help with z axis (solved)

hi all i am very new to this whole cnc thing. i have the basics down and i am using google sketchup with the sketchucam plugin to generate my g code.

so my machine is a hobby cnc 3 axis router. each axis is driven by one stepper motor and lead screws. i did not build the machine so im not exactly sure what pitch the lead screws are but i think i got them all pretty close. i have confirmed that my x and y axis are scaled correctly but my z is not. for the most part i have just been doing mdi commands to test my axis. so when i jog down to what would be my material thickness and home the z axis which would for this instance make my tool 0.25 inches off the bed. if i enter G0 Z -0.25 my tool moves exactly -0.125 inches which should have been -0.25 inches. what would i need to do to fix this problem?

for the lead screws i tried my best to count the threads per inch and i think im at about 17 for the z axis also when i did the test this axis command in step config i noticed that if i gave the distance of .5 inches the z axis moves about .25 either way

here is what the step config generated file shows for my z axis

[AXIS_2]
TYPE = LINEAR
HOME = 0.0
MAX_VELOCITY = 2.2
MAX_ACCELERATION = 44.0
STEPGEN_MAXACCEL = 55.0
SCALE = 6800.0
FERROR = 0.05
MIN_FERROR = 0.01
MIN_LIMIT = -1.0
MAX_LIMIT = 0.01
HOME_OFFSET = 0.0

thank you for your help

Hi

The scale figure controls how many pulses equal 1 unit of movement, in your case 1"

If you are moving 75% of the distance you command, the calculation is quite basic
(6800 / 75) x 100 is your new scale figure, so try that.

See linuxcnc.org/docs/2.5/html/config/ini_co...d_sub_axis_section_a

regards

ok i just looked at my driver board and there are a series of jumpers for each axis that tells the board what micro steps to use. the jumpers are set to 1/4 micro steps. in step config i told it 2 and it is working for the x and y axis. so do you think i need to just play with the axis scaling untill i get it right or should i try again with the lead screw counting?

this is my driver/ break out board
www.hobbycnc.com/products/hobbycnc-ez-driver-board-kit/

these are my steppers
#23-205-DS8 285oz-in bipolar rating, 205oz-in unipolar rating. 3v, 3A, 200 S/R, 2.2mH, Size #23, Dual Shaft, 8 wire faster than the 305’s. Good all around choice.

when i do the test this axis command in step config when i tell it the safe move area is .5 inches is this 0.5 to -0.5 for a max travel of 1 inch?

sorry for all the noob questions

hi guys i solved my problem with the z axis on my machine.

andypugh you were right the tpi of my z axis was not 17 it was 34.

first i tried just doubling the axis scaling and this did fix the problem but it created a new problem and that was if i did a g0 jog from -0.5 to 0 i would get a joint following error. so i went back into step config and played with the tpi option till my scaling read 13600 which is the scaling i changed earlier then retested the axis to set my velocity and acceleration and this fixed my problem with the joint following error.

i do have another question though in regards to actually using the machine.

with my machine the x and y home will always be the same regardless of material thickness but z will be changing depending on the size stock i use. so should i just home the axis at my material thickness or should i home my z axis at machine 0 and touch off at my material thickness? thanks for your help

it's usual to home machine , and then use touch off at top surface of material and set z to zero

thank you for your clarification. that is exactly what i was planning on doing. i think im going to try and get home switches working but my machine is finaly in a configuration that i can actually use