Carriage stops

April 2017

I had seen carriage stops on other small lathes and could see the usefulness of these little devices – as an aid to accuracy, a safety feature, and also time-saving.

Plans for the parts are here.

Typically I set the stop about 0.1mm short of the required depth (using my carriage DRO), and then remove the stop for the final pass. This improves accuracy and is also time-saving as less effort/concentration is needed to avoid overshooting the target depth.

I completed the stops and have been using them continually over the past couple of years. I am very happy with the design and use. If the bar gets covered in swarf then it’s necessary to give it a quick brush off before moving the stop, otherwise it can jam on the swarf. I don’t really use the tailstock-end stop.

Design

I opened my CAD model of the MDF65 and started messing about. The natural place for such a device is the bar that runs over the top of the leadscrew. However, I wanted to make sure that the stop would not interrupt the full swing of the lathe.

I added a swing arc circle to my CAD model, and fiddled with the design until it (just) cleared the swing. I have had no issues with the stops getting in the way after more than a year of use.

The carriage stop is bottom left in blue, the max swing is the blue 130mm diameter circle. The swing just kisses the stop, but in practice I would swing something less than 130mm to reduce risk of hitting the ways.

My other design requirement was that the stops can be moved the full length of the bar without fouling either the tailstock or my fixed steady. There is something less than 1mm clearance between the stop and the tailstock body.

Lastly, I wanted to avoid raising burrs on the bar due to the clamping screw. To achieve this I added a small sliding angle between the clamping screw and the bar – you can see this in the pic above.

Fixing the bar

The only problem with the design was that this bar is not fixed in place, rather it is held by pins at either end and is free to move about a little.

Therefore the first job was to fix the bar in place. I did this by drilling and tapping a pair of M5 holes into the head, to hold a small piece of angle which was in turn bolted to the bar.

The angle is 15 x 15 x 2mm bright mild steel, 95mm long.

I chose the positions of the holes in the head carefully to be both wide enough to provide a strong structure, and also to avoid anywhere that may be structurally critical for the head.

First I cut and shaped the angle, and drilled the four mounting holes to 2mm to act as punch guides (two to the head, two to the bar). Next I set the angle on top of the way, positioned it and centre-punched the two holes in the head. Drilling the holes was nerve-wracking as it is very difficult to get a drill in there due to clearance issues with the way.

Once the angle was in place, I transfer punched the positions of the mounting screws into the bar. The two small screws attaching the angle to the bar are M3 CSK. I used M5 capscrews to mount the angle to the head, as this gave some ability to adjust the positioning – I thought I might need this after the difficulty I had drilling the holes in the head.

Once all the drilling and tapping was done, it was a simple job to screw it all together.

Carriage stop body

The body of the stop is comprised of three pieces, which are brazed together. The material is bright mild steel.

Machining is straightforward, the only advice I have:

  • I used a T-slot mill to machine the undercut in the angle piece that clamps to the bar
  • in the top section with the stop screw, I drilled a 3mm pilot hole before brazing – I drilled the hole to final size and made the slit after brazing, this was to improve the final appearnace of the part
  • I would suggest making all pieces 0.5mm oversize, and machining to final size after brazing – again to improve the appearance of the finished assembly

Finished pieces before brazing.

Set-up for brazing.

My first attempt at brazing – I didn’t realise that it would make such a mess of my lovely shiny parts! The result was not pretty as you can see, but a quick visit to the mill and some rubbing with sandpaper on plate glass soon turned my ugly ducklings into…

…beautiful swans. Proof, if it were needed, that you CAN polish a turd if you have the right tooling.

Nuts & bolts

Final finishing involved adding the clamping slit, and drilling & tapping the various holes. I used teeny-tiny M3 capscrews to hold the movable clamping angle in place – these are not tightened, allowing the clamp angle to move.

Underside showing the M3 capscrews. The clamping capscrew is M5 – oversize for this application but I used M5 as is the same allen key as the toolpost clamping capscrews.