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Differences in Drill Finishes

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  • Differences in Drill Finishes

    I am interested in buying an American made drill index for general use in steel, aluminum, brass etc. I often buy individual bits for special projects, but sometimes miss having a decent selection of bits on hand for the occasional unplanned odd job.

    If I buy American (Hertal, OSG etc) is there a particular difference between a bright finish and a black oxide finish?

    Any information or a link explaining such general concepts would be appreciated.

    Best Regards,


  • #2

    The finish on drills is one way to improve performance in various materials in process.
    Many factors are involved.

    Bright drills let some swarf slide by more easily than black oxide drills. the reverse is also true with different materials.

    example - -Black oxide will let hot steel swarf flow a lot better than bright. Aluminum seems to flow better with bright.

    Black oxide and a little oil also resists rust better than bright.

    There are other coatings and coolants that have their effect too.

    Then comes point geometry. yada yada.

    HTH Ag


    • #3
      I can't say I've ever noticed much difference between bright and black finish. I do like the fact that Dormer uses black finish with the information laser engraved in gold on the drills. It makes it easy to read.
      Free software for calculating bolt circles and similar: Click Here


      • #4
        Hi Sandy,

        Go to the McMaster-Carr website and enter "twist drill" in the search box. They have a nice tutorial on the different coatings and different point geometries.
        The entire content of this post is copyright by, and is the sole property of, the author. No assignment
        of title nor right of publication shall ensue from presentation of this material on any computer site.


        • #5
          Geometry is the more important factor for a casual user. Coatings come into play when you get into production of a lot of holes. Just plain bright finished HSS or Cobalt will be fine for 98% of what a HSM will run into.
          Time and money are better spent in learning how to sharpen dull drills.My opinion only.


          • #6

            Thanks for the replies and the link. In my past experience, I noticed that the black oxide bits I have would get 'rubbed' with aluminum when I used them. I also recall the plain bits seemed to bite a little more in steel. Honestly, I don't know the pedigree of the materials or the bits, though, so it could be an unrelated issue.

            I'll go with the bright finish, as I plan to be doing a little more with aluminum, as I got some good stock from the scrap yard and it is nice to work with. It seems that it should still be fine with steel and brass as well.

            I also agree that drill sharpening is a key issue. I was recently saved a lot of grief from reading about techniques here in the past. I had to drill out some pins which were welded into a flat bar. Of course, it killed bits pretty quickly and instead of just giving up for the evening, I went over to the grinder and put a new edge on the bits used. The smallest one was probably my worst, but the larger two of the three I think turned out pretty well. Not bad for my first try. I have more practice needed, but will get there.

            Thanks for the advice.



            • #7
              Warning - quite long rant - get a coffee

              Point geometry IS important.
              I do a *LOT* of drilling, probably not as much as Evan did when he was pop riveting but I only have two CNC doing this, no hand drill

              This is a day and a half's work.

              Because the time taken per plate is more important than the material or drill it's imperative I don't wast this by breaking a drill so quality drills are important.
              Over the years I've experimented with all types, grades and materials and found that a good brand new drill RE-SHARPENED from the offset was the best.

              This is where I got a lot of flack in previous posts but I still stand by what I posted then.
              I was told buy goods drills and throw them away after a time, I tried that, buying Guhring drills but after only one plate the web would be totalled. Often didn't even last a plate hence the reason for the original post.

              So I bought a Meteor drill grinder off Ebay, now new these aren't cheap, around $12,000 for a machine that fits inside a shoebox. The Christian is about $18,000 so they must work if they are still selling them as they do.

              Now these are precision bits of kit, you grind under a microscope and the cutting edges are spot on and intersect EXACTLY.
              One point I noted is they only do 4 or 6 facet drills and not sweep grind like an off the shelf drill.
              But anyway they work and a new drill reground or even the old drill reground will do 4 plates before showing web wear which justifies getting the machine, incidentally I paid about $70 for it off Ebay

              A couple of weeks ago the local tool rep called and said they were doing some new drills so I ordered a few of each size I use and gave them a try.

              Did one plate and checked the web, no wear, did 4 plates and checked the web, again no wear so I let it run the series of plates thru, 36 plates to see how long it would last. It did the whole set with only slight wear to the web. That's 18,400 holes with a 2.5mm drill [ 3/32 approx for the metrically challenged ]

              On closer inspection I saw it was ground 4 facet with only the narrowest of cutting edges on it.

              Then the penny dropped. For years commercial drills have been swept ground as it's a simple operation, quick and cheap. Now with the advent of fancy CNC grinding they can mimic what the hight end grinders can do but be commercially viable.

              This is why the Christian and Meteor grinders can't swing grind as it's an inferior and cheap way to produce a cutting point.

              Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


              • #8
                "Evan did when he was pop riveting..."

                They aren't called pop rivets, the equivalent on aircraft are Cherry rivets. Have a look at an airplane someday and try counting the rivets. I used to have dreams where the sound of the rivet guns would awaken me.

                A Cherry structural rivet doesn't work the same as a pop rivet although it is similar in principle as it is pulled from one side. When pulled it looks like this:

                The use of such rivets is highly restricted and they are not allowed as a total replacement for solid rivets. They may be used for certain repairs and only as a certain percentage of the total rivets in an area.

                For the drilling of many thousands of holes in aircraft skins when just one single slip of the drill could scrap an entire part we often used split point cobalt alloy drills. They last nearly forever even in high silicon aluminum alloys as used in aircraft. These drills are not considered resharpenable as they have a web that is thinned near the point and thicker just slightly back from the point. They actually can be resharpened perhaps once or twice but great care must be taken not to remove too much material from the tip as you quickly are into the thicker web.

                Resharpening split point drills is interesting when done by hand and I use a 4" CBN wheel with a very sharp corner to do it. It has the advantage of keeping its shape for a very long time as it basically doesn't wear in use. They are rather expensive but I have a good source for such products.
                Free software for calculating bolt circles and similar: Click Here


                • #9
                  Eh, that faceted grind sounds interesting. Also sounds easier to do than a full sweep..... if not quite as fast.

                  Makes a lot of sense, since it is the same grind as a milling cutter. More facets than typical there, though. The shallow first facet would put more metal behind the edge, supporting it. I guess the others could be or not be faceted, as you please.

                  A full sweep grind takes off what would otherwise be the first little corner of the first facet, and remove actually quite a lot of the support behind the edge.

                  Now, I'm trying to figure out why in the lots of drills I have that are ex-McDonnell-Douglas (now Boeing) there are basically NO faceted grinds. You'd think they would have bought the best grinders, and I am pretty sure they re-sharpened, based on the drills.

                  Keep eye on ball.
                  Hashim Khan


                  • #10
                    Most twist drills web will be thicker the farther away from the tip you get. Web thinning and grinding of split points are standard operations in drill grinding.

                    Most shops that regrind don't concern themselves with the faceted grinds as they are time consuming, hence more expensive. It is easier and quicker to swing grind and replace as they dull. It is only in applications such as Sir John's that the added cost of the faceted grind would prove economical.
                    Jim H.


                    • #11
                      Well I'm sure glad John posted that info.

                      Compared to many here my experience is miniscule, but to me it has always made more sense to grind faceted points, rather than the sweep grind. After all, endmills have flat facets. Why not drills?

                      It hadn't occurred to me that the factory ground a rounded relief simply as a matter of expediency. But I certainly get better results with the flat facets.
                      Lynn (Huntsville, AL)


                      • #12
                        Differences in Drill Finishes

                        Point geometry makes all the difference in the world. Different angles and mor or less rake angle will allow you to drill either soft and gummy material or tough and dense material. In tool and die we all learned how to hand sharpen drills adequately for the job but machine sharpening makes a world of difference in the quality of the hole you drill. Many of the manufacturing plants that I worked at used cobalt drills to do production drilling of parts, mostly hard materials like 4130 tubing, and just threw them away. I practiced trying to sharpen them and split the point but it was not as easy as I thought. It wasn't until I got the Drill Doctor that I could reliably sharpen them and continue to use them over and over. (flame away as I know what results I get with the little ugly machine) It took a little trial and error to get the hang of it but once understood, the little plastic box makes new drills out of old stubs. Sorry to get side tracked, back to the original thread, Plastics that tend to auto feed with off the shelf bits will drill beautifully if you decrease the rake angle to the point that the heel of the bit just barely drags the work. Harder materials need more strength and supprot for the cutting edge, and that is why the faceted points work so well. I would love to get one of the more expensive sharpening machines but they are far to pricey to justify at this time.

                        Just my humble opinion.

                        Jim (KB4IVH)
                        Jim (KB4IVH)

                        Only fools abuse their tools.


                        • #13
                          It's true about different rake for different materials.
                          Last place I worked was a piano factory, part of this was a rail section where they drill the aluminium rails that held all the action.
                          Special extrusion made for us and had about 200 holes per length.
                          Now this stuff is gummy and I mean gummy. we have all come across this 99% pure alloy that just wants to weld itself to the drill and basically punches it's way thru.

                          Well these holes had to be perfect, clean and no burrs on the backside.
                          The guys who ran the machines found out that if they hand ground their drill bit using the edge of a radiused wheel so the drill looked more like a corner radiusing mill cutter than a drill they would fly thru and leave a clean hole at the rear.

                          It was only experience that found this, when we used scrap bits of rail for odd jobs and used ordinary drills we had massive ragged mushrooms on the back.

                          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


                          • #14
                            Differences in Drill Finishes


                            I can relate to that. We had some aluminum extrusion that was condition T-0, dead soft, and it was like working chewing gum. Over the years you remember a lot of the little tricks in grinding drills and "special tools" and that is one of the things that really makes the job enjoyable. This trade is just a process of gaining knowlege, either by personal experience or from the help of others who have been there and done that. THat is what makes this forum a real good resourse.

                            Best regards,
                            Jim (KB4IVH)
                            Jim (KB4IVH)

                            Only fools abuse their tools.


                            • #15
                              Originally posted by Scishopguy
                              This trade is just a process of gaining knowledge, either by personal experience or from the help of others who have been there and done that. THat is what makes this forum a real good resourse.

                              Best regards,
                              Jim (KB4IVH)
                              Very true and you never stop learning.
                              I go into quite a few different works doing odd job, collecting parts etc and I always try to get a look round at what they do if possible.
                              It's surprising what can be 'adapted' to another trade or remembered for another day.

                              Here's an example, I went into a lace factory one day and they were scrapping an old lace machine. At the rear of this machine, cast into two extending arms were a pair of 3 point steadies.
                              I asked what they for for and told they fitted a dummy roller in the steadies, drove it with a big O ring and it wound the unused threads up and then was thrown away.

                              I asked if I could buy the steadies and told to just take them.
                              They have done me very good service over the years.

                              Tel make a very good remark in the Ring Roller thread where he said roll the ring alongside a longer piece so as not to get flat ends.
                              I mentioned this yesterday to the sheet metal guys I use and they hadn't heard of it and they have been in the trade all their lives.

                              As well as learning though we also need to share otherwise this information is going to be lost.

                              Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.