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### Author Topic: How to calculate steps per unit when ball screw values and other info is unkown  (Read 6670 times)

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#### rodjava

• 9
##### How to calculate steps per unit when ball screw values and other info is unkown
« on: October 24, 2012, 11:26:04 AM »
Hi,
I build a cnc router with ball screws and nuts from Ebay. Unfortunately, they did not come with any documentation.
I am also running servos with 14bit absolute encoders.

I have calculated my backlash and entered the values in Mach3.

When I calibrate my x and Y axis using Mach3, I have the following values per.  x 6600.500932 any y 6602.946044

When test cut a 3.00 circle, it is 2.9850 in diameter.

What is the formula to increase .015 to make it 3.00 diameter?

Any help would be appreciated.

Rod
San Francisco

#### tripleblack

• 128
##### Re: How to calculate steps per unit when ball screw values and other info is unkown
« Reply #1 on: October 24, 2012, 12:17:24 PM »
take your needed which i 3 and divide by what you have 2.985= 1.0050251. multiply unit value

#### rodjava

• 9
##### Re: How to calculate steps per unit when ball screw values and other info is unkown
« Reply #2 on: October 24, 2012, 01:29:32 PM »
Hi Tripleback,
Thanks, but I can't seem to wrap my head around your answer.
What do I multiply 1.0050251 with?
I'm sort of dumb.
Can you give me a example?

Rod

#### Hood

• 25,838
• Carnoustie, Scotland
##### Re: How to calculate steps per unit when ball screw values and other info is unkown
« Reply #3 on: October 24, 2012, 02:14:40 PM »
First thing is disable backlash as it may be affecting any values you have calculated.
The number that Tripleback has mentioned is the value you need to multiply the steps per unit with to make it correct but as mentioned try first with backlash disabled and just do a one axis move and measure, making sure you have moved in that direction previous to a commanded move to eliminate any backlash you may have.
Hood

#### rodjava

• 9
##### Re: How to calculate steps per unit when ball screw values and other info is unkown
« Reply #4 on: October 24, 2012, 03:42:23 PM »
Hood,
If I understand you correctly for my x axis,
1.0050251 x  6600.500932 (x)= 6633.66910923

I will give it a try. As well as disable backlash

Thank you all in advance.

Rod

#### Allstar1

• 41
##### Re: How to calculate steps per unit when ball screw values and other info is unkown
« Reply #5 on: October 29, 2012, 09:22:22 AM »
It's easy to calculate thread pitch. Either count the number of threads per inch if standard or mm per revolution with calipers (how much the nut moves with one complete turn). Most Asian screws will be fairly standard pitches like 2-5 mm or something even. Also measure with calipers center to center of each thread. You gotta know this going in brother. Program can calculate it but don't depend on software 100%.

Robert