Hi, Sounds like you guys are talking about the tool known as a combination center drill and countersink. Most CNC operations use a spot drill as a precision starting guide for the drill point entry. I don't know about the precision differences between the two styles but if you really require a close tolerance hole position you should use a more involved process than just a spot (or center) drill and drill. Drills walk. The main reason to use a spot drill is time. A spot drill is faster.
regards Stuart On Sat, May 11, 2024 at 1:50 AM <marcus.bow...@visible.eclipse.co.uk> wrote: > On 2024-05-11 03:32, andy pugh wrote: > > That's it. really. Why? A large-diameter one with a small drill point > > could > > make all the sizes. as far as I am aware the drilled hole is > > irrelevant. I > > suppose it might have mattered as a reservoir for the white lead in the > > days of solid centres. > > Interesting question! > The pilot (parallel) drill creates a hole for the very tip of the mating > centre. If the drill had to cut the exact shape of the conical tip of > the female hole, it would struggle to have proper cutting edges and > still make the true cone. > You are right that the conical end on the body of a large centre drill > (ignoring the parallel tip) could cut all sizes of cone. But there might > be some reasons for the different sizes: > > 1. very small centres would enter the parallel drilled hole, but miss > the conical part. I have just been trying to clean some small center > holes in a clockmaking tool designed to support the ends of small > diameter clock shaft pivots which run between two female centres. Those > pivots would completely miss the conical part of the female hole which > had anything but the smallest parallel section at the end. > > 2. for male centres which will take a decent load, the end needs to bear > inside a large (deep) female hole, so a large drill can be used here, > especially when pushing drilling feeds and speeds. The flip-side of > that is that allowing the body of the centre drill to set the final > diameter provides a small amount of parallel recess at the outer end of > the centre, which will foul a centre and hold it off the female cone. > Centre drills do exist to create 'protected' centres with a larger > parallel recess just at the entry to the female cone. Those drills have > a short stepped-out section of cutting edge of larger diameter than the > largest end of the female cone. > > 3. In pre-CNC days, the best way to set the size of a hole would be to > have a drill of the correct diameter(s) mounted in a turret with stops. > The operator then would not need to think, but could just pull the > lever. Aside from the problem in (1), you could, of course, set the stop > to make a large drill create a small diameter centre. Which makes CNC > an obvious advantage, of course. > > Marcus > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Addressee is the intended audience. If you are not the addressee then my consent is not given for you to read this email furthermore it is my wish you would close this without saving or reading, and cease and desist from saving or opening my private correspondence. Thank you for honoring my wish. _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
