Workshop made grinding tool rest.

Posted by on Nov 16, 2016 in Articles, Blog, Newsletter articles | 2 Comments

On my courses students learn how to grind a primary bevel using both a dry grinder and a wet stone grinder. For the wet stone we use a Tormek which is a very good tool. For dry grinding we use  a rather old and decrepid  double ended grinder. A couple of weeks ago I decided to up grade this to a Creusen HP7200T. I liked this model as the wheels have a wider diameter 200mm as opposed to the more common 150mm. I believe the geometry of the extra diameter gives a stronger edge. Also Creusen have a reputation as a good quality tool. However I was very disappointed by the woefully inadequate tool rest. It was flimsy, difficult to adjust and the retaining screws seemed to interfere with the line of the edge when sharpening wide plane blades.  I believe this is a common problem with many  modern grinders, but I expected more from Creusen. After fiddling with the Creusen rest sfor a while I decided to ditch them and make my own more robust rest.

It was quite tricky to work out the geometry of the rest, the grinding angle is affected by two factors, the angle at which the tool is held and the position it is held on the wheel, moving the tool higher on the wheel will reduce the grinding angle. Although the rest is adjustable I needed to know approximately the required angle and height of the rest. I worked this out by trial and error, holding a chisel I knew was ground at the correct angle (25 degrees) against the wheel so the bevel rested on it’s circumference. The position of the chisel and wheel were recorded against a board offered up to the wheel. The geometry of the tool rest could then be picked from the board.

The rest design has two brass bearer plates that the tool rests on. The back plate is adjustable, being raised or lowered to change the grinding angle. The core of the rest is laminated from four layers of birch ply, the wider sides being glued to the sides of the core to form a recess for the adjusting assembly to run in. The adjustment is fixed by tightening the retaining  machine screw, the screw runs in a threaded insert fitted to the core piece.As with all jigs, which is what the tool rest is, accuracy and precision are vital. If the rest is not square the ground edge will not be square, so each cut was checked for squareness as work progressed.

When finished the rest was screwed to the grinder base board positioned as indicated by the original trial and error recording, being careful to get it square with the wheel

I have to admit that I am always a little surprised when my jigs work first time! I was very happy with the edge produced using the rest. Looking at the design I realised that I could add a fence which would help students to grind a square edge. The recess between the two brass bearers could accomodate a sliding plate with a 90 degree fence. This was made using 3mm perspex sheet. The plate is retained by a screw that runs in a countersink groove, this ensures the plate runs true between the bearers.

If I make a second rest, which I probably will do for the other wheel, I think I will reduce the dimension front to back as the current dessign may not work for short chisels that have been reground many times.

If you would like to make the rest you can download a sketchup drawing of mine here:

http://www.christribefurniturecourses.com/Tool-rest.skp 

 

However your rest design may require adjustment to allow for the different configuration of your grinder eg diameter and height of wheel. All the elements of the design are components so you can use the hide command to see hidden parts. I have not drawn in the various screws (my patience with sketchup does not extend to that!), but I have indicated the position of the holes.

If you have a go at making it but run into problems feel free to contact me.

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Thankyou, Chris.

 

2 Comments

  1. Luke Mackenzie
    14th June 2018

    Hi Chris. I would like to have a go at making this but could you elaborate on your trial and error method for establishing the correct angle?

    “I worked this out by trial and error, holding a chisel I knew was ground at the correct angle (25 degrees) against the wheel so the bevel rested on it’s circumference. The position of the chisel and wheel were recorded against a board offered up to the wheel. The geometry of the tool rest could then be picked from the board.”

    Couldnt the chisel rest anywhere on the wheels circumference? How did you ‘record’ and ‘pick’ these measurements?

    Reply
    • Chris
      Chris
      15th June 2018

      Hi Luke

      I’m pleased to hear you are making the tool rest. The problem with the tool angle is that the eventual bevel on the blade is determined not just by the angle of the blade in relation to horizontal but also by the height of the rest and also how close it is to the wheel.

      To overcome this problem without complicated geometry I used a small board with the proposed height of the front rest marked on it. This was positioned so the front edge was in the same position as the front of the rest would be in use. A chisel ground to the correct angle (25 degrees) was offered up on the rest height line so the bevel rested on the circumference of the wheel and the position was marked off on the board (see picture one). The marked line of the chisel could then be used to determine the position of the back rest. Hopefully picture two explains the markings.(Edit. Actually looking at the second picture I think “Angle between front and back rest” is misleading, I think it should say “Angle created by positions of front and back rests”. Hope that’s clearer).

      The position of the back rest is not critical as it is adjustable, as long as it allows adjustment either side of the 25 degree mark. The height of the front rest may need to be amended depending on the height of your wheel. You can play around with these variables using the marking board until you find a happy medium.
      I hope that answers your query, please let me know if you have further issues.


      Reply

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