Thursday, 14 July 2016

Super Cheap Digital Read Out (DRO)

So, a DRO for machine tools (lathes, milling machines etc) is a digital read out that tells you the position of or how far a given axis has travelled. Obviously this useful if you want to make a part or a cut into a component to an accurate size, the trouble is... they can be very expensive! Even a hobby level DRO setup for say a milling machine can run to hundreds of pounds.  One common workaround (apart from using the engraved hand wheels) is to use a Dial Indicator set up to indicate how far a given axis has moved and indeed I've used that technique a lot. (it's a nice technique as it automatically cancels any error due to backlash as it isn't linked to the lead screw and always tells you the exact position of the axis). Still dial indicators and requisite holders/clamps can cost a few quid and can be awkward to get into a good position etc... so when I saw this hack to convert a tyre tread gauge into a micro DRO... I knew it was for me..

So these digital tyre tread indicators are super cheap... I paid £3.99 for one and could have spent less if I was prepared to wait for one to be delivered from the far east. They are essentially a short travel version of the cheap digital callipers that are widely available ( and indeed have the output slot that people have exploited to create data logging calliper )..  So the modification consists of a couple of different things... firstly you need to reduce the drag on the calliper slider which is created by a small steel slip inside the device.. this is for when you measure a tyre tread you can position the indicator then remove it from the tyre to read the result and the slight friction on the slider keeps it in position. For the DRO hack we need it to be as free to move as possible. Once this is removed I turned a small collar for the end of the probe from some Delrin which was drilled to fit the probe end snugly and the other end drilled to house a cheap but powerful 3mm diameter neodymium magnet

For the next part I whipped up a quick drawing to be able to CNC rout a plate for the base that would house the larger magnets. I made the toolpaths tight to the 8mm diameter of the magnet so that they were an interference fit and required pressing in on my small arbour press (also for added security I put a blob of superglue under each magnet). I then glued the magnet base to the base of the unit and hey presto... a portable quick fit DRO!

It works great, I am really pleased with it and will probably for the price make a couple more.. Its also great that the 8mm magnets are enough to hold the device firmly even if only 2 magnets are attached to whatever it's clipped too. The zero function of the device means that even though it only has a small throw (27mm) it is easy to sequentially move the DRO and zero and keep a running count of position over a longer length. 

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