DRO output for CNC - basic question(s)?

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Dale Grice
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DRO output for CNC - basic question(s)?

Post by Dale Grice »

Hi, newbie here.

I have a Bridgeport series I mill. I would like to install a DRO. Wile I dream of a CNC install, that would be in the future. I have learned by reading that a DRO output can be used as an secondary input to CNC.

When shopping for a DRO system, are all capable of doing this? Is there a "term" for this interface? Was leaning toward yadro. Thoughts? Suggestions on which DRO to dive into the waters with?

Thanks,

Dale Grice
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1.6 CB&Q O1a Mikado
1.6 CB&Q NE7 Waycar

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sch
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Re: DRO output for CNC - basic question(s)?

Post by sch »

You may or may not be aware that a cnc system is its own DRO, ie the computer/software can use the pulses generator by or for an axis
drive motor either stepper or digital servo to give you a measurement of axis motion so later install of the cnc makes the dro redundant.
Possibly a consideration at the start as even cheap dro are $300+ or ++ or good dro perhaps $1000 or so. Yadro is a primtive variant on
this cnc as dro approach using the cheap far eastern scales, but it is only a little step up to using the CNC software with the motors to
do the same.
toddalin
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Re: DRO output for CNC - basic question(s)?

Post by toddalin »

sch wrote:You may or may not be aware that a cnc system is its own DRO, ie the computer/software can use the pulses generator by or for an axis
drive motor either stepper or digital servo to give you a measurement of axis motion so later install of the cnc makes the dro redundant.

Ever heard the saying that a man with two watches never knows what time it is?

I have both the CNC software and a Jenix DRO and while I calibrate the software to the DRO, after a bit they always seem to be a bit off of each other.
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Bill Shields
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Re: DRO output for CNC - basic question(s)?

Post by Bill Shields »

Many industrial grade CNC systems (Fanuc / Fagor / Mits / Siemens) have the ability to input either the encoder attached to the lead screw and / or input from a glass scale.

It doesn't matter AC or DC or Digital servo system the end result is the same. If you are reading motor rotation, then you are depending on the accuracy of the ball screw to 'know' where you are.

The ball screw / encoder takes no account of backlash and / or variation in the actual pitch of the screw, so it is a 'theoretical' position, instead of an 'actual' position. Some CNC systems allow a 'variation' parameter to be input for ball screw 'ranges' but for any given exact position of the table, it really does not know.

Steppers, without encoders can potentially worse - miss a 'step' because of a high load, and you have no idea where you are, unless you have an encoder or scale for feedback. Granted stepper systems have come a LONG way in the past 20 years, but without 'feedback / closed loop', you just do not know for sure.

The glass scale would give 'actual' position.

What you need to be concerned with if you put in a glass scale - does it have 'encoder equivalent / quadrature' output capability which is what most CNC systems are looking for. This would / could also include a 'zero pulse' if the encoder output is incremental (not know where home is on power up), as opposed to encoders that are absolute and know where they are the minute you turn on the power.

Many of the less expensive systems being sold today do not have 'encoder equivalent' (quadrature) pulse output capability, so they CANNOT be used with a CNC system without some sort of conversion.

If you are considering a CNC with lead screws that are NOT BALL SCREWS, then you should really be concerned about 'where I ACTUALLY AM' vs. 'WHERE DOES THE CONTROL THINK I AM", because the two may be quite different.....backlash is a significant concern.

None of this applies if you are looking at systems with linear motors and no ball screws. LINEAR motor systems all require glass scales and are not something that the hobby guy retrofits to a Bridgeport.
Too many things going on to bother listing them.
hwboivin3

Re: DRO output for CNC - basic question(s)?

Post by hwboivin3 »

Agreed with Bill 100%.

Also, in case you go with the Mach3 cnc control, Mach3 will also double as a read out. You just hook the glass scales to a control box and to the computer via parallel port.

But the biggest problem is backlash! Mach3 has a backlash adjustment setting that can be tweaked in. Although I don't know how well it works with a lead screw.


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Dale Grice
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Re: DRO output for CNC - basic question(s)?

Post by Dale Grice »

Thanks for the response(s).

I had looked at Mach3 before and did some more looking. I also joined the mach3 yahoo group and looked around in the files section. I do have ACME lead screws and realize that ball screws are much preferred in a CNC machine.

I can put together a PC and understand the basics of scales. So if I went with Mach3 and used it as a DRO readout, what is the device that goes between the parallel port of the PC and the scales? I also realize that I won't be using mach3's cnc capabilities. But I also can't see getting a DRO display if it is only going to be replaced by Mach3's display in the future.

Any suggestion on where a beginner can learn more about how to build / buy the interface that goes between the PC and the scales? I search around, but get overwhelmed. I found modbus (sp?) But I did not find an explanation of what it actually does.A primer would be nice.

Thanks again,

Dale
Projects:

1.6 CB&Q O1a Mikado
1.6 CB&Q NE7 Waycar

RR Supply 90 ton Logging Mikado
toddalin
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Re: DRO output for CNC - basic question(s)?

Post by toddalin »

My ShopMaster uses the Mach3 software.

The best I can tell, there really is no "interface" of which you speak. The computer communicates with the three Gecko drivers located within the machine and these drivers send output to the three stepper motors for move the X, Y, and Z lead screws.

You can calibrate the software by telling the quill (table) it to go a certain distance, then measure the actual distance, key this into the software, and let the software perform a "compensation." But other than that, when you tell the quill to "go someplace," the software assumes that that is exactly where it went and reports that value regardless of where it actually is.
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Bill Shields
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Re: DRO output for CNC - basic question(s)?

Post by Bill Shields »

which is what is called 'open loop'.

If something binds and the table doesn't move, you have no idea where you are.
Too many things going on to bother listing them.
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ng_2-4-2
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Re: DRO output for CNC - basic question(s)?

Post by ng_2-4-2 »

If you want a DRO now and a CNC later, there is the advantage of having the DRO when using the mill manually. You will be able to interface almost any DRO system as the feedback loop for a servo system CNC. If you plan to add Mach3 with stepper motors without encoders as 90+% do, then you would use the Mach3 display when using the machine in the CNC mode, and the DRO readout when using the machine manually. It would be doubtful that you could ever get the two to independently agree with each other. So, for now, get a DRO. When you are ready for CNC, install the CNC and use both independently. The advantage of having the DRO when installing the CNC is that you have the DRO to use as a reference when tuning the CNC.
hwboivin3

Re: DRO output for CNC - basic question(s)?

Post by hwboivin3 »

For future thoughts, check this out.

http://home.comcast.net/~chrisbruno/cnc_bridgeport.htm


I bought the $30 digital set of prints. He has done an excellent job of documenting the hardware aspect of a cnc bridgeport conversion.



Harry
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Bill Shields
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Re: DRO output for CNC - basic question(s)?

Post by Bill Shields »

be careful about 'DRO INTERFACE TO CNC'.

They don't always work.

I have a DRO on my south bend lathe that would have a great deal of trouble interfacing to a cnc.

Not all signals are 'universal'. Many of the new companies are using stuff that is quite proprietary
Too many things going on to bother listing them.
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