Carriage DRO

September 2015

I decided to add a DRO to the Z-axis of my Hobbymat / Prazimat MD65 lathe as I often found it difficult to accurately measure along this axis. Also measuring can be time-consuming, so I added the DRO as a time-saving tool.

This is not a large lathe, and getting a digital caliper in there can be difficult especially if the tailstock is in use, or I don’t want to move the cross-slide. Setting up a DTI is feasible but again often difficult as most of the lathe is aluminium, making it more difficult to use a magnetic base.

Unlike most of my other projects, I didn’t CAD this up before I started. Once I’d settled on the design, construction was a matter of offering materials up to the lathe and marking cut lines and holes.

Like the DROs for my mill, I bought a 500mm unit from machine-dro.co.uk – their service is excellent and the DRO arrived in a sturdy wooden box (a short custody battle ensued with Mrs Naut, who has a bit of a thing for boxes).

Moving the switches

In order to get the scale to measure the full length of travel (important for me), I had to move the switches a bit to the left. I added a chunk of ally under the section where the DRO end attaches, to give it some extra rigidity.

Fortunately the existing holes in the lathe baseplate are conveniently spaced in whole centimeters, making it easy to drill and countersink the mounting screws.

I also had to move the switches out a bit (~10mm) as they fouled the electrical connection block underneath the lathe. To do this I milled a rectangle out of a block of 10mm ally, and made up a faceplate out of 1.5mm ally sheet. I used a step drill to drill out the holes for the switches.

I really should get some new switches as the original ones are quite doggy, but there was enough to do on this project already so I kept the originals (also I have grown attached to the them). The 22mm mounting hole for the switches is a standard size so I should have no trouble finding replacements in the future.

Z-DRO switches new plate

Not the best picture… you can see the yellow-shrouded wires going through the rectangle milled into the 10mm ally plate, and to the right of this a “chunk-o-ally” to which the DRO end is screwed. The switches are mounted to a piece of 1.5mm ally plate, which screws into the existing mounting holes in the lathe baseplate.

Z-DRO switches completed

The completed switches and DRO mount, with the original switch mounting plate included for reference.

Swarf protection

I spent a lot of time thinking about how to protect the scale from swarf and oil. Sliding plates, a labyrinth-type seal, brushes etc etc – in the end I decided to protect the part of the scale that is directly under the cutting tool. My reasoning is that this will protect the scale for 90% of jobs, and for the other 10% the scale is easily reached to give it a clean.

The design of the MD65 offers a natural home for the long part of the cover – it just slides on over the flat surface directly above the switches.

A more complicated arrangement would better protect the scale, but would also involve a lot more work…

The clear cover is 1.5mm perspex, mounted on a 290mm length of 3mm aluminium angle. The gap between the angle and the lathe baseplate is filled with a strip of felt (approx. 5mm thick).

The aluminium angle is attached to the carriage via two M3 screws. The perspex is screwed to the angle also using M3 screws.

There is a 40mm gap milled in the horizontal part of the aluminium angle, to accommodate the top mounting bracket for the reading head. This allows adjustment of the reading head brackets without the need to loosen the aluminium angle.

Z-DRO felt runner

Perspex cover – it will never be this clean ever again. There’s the felt under the aluminium bracket (orange arrow). “Is that felt?” asked Mrs Naut, rubbing the material between her fingers. “It is now”, I replied suavely.

Mounting the scale

(The scale is the long part with all the numbers on it. The reading head is the plastic box that slides up and down the scale and has the digital readout on it).

Because the lathe baseplate has a slight angle on it (i.e. not at right angles to the flat top of the baseplate), I milled a couple of wedges for the scale mounts. The goal was to mount the scale as near to vertical as possible. As it turned out, my angle brackets to mount the reading head to the carriage were not square, so I ended up reversing the wedges.

I drilled and tapped a small (M4) hole in the end of the lathe baseplate to mount the right hand side of the scale. The left-hand side is mounted on the modified switchgear block.

Z-DRO wedges

The thicker spacer is a wedge, the thinner one is 1.5mm of ally to match the 1.5mm ally that the other end is mounted to. The side of the lathe baseplate slopes out at the bottom, so this wedge is actually upside down (but matches my not-square reading head mounting brackets… I’m calling it a feature).

Attaching the DRO reading head

I attached the reading head to the carriage via two ~2mm thick, 40mm wide sections of mild steel angle plate. The standard bracket was too short and does not allow for easy adjustment.

I milled a slot in the aluminium angle so the reading head brackets can be adjusted up and down without the need to loosen the aluminium angle. The screws that mount the top bracket to the carriage (the two larger cap screws) go through the aluminium angle and into the carriage.

Z-DRO reading head brackets

After mounting and leveling the scale, measure the required dimensions for the two angle brackets and make up the blanks. The steps to mount the reading head are straightforward – I used 2.5mm pilot holes to mark out transfer holes between the two brackets and the carriage. The top mounting bracket is screwed to the carriage via two M4 screws. The bottom mounting bracket is screwed to the back of the reading head with four M3 screws.

(Let me know if this process needs further explanation – I didn’t take many pics for this project and a written explanation is likely to be long and tedious to write and read.)

Z-DRO reading head bracket

The bottom mounting bracket attached to the back of the reading head. In the top part of the angle you can see the two M4 tapped holes where this bracket attaches to the top bracket.

Leveling the scale

I used a DTI gauge on a magnetic base, mounted to the carriage. Line the DTI up on the scale, and crank that handle to move the carriage up and down the lathe. Adjust one end of the scale as necessary until the scale is level to within 0.05 or so.

Do this on the top edge of the scale (as shown in the picture below), and also the outside (vertical) edge. All of the scales I have bought have a slight amount of bow in them, so don’t tear your hair out getting it absolutely level (if you are lucky enough to still have some hair that is…).

Z-DRO levelling

Leveling the scale prior to marking out holes for the mounting brackets. Repeat after you have made up the brackets and attached the reading head. I am not sure what is going on with my camera – the scale does have a slight bow in it but is not wavy like it appears in the pic above.

Next steps

I am planning to attach a remote display to make the DRO easier to read, but it’s fine for now. I have also bought a 50mm digital DTI which I am going to attach to the cross-slide as a X-axis DRO – more on this later…