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Milling machine – how to make – graduating
Graduating is often done on the lathe. With round workpieces this makes sense. The surface is turned in the chuck using some sort of indexing system. The graduations are then cut with a very thin ground point made of HSS held in a tool holder. If the point breaks or needs to be re-sharpened it is very difficult to make it exactly the same size as before.
With a milling machine the possibilities are more numerous and the ways of doing the graduating more interesting. Not only can we graduate round surfaces but we can also graduate linear scales on flat surfaces.
Instead of using a piece of HSS we can use a rotating cutter – usually a slitting saw. If it is reground to sharpen it, the width of the cutter, and hence the slit cut, will still be the same.
What appears to be a problem really is the assumption that what matters for accuracy is the thinness of the line. The thickness of the line matters if we are trying to estimate the position of one line on one part of a system to a space on a scale on another part of the system. But, in practice, if accuracy is important, what happens is we have one scale and a Vernier to compare the scale against. We do not look and estimate spaces. We look for where two lines match up. What is needed for this are lines of very constant width.
Comparing the position of lines is much easier if the thickness’s of all of the lines on both the scale and the Vernier are the same width.
Usually, when using a vernier, one part is fitted with the vernier, the other part is fitted with a normal scale. consider the movement of the stage on a microscope.
fig scales on a microscope stage
The scale is in 10’s of millimeters. The Vernier is a small scale with just 10 spaces but these spaces add up to just 9mm on the other scale.
The resolution of this system is size of the space on the first scale divided by the number of spaces on the Vernier. On the height gauge shown the scale is, again, in millimeters but the Vernier has 50 spaces so the scale can be read to an accuracy of 0.02mm.
The limit is where it is not possible for the user to find only one pair of lines that match up for any position of the scale against the Verner.
Setup for graduating with a slitting saw
The slitting saw has to be held on an arbor. It can either being on a horizontal arbor fitted to the horizontal spindle. Alternatively, it can be fitted to a stub arbor. This can then be held in the vertical spindle.
Graduating – using the vertical socket
In this situation a round workpiece could be held using a dividing head.
A linear scale can be held on an angle plate – covered later.
Graduating – using the horizontal socket
This is the natural way of doing most graduating problems. As a round shape would be held using a dividing head – see later. A linear scale would be done with the workpiece clamped to the table.
Though this is the most straightforward setup the maximum length of the scale that can be cut is the length of the y travel which is always less than the x travel. (See later).
To cut a long scale would require the workpiece to lie along the x-axis with the cutter to be in the y/z plane. It will be seen that this problem is the same as cutting a long rack. This is covered later.
For long scales it is always possible to use off the shelf metal rulers. If we want accuracy what we need in addition to this is a Vernier. This only has to be a small fraction of the length of the full scale. If we do this the width of the graduations must match those on the ruler. So we can cut this in any of the three x, y or z-axes or a circular one without any difficulty.
For a scale to be easily readable, some graduation will be longer than others. On a decimal scale the longest lines will be on 0, 10, 20, 30 etc. The next longest will be 5, 15, 25, 35 etc. Then the shortest might be a 1, 2, 3, 4, etc. The easiest way to do this is to use a stop to set the length of the graduation. The most foolproof method of getting longer graduation in the right place is to do all of the graduations as short ones first. Then move the stop and do the next length, say every 5, for this length. Then move to stop again and do the longest graduations at every tenth line.
It will be found that the longer cuts will coincide with the existing short cuts so well that it will not be at all obvious how it was done.
If the scale required is unusual, for example for a Vernier, or if the milling machine has imperial lead screws and the graduations needed are to be metric then the safest way of getting it right is to use a spreadsheet to list the DRO reading for each line.
Fig. 288 – listing of readings needed for a metric Vernier – xxx
With the slitting saw it is easy to cut a slot deep enough so it can be filled with coloured wax. On the other hand, if the cut is too deep and can be seen on the edge of the graduated surface this might not be thought of as desirable.
Making the Vernier scale