Drill grinding jigs – optimising the use of

[Martin of WickParticipant @martinofwick]

using the Hugel spreadsheet model

[Martin of WickParticipant @martinofwick]

For basic drill sharpening I use both the old style Picador vertical spindle jig and a properly made (apparently) slant spindle clone drilling jig. much has been said about both in the forum.

In their unique way they both deliver reasonable looking drill re-grinds with satisfactory point angle and what appears to be an acceptable heel clearance (as long as some care is taken) . Best results are usually obtained with a clamp bar on the drill so each side is ground at a true 180 degrees where possible.

To improve repeatability, I thought the spreadsheet model below might be helpful (available from this site).

Joerg Hugel's Drill Performance Tables – Processes (model-engineer.co.uk)

The idea was to be able to generate a table of drill lip projections for differing diameters at differing point angles for the jigs (118, 136, and possibly 98 degrees). Hopefully, I could then make a small gauge to consistently set the point projection and lip angle and then produce grinds where relief is within the optimum range on a consistent basis (without the usual trial and error each drill grinding session). Optimum would be a projected clearance that increases from between 12 to 15 degrees at the outer edge rising to between 20 and 30 degrees at the centre.

I was wondering if anyone had investigated this spreadsheet model and attempted to ground truth it?

The first issue I had (after measuring up the jigs and calculating their geometry parameters) is that the two variable parameters that actually define the grinding cone are not direct entry – you have calculate them by an intermediate step then check to see whether you have got close to your desired axis offsets- Doh!

I was also wondering how reliable the results are? modelling the Picador (easier because the cylinder axis is vertical) the results sort of confirmed what I was getting – ie the picador suggested settings generating rather too much clearance (specially in smaller sizes).

For the clone jig, the model results suggested that an extra 5mm of forward projection was needed on top the projection required for the diameter (forward projection of 1 Dd in the clone instructions, the sideways projection is fixed on these jigs). I find with the clone jig that I usually need less rather than more projection, but that may be because I can only eyeball the finished product, which may look OK but is actually sub optimal.

Predicted chisel angles could quite interesting too…

It is quite a useful paper and spreadsheet model, well worth a look and I would welcome input from anybody that has reviewed it or attempted to use the results to finesse their drill grinding activities!

M.

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