Hi Sorveltaja,
as I’m concerned with the drill subject too I will take the chance and try to explain a little bit about the mysteries of the helical tool called drill, hoping not to jam your interesting and already started article here?
Your idea of using the pitch of a special thread as a guide for grinding the relieved surface of drill tips sounds interesting, some commercial drill grinder use a similar technique in form of axial control curves embedded in the drill fixture bearing. But I think it is a little bit too complicated, there are much more simple techniques to achieve a really good performance at home.
But first of all, the pitch of the flute (helix) has nothing to do with the relief angles, it only generates the rake angle (if this angle is not corrected or changed by extra grinding the cutting face for non standard boring tasks). This will come obvious if you compare a drill with the front cutting edges of an end mill, end mills are only the special case of the hole system “helix tools”, the case with the point angle 180deg.
The relief or clearance angle is more or less independent of the drills geometry, naturally this angle must nevertheless follow the rules of making chips….a first clearance angle in the range of perhaps 8 to 15 deg makes sense for our several metal drilling jobs.
In principle there are two major techniques for grinding drills, the four or more facetted relief flanks method and the conically shaped flanks method, normally you will find the conically type on most of all standard drills at your tool store.
But the conically drills don’t make a better job, I think it’s easier for industrial manufacturing having only one process on each cutting edge than several passes like you will need on the more facet types. So to my opinion, it’s up to your free choice what grinding technique you will prefer at home.
If you like it conically a simple swinging fixture will work fine. In fact the cheap fixtures from your tool warehouse use the correct technique, but most of them are not build precise and tough enough for holding the drill exact in the axis of rotation. And they have problems with indexing the both cutting edges precisely from 0 to 180 deg when changing to the other cutting face…..therefore a lot of frustration for there users is preprogrammed….ha ha ha….
So you can buy a reliable and expensive fixture from a serious tool grinding machine builder like Cincinnati or Deckel or you have to build your own device.
To achieve the needed clearance face by swinging the drill in front of a grinding disk the rotating base of the tool fixture has to be arranged with a defined offset distance to the axis of the drill, the shifting vector of this off set is pointing in the direction away from the cutting edge. The principle of the conically method is easier to present by changing the moving systems, here, contrary to reality, the drill stands still and the grinding disk is performing a conically rotation:
The drawing shows two of many possibilities for arranging the rotating base in relation to the disk surface (90 and 70 deg), they both works fine and only need some different calculation of the setting parameters.
This is a drawing from the last year when I was starting to design my second grinding fixture:
And here an intermediate result of this in the mean time further enhanced fixture:
Some details like the graduation of the rotating base and the fine indexing of the tools axis are not really necessary for only drill operations, but I designed this for additionally grinding all forms of d-bit routers.
But very important is the precise and rigid mounting of the tool, my choice was an ER20 system. And we have two linear adjustments, in tool direction for setting the cone radius in relation to the disk surface and perpendicular to this axis the off set value, together both parameters define the relieving characteristics of the desired clearance.
A good method for evaluating the clearance angle is having a frontal look at the drills chisel. As evident the chisel line rotates clockwise in direction towards the cutting edge, more rotation means a higher value of clearance. Number 1 and 2 are related to the numbers in the first picture above, as you can see, the different setting angels of the rotating system make no great difference to the result of the clearance value.
Okay, further details and the description of the other method (four or more facetted) could develop in to a really large article and my concentration needed for writing and translating at the same time is going to fade away for this night…..ha ha ha…..
I will try to continue as soon as possible in the next days, and I hope the stuff is not too dry presented, don’t wont to bore you with too much theoretic.
Bye from Achim
