DRO -- Educate Me Please

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Rick_B

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I am leaning strongly towards a DRO purchase for a Bridgeport mill. The mill is a Series 1 J head with a 9 X 32 table and X axis power feed. With the power feed the table movement is 16-1/2" in the X direction and 12" in the Y. I know VERY little about DRO's so I thought I'd start a discussion relating to some high level questions and working down into the details. Please don't consider any of the following as factual statements - they simply represent what I think I know today. At a 50,000 foot level:

What a DRO isn't - it is not a CNC type of functionality. It cannot be programmed to perform a series of steps that can be executed by the push of a button.

What a DRO is - in simple terms it is a tool to measure and monitor table movement but the movement is executed by either manually using the the lead screw or a power feeder in one or more axis. In other words a DRO replaces the standard lead screw dials as a means of measuring how far the table has moved in any of one, two or three directions (depending on what you buy). As a result, any lead screw backlash is no longer significant because the DRO is measuring only actual table movement where as the dials are factoring in any backlash in the lead screw/nut configuration as well as table movement.

If the above is correct then let's drop down to 5,000 feet
the DRO uses a scale on each axis to measure movement and reports that back to a display - Question - how does the scale actually measure the movement?

When using a DRO I believe you can set a zero point and then measure from that zero point and return back to the zero point. question - does the zero point represent the tool center - in other words can the tool diameter be reported to the DRO and then it compensates for that diameter or is that an operator calculation?

What can I do with a DRO beyond the elimination of backlash issues. Some examples - touch off the end of a work piece and measure the distance a slot has been cut, touch off the work piece and find its center, touch off a work piece with a hole and find the hole center. Are these typical DRO features? What else can be done with a DRO before you need to consider full blown CNC functionality?

That's probably enough for now - thanks for any thoughts/comments you may have.

Rick
 
Try looking at DROPros.com for a good introduction into the differences between glass and magnetic.
They are very helpful in selecting the DRO that best suits your needs and equipment.
 
Well, for starters, it is more accurate than the mechanical scales. I know there are two basic types, the magnetic and the glass
scale type. Supposedly the glass scale is slightly more accurate. You can set your zero point anywhere you want to to establish
edge surfaces or hole centers. Most will do bolt circles which gives you numerical coordinates of each bolt hole center. I often use
an edge finder to set my DRO. You find the edge of the enge finder tool and then move the table half the diameter of the edge finder and
you are at the edge of the part. Then you can reset the DRO to zero. You can also edge find both ends of a part and determine the
center. I do that a lot and work from the center for easy symmetry. I'm not an expert on this subject, just having learned from doing.
 
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The DRO scale is typically optical, magnetic, or capacitive. It consists of a repetitive pattern on a linear scale and sensor which can detect that pattern. The frequency of the repetition determines the resolution of the DRO scale. Generally , the DRO uses a detection scheme which can determine the direction of movement.

The DRO simply determines the position relative to a reference position. It doesn't compensate for tool diameter. On a mill, the table moves underneath the spindle. Setting a zero reference position can be any point. Typically, one would refer to the spindle axis but you could just as easily bring a tool up to the edge of the work and set a zero there. Substituent milling into the work on that axis with that tool would be relative to the edge.
 
Many better DRO's have functions for bolt circles and step measurements. After the first position you push a button, and the next position will then become zero, zero. Go there. Repeat until done.
 
Rick, you seem to have a good handle on the use and benefits of DROs.

Personally I have not made the jump to DROs yet.

Many people like the little igaging LCD units, and the price is certainly right.....
12 inch: http://www.busybeetools.com/products/digital-remote-readout-0-12in-in-metric.html
24 inch: http://www.busybeetools.com/products/digital-remote-readout-0-24in-in-metric.html
36 inch: http://www.busybeetools.com/products/digital-remote-readout-0-36in-in-metric.html

However I have two things I do not like about those:
1) the small size of the LCD, and
2) the lack of bolt circle and other functions because the axes are independent.

Sure you can work-around those issues by mounting them on a swivel arm with a light and you can work out the coordinates of each manually or with the help of your Machinery Handbook, but if(when?) I pay for a DRO unit I want to realize the biggest benefit!

-brino
 
Thanks for the info guys - this may be a short lived discussion. I just took a look at pricing and now have sticker shock. If I can't get into the DRO game in the $300 range I will likely seriously reconsider the jump to DRO's.

Rick
 
You might take a look at Yuriystoys.com He has a do it yourself system that comes as a parts only system or he also provides fully built modules
for reading out DRO's on large (inexpensive) tablet screen. His site talks about the differences and accuracies of the various sale types.
 
There are many 2 axis Sino DROs in the $200-300 range, a 3 axis is not much more. You could get by with a 2 axis with a a separate spindle DRO (about $50). I consider the Igaging Absolute scales much less affordable these days and there accuracy is not up there with comparable glass scales. Glass scales work just fine, I do recommend a scale cover for longevity of the scale. Glass and newer magnetic scales have comparable accuracy, the magnetic scales are less prone to fouling and a bit easier to install, but much more expensive.
http://www.ebay.com/itm/2-Axis-digi...ne-with-precision-linear-scale-A/291767846797
http://www.ebay.com/itm/2-Axis-digi...ne-with-precision-linear-scale-m/331859638720
 
For the best prices, go to Aliexpress. There are the same three axis DRO's there for less than $250, shipped.
 
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