Mini-lathe tools « Start Model Engineering

Mk lll micro DRO

Made from an inexpensive digital tyre depth gauge available on eBay this portable DRO shown in position to measure the travel of the cross slide on a mini lathe.

The Mk lll mini DRO is an evolution of a brilliant idea from Rob in Australia. Up to now setting up a digital read out system has been an expensive and labour intensive undertaking with questionable reliability. This little gizmo can be put together in around one hour at a cost of less than £10. It snaps into position thanks to the powerful rare earth magnets (neodymium) when required and can be removed just as easily.

I regard this DRO as ideal for use on the lathe to accurately measure travel of the cross slide and carriage. We can all get within a few millimetres when turning or boring but its that last few mm or thousands of an inch that gives us either the ‘spot on’ size we are looking for or relegates the workpiece to the scrap bin. No doubt with imagination this unit can be used equally effectively on the mill.

In Mk l and Mk ll versions a spring was an important element in the operation of this little unit. However in an Eureka moment my friend, also from down under, Aussie Jim, suggested scrapping the spring and using a small magnet on the tip of the probe.

Brilliant ! The following is a step by step guide to making your own DRO unit.

: For around £5 buy yourself a digital tread depth tyre gauge, there are loads of them on eBay.

: Remove the sticky label off the back and clean up residue with a solvent like brake cleaner.

: Remove the CP2032 button cell and remember to buy a couple of spares at the next autojumble.

: Take apart and relieve the pressure of the strip metal tension spring by straightening.

: Turn up a 3mm dia steel sleeve (not stainless) to slip over probe and take two 3 x 1.5mm rare earth magnets.

: The sleeve and the magnets are held in position with a dab of Araldite. Note the magnet protrudes slightly from the sleeve.

: A 2mm thick 1″ square steel plate is recessed to take four 9 x 1.5mm rare earth magnets.

: The DRO in position to measure the travel of the cross slide.

: Mk l with external spring, Mk ll with internal spring and Mk lll with no spring at all !!!

Mk ll version of Micro DRO

Tension version of Micro DRO in position measuring travel of cross slide

One criticism of the earlier version of my Micro DRO is the reduced length of travel resulting from the space required for the compression spring on the probe.

This effectively reduces travel by something like one third. The way round this is to use some form of tension spring or even elastic to ‘push’ rather than ‘pull’ the probe out.

A while back I bought a selection box of coil springs and this contained a number of unusually slim tension springs of just the right length. If anyone is stuck in obtaining a suitable spring email me and on receipt of a SAE I will send one on.

BTW those who may be curious about rare earth magnets according to Wikipedia Rare-earth magnets are strong permanent magnets made from alloys of rare earth elements. Developed in the 1970s and 80s, rare-earth magnets are the strongest type of permanent magnets made and have significant performance advantages over ferrite or alnico magnets.

There now, you learn something everyday !

: Micro DRO stripped down, flat tension spring flattened further and ‘wings’ removed from probe.

: Tension spring into position and held by small self tapping screws (ex discarded digital read out head).

: Horseshoe shaped steel magnet carrier plate araldited into position.

: Rare earth magnets fixed with araldite. Holding the little beggars in place isn’t easy !

: Completed assembly of tension version of micro DRO

: First compression spring version top with Mk ll tension spring version below.

Micro DRO for less than £10

Reading carriage travel - range is 0 - 13 mm

A digital read out unit that will improve your machining accuracy for less than £10

I can’t claim to have come up with this neat idea and credit where credit is due. I came across this tutorial via MadModder forum at users.tpg.com.au/agnet/micro.html and it really is worth passing on to you.

The idea is based on a digital tyre tread depth indicator currently available for just £4.99 on eBay. With a few simple modifications the unit will attach to your lathe as and when required to measure cross slide travel and carriage travel to an accuracy of 0.01mm. The unit can also be switched to imperial mode. It also has potential to measure travel on your milling machine.

You will also require some 10mm x 2mm earth magnets and I bought 10 for less than £4.00, again on eBay.

So how does it work ? I hear you ask. Well, lets suppose you are turning down an aluminium round, perhaps to make a 10mm diameter piston where precision is essential. You have turned it down to around 11mm and getting close to the finished size. Right so here is what you do.

1 Using a normal digital caliper you have determined that you have turned down to say 11.10mm.

2 Deduct the finished size of 10mm from the 11.10mm and you will see that you need to reduce the diameter by a further 1.10.

3 This equates to 0.55mm radius.

4 Position your Micro DRO so that the probe is just starting to be depressed and zero the readout.

5 Wind in your cross slide until the reading is 0.55mm and take your final cut.

It really is that simple.

: Digital Tread Depth Gauge cheap as chips on eBay for £5 and that includes p & p.

: Remove sticky label, and you can remove sticky gum residue with brake/clutch cleaner.

: Remove cell battery CR2032, larger than usual digital caliper battery.

: With label removed you can now undo the four screws on back of unit.

: Remove and discard the flat metal tension spring from the side of the slide.

: I superglued a 1/8th brass pipe nipple to retain the compression spring.

: I uncoiled the spring at the base of the probe to prevent it entering the unit body.

: A square steel plate was araldited to the body as the magnet mount.

: 4 x 10mm earth magnets secured with Araldite on steel plate.

: Monitoring movement of the cross slide for precision turning.

: Reading carriage travel – range is 0 – 13 mm

Parting Off

A parting tool with replaceable HSS blade copes with most of my parting off requirements though after reading Bog's article I shall be looking at further options.

The selection and use of cutting tools for the newcomer to model engineering can be a real problem. No moreso than when it comes to parting-off the machined workpiece from the bar stock in the lathe. To me, when I first set out, parting-off in the mini-lathe was a nightmare and I frequently avoided the issue by transfering the job to the vice and using a hacksaw.

It seems I am not the only one to find the parting-off operation tricky. The mini-lathe whilst being a versatile and affordable introduction to model engineering lacks the power and stability of its larger bretheren. I recall my delight when upgrading to a heavier more robust lathe the joy in being able to part off with comparable ease.

Now I am not suggesting that you should abandon your mini-lathe for something better as there are ways and means of significantly improving the outcome of your parting-off activities.

Bogs has written the first of a two part article which provides an overview of the various types of parting off tool currently available. Part two will cover machining techniques with useful tips on improving parting off results.

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Mk lll micro DRO

Mk ll version of Micro DRO

Micro DRO for less than £10

A digital read out unit that will improve your machining accuracy for less than £10

Parting Off