DRO test

D

dpb

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H-M Supporter Gold Member
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Looking for a method of testing my mill DRO vs. reality. I can think of several ways to do this, but I’m looking for ideas and opinions, to eliminate as many sources of error as possible.
I have:
A mill with functioning DRO (the one being tested)
Gauge blocks
1-2-3 blocks
Parallels
Kurt vise
Dial indicators
Test indicators
Edgefinders
Various measuring tools

I don’t necessarily need aerospace level accuracy, but +/- .001” over 6+ inches would be ok.
 
That's a good question not seen on here lately and it depends on your bed length .

Do you have a long piece of stock , long gage blocks or jig bore rods that you feel is " calibrated " to a given length ?

You would clamp this down against your T slot bocks and tram one end . Set 0 . Then tram the other end in and compare readings of the DRO vs. the given known length of the part .

If not splitting hairs , an edge finder may be close enough for you .
 
Unless the DRO has linearity correction in several sections of it's travel, I'd suggest doing that procedure in the area of your table that gets used the most, not in the far corner of the table that see little use.
 
I tested X & Y by clamping and dialing in a ground V-block on my table then pushed a pair of 123 blocks against it. Indicate both blocks, one block, no blocks. Mine was out .0004 on 6 inches and some of that was likely the 123 blocks. I tested Z by just stacking up the blocks. Happy as a clam with the DRO but I was really surprised at how far off the hand wheels were.
 
If you have or can get one, a 6 in. micrometer standard is a good choice for this job.
 
If your scales are aligned within their tolerances (i.e. 0.003" from end to end) you should be able to get WELL within 0.001". Probably close, if not to, the resolution of the scale/head (i.e. 2 tenths). My understanding is that glass scales have linear error while magnetic scales have non-linear error. There's lots of info on the web about the differences and advantages/disadvantages of both types.

I went though the calibration routine of my two DRO glass scale systems using a 1-2-3 block as the instruction suggested and they all repeat to within a couple of tenths during the calibration process. Since the glass scales have linear error, in theory, they should be accurate over their complete range of travel. Not like magnetic scales.

Lots of info covering this and more on-line.

Ted
 
My Grizzly DRO scales came with plots of the errors over the scale position and they varied with position although worst case at the end of travel on the y scale and about 7 microns (.0003"). The other two scales were 5 microns (.0002") at their worst position.

The procedure for calibrating the Grizzly scale is different from the above video in that the error divided by the traveled distance is entered rather than the actual distance vs the entered distance for the TouchDRO. Readouts will vary in the calibration procedure with brand and model so follow the instructions for your readout.

As to the actual procedure used, I clamped two 1-2-3 blocks at right angle to each other and parallel to the x and y axis, I then used a 6" parallel for the step block. The parallel was measured with a 6" micrometer to determine its actual length. I preferred this to using a micrometer standard because I could lay the parallel on the table in a stable position and not have to be concerned with any twisting while making the measurement. For the z axis, I used a 1-2-3 block as my z axis travel is less than 6".

If using a 1-2-3 block for the step block, be aware that they are not necessarily the nominal dimensions. Even name brand blocks will be out. They usually are a small amount oversized to allow for lapping.
 
Good thread and answers. Just something to keep in the back of your mind, especially on bigger mills, is that there can be wear and movement between the knee and column, knee and saddle, etc that the DRO scale will not see. So if you are seeing a lack of repeatability, keep that in mind. Normally I see that in bigger mills, BP Series II's and the like. But I have seen it on smaller table sizes too.

Jon
 
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