## Saturday, July 30, 2016

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Recently I've been trying to measure the shape of a vintage plastic cupboard handle.  For those that aren't up to speed, check out my previous post on how to use a radius gauge, where I explain the motivation for the project.  I was able to determine the basic shape of the profile on the face of the handle, but wanted to quantify it a little more.  After a lot of thinking I figured out a way to do this using a dial gauge with a magnetic base attached to the carriage of a lathe.  It takes patience but gives great results.

 Measurement Setup
The first thing to do is rigidly mount the object you want to measure in the toolpost of a lathe. In this case I've fitted the handle to a thick sturdy bracket.  The screws in the toolpost weren't long enough to reach it so I've used a lathe tool between the bracket and the centre screw to space it out.

 Handle Mounting
The next step is to place the magnetic base and dial indicator in a position on the lathe carriage so that they won't get hit if you move the topslide.  Do some tests to make sure that the tip of the dial indicator can reach all parts of the surface you want to measure.

 Probing The Handle
Then comes the tedious part.  Position the handle so that the indicator tip is just to the side of the surface you want to measure.  You then raise the indicator tip, move the topslide a set amount by turning the dial, lower the indicator tip, and then take a reading and repeat.

In my case I decided to take a measurement every 0.1 mm.  In the image below, this means that I had to turn the dial 5 divisions between every reading.  Make sure that you always travel in the same direction when doing this, otherwise the backlash in the topslide will make the measurements worthless.  It's also important to lift the indicator tip when moving the topslide.  If you don't, the surface will push the indicator tip to the side, once again making the readings meaningless.

 Cross Slide Dial
When taking readings off the dial, estimate the position of the needle if it's between marks.  It may not be exact but it's better than rounding.

 Dial Indicator
Depending on what dial indicator you have, you may need a new tip for it.  The one I was able to use had a tip that was very round and wasn't sharp enough to access all the areas of the surface I wanted to measure.

 Dial Indicator
You can see the difference between the tip I used in the above process and the one that comes with the indicator.

 Contact Points
I was able to get a set of dial indicator contact points on eBay for 20 dollars Australian.  They aren't the highest quality things I've ever seen, but they do exactly what I want and that's all that matters.  If you're looking for a set yourself, search for "indicator contact tips" or "indicator contact points".  Searching for 4-48 UNF threads also leads you to these as it seems to be pretty much the only thing that thread is used for.  Some dial indicators use an M2.5 thread, so you may need an adapter.

 Contact Point Set
The point of the tip I used looks to be round, but using radius gauges I was able to tell that it had a radius of less than 0.3mm.  I'm fine with that.

 Sharp Contact Point
When performing measurements I wrote down my results as I went.  I then went way back to the beginning started again and took a measurement every 2.5 mm to make sure that the dial indicator didn't move during the process.  This was just to verify the setup and are the circled data points.

 Data
The data was then plotted to reveal the shape of the handle.  When looking at the shape it's important to remember the limitations of the tip geometry.  The three sharp crevasses will appear round because the tip is slightly round and can't get all the way to the bottom of them.  As the tip is conical it will also have trouble measuring the vertical part to the left and right of the central bumps, so these sections will also be slightly distorted.

The red dots in the image below are the verification data points.  This makes me confident that the setup didn't move during the measurement process.

 Profile with verification points
Another thing to look out for is that you might not have mounted the object perfectly flat.  Because my object is symmetrical. I can mirror the data, overlay it and see that I do indeed have a small misalignment.

 Profile with its mirror image
By lining the two up, I was able to determine that the part was off flat by about 0.5 degrees.  I can now compensate for that later.

 Profile with its mirror image rotated 1 degree
Another thing that may happen is that the handle is not mounted perpendicular to the motion of the top-slide.  This would have the effect of stretching the shape side to side.

To be honest this turned out a lot better than I expected.  I need to do more work to clean up the data, but I now have a good reference to work from.