Tools (8) : making lathe tools my way

Page created on 10/10/2024; updated on 24/10/2024.

(Mini) lathe tools

NOTICE! The lathe pros are probably going to jump out of their skin when they read the following. I beg their kind indulgence…

My need

In fact, I have a very specific need: the manufacture of insulators for the roofline of the Lima and Piko BB 17000. I haven’t found anything similar on the market. I’d thought of asking REE for a BB 16000 part, but there are too many differences.

Problem: I don’t own a lathe, and even if I did, I haven’t found a lathe tool that would be suitable for this application, in particular, a tool that can make triangular grooves with an inclination of about 22°. See this catalogue page, for example.

I only own a mini drill and a milling machine. The main disadvantage will be the lack of rigidity, implying a risk of vibration. Regarding the modified mini drill (see this page), it needs to be placed horizontally in its support. The tool will be fixed to the cross table. The X axis of the table will be the “turning” axis (so in fact the Z axis in turning terms, if I’ve understood correctly), the Y axis of the table being the one for facing, cutting, etc. (so in fact the X axis in turning terms, go figure…). In the case of the milling machine, I’ll be able to use the Z axis for longitudinal turning and the X axis for the other operations. The disadvantage of the latter solution is that collets are not well suited to gripping short workpieces: there will still be a fair amount of wasted length.

My idea is to use broken carbide 3.2 mm bits – I’m not short of them – ground to suit my needs. I’ll also need a sufficiently rigid toolholder.

The toolholder

The toolholder is based on a steel ballast block from an antique BB 17009 Lima purchased in 1972. And so we come full circle! For some reason, I had cut the block into two equal halves. We’ll see that both parts will be useful.

First, I have to properly straighten the faces of each block on my milling machine, using an 8 mm cutter, in 0.1 mm passes. The material is soft enough and poses no difficulty. However, I lubricate it with WD40.

One block is then drilled and tappered according to this plan.

  1. Hole for tool.
  2. M4 tapped holes for clamping screws.
  3. Hole for M6 fixing stud on cross table.

Making a tool

Starting with a broken carbide drill bit, using a diamond disc, I begin by machining a flat on the shank, which will serve as a reference. Grinding depth: 0.5 mm. It could be more.

Click on the photo below to zoom in.

Broken carbide drill bit

Making a flat on the shank

Note: the toolholder used here is an unsatisfactory test which doesn’t conform to the plan given above. In particular, it has only one clamping screw.

The future tool is turned over and locked by the clamping screw on its flat. With the toolholder turned 90°, I machine the first side face to a depth that depends on the desired thickness. Here, for a final thickness of 0.3 mm, on a 3.2 mm drill, I need to remove 2.9 / 2 = 1.45 mm on each side. I proceed in passes of 0.2 mm.

Machining the first side face

Click on the photo to zoom in.

The grinding of the opposite face is not shown.

Grinding the end, for a clearance angle of around 7 to 8°. Note the inclined holder for this purpose.

Grinding the end

Click on the photo to zoom in.

Grinding the cutting edge, again with the holder tilted to give a rake angle of just over 90°.

Grinding the cutting edge

Click on the photo to zoom in.

Here are some of the tools made. From left to right, three straight tools 0.3, 0.5 and 1 mm wide, then two tools with a 22° inclined face for machining insulators. They don’t look great, but if they do the job… That’s what we’re going to check.

Presentation of the tools

Application and testing

Here’s the general setup, with the drill swivelled 90° and lowered enough for the tool to be at the correct height. This is obviously only possible because I’ve fitted a Z-axis adjustment to the support.

General setup

Because the drill was coming up against the table, I had to raise the toolholder using the other half of the ballast used to make it, the half I used to machine the tools. I milled a groove the width of the toolholder, and 0.5 mm deep, so that the two parts are correctly aligned: this makes squaring on the table easier. Of course, a one-piece part would have been preferable. On the plan, the height of the toolholder is sufficient so that it doesn’t need to be raised.

You can see the fixing stud, taken from an M6 threaded rod, because the studs supplied with the table were too short.

Another view of the device.

Turning

Click on the photo to zoom in.

Machining parts

To make things easier, I dimension the part as shown on the drawing (example: line insulator). This way, the values correspond directly to the graduations on the cross table.

The Y axis zero is set flush with the workpiece, in this case a ⌀ 2.5 brass rod.

I proceed as follows, with only the 22° tool. Given that this tool is far from perfect and the fixture lacks rigidity, I can only proceed very carefully, in 0.1 mm passes.

  1. Longitudinal turning the foot over 2 mm, depth 0.6, to obtain diameter 1.3, then the tenon over 1.5 mm, depth 0.75, to obtain diameter 1 mm which will then need to be checked and rectified.
  2. Turning the “plates”, depth 0.6.
  3. Turning the upper collar to obtain diameter 1 mm.
  4. Reducing the diameter of the plates, from left to right, successive depths: 0.26, 0.18 and 0.09 (all theoretical values!).
  5. Refining the reduced plates by taking up the grooves at depth 0.75 and moving back slightly.
  6. Using 400-grit sandpaper, folded in 2, to deburr and round off the corners a little.
  7. And finally cutting the finished piece with a jeweller’s frame saw.

I successively made four line insulators, two circuit-breaker supports and a roof penetration, without too much difficulty.

An example of realization

This example will be described in detail in the future article on BB 17041 Lima. This photo shows raw parts, that have just been soldered.

View of completed parts

Turning inserts, length 24 mm.
Bimu turning inserts 040line.

Proxxon Micromot 50/E +
Support MB 140/S +
Table KT 70, étau MS 4.

Milling machine BFW 40/E +
Primus 75 vice.