Make a Micro DRO/lathe travel indicator for less than $10

Magnetic base with 10 mm travel and 0.01 mm accuracy
 

micro DRO


Want a digital read out that doesn't take over the lathe, is easy to make, simple to attach, and costs peanuts ?

I often wished I had a small magnetically backed probe or travel indicator which would easily attach to the cross slide and give readouts when setting the depth for gear cutting.

It didn't have to read out the whole cross slide travel, just the immediate range that I was using.  Reading out carriage travel for milling would also be handy.  0 - 10 mm range would be plenty.

I have a dial indicator, but it has the usual long probe housing, and takes time to set up.  I just wanted something compact I could toss in the drawer and use for quick readings as required.

Looking on the web didn't show anything suitable, so I decided to make my own - as cheaply and easily as possible.

The digital tyre tread depth indicators on Ebay looked to be suitable candidates, so I forked out my hard earned cash and eagerly waited for the post man to deliver. 

The unit was made of ABS plastic and seemed to be reasonable quality for the money. The only issues to address were how to put tension on the probe, and remove friction from the slide.

After thinking about the possibilities, I decided to keep it simple and sacrifice a bit of travel by putting a spring on the probe rather than connecting it to the slide.

The downside is that the probe must project the length of the compressed spring, requiring a larger area to attach the unit to.

You can get around this by mounting a spring in the base, but it complicates construction a lot more.   I did this later - see details at the bottom of the page.

What you need and how much it costs

One digital LCD car tyre tread depth gauge    -  $5.85 AU off Ebay (delivered),

Rare earth neodymium magnets (10mm x 2 mm) - $3.74 AU off Ebay (pack of 10 delivered),

4 small self tapping screws from an old audio casette case (optional),

A small metal washer,

A small compression type coil spring,

A small circlip,

Two pack glue such as Araldite,

A steel plate about 25 x 25 x 5 mm.

Making the DRO

Remove the battery.

Peel the sticker from the back of the readout to expose four small screws.

Remove these and the unit comes apart.

Remove the flat metal spring that applies friction to the slide and discard it (see above).

Alternatively you can flatten it so that only minimal pressure is exerted on the slide - this may improve reading repeatability as it will reduce any tendency for the beam to move sideways.

Wash the glue residue off of the ABS plastic back with some petrol (benzine).

Pull out the slide and file back the taper at the base of the probe so it's all the same size.

Cut a small groove just back from the tip of the probe for the circlip to lock into.

Cut one (or both) wing(s) off of the black ABS base to reduce size.

Fit the washer, spring and circlip to the probe.

Put the slide back in and test the spring for tension.  It should move easily.

Put the unit back together.

Cut a steel plate to the dimensions of the black plastic section adjacent to the readout unit.

Recess two 10 mm depressions into it for the magnets (optional). 

Araldite the plate to the black ABS back (or use the four small screws to attach it).

Araldite the magnets into place.

Let the glue harden for 24 hours.

Job done.

Use

The DRO is a 3 volt unit and has a noticeably faster refresh rate than the 1.5 volt caliper seen above.  Lag is minimal.

The display is easy to read and the scale is reasonably well shielded from contaminents.

The modified travel range is actually 0 - 14 mm.

Here are some shots of the DRO in use.

cross slide read out

And reading carriage travel.

DRO

Notes

I attached the plate with screws as I was unsure if Araldite would stick very well to ABS plastic.  I have since discovered that it will. The screws are therefore not necessary.

Four magnets in a square is a much better setup.  It's also advisable to mount the magnets on/in steel as this gives a much greater magnetic effect than with aluminum or plastic.

As I primarily made the unit to readout the cross slide, I kept the weight down as much as possible and have the maximum magnetic effect at the probe end so it can attach to the smallest possible area.  It's also why I made the later version with a flush fitting probe.

Make sure you have the magnetic poles all pointing the same way.

The magnetic grip is very strong onto a bare metal (steel or cast iron) surface.

However, the magnets do NOT grip very well onto a slippery enamel painted surface such as the tailstock.  In this situation it would be advisable to mount a steel plate it can grip to.

Other versions of the Micro DRO concept

Several adaptions of my original Micro DRO have followed from other builders, including a version which does away with the spring altogether and uses a magnet on the tip of the probe to maintain contact.  So plenty of options to choose from if you search the web.


After proving that the original concept worked, I reworked the design into a longer travel unit with the compression spring housed in a lightweight alloy base.

This is a much better unit, but more complex and significantly harder to make.

cheap DRO probe

The main advantage is that the probe tip can come flush with the face of the unit.

This allows it to be attached to a much smaller area.

Seen below clinging to the last 15 mm of the exposed cross slide base.

home made DRO

It works extremely well, and has the same responsive readout. 

Here is a short video of it in action.


The improved layout of the magnetic base is more powerful.

It also gives a greater measurement readout (0 - 18 mm with this version).

Click here to see construction details on the next page.


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gloaming_agnet@hotmail.com

South Australia

2017