I had an interesting find the other day when tuning and test cutting my last four MPCNC builds. I haven’t seen anything on this exact topic after a few hours of digging through the forums, so if there’s another post on exact subject my apologies.
First off some project details. I’m using the Rambo 1.4 with dual end stops on all Six builds. I purchased the boards from V1 all with in 3 or 4 months of each other. The seemingly small difference is, the first two builds were using complete kits from V1 with the 76oz stepper motors. The other four builds with the 84oz comparable (17HS19-2004S1) from Amazon.
So after some test cuts on the latest builds I noticed the machines were all skipping some steps cutting the same plywood material with same speeds and feeds with a sharp bit as my first two builds were cutting with no problem. This piqued my curiosity so I ran each machine in their X and Y axis and stopped the movement by hand comparing the relative amount of force it took to make the stepper motors skip. While I have no way to measure it at this moment( a fish scale crossed my mind) there is a noticeable difference, in the order of twice as much force to stop the 76oz steppers purchased from the V1 store as opposed the the other 84oz steppers from Amazon.
At the moment it looks like I need to get a cooling fan for my control box’s and follow the instructions to increase the power to the steppers. I figured since I’ve tested all 4 new builds vs the 2 previous with the same results its gotta be a that simple. Simple-ISH…
After going way out of my comfort zone I followed the instructions on the V1 site to increase the rambo 1.4 stepper motor amperage via the digipots. Pleased to report that it all worked!
My buddy loaned me a digital fish scale and what we found that the 76oz steppers took about 27lbs of force to move the x or y axis with the steppers engaged. The 84 oz steppers from amazon took about 11lbs for the same test before bumping the digipot amperage values.
After reflashing the firmware a few times starting at 185 (1Amp seemed like a reasonable starting point for a 2amp stepper) it seems that the following settings gave us comparable holding power on the 84oz motors.
Once I get some fans installed I’ll run some further tests to make sure its all as it seems.
Anyhow hopefully this will be helpful to anyone that gets the 84oz motors. You’ll need to do this if you want to get your MPCNC running anywhere near what it’s capable of for routing operations.
That’s still pretty impressive. That’s like five bags of sugar hanging sideways off the router. I certainly don’t push that hard when I’m using it by hand!
Can somebody help me see if I have this correct? Pulley is about a half inch diameter (12.2mm according to the Google). Assume force is applied equally to both steppers. 13.5lbs=216oz. Multiplied by the radius of the pulley (0.24in) =52in-oz?
That’s a little over 60% of the rating. At a little over half the rated current. I know it’s not linear, but I was expecting less and this looks like a great result to me.
Thanks!
who knows if the fish scale is accurate or if I caught the exact moment the motors skipped but the 27lbs should be considered ballpark. cool to see the math on it
Even as a ballpark, that seems pretty good to me. Maybe I’m just jaded by all the over rating that happens with marketing and power numbers.
Hopefully those equations are right, lol.
You’ll want to do a test for a little while to make sure the cooling you have is sufficient. The drivers will turn off if they get too hot. They just stop moving for a few minutes. The motors can heat the mounts up enough to bend. The number I’ve heard from Ryan is to keep the motors lower than 50C.
Noted, Thank you. I’ll be watching the motor temps with an infrared thermometer. Should have the fans rigged up mid next week. I’ll update after some testing. Hopefully It’ll all stay 50c or below.
Used PETg on all builds so hopefully that’ll help resist any deformation if things get a little too spicy
Ok so here goes. first things first I forgot my damn infrared thermometer at home. The good news, ran two thirty minute programs designed to keep the axis moving and then followed with an actual job that was 41 minutes. The job was trochoidal milling in 1"+ Rustic white fir full depth at 15mm/sec 30% step length using a dewalt 660 and rotozip SC4 bits. Even gnared through some huge knots with out skipping a step. The steppers were kinda of hot but not to hot to leave your hand on comfortably. That really means nothing though. I’ll get stepper temps on Monday. Thankfully the drivers didn’t turn off. Things were definitely learned today and one of those things is that the SC4 bits from rotozip were absolutely amazing for this application.
They draw the same current when holding, so once you power them on they’ll warm up the same even if they don’t move. At least until they power off. I forget what the default time is, though.