Sometimes a workpiece will already have a machined bore, spigot or a vertical face
and a DTI will be used for checking that these are correctly positioned for the required
machining to take place. This can be a part on the lathes faceplate, in the four
jaw chuck or on the milling machine table. In other cases though, one or more bores
may have to be made and positioned, very precisely in some cases, relative to some
other aspect of the item being machined or to each other. This requirement is where
toolmaker's buttons come into use.
If these are new to you then they are small drilled posts, made to a very precise
diameter. How then are they used? Taking first the need to accurately position a
single hole in a workpiece relative to some other aspect of this. The workpiece is
carefully marked out and drilled and tapped where the bore is eventually to be made,
normal accuracy for this process is all that is needed.
The toolmaker's button is then fitted using a screw into the tapped hole and its
position adjusted so as to accurately set the location of the hole to be bored. This
adjustment is possible as the hole in the button has a very generous clearance over
the diameter of the screw being used. Then, with the workpiece now on the faceplate,
four jaw chuck or milling machine table, its position can be set using a DTI so that
the button runs true, it is then removed and the required bore made.
Another application is to set hole centres, say for a pair of gears. If 10mm diameter
buttons are being used and the centre distance required is 87.5mm the side plates
which carry the spindle bearings would be marked out at this dimension and holes
drilled and tapped, again accuracy is not critical. The two buttons would then be
fitted and set across their outer diameters to 97.5mm (centre distance plus the diameter
of one button). One button would be set to run true, Photograph 1, then removed and
the first bore made, Photograph 2. The workpiece would then be moved and the operation
repeated for the second button thus ensuring that the required centre distance has
been achieved. Note the second button seen in both photographs.
Toolmaker's buttons have precise diameters, probably within 0.002mm, but this level
of accuracy is rarely required in the home workshop and they can be another candidate
for a mini project, especially as it is not that easy to find a supplier. Normally,
commercial items would be hardened and ground but for the limited use in the home
workshop mild steel buttons should suffice provided they are treated with care.