Lazy Sharpening Jig / Granite Lapping on CNC

I bought this granite slab. It’s what is described as a work surface saver, and if you have melamine kitchen top you can buy something like this and use it to put hot stuff on, even though the melamine would still suffice.  I’m going to use it as a lapping plate because I’ve done something a bit stupid – I’ve managed to scratch my block plain on a screw or nail that I didn’t see in some material. Really stupid.

This isn’t too bad. If I put a straight edge on here and a torch underneath. It’s pretty impressive considering that only cost me £10.

I’ll give everything a bit of a clean.

Ok, so I’m going to use my CNC machine to pull my block plane backwards and forwards on the lapping plate. I’ve cut two strips out of 18mm spruce ply, which is 4cm wide and about 27cm long. With a 45 degree bevel at the end.

On the block plain, I’ve cut another piece of ply the same width as the spindle holder with a hole in the centre, which bolts onto the threaded hole which normally holds the blade flat.

Now I just need to drill some holes and screw, or use an insert nut to attach the clamping block onto these arms.

So I’ve just drilled some holes into this section here, and onto the arms. What I imagine doing in the future is actually cutting a groove out along here and having some insert nuts along the side, so I can actually adjust this a little bit. But at the moment I don’t have the right size so this is more like a prototype and if it works, I’ll rebuild it.

I’m just going to give this a wash, so there’s no sawdust or anything on it which might affect the sanding process.

Now I just need to turn the machine on.

So what I need to do now is edit some g code so I don’t move the z axis down at all, and run that simple motion of 100mm backwards and forwards along the y axis.

Ok I’ve just edited some g code – I am not sure if it going to work but I’m going to give it a go now and see what happens.

So the code is very simple. I start by setting the feed-rate which is 1000mm/min and I’ve deleted everything out of the code and just kept two movements which are backwards minus 50 and forward plus 50, or just 50. And that seems to move it the correct amount. And the code is simply G0Y50.000 G0Y-50.000

So it looks like it’s nice and flat. I am using universal g-code sender at the moment.

I’ve had that running for 10 minutes roughly. I’m going to see what the, it’s not bad, it’s getting there, I am using 320 grit. I might need to get a bit coarser.

I think I need to put a nut under this section here, just so the pressure is maintained, at the moment the machine screw is rising. I’m going to use this pulley as a weight.

It’s really taking ages, I haven’t got anywhere near the scratches – maybe a bit here but it’s mainly doing this section here and over there. Not sure how long this is going to take – it’s been running for about an hour now.

I am going to swap over to the actual plan iron now and see if I can take a little bit of this and get rid of the chips.

I’m going to have to chop off a bit here to do this.

So I’m just changing the g code so the plane iron moves a lot further backwards and forwards. I’m just going to send it to the controller now.

Ok I’ve got bored of this.

So I’ve come to the conclusion that this works in principle, but the jig needs to be developed. Looking at it I can tell I haven’t assembled it square, and it’s actually slightly twisting in the position I have attached it to the spindle mount. I also can’t get the angle of this part here flat enough without the bottom of the arms actually hitting onto the sand paper that was on the granite work surface.

What I would like to do is redesign these and cut it out using the CNC machine itself, and come back to this again to finish this particular task.

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