A HANDWHEEL DIAL FOR THE MINILATHE

By Graham Meek. Part one.

Like many of the lathes on the market for the home machinist the Minilathe has no means of measuring off turned lengths easily. No doubt many users are quite content to make do with a rule measurement but this then restricts the potential that these little machines can be put to. While I have seen many of the cheap digital calipers cannibalised and fitted to these lathes to perform this function there is a trade off in the amount of room that these attachments take up. While it is a perceived view I cannot help feeling that these add on’s must get in the way at some point or other, plus keeping coolant and debris away from the workings of the caliper must also present some problems.

When my friend started using his Warco version of the Minilathe in earnest, this measurement shortcoming started to be a real problem and it was not long before he popped the question about adapting an earlier Myford design to fit the Warco. Unfortunately, it is not that simple as the Myford travels approximately 0.8662 or 21.99mm for one complete turn of the hand wheel whereas the Warco travels 19.28mm per handwheel revolution.

Originally when the design for the Myford was being considered, a double reduction train was chosen to get the desired reduction ratio and to give a dial that visually looked as though it was part of the original fitment. Using the same approach with the Warco would have lead to the dial being considerably bigger than the handwheel itself. Discussing the matter over with Anthony Rhodes in Berkley, California resulted in a much more compact approach simply by using one simple reduction and a pair of 1:1 gears.


This, therefore, is a principle that can be applied to any lathe that requires a handwheel dial, if the distance travelled for one complete revolution of the handwheel is divided into the desired travel of the dial then this will give the required ratio. To get a more accurate reading of the distance travelled per handwheel revolution it is advised that a multiple of handwheel turns is used and this multiple is then divided into the measured distance travelled.


Thus taking the distance travelled for one revolution of the handwheel on the Mini Lathe as 19.28mm and dividing this into 25mm for the distance travelled for one complete revolution of the dial we get 1.296. It does not take long with a calculator or spread sheet to work out that a 27 tooth gear driving a 35 tooth gear will give a reduction of 1.296296, if this is multiplied by 19.28mm, then the actual distance travelled for one revolution of the dial is 24.992mm, in other words although our dial shows we have travelled 25mm we are in actual fact short by 0.008mm. Working through the above but for a 1.000 inch travel per revolution of the dial we find that a 25 tooth gear driving a 33 tooth gear will give an actual distance travelled as 1.0019”, which is not quite as accurate as the metric version but none the less nearer than I can get with a rule these days.

The next thing to decide is the DP or Module of the gears as this will have a direct bearing on the overall size of the dial, while I have 0.5 MOD cutters it was felt most home machinist’s would be more likely to have 40 DP, especially if they were building any of the published designs on internal combustion engines by ETW. The 40 DP cutters are available on the Internet at very reasonable prices in 14.5º Pressure Angle, (PA), which is absolutely fine for this application. Once the size of the gears is established the design proper can start, while it would be nice to have a handwheel pinion shaft going all the way through to the handwheel from a strength point of view, this does make for a more involved construction that requires some dismantling of the lathe in order to fit the dial, this method of construction I considered would put many constructors off.

I felt an ‘add-on’ dial would also be of benefit should the reader decide to upgrade his or her machine at a later date, the dial simply being transferred to the new machine. Therefore, the design that was settled upon follows closely that of the Myford Super 7 in that the extension pinion shaft is pressed into a radial ball bearing, (68022RS), mounted in the dial backplate, which is retained to the apron face with two M4 cheese head crews. The use of the radial ball bearing has several duties in that it holds the two components together both concentrically and squarely, it also negates any wear in the apron handwheel bearing that might be present, which would cause no end of trouble with the internal gearing of the dial as the saddle was traversed up and down the length of the bed, which would also do little as regards the accuracy of the dial.

Two M4 grub screws at 90º to one another lock the extension shaft onto the existing handwheel shaft, but the size of the shaft needs to be checked as I have found that the Warco has an 8mm diameter shaft while the Sieg C3 has a 9mm diameter shaft, (this size being shown on the drawing), two dimples or flats for these grub screws to impinge on will be found advantageous, as well as applying some thread lock to the grub screws on final assembly.

I did consider the use of a cross dowel through the extension and handwheel shaft as a more positive means of fitting the extension shaft, but to perform this operation effectively would mean dismantling the apron to be able to mount the parts in a machine vice, again I thought this would put potential constructors off, but if the reader feels confident about this operation then this is a worthwhile upgrade and the grubscrews can be dispensed with.

As regards trying to do the cross drilling operation free hand, this is fraught with problems and on no account would I advise this method, or the substitution of the solid cross dowel with a Roll or Seloc Pin. With the constant back and forth motion of the handwheel the Roll Pin would distort or even shear under heavy usage.

The dial is mounted on the sleeve gear such that a friction spring allows the dial to be zeroed at any convenient point. In an attempt to keep costs down it was decided to make the sleeve gear from aluminium instead of phosphor bronze (PB), the aluminium is more than suitable for this application but if it is felt more appropriate to use PB or brass then I leave that decision to the reader. The dial itself is graduated with 100 divisions giving 0.25mm or 0.010” per division depending on which dial is under construction.

As with all these dials concentricity of the relevant parts during manufacture is essential, but this is not as bad as it sounds especially if a little attention to the steps of construction are followed. Before work proper can start it would be a good time to make a 22mm diameter plug gauge as this will be needed to size the backplate and the dial. George H Thomas (GHT) gives an excellent account of the preparation and use of plug gauges in his The Model Engineers Workshop in Chapter 7 on Boring tools, page 97, (in my first edition), and also in his companion volume in Chapter 8 of the Universal Pillar Tool.


The backplate (front view above) was the first part to be made; this started off as a piece of 2.625” diameter EN1A Pb (Leaded), the blank was faced off and bored to 22mm diameter. The part was then held in the reverse jaws of the three jaw chuck with the machined face hard up against the jaws and the overall thickness brought to 7.5mm, this needs to be kept parallel to within 0.05mm or 0.002”, for the time being this part is put to one side.

The dial blank was the next item to be tackled again made from the same material; the blank is faced off and bored through 22mm to the previously made plug gauge, while at this setting take a skim over the outside diameter to bring it to 66mm. Again reverse in the jaws and bring the overall width to 20mm, the outside diameter can be turned down to 65.50mm diameter by 11mm long, but no longer, (see enlarged view of undercut detail). The internal recess can now be formed 8mm deep by 60.5mm diameter, this diameter can with advantage be made bigger by up to 0.25mm to ensure that it clears the idler gears.


The dial is then reversed in the chuck jaws and gripping on the 65.5mm diameter such that the 65mm diameter step is machined 3mm long as well as the 1x45º chamfer. At the same time a groove 1.25mm wide by 64.5mm diameter is machined 10mm in from the outer face. The chamfers at the junctions of the knurled portion and the 65mm diameter as well as the 64.5mm diameter can now be machined at 45º. The final operation at his setting is to produce the straight knurl, this is best carried out with the straight knurling wheel slightly skewed towards the tailstock by about 2 to 3º such that only the leading edge of the knurl contacts the work. When a satisfactory depth is reached then the tool can be repositioned square on to the work and a couple of very light burnishing passes taken to crisp up the knurl.

A mandrel now needs to be turned to 22mm diameter with a central M6 tapped hole, the spigot needs to be about 10mm long, this is best turned out of a short bar end of about 38 to 50mm diameter. The reason for this being that a larger diameter then offers some additional support in keeping the part square on the mandrel. You will also need a clamping washer about 25 to 38mm diameter by 3mm thick with a 6mm clearance hole. I usually start such mandrels by turning down a spigot portion that is slightly over the length of one of the steps in the external jaws, the diameter of this spigot is usually made a diameter that matches the curvature of one of the internal diameters of the external jaws, before reversing in the chuck to do the mandrel proper, it is also good practice to mark with a felt tip marker or a metal marking stamp some reference point such that this mandrel can be returned to exactly the same position in the chuck, thereby ensuring maximum concentricity.

First up is the backplate, this will need a tube spacer and the clamping washer to secure it to the mandrel, this is turned to 65mm on the outside diameter and a 0.25mm by 45º chamfer machined on one edge, the opposite edge needs the sharp corner taken off with an India slip stone, this now completes the turning for the moment on this part.


Addendum

Since writing this article it has been brought to my attention by an enthusiast who built an Imperial version of this dial in advance of this publication for his Clarke Mini Lathe, that it has a different distance of travel for one revolution of the handwheel, which in his case was 0.766” or 19.456 mm. His gear train solution for the imperial dial was two gears of 30 teeth to transmit drive to the idler shaft and a 26 tooth gear on the idler shaft driving a 34 tooth sleeve gear. This gives for the 01” dial version a theoretical travel of 1.0016” per revolution, an over travel of 0.0016” and is marginally closer than my first design.

I have also worked out an equivalent metric version for this lathe and the gear train is as follows, two 24 teeth gears transmit the drive to the idler shaft and a 21 tooth gear on the idler shaft driving a 27 tooth sleeve gear, the bore of this sleeve gear would need to be reduced to 15 mm diameter and the journal on the extension shaft also reduced in order to give sufficient strength to the sleeve gear. This gives for the 025 mm Dial a theoretical travel of 25.01 mm that is again an over travel of 0.01mm, which is not quite as close as the original design, but close enough not to split hairs over.

I need hardly say that the original gear centres on my back plate drawing now need modifying, the coordinates for the Imperial version are now 13.47 by 13.47 mm and the metric version 10.77 by 10.77 mm A check was done after the dial was mounted on the lathe and over an indicated 1” travel on his dial the lathe carriage actually moved 1.002”, this goes to show that for a very little outlay a high degree of accuracy can be achieved.

Can I, therefore, urge the reader to check his lathe before he commences work on this dial, as it appears there are many more anomalies associated with these Mini Lathes than I was first aware of.


PART TWO HERE


Graham’s description of the dial for ML7 series lathes can be found in his book: Projects for Your Workshop from TEE Publishing.