# 3DR Build Update

Just ordered the stepper motors.

As I am such a cheapskate I ordered four of these...

ausxmods.com.au/index.php?main_page=prod...ath=2&products;_id=74

These are a 35 ounce / inch Nema 17 form factor steppers (2kg/cm). The best thing is the price ($10) and they are Australian stock so postage will be fast.

Most steppers are in the range of 3 - 4 kg/cm but after doing a few calculations I decided to give these a go as I'm pretty confident that these will be fine.

Coincidentally I had been weighing the parts that made up my rods to try and figure out how much of an effect the larger ball ends would have. The complete rod weighs in at 7 grams including a 3mm nut and bolt at each end. What I had figured out was that by using this setup and crimping the ends as outlined in my earlier post you could get rid of the backlash springs (about 2 grams per axis) . Also as the rod connectors we not used in this design there was an additional saving there (4 grams per axis). I figured out that the additional weight of the larger rods was a non-issue when considering the weight that had been removed by the design change.

Anyhow, back to the stepper motors....

So to calculate the torque requirements for the motors you can use the following formula (vertical linear motion). It is a simple formula based on the weight being suspended from a pulley attached to the motor - exactly the same as the 3DR. It does not take into account transmission losses but for the sake of this exercise I chose to ignore them.

The formula is as follows:

T = 1/2 D x W

Where:

T = Torque in Kg / cm

D = Diameter of pulley in Metres

W = Weight of load in Kg

First off you need to calculate the load. I did this as follows.

50g - Weight of extruder and fixings

7g - weight of centre plate

12g - weight of extruder mount with fixings

69g - Total

This has to be divided by three as this load is shared between the three axis.

...so 69 / 3 = 23g per axis

Next i added the weight for the bearing carriage, bearings and rods. As I had weighed my rods knew that they were 7 grams each

14g - two rods

7g - bearing carriage including zip ties

20g - two bearings (this is an estimate as I do not have my bearings yet)

41g - Total

Add this to our carriage weight to get the total load per axis

41 + 23 = 64g

(I chose to ignore the weight of the spectra line.)

So putting these figures into our formula we get the following

T(^ -3) = 1/2 18 x 64

Therefore

T(^ -3) = 1/2 1152

Therefore

T(^ -3) = 576

Therefore

T = 0.576 kg/cm

So as the motors have a rated torque of 2kg/cm I do not think that they will have an issue.

I havent calculated the torque requirements for the extruder as there are some forces that I do not currently know but given that it has a very low gear ratio I don't expect this to be an issue either.