Set up AMB Spindles...
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Set up AMB Spindles with Digital Control in Openbuilds Control

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Hi all,

After spending many hours figuring this out, I would like to share it with anyone else who needs this information.

When connecting the AMB Spindles with Digital Control to the Openbuilds Blackbox to allow the 10v analog signal to control the spindle speed, there are some things to consider.

Conclusion (TL/DR)

Even though the spindle is rated as e.g. 3500rpm to 25000rpm, you should set #31 Minimum spindle speed in Openbuilds Control to 0rpm and not e.g. 3500rpm. This will ensure the 10v signal is linearly calculated. If you set the minimum spindle speed to e.g. 3500rpm you will get wrong voltage and therefore wrong spindle speed.

The spindle speed is still a bit off due to AMB Kress spindles operating from 1v to 9v. 0-1v means not running at all and 9-10v means full speed. Openbuilds Control cannot account for this.


I bought the AMB (Kress) 1400 FME-P DI rated 3500rpm to 25.000rpm and the Openbuilds Blackbox. I am using the Openbuilds Control software (latest version at the time of writing v1.0.330).

Going into the software to set up my Piranha CNC I entered 3500rpm in field #31 (minimum spindle speed) and 25000rpm in field #30 (maximum spindle speed). I then turned on the spindle to check the control voltage.

I got the following readings when testing with a voltmeter:

100% speed - 10.5v (should have been 10v)
50% speed - 4.45v (should have been 5v)
25% speed - 1.42v (should have been approx. 2.5v)
10% speed - 0.139v (should have been approx. 1v)

Openbuilds Control told me that 50% was 12500rpm, 25% was 6250rpm and 10% was 2500rpm (which is under the limit of the spindle).

I was very confused. I could not trust the spindle speed at all. After speaking with Openbuilds support I now know that Openbuilds Control actually try to calculate the right voltage. As I entered 3500rpm as minimum, Openbuilds Control assumed that just above 0v would be 3500rpm. It tried to calculate the correct voltage based on this assumption.

After setting #31 minimum spindle speed to 0rpm, adjusted the 10.5v to 10.0v with the trimmer and then measured the voltages again I got correct readings:

100% - 10v
50% - 4.96v
25% - 2.45v
10% - 0.9v

Okay, while this is quite accurate for an analog signal, this actually does not translate well to the AMB spindle as the spindle is controlled in the range from 1-9v. The following image is from the manual:

AMB manual

I asked Openbuilds support about being able to adjust this in Openbuilds Control, this is their reply:

Hi Martin,​

You are most welcome!  Glad to hear it worked!​

According to the datasheet the input is still 0-10v
0-1v does not enable the spindle
1v to 9v = 3500-25000rpm
9-10v stays at 25000 rpm

It does make things a little non-standard (a shame really as the 0-10v industrial standard is quite well spec'ed and implemented everywhere else, eg DC and VFD spindles etc) 

I think the best thing to do is just to ignore the slight scale offset. On a hobby grade machine with a Induction motor spindle like a Kress, things just aren't THAT precise.  You'll never dial in cutter RPMs down to the closest 100rpm anyway. You'll see when you start using the machine -  That doesn't have to be that exact ever. During the cut the spindle will ramp speed up and down by 2000rpm easily as it bog down in plunges and corners etc. 

Chipload calculators has no real application in a machine like this. It's not an industrial cast iron beast with a closed loop VFD spindle. You'll get deflection and chatter in ways that the calculated chiploads just cannot help with. Your cuts will be conservative. I wouldn't exact RPM, and just start making stuff (:  - close enough like this is already margins better than a router with a 1-6 speed dial and that works just fine too.​

Hope this helps and thank you for your support! 

Thank you, 

OpenBuilds Team

I actually think this is a fair answer.


I hope you are now wiser on the Digital Control and Openbuilds Control software.


/Martin Verner

Topic starter Posted : 05/08/2022 6:33 pm
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