New Myford 7 DRO article

[JasonBModerator @jasonb]

Chris has just let me know that he will have a stand at the Midlands MEX so anyone else with questions will be able to ask in person.

He also tells me that he is happy to sell sensors and tapes to those who don’t want to fit the product to a Myford and that he is also working on a simple to use but fully featured display console that won’t require a PhD in Chinese -English to understand, hopefully a prototype will be on display at the show.

[Michael GilliganParticipant @michaelgilligan61133]

A very welcome posting, Chris

Thank You

MichaelG.

[JasonBModerator @jasonb]

So Bob and Robert, now that your queries about accuracy have been answered what are your thoughts?

[Robert Atkinson 2Participant @robertatkinson2]

Chris’s response on accuracy seems entirely reasonable. It is hard to quantify accuracy at these levels without access to sophisticated equipment. I’ve designed equipment using 1um optical encoders that we checked with s Renishaw laser interferometer. A DRO can improve the “accuracy” of a machine particularly if care is taken to locate the scale in relation to wear and lack of rigidity.
I think it would be good if he put a similar explanation on his webpage.

The other question I had was about CE/UKCA marking which remains unanswered.

Robert.

[Bob WorsleyParticipant @bobworsley31976]

Ah, not quoting an accuracy or repeatability, no one can complain then. Why not? Well, you tell me. These figures are the basic information about the DRO, it is irrelevant how they are mounted, it is the best possible figure for the DRO its self.

My 2 thou was an error band of +/- 2 thou. Because I used a drum with wire wrapped around then the predominant error was cyclic, so I tested every sensor over 400mm to ensure this band was not outside +/-2 thou, so error could be over 10thou, o 10 inches. Repeatability is another measure, and has to be guaranteed by the design and manufacture, this is why I mentioned the Price book. I note that Chris hasn’t done any tests and provided the results.

So the weasel wods about accuracy depends on the fitting of the DRO and the state of the machine are just that, weasel words, and mean nothing. What I wannt to know is what the best possible accuracy and repeatabilty of the DRO, not the machine, produce. This requires something beyond the Myford 7 lathe.

Do you understand the tests that gave a figure of 0.5um? No, I don’t either, has the method been described to allow you to repeat the tests? No, of course not. Do I? Well, I pointed you at the Price book and the two step test piece, what I forgot to say was that each step was machined by power feed.

Using slip gauges to measure a dynamic measurement is to fail to understand what slip gauges are used for, static mesurements, not dynamic measurements. A pile of slips are wrung together to produce a fixed distaance to very close tolerances, less than 1 micron? Might be optimistic there. A DRO measures dynamic distances, not static distances.

Another problem is that the sensors seem to use magnetic sensing. Mention is made of the Newall ball bearing sensors at 5um. There is a problemm here, assume a ball bearing diameter of 5mm, so to get to a resolution of 5um than each 5mm needs to be interpolated, interpolated, not measured, by a factor os 1000. My sensors use a 22mm diameter slotted disc with 120 slots, I then use a 96 times interpolation, so each 90 degrees of the sine wave is interpolated by 24 giving about 6 microns resolution. I know, having done it, that this is an interesting problem because the two sine waves are not at 90 degrees +/-6 microns. I also know, having done it, that i then have to stitch these two sinewaves together to produce a smooth output that doesn’t jump when moving from one sinewave to the 90 degree sinewave as the sensor moves. This is not easy at X24, so how is this done at X250, answers on the back of a postage stamp please.

To expand on this. You have two sinewaves at 90 degrees, but you only use one, then the other, for position measuring. Why? Because as the sinewave goes over 60 degrees then the slope of the wave reduces rapidly, and moving from, say, 75 degrees to 76 degrees then the wave only changes by 0,004 so interpolating that to a level of X250 is, tricky. So the sinewave is only used from 0 degrees to 45 degrees but then the position sensing has to shift to the other sine wave, now at 45 degrees and moving down to 0 degrees. This is where the stitching together of the two sinewaves has to be done, This also has to be done by every interpolated sensor, whoever makes it, including me. It is a bugger! This is why you need static slip gauge measurements to move the sensor acrosss this interpolation distance and just see what happens, and weep. Now you can see why having very large interpolation values are a two edged sword, you cab gain ny amount of reolution, as here, but the accuracy is left in the dust, possibly here as well but no information is provided, unlike me.

So, still not impressed because Chris has answered none of the questions, and has not even done somme basic tests to give the results, I have.

As I said, a DRO is trivial, it is the sensor that is the problem. I started out with a Heidenhain 2um spar as my calibration standard, and using slips as a fixed distance standard confirmed that the calibration rig as a whole could be believed to +/-5um. My catalogue didn’t say what grade 2, 1 or reference slips accuracy was, but I recall is being less than 1um, so where Chris’s 0.5um came from who knows. It is woth pointing out that 0.5um is the wavelength of orange light, net really very much at all.

 

[Chris IrlamParticipant @chrisirlam72008]

Robert, thanks, and I agree. This thread has changed my mind about how I communicate accuracy. When I get some time I’ll add a page to the website explaining what’s set out below, including the raw test results for anyone who wants to interrogate the numbers, along with the caveats around machine wear.

Apologies for missing your question on CE/UKCA marking. Yes, the kits are CE compliant. UKCA marking isn’t currently required, as the UK has extended recognition of the CE mark indefinitely.

Bob, I’ll take your points in turn.

Method. Each reading was taken by zeroing the axis against a hard stop, winding it back, placing a slip gauge stack between the axis and the stop, and winding the axis up until the gauge was held, so every reading is approached from the same direction and backlash and hysteresis drop out. 71 positions from 0.5 mm to 100 mm, including 0.05 mm steps within a 2 mm magnetic pole pair to look at the interpolation error directly.

It isn’t a gold-standard laser interferometer measurement, but it is a simple, repeatable test against a good length standard. I’m happy to share the raw results with anyone who wants them.

I have a demonstration rig set up the same way and I’m bringing it to the Midlands Model Engineering Exhibition, so anyone can put a gauge in and see the reading for themselves.

What the numbers mean. As in my previous post, the +0.5 µm is the mean (or average) of the 71 errors. That is the measurement bias, not the accuracy, and it is small because positive and negative errors cancel. The figures that describe the accuracy of the read head are the standard deviation of 3.6 µm (0.00014″) and the extremes of −6/+10 µm (−0.00024″/+0.0004″). These were all in my previous post. I’ve given every number rather than a single headline so nothing is hidden, but that does mean each one needs reading for what it represents.

The sensor itself. The encoder IC manufacturer specifies a best case of ±5.6 µm under ideal conditions, increasing to ±10 µm in real-life conditions. My test results agree with this.

Regarding interpolation, this is done inside the encoder IC, so it is already included in their published accuracy figures. My understanding is that it reads sine and cosine together and takes the arctangent, so there’s no stitch point.

I don’t offer this product as any kind of criticism of what you built – your product is objectively impressive and it’s interesting to hear how you achieved it. But please give me the benefit of the doubt and allow me to provide the explanation / data before jumping to conclusions about my product.

Chris

[JasonBModerator @jasonb]

The sensor itself. The encoder IC manufacturer specifies a best case of ±5.6 µm under ideal conditions, increasing to ±10 µm in real-life conditions. My test results agree with this.

I think that says it all in response to “What I wannt to know is what the best possible accuracy and repeatabilty of the DRO”

As you are really only reselling a sensor and tape together with a bit of hardware and drawings for working out how best to fit them the DRO accuracy is going to be as good as the maker of the components has tested as they are the two bits that make up the DRO.

As I said before must people don’t seem worried about knowing the accuracy if sales from the likes of Machine-DRO are anything to go by who don’t state accuracy so quoting 10um could be considered just as misleading  as not stating it at all.

[Author]

Posts