Shown below is the result of a few different design iterations, researching similar designs, and lots of subconscious thought processing. Many an hour of sleep has been lost thinking about this project, for better or for worse. It has been a bit consuming, but I like when a project draws me in.
Related threads:http://madmodder.net/index.php/topic,11999.0.htmlhttp://madmodder.net/index.php/topic,11977.0.htmlIf you just want to see pictures and comment on what you see, skip past my ramblings below. However, I may have addressed some design features or pitfalls already and you could avoid looking silly by reading everything before you comment.
I'm looking for input on problems I may not have considered, and things that could be done simpler/easier/faster/cheaper/better. Maybe there's a $50 widget I can buy that does everything this fixture does and more - but I sure haven't been able to find it!
The design goal, was to create a fixture to grind a four facet point on drill bits using a surface grinder. I decided on using ER25 collets, which limits the range from about 2mm to 16mm (5/32" to 5/8"). I wanted to make it accurate, fast and easy to use, and rigid as possible.
In my research, and a question or two on this and other forums, I was not able to find any existing fixture design that holds the point of the drill stationary between grinding the primary and secondary facets. This means, for all I can tell, that you need to carefully sneak up on getting the point to match in the centre without going past. If you do, you have to grind the primary facet again, trying to get it
in the centre. If there's nothing I'm missing that makes this process fast, accurate, and easy then it's no wonder the go-to answer is just to sharpen your drills by hand on a bench grinder. (Yes, there are times this is the answer, but there are times when more accuracy and repeatability are needed)
The fixture I've designed (actually it isn't quite finished yet) allows the drill bit to orbit around the point. Once you have the axis of the fixture set up to the face of the grinding wheel, you leave it there throughout the grinding process. The best way to use it would be to have a bunch of drill bits to sharpen, as aligning the fixture centre point to the wheel is probably the most time consuming and most critical part of the process. Not that it's difficult, just that how well it is aligned dictates how well the point is formed. Any material removal adjustments are done by adjusting the fixture, as are the angles to be ground. Indexing is also provided for by the fixture, so after grinding one facet it is easy to rotate exactly 180° to grind a second cutting edge.
Throughout the iterations, I tried to keep my focus on the design goal, but clearly this fixture has other potential fixturing uses on and off the grinder. I am keeping those applications secondary at the moment so as not to distract from the original intent.
Some of the things I'm not completely comfortable with in the current design iteration:
- Rigidity is questionable. I plan to add stiffening connections between the linkages, but I'm not sure this will be rigid enough. Unless another solution presents itself though, I'm willing to try building it.
- Holding the drill in an ER25 collet is simple and uses off-the-shelf collets, which is a big plus. However, this may not provide enough clamping surface on the margins of the drill. There's a chance it may cause excessive wear or damage, but I think this is remote.
- With small diameter drill bits there is probably not enough clearance past the collet nut for the wheel. A pin vice or similar device could be used here to stick it out farther without excessive overhang.
- Locking all of the linkages in place in a rigid and repeatable way may be difficult, though I have some ideas to try out here.
- Grinding dust will probably get into everything. Maybe it won't be an issue, but maybe it will. It might just be something I need to try.... And finally, PICTURES!My thanks for your constructive critique.