OAK linear encoder

[Martin Zarnay]

Hi guys,
I've to build a CNC machine with Acorn, now I'm planning to build another, with much precision that this one.
I'm definitely will use the Centroid OAK system, but want to use linear encoder too... as the ball screws that I've bought for project si C5 ground (0,025 +- mm) and when I'm machining I need total precision. My question is,

CNC controller and machine position are driven by those scales and servo commands ale adjusted by those scales? and is there some tuning procedure?

[tblough]

What kind of machine? What kind of servos? What linear encoders? The scale setup has adjustments for deadband and acceleration, but adding scales won't automatically increase the precision of the machine. You need decent iron with minimal axis slop to begin with, and the C5 screws are a good start, but the screw diameter (and therefore the ball diameter) has a significant effect on stiffness as well as the ballscrew mounting.

If you use $20 ballscrew mounting blocks, you're not going to get the same precision as $400 bearing blocks. There are a lot of factors that affect positioning accuracy and linear scales are not a cure for crappy components. You'll need to provide much more detail on the machine and components if you want recommendations on linear encoders.

[Martin Zarnay]

yep your totaly right, im still thinking if it is important,
Im using ballscre : https://www.steinmeyer.com/en/products/ ... -4-25-3-4/
linear rails : https://www.tuli-shop.com/linear-motion ... il-systems
DMM servos , with 25600 ppr encoders ,
all mounting blocks for ball screws are custom made blocks of cast iron and angular contact bearings and are temperature controled.
linear rails are also temperature controlled with separate coolant loop systemwitch goes under rails...

all frame of machine is from cust ironstructure, and is surrounded by, 800 KG of granite.

machine is also not too large with travels of X400 Y300 Z200 mm
I've forgot, company recommended by centroid for linear encoder is :
http://www.zsinstruments.com/Products/Scales



One way is add linear encoders , another is change ball screw to C1 or C2 precision

[Martin Zarnay]

On centroid manual thee is link for company what is providing linear encoders for centroid OAK . I've been writing to them just now about linear encoders, what kind they would recommend to me ... defenately i don't want som crapy encoders from ebay ... nono

here is some good reading
http://www.auto-met.com/Globalspec/Line ... 0Tools.pdf

[tblough]

Acu-Rite (Heidenhain) has 0.5micron glass scales. RSF Electronic MSA170 and MSA770 series come in 0.1micron versions. I've used both with Centroid controls. You will have to make your own DE9 connectors with the Centroid pin-out.

Also realize that while the resolution of the scales is sub-micron, the accuracy of the glass scales is either 3 or 5 micron. You want linear encoders with AT LEAST 1/2 the resolution you are shooting for and preferable 1/10 the resolution. The way glass scales work it the control moves the motor back and forth (dithers) around the scale reading. Therefore, 2X the scale resolution is the maximum positioning resolution you will get. Accuracy will be worse than this.

[Martin Zarnay]

What ive read on page where i found link to this company ( http://www.zsinstruments.com/Products/Scales )they sad that they will prepare DB9 connector for Centroid so it would be just plug and play system.but for sure pricing would be plus 1500 dollars for two scales, that is what I'm counting with ...
Maybe you asking why I need so much accuracy , it is for probing - a lot of probing . you sad you used it, do you recommend it ? i mean all linear encoder add on ?

also how I do milling is that machine goes for one our and than its wait for another ... and termal grow in this style of machining is crazy .
that's why I want to go for it ...

my tolerance is +- 0,0025mm
this is page where I found that :
https://www.centroidcnc.com/dealersuppo ... ds/292.pdf

[martyscncgarage]

Remember, there is a cost preminum TO Centroid for the linear scale addition option...
PIctures and description of the machine would be helpful to see what you are working with so you can be given better advice.

Marty

[BodeRacing]

my tolerance is +- 0,0025mm

If I did my math right that is less than 0.0001". I am afraid that you will never get there with linear scales, although an explanation of what that tolerance indicates would be helpful. Is that the measurement uncertainty tolerance and the part tolerance is +/- 25 microns or is the part tolerance +/- 2.5 microns? Makes a huge difference. Temperature alone can knock you out of tolerance in a heartbeat. How are you planning on keeping everything at a constant temperature?

For me linear scales on a CNC are a bandaid for poor mechanics not to improve accuracy. Do you see any linear scales on a high end CNC machine? I don't think so.....

[tblough]

Mazak has linear scales as standard, Okuma offers them as an option.

[Martin Zarnay]

Guys , maybe im over shooting with tolerances a bit. +-0,0025 it would be part tolerance. I just want to take part to hands every time and see 300,00 mm everytime, with no worried it can change. im shooting to 0,0025mm (0,005 mm max) not for small parts ( 20mm ) but in in all travel of machine 300x400mm. Bal screw C3 is rated at 13um ( 0,013 ) in 300mm it means that if I will machine 300mm part it could be 300,013 up or down in length and I would need to adjust for error, that's why linear scales .

In shop my friend works, he has DMG mori relatively new one. and in longer parts it can finish part with tolerance +-0,005 sometime part longer, sometime shorter. Of course , I know there are many many more factors to influence this I know . If is bad machinist - even linear scales wont help keep tolerances ...
I just wont to build best accurate machine I can, with very good ballscrews, if ball screws will be accurate enough for my needs good, and encoders will be safety measure.

Ive got email from zsinstruments.com so I will chat with them and will see. will share my results