Harold Hall


Because of the unconventional design a little explanation will obviously be appropriate. Basically, the method is to locate a half round groove in the holder onto the sides of a round key on the body, this is best understood by reference to Sk. 1.  We immediately though come to the problem of manufacturing  tolerances. If the key is marginally larger in diameter than the groove then there will be a tendency when the holder is clamped into position for it to grip the key, possibly making it difficult to remove.


As a perfect fit (+/-0.0000) cannot be guaranteed the key has therefore to be very slightly smaller in diameter than the groove in the  holder, say nominally - 0.005mm. This though will result in the holder locating only on the crest of the key giving a less than ideal location Sk. 2. This showing it very much exaggerated of course.  However, this is overcome by the addition of the flat on the top of the key resulting in the holder locating on the sides of the key  Sk. 3. This  basically giving an inverted dovetail with slightly curved faces, and importantly, a large length to width ratio, a major factor in achieving an accurate location repeatedly.


Another factor is that at the edge of the half round groove the angle of the contact surfaces is 90 degrees and it is therefore almost impossible for the key and groove not to align perfectly.


How does this compare then with conventional holders having a dovetail locating method. Commercial holders will have a very good finish on the dovetail surfaces, maybe ground and hardened. This will enable the two parts to move easily over each other achieving accuracy despite their relatively small length to width ratio. My design, as made, should get close to this but I would not suggest it would be better.  


Workshop Projects

If though the workshop owner intends to make his or her own using the common large dovetail design, it is unlikely that the dovetail surfaces will have such a good finish. This, together with unhardened surfaces will make it more difficult for the two surfaces to move on each other and pull into line. In this case, I feel that my design will give results at least as good as the conventional shop made ones and probably  better.  If then you would like to make your own then do give my method consideration, it will certainly be quicker to manufacture, especially if a largish number of holders are to be made.


Mk1/Mk2 The differences.


Apart from minor dimensional changes there are two main ones and one optional one. The two main ones are that I have increased the diameter of the Locating Key (1) from 8mm to 10mm and moved the Clamp Assembly (S1) nearer to it, resulting in a greater clamping force at the key. However, as some viewers may have made the holders to the original design and now wish to make a few more holders then I have retained the details for the original elsewhere on the site.


The optional one is that I have made, and drawn, the base to take just one holder, rather than two on adjacent faces. This, because I found I very rarely had two holders fitted at the same time. Should the reader prefer the system to have two holders it is very easy to make this provision by doubling up a few parts and repeating the facility, exactly as drawn for the first face, onto the second face.


The main change to the methods used to make the QCTH relates to the half round groove in the sides of the tool holders, but more about that later.