Member postings for Peter Tucker

Hi Lawrie,

I’ve been cogitating on your problem and I think it is more complex than you imagine, let’s start simple. Imagine you have a cylindrical D bit of diameter ‘D’ which you want to use as a sort of single edge end mill so you put it in your eccentric and grind a relief from nothing at the cutting edge to an amount ‘A’ on the other side. The Sine of the relief angle is equal to A divided by half the circumference of the D bit. A 10mm D bit ground to ‘A’ = 1mm would have a relief angle of approximately 3.65°; 7mm dia, same A , relief angle approx. 5.2°; 4mm dia, same A, relief angle approx. 9.2°.

However you are wanting to grind a relief angle on a tapered engraving tool so no one eccentric setting will give the same relief angle all the way down. If you only need to cut on the tip then a relief amount ‘A’ could be calculated to suit the tip diameter, but if you need to have the proper relief angle all the way down the cutting edge then I can only think of two possibilities. 1. The bit must, when it is being ground, move in such a way that it’s axis travels through a conical path, the center of rotation when grinding should be parallel to the cutting edge (I have not attempted to calculate the offset for this axis). 2. Using a cup stone whose inside taper closely matched that of the relief wanted to be ground. Number 1 I think would need a grinder of the complexity of the Quorn with special tool holders, number 2 I think one would be most unlikely to find a stone to suit.

If you want a formula for setting your eccentric offset to give a relief angle at the tip I think I may be able to work that. If on the other hand I am completely wrong I’m sure someone out there will point out the error of my ways.

Good luck (I think you’re going to need it)

Peter.

Hi John O.,

Take your worn blade to the saw doctor and have it sharpened.  I took my broken blade to the local saw doctor to weld and he threw in a sharpening.

Peter.