Saturday, July 30, 2016

Using A Dial Gauge To Measure The Shape Of A Surface

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.

Lathe Cross Slide
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.

Lathe Cross Slide
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.

Contact Tip
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.

Lathe Dial
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
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
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 Tip
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 Tip
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.

Contact Tip
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
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.

Graph
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.

Graph
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.

Graph
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.

2 comments:

  1. Outstanding methodology, from ''how ma I gonna'' to interpreting the results. At least one other factor can introduce variation you see. Assume perfectly rigid set up and consistent ability of indicator to maintain the alignment of the shaft travel, point and indicator body, different from position of itself remaining as set. It's the plastic itself. Injection molding produces near identical parts. Once out of the mold, cooling in a crate with a few hundred kin induces change, usually at the sub-cosmetic level, especially in thermo-plastics. Look at the back. In the middle will be the stub of sprue or tree. Nearly the same thing, varied by how many cavities and layout the mold has. I'm guessing the parting line is on one side. The closeup reveals a lot of nice detail from the mold itself, and may be both sides. The back will exhibit slight depressions, especially in the vicinity of thickest areas, where cooling is uneven.
    I expect you are planning to replicate with 3d printing. Ideal for the art deco motif, color and all. I' program at standard deviation of the 4 ribs. Bet these are readily available nowhere, even the deco stronghold, Miami FL, USA.
    I hope what looks like a restoration project runs well, in keeping with the dedication shown in detail here.
    Found your post at homemadetools.net
    TM51

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    1. Thanks for the tips. It explains a few of the things I'm seeing. I haven't exactly determined what I'm going to do with the handle. 3D printing has crossed my mind, as has some sort of milling operation. These handles are hard to come by and I just like the look of them. Made by a small Australian company that went out of business 50 years ago. My main goal at the moment is just getting everything measured, that way I can make one in the future or someone else can.

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