Not a novel technique, but new to me as a beginner at lathe work. This instructable just shows how I made a series of radial marks on a couple of aluminium alloy discs on my Myford ML10 lathe. The discs in the photographs are 25mm (1") in diameter.
I do not have a dividing head or indexing plate, so I needed an alternative method. Here I use the thread-cutting gearwheels that came with the lathe.
The purpose was to roughen its surface so that it would grip onto some cord (the discs form a pulley wheel). This application is probably not very interesting to others, but (more interestingly) the technique is normally used to engrave scale markings on dials for tools and equipment.
Here I simply use the easiest number of radial lines (25), but the technique could be adapted for a wide range of line numbers, by using the 'normal' thread-cutting change wheels (gear wheels).
Everything is done by hand, with no electrical power applied at all. The engraving cuts are made by digging a cutter into the aluminium (using the cross-slide) and pulling it through the aluminium (using the top-slide). Everything is 'done by numbers' (using the slide dials and a change-wheel gear).
Step 1: Set Up for Achieving Equal-angle Steps
The spindle on my lathe has a chuck on one end, and a 25 tooth gear-wheel (cog) on the other. The gear-wheel is used to make sure that there are 25 equal-angle steps.
A springy strip of steel was clamped (with a G clamp) onto the lathe's cover support-rod. The steel rested gently on the gear teeth and the spindle (ie the chuck) can then rotate freely in one direction, but is stopped from rotating in the opposite direction by the springy steel strip catching on one of the teeth. Almost any bit of thin metal could be used; there is virtually no force on it.
The process of creating the engraved lines is:
- Rotate the chuck until the strip drops off a gear tooth into the gap
- Engrave the line with the cutting-tool (see later)
- Back-off the cutting-tool
- Rotate the chuck until the strip drops off the next gear tooth into the gap
- ... ... etc. etc
Step 2: Set-up for Engraving the Lines
I removed my 4-way tool-post and went back to my 'elephant foot' tool-post.
I made the cuts by putting the cutting tool (a sharp HSS 60 degree threading tool) on its side in the tool-post, digging the tool into the aluminium (by 20 divisions on the top-slide dial) and pulling it away from the lathe axis (ie towards me) by rotating the cross-slide handle.
I was not very concerned about the engraved lines being exactly radial, so I did not spend much time setting the point of the cutting tool on the lathe centre-line. To get radial lines, this alignment is critical.
The whole process was done with reference to the cross-slide dial and the top-slide dial. It went something like this.
- Zero the top-slide dial to the surface of the aluminium
- Zero the cross-slide dial to the smallest radius required (the inner-most point of the engraved radial line).
- Make sure the chuck is touching a gear tooth (ie the backlash is taken out of the rotary movement).
Then, for each tooth/line:
- Dig the tool into the aluminium by rotating the top-slide by 20 division on the top-slide dial.
- Cut the line towards me, by rotating the cross-slide handle until the tool leaves the aluminium.
- Move the tool away from the workpiece (by rotating the top-slide handle through about 30 divisions).
- Move the tool back towards the centre of the workpiece until the dial is zeroed.
- Rotate the workpiece by one 'gear tooth'.
- ... ... Keep going for all 25 teeth/lines.
Control the backlash
To get an accurate set of lines, it is important to take the backlash out of each of the three movements.
- The top-slide is obvious - you always approach the workpiece from 'above' the surface and this takes up the 'slop' (if for any reason you have to back-up the tool, it must be backed-out by a big distance, and brought in from 'above' again).
- The cross-slide should first be taken in towards the centre-line and then brought-out (towards the operator) until it is zeroed.
- The chuck should be rotated until the steel strip drops between the teeth and then rotated back until it jams up against the tooth.Cutt
Step 3: Engraving Lines on a Sloping Face
The same basic method is used for this piece, but there are several differences due to the different shape of workpiece. Take a look at the photos to see the differences:
- The tool was dug into the aluminium using the cross-slide.
- The cuts were made by moving the angled top-slide inwards towards the lathe axis (away from the operator).
- Vertical alignment of the tool with the lathe centre-height is critical, if perfectly radial lines are needed (I was not bothered).
- Controlling backlash is vital.
Step 4: Different Number of Divisions + Finished Result
Different numbers of engraved lines
One way of doubling the number of lines would be to cut the first 25 lines and then slacken off the chuck, rotate the workpiece until the cutting tool was half-way between two engraved lines and clamp it up again. Then repeat the next 25 lines using exactly the same method as the first set. This would not be a very accurate halving of angle, because it would depend on the accuracy of the initial positioning.
Accurately increasing the number of divisions (25 to 60)
This could be achieved by arranging for the steel strip to rest on one of the other gear-wheels in the leadscrew gear-train. It would need some ingenuity to position the strip, and backlash would have to be removed in a similar way to that described earlier.
On an ML10 it is not possible to increase the number of engraved lines (ie decrease the angle between them) by a large amount. With the standard set of change-wheels 75 divisions is probably the maximum sensible number. (A larger number of divisions would need gearwheel pairs on both studs on the banjo, with the primary pair having a small gearwheel in contact with the 25T spindle gear, and a larger gearwheel linked to it on the primary stud. This combination is not possible, because the 25T spindle gear would foul on the larger gearwheel).
In fact, the simplest way (possibly the only sensible way) of changing the number of lines is to have the gearwheels positioned as shown in the photos. The 25T spindle gear drives through an idler gearwheel (any number of teeth) onto the secondary stud which has a 25T gearwheel in contact with the idler and a 50T (say) gearwheel linked to it on the stud. This combination gives 50 divisions. The number of teeth gives the number of divisions. Ie 50T gives 50 divisions, 40T gives 40 divisions, etc. It seems to me that the useful gears from the standard set are 20, 30, 40, 50, 60, 70 (I can't see much use for the others 21, 35, 45, 55, 65, 75 except for their divisibility when producing small numbers of engraved lines - see below).
Accurately decreased the number of divisions (2 to 25)
Fewer numbers of divisions are achieved by using the method above and skipping teeth. For example, to get 4 divisions use a 40T gearwheel and only engrave using every 10th tooth. (As far as I can tell, the following numbers are not possible using this technique 13, 17, 19, 22, 23; perhaps another arrangement of the gear train might make some of these possible).
Step 5: Another Solution
Using the bullwheel (the large gearwheel just behind the chuck - used in conjunction with the backgear to make the lathe rotate very slowly) is evidently another solution. For the Myford ML10 the bullwheel has 65 teeth and the backgear cluster (which engages with it) is a combination 21T/56T gearwheel - neither of these look much use for the task in hand. However, in other machines, this could be very useful (eg a 60T bullwheel could be used for 2, 3, 4, 5, 6, 10, 12, and 15 divisions).
The two images above came from google images.
The first from madmodder.net (title "Reparing a Cub Lathe") - the original images seems to be unavailable
The second from homemodelenginemachinist.com (title "Mods to a Super 7").