If you resharpen a drill bit many times the web is thicker and the point should be split even on 118 degree points.
One thing to consider about drill point angles (as opposed to relief angles) is the 118 degree has a longer cutting edge in contact with the work than a 135 degree point. So, in most cases this means the 135 degree is going to require less pressure to drill.
For instance, on a 1" diameter drill the 118 degree has a 1.167" length of cutting edge in contact. A 135 degree has 1.082" in contact. This based on a quick CAD view of the points ignoring center web, etc. Still you have less cutting edge working which means less pressure to drill. This analysis ignores the advantage of the split point, that makes drilling even easier.