THERE were two things that bugged me when I started using my new Emco Maximat Super 11, one was the fact that the zero plate fitted to cross slide dial of the lathe was on the skew and the other was the friction setting dials were very stiff and would often move while I was trying to change the setting. It was, therefore, decided after a short period of ownership to do something about both these problems. Some might say that the askew zero plate is not such a big thing and could be lived with. I must agree. I thought that at first but it is surprising how this just keeps niggling away until finally it must be rectified.
While tackling the zero plate problem I thought I could possibly address two problems at once. The standard graduations on the Maximat are 0.05 per division of the dial off the diameter, taking 0.025 off the diameter of the work is easy as it is just a simple sub-division. Now let’s suppose I want to take 0.03mm off the diameter. Half a division as pointed out is 0.025mm that is not enough, three quarters of a division is 0.0375mm which is too much, dividing the dial division into less than this becomes pure speculation.
It was therefore decided to make a Vernier scale to replace the askew zero plate, this would then give a positive displacement of the cross slide instead of the hit and miss set-up that then existed. Now there are two ways to approach the manufacture of such a scale. A short piece of material can be rolled into a curve with some bending rolls to fit the cross-slide feed screw bracket, this can then be mounted on a suitable mandrel and the divisions put on for the vernier scale by pure indexing. The second alternative is to make the vernier scale in the flat and then roll this up to fit the curve of the feed screw bracket as in the first case.
The dial having 80 divisions for a 2mm pitch leadscrew means that each division represents 0.025mm distance moved or 0.05mm off the diameter. If the vernier scale were to fit within the confines of the existing fixings of the current zero plate then the scale would only be able to encompass four dial divisions either side of zero or 0.1mm of actual cross slide movement. Therefore, if these four divisions were further subdivided into five then 0.1mm divided by 5 equals 0.020mm this then gives a difference between the dial division (0.025) and the vernier division (0.020) of 0.005mm, which is 0.01 off the diameter. Thus if one wishes to take 0.03mm off the diameter one merely moves 3 divisions on the vernier scale, and if I find I need to take another 0.01mm off then it is simply 1 division, with the original set-up this was done more by luck than judgement, what the vernier does is take out the guess work, one cut instead of several.
Cutting the 5 divisions of the vernier in the first method merely means setting up the dividing head to index 100 divisions, 2mm divided by 100 equals 0.02mm. As it is necessary to use the cross slide in the opposite direction when boring etc, then the vernier scale would benefit from 5 divisions either side of the zero line making 10 in all, if the reader looks at the vernier scale in the photograph supplied and the drawing then what is proposed is quite simple.
The centre of the plate is brought to the centre height of the dividing head by using a height gauge set to the dividing head centre height dimension. Once this has been achieved the dividing head is indexed through 90° in order to bring the centre of the plate to top dead centre. Then using an engraving cutter, mounted in the vertical head, similar to a V-form screwcutting tool that is set in-line with the dividing head axis but with an included angle of 40° and with a small flat on the tip of about 0.05mm. The zero line can now be engraved about 0.15mm deep by 5mm long, the dividing head is then used to index the remaining vernier lines which are 4 at 3mm long and the final line again 5mm long, this work can of course be accomplished in the lathe if a suitable indexing device is available, such as a 100 tooth change wheel.
There is no reason why the zero plate cannot start life as a turned ring, at least with the turned ring method there would be a chance of several bites at the cherry should a mistake be made.
Engraving the divisions in the second method is a little more involved, and as I always like a challenge this was the method I decided to choose, this is because the engraving is done by linear displacement and before the plate is rolled into shape. At first sight not too much of a problem except that the material on the outside of the plate is going to be stretched due to the rolling process and therefore each division will elongate from the original value to such an extent that the vernier scale might now occupy 4 and a bit divisions on the dial which is not really useful, unless some compensation is made before hand.
The linear displacement is easy enough to work out, it is simply the diameter of the dial which in this instance is 64mm multiplied by π to give a circumference of 201.06mm, this is then divided by 80 to get the linear length of one dial division, which is 2.513mm. Multiplying this by four will give the total length of the five divisions of the vernier scale, which is 10.05mm, this is then divided by 5 which makes each vernier division 2.01mm wide, if we ignore the 0.01mm for a moment this is a nice convenient figure, but what about the material stretch during rolling.
A similar problem was realized by George Thomas in his book Dividing and Graduating in chapter 5 and more specifically pages 46 and 47, the sub paragraph entitled “The Vernier Scale”. In this case George is wrapping a 0.060” thick by 0.750” wide strip partially around the table of his Universal Pillar Tool.
He says that the outside of the curve is 1.0102 times the length at the neutral axis, quite where he obtained that value I do not know, and I would welcome any information regarding on how to obtain this figure. However taking the circumference at the outside diameter and dividing this by the circumference at the neutral axis I managed to get 1.008, which is not too dissimilar to George’s value.
Dividing my 2.01mm dimension by 1.0102 we get a figure of 1.9897mm for the pre-rolling width of each vernier division, dividing the original 2.01mm dimension by the value that I obtained the reduced dimension is 1.9936mm. Multiplying both by 5 for the overall length of the vernier we get 9.9485mm and 9.9682mm respectively, which if taken away from the original 10.05mm gives a total shortening of 0.1015mm at worst and at best 0.0817mm.
However working to an engraved width of 1.9897 on the milling machine would be difficult on a dial graduated in 0.025mm per division, the nearest I could hope to get to with any degree of confidence was 1.9875mm. Not knowing with any certainty how George had arrived at his figure and not trusting my own reasoning I erred on the side of caution and decided to go with my Mentor, but using the revised reading of 1.9875mm.
As can be seen from the photograph the resultant vernier scale came out just as I had hoped with the lines of the vernier scale lining up exactly with the fourth line on the dial either side of zero.
Making the zero plate presented a few difficulties in that the difference in diameters of the dial and the cross slide feed screw bracket was such that the SWG gauges of plate material were either too thick or too thin and both these factors would introduce further problems to the stretching problem, which in turn would mean more calculations.
It was therefore decided to make a piece of material out of free cutting stainless steel, using stainless plate material was ruled out on the grounds that the plate would be difficult to form and machine. A 50mm diameter piece of stainless was faced off in the lathe and a disc parted off 1.5mm wide, cutting this disc across the diameter meant that I could get two vernier plates out of each disc.
A piece of gauge plate or ground flat stock was placed against the fixed jaw of the machine vice such that there was enough material protruding above the vice jaws to permit the width of the vernier plate to be machined. The half circle of stainless was then placed against this with the faced off side next to the gauge plate and such that the sawn edge was just proud of the gauge plate, the vice was then tightened.
The top edge of the half circle was climb milled with an endmill that way all the cutting forces are transmitted into the gauge plate and a good milled edge results. The endmill was then lowered sufficiently to cover the width of the plate plus a bit more for cleaning up, the side of the endmill was then used to bring the overall thickness of the material to the required dimension. Which using a micrometer is the required material thickness plus the gauge plate thickness, again it will be found beneficial to climb mill this material as the cutting forces tend to force the material into the gauge plate, milling this conventionally the material will constantly be plucked from the gauge plate and may even break the cutter.
If there is sufficient material above the vice jaw the material can be severed from the parent material with a thin woodruff cutter, but do no go to full depth leave about 0.05mm and again it would be best to climb mill. Alternatively the embryo plate can be cut from the stock with a hacksaw and with the previously machined edge resting on a parallel in the machine vice the sawn edge can be milled to size, but only have the bare minimum protruding above the jaws and do not use a blunt cutter as at this thickness the material will rather distort than cut.
Gripping across the width of the plate the ends were brought to length and the two countersunk holes machined. Then with the plate resting on a thin parallel which is gripped along the two edges in the machine vice such that the edge of the vernier plate was parallel to the vice jaws and the two ends of the plate were held with toolmakers clamps, the centre of the plate was found using an edge finder and the engraving cutter again mounted in a dead vertical spindle and the engraving carried out to the above dimensions. The plate was then rolled using a set of GHT’s Bending Rolls, as these do not produce a straight portion on rolled material if used correctly.
By carefully teasing the old index plate off I was able to extract the drive pins retaining the original plate without damaging the cross slide feed screw bracket. As one of the holes was off centre this was plugged with a turned piece of aluminium and secured with Loctite 603, the vernier plate was then attached using the existing hole and a countersunk self tapping screw salvaged from a dead video recorder. With the one end secured and the vernier plate at last aligned correctly a toolmakers clamp was put across the plate and the bracket whilst the pilot hole and hole for the self tapping screw were drilled using a hand brace, the second self tapping screw was then inserted and finally the toolmakers clamp was removed.
I can honestly say that the work involved in making this little item has paid for itself many, many times over and the lathe regularly removes exactly what I ask of it.
When I had my Myford Super 7 lathe many of the enhancements advocated by George Thomas were incorporated, with the exception of the non-friction micrometer dials. The reason for this was I had incorporated a modification of my own that did away with having to dismantle the cross-slide feed screw bracket each time when using the taper turning attachment.
I did, however, fit a cross slide feed screw lock, to use in conjunction with the friction dials, which proved very satisfactory. When it comes to fitting this type of lock to the Maximat there is a radial ball bearing in the way. A sub-plate between the dial and the cross slide feed screw bracket was considered but that would involve moving the vernier scale across to the sub-plate and also a new cross slide feed screw as this needs to be longer to accommodate the sub-plate.
This option was soon becoming a major work, a simpler alternative was needed, and it came out of the blue as often these ideas do. While adjusting the Verdict clock in my Starrett surface gauge I saw the solution, what we in the toolmaking trade call a ‘monkey’, the adjustable piece that goes up and down the pillar of the surface gauge and can be locked where it is needed.
If the reader looks at the photograph of the cross slide feed screw bracket there is a large void in which the revised monkey could fit, note also the two M4 tapped holes to mount the plate of the spindle lock. These tapped holes break into the radial ball bearing housing so this must be removed to carry out this operation, and carefully de-burred afterwards. The feed screw at this location in the void is wasted from 10mm diameter that the ball races fit on down to 9.5mm diameter; therefore any monkey that works on this reduced portion must have a 10mm hole in it to allow this to slide over the bearing location.
The mild steel thumb screw applies the pressure and the actual locking is carried out by a brass pad bearing on the wasted portion of the feed screw forcing the feed screw against the opposite side of the 10mm reamed hole of the mild steel monkey. If the 10mm hole of our monkey were allowed to
