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An idea for drill sharpening jig
sorveltaja:
Yes I meant the screw pitch, that is to be used on the jig. My use of the term "pitch" might not be the most correct way, when describing things on a metric system.
tinkerer:
I am sorry, I was editing the previous post as you were answering. A drill bit does not have pitch from one side to the other. Each flute forms an equal angle from the point that is measured from the center of the bit. Any deviations in angle from one side to the other will result in only one flute cutting and the other just following along. Swarf (I love that word) should be removed equally down each flute when cutting.
I may still not understand the process and will follow with interest. I do believe you have a great design in the tooling if you take the pitch out and just provide rotation. It may take two grinding heads, one for the angle and another for the rake and that could be difficult to incorporate.
sorveltaja:
The brass piece, before I turned the 4.7mm pitch threaded part off:
Next I made 2 point thread with 1 mm pitch to it. It's barely noticeable in the photo, so demopicture instead about it:
With that, test-sharpening a 3mm drill gave a lot better result. The angle (in the picture) was quite small, allowing only slow feed, when I test-drilled aluminum, brass and mild steel.
Next thing to test would be to increase the jig's guiding thread's pitch to 1.5-2mm, to get closer to average cutting speed, when drilling.
Joachim Steinke:
Hi Sorveltaja,
I spent some time over the weekend thinking about your helical grinding method, but I don’t know if something wise had come out of my brain….ha ha ha….okay, let’s try it….
First of all your pitch problem, you wrote that your last testing version made too little relief angle, as you already mentioned the helix pitch is too low.
The calculation of the helix angel is still quite simple, you only need to unwind the surface of the cylinder and so the angel for a given diameter (D) and a give pitch (s) follows the equation
tan alpha = s / (pi * D).
So one and only helix curve of a multi purpose grinding fixture should become a problem, variations of the drill diameters will require a wide range of pitch value. Here for example the pitch table for a relief angle of 10deg at the circumference of the tool:
And here, for a constant pitch of 2mm the table of the variations of relief angles you would get for the various diameters from 1 to 10mm, you see, the range is really wide:
As evident, alpha is inverse depended to the helix diameter, large diameter means a small helix angle. This in fact also will create an increasing relief angle from the circumference to the center of a drill. Within limits this effect is helpful and even desired, but if the value of variation on one and the same drill is too heavy it might become a problem.
So I consulted some articles from the web and then tried too visualize the theory for Helical Drill Point Grinding by my self a little bit further. What came out is a schematic model for a Universal Helical Grinder System, which, depending on the highly manipulable axis system can create all imaginable relief surfaces on a helical drilling tool.
This model can create all sorts of grindings, quadratic surfaces as well as planar like the multi facet method, so planar is only a special case of the whole process.
To simplify the system, some parameters like the distance S and B can be fixed during the operation. But the problem of rotate the tool along the pitch of a helicoid (H) under the implemented angel PHI and with the distance S relative to the tool axis (index of rotation) can not be solved with a collet fixture collinear to a single helix curve like this:
If the axis of rotation and indexing (toolholding) coincide you will have no influence on the relief characteristics distribution on the flank surface from center to circumference. But I might be mistaken, that is only a first glimpse on this geometrical problem. For a real calculation it is really not quite a simple task, for it is complex vector analysis of a multi determined 3D system, I don’t do that normally every day…..ha ha ha……
I just tried to simulate the simplified process in AutoCAD and the results were not satisfying, had some problems with boolean operations on complex sweeped bodies. So I should try it with Inventor in the next days.
Bye, Achim
tinkerer:
Achim,
Kind of what I was saying, only with science to back it up. Try with two grinding heads, one for the angle and one for the rake. May be too close proximetry to accomplish. Take the pitch on rotation out. Another solution could be two seperate tools, one for grinding the angle and one for grinding the rake. The back cut can be done by hand, as it is only needed on larger diameter bits.
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