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DRO Help?
#1
First I am new to this site. Second I am almost totally ignorant of electronics. Third, I am a very good machinist (30+ years)

I recently bought a Chinese made DRO kit to mount on my lathe. It's so easy a 10 year old can do it right? Well, since I don't have any 10 year old's at hand, I decided to take on the task my self. I figure at 72 I should be at least as smart as a 10 year old. Hmmmm?

The Glass scales provided work very well and are as expected BUT there are several small problems. 

The only one that matters is that the X-Axis scale covers the carriage lock and the lubricant reservoir of the carriage. being somewhat of a Oilccan Henry type, I feel the this is not acceptable. 

What I would like to do is replace the glass scale(s) with Magnetic scales, the Magnetic scales have a much slimmer profile and even if mounted where the glass scales are, would uncover the needed features.

My question is can I switch to Magnetic scales? If a I can, (both of the scales use db9 connectors) do the db9s have the same connections ie: are the pinouts the same and what other problems might I have? How do I identify the problems and can I correct them so I can use Magnetic scales?


Help?
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#2
Well, it's complicated ...
It depends on what the input circuit of your DRO is and what scale you end up buying. Both types of scales use the same communication protocol and signal level (5V) but:
1. Pinout will very likely be different so you will need to figure out what it currently is and resolder the connectors on the new scales
2. Magnetic scales (and definitely the ones from China) output really dirty signal; glass scales output nice square wave. If you got one of the no-name DROs for $300-$400, there is no input signal conditioning stage to speak of. There is better-than-average chance you will have at least some problems with noise and a non-zero chance the scale will fry the microcontroller.
Hope this helps
Yuriy
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#3
I guess that helps. I have also looked at the igaging dro's but the cable they have doesn't appear to be protected in any way from hot metal chips. 

I guess I could sell this one to somebody and recover part of my cost and then re-invest (LOL) in another with magnetic scales.

thanks

Jim Wright
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#4
Jim,
Have you considered making a braket to move the DRO scale up or down to give you access to the hole?
Regards
Yuriy
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#5
Where I had an access issue I used a Delos DMA magnetic scale, the one I selected outputs EIA422 so has A & B plus -A & -B which is handled by Yuri's Glass scale boards and is very stable. I used Delos glass for the other axis.

https://www.alibaba.com/premium/delos_linear_scale.html

Not looked at the shape of the output to know if its as square in shape as glass scales but the DRO display shows no jumps and accuracy of cut is only an issue if tool wear is high.
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#6
The main problem is that many Chinese magnetic scales (Ditron being the worst offender) tend to have sizable positive spike at the "going high" edge and negative spikes at the "going low" edge of the pulse. I don't know what they use internally;Hall effect sensors should not do this, but they could use induction to detec magnetic poles, in which case the spikes make sense. I had a gentlemen send be a scale to test and negative spikes were as low as -5V.

I use magnetis cales on my lathe's Z axis and mills X axis myself. TouchDRO board handles magnetic scales pretty "OK". I used proper differential signal decoders on the inputs stage that are rated to handle up 7V delta, so -7V is still safe, and depending on the individual IC, probable more than that. The two Chinese DRO heads I took apart so far had scale input connected directly to the microcontoller. -5V on a GPIO pin will not do it any good.
Regards
Yuriy
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