Drill Sharpening Jigs - Advice please.

Edited By ady on 24/09/2010 10:53:02

Far as I can see the Potts has the same set of fundamental problems that afflict all the simple swing jigs.  It seems asking for trouble necessitate simultaneous setting of angle and projection separately to do both edges.  Given the space available effective accurate stops are probably impossible to achieve.  The complex geometry involved, which is never explained, doesn't help.  Its take me about 1/4 of a century to finally twig WTHIGO with the common Picador / General et clones and why one has the edge at an angle and one has it vertical.  These days I can get decent results from my Picador although I still need to put a screw depth of cut adjustment on my improved base mount.

I suspect this http://www.practicalmachinist.com/vb/general/who-makes-drill-sharpening-jig-210357/ is about the best type for the home shop, at least it has a calibrated adjustment.  If you are careful the PlasPlugs device works unreasonably well for cheap and plastic.  At least they have made an effort to give you projection and edge angle gauging as well as providing a flip over drill holder so you only have to set it once.  I suspect that improved versions of the setting gauge and drill holder made accurately in metal would transform the performance of the swing systems.  Prolly need about three holders tho'.

Clive

I’m very much a novice at all aspects of model engineering, let alone the ‘black art’ of drill sharpening. As I have inherited/scrounged a serious collection of blunt drills and an old swing-type Reliance jig – which I’m relieved to find others have difficulty with – I’m very interested in this thread. However no one has mentioned drills less than 1/8 ins dia, except for Stub Mandrel’s passing reference to wishbone sharpeners. What are these please? Also have you any other advice and guidance re small drills? Thanks very much. Bill.

Drill sharpening on an offhand grinder is very much a skill. Until I built a tool grinder I used a Picador ‘Swing jig’ (I still have it). The trick is setting the drill in it in the danged thing. For drills under 3.2 mm (1/8” I used to use and often still do a block of square steel with ‘V’s files across the diagonals and a front clamp to hold the drill. It is then rubbed on a diamond plate. I know it is not exactly correct but the drills cut! I also built a 4 facet hone which was written up in Model Engineer somewhere in the late 70’s to 80’s. Again the problem is setting the drill in it. I saw a ‘wishbone’ sharpener but it was beyond my skills to use.  Over here I often make and use diamond pointed drills which I make with an old lump of ‘hex’ sawn at 64 degrees across the diagonals and rubbed on a diamond plate. I think I saw this at an ME exhibition at Wembley on the SMAEE stand. These drills are very good for sheet metal as they do not ‘pick up’

John

My take as to WTHIGO with the two different varieties of the Picador / General and clone swing type drill grinding jigs goes like this :-

1) The clearance angle behind the cutting edge of standard point drill runs circumferential at all radii of cut.  The natural way to generate this clearance angle is by twisting the drill during the grinding process.  Correctly timing and synchronising this twist with the basic grind is the hard bit of learning to do it by hand.

2) Swing grinders move around a single axis.  The clearance angle runs straight across the drill at right angles to the swing axis so the grinding depth at the trailing edge of the land is less than when the drill is correctly twisted whilst grinding.  When correctly set up a swing jig generates an increasing depth of cut  to produce the circumferential clearance angle by geometrical means.

3) Consider the vertical spindle Picador type jig.  If the drill cutting edge were also vertical and aligned with the pivot axis the swing would grind a circular arc shaped face across the drill, the radius being set by the projection of the drill relative to the pivot point.  Having no clearance  angle the drill won't cut.  Offsetting the cutting edge so it trails behind the pivot axis grinds a clearance angle so the drill can cut.  This offset also increases the effective depth of grind as the swing proceeds giving a better approximation to the desired circumferential clearance angle.  Clearly if the swing were 90 degrees the increased grinding depth would equal the offset of the drill cutting edge relative to the pivot.  It appears that in practice offset alone doesn't increase the depth of cut sufficiently as the grind moves across the land.  By rotating the drill cutting edge away from the vertical the trailing edge of the land is effectively put further behind the cutting edge increasing the grind swing angle and therefore the depth of cut.  This rotation effectively reduces the offset of the drill point relative to the swing axis so clearly there is a pretty fine balance between offset and rotation angle if clearance is to be ground along the whole cutting edge.  The grinding swing is no longer at right angles to the cutting edge which makes it harder to follow.

4) The spindle of the General type jig is angled back away from the grinding wheel.  Effectively the lower edge of the drill is closer to the grinding wheel than the point so it travels on a slightly smaller radius during the swing thereby increasing the depth of cut at the trailing edge of the land despite the cutting edge being vertical.

Clive