Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: BJenkins on June 19, 2009, 12:44:18 PM

Hi,
Go to X zero Y zero question:
I use a screen set that has an XYO button. If I zero the X and Y DRO’s and then jog off and click on the XY0 button the machine returns to zero for both X and Y.
My problem is when I enter adjustments for touch plate thickness and half the diameter of the bit into the X and Y DRO’s and then jog off and click on the XY0 button, the machine moves back but the DRO’s show a number like .0004 when I asked it to go to zero.
Here is my touchoff technique:
1. I touch off of the bottom left Outside corner of X.
2. I make the correction for the plate thickness and half the bit diameter.
3. I select the X DRO and enter the value with a minus sign in front of the number.
4. I repeat this for Y.
Example: I touch off using a .062 bit and a .01 thick plate.
1/2 of .062 = .031 + .01 = Total .041 DRO adjustment in the Minus X or Y direction. (.041 Enter) I'm not sure if I'm using a bad technique or I have a setting issue with Mach3.
I was also thinking that it might not make any difference if I select Cycle Start from a point other than X and Y zero. (???) No matter, it still bugs me that the machine is not moving to the position requested.
I hope this makes sense. Any ideas?
Thanks,
Bill

What is the resolution (steps per unit) of your machine?
Hood

I don't know. Can you tell me where I can find the steps per unit?
Thanks for the response.
Bill

motor tuning page.
Hood

Thanks Hood,
I will drive out to the shop and find out. I'll report back in about an hour.
Bill

The Steps Per are 714 for both X and Y.
Thanks,
Bill
Also:
Velocity = 60
Acceleration = 30
G's = .077705;
Step Pulse = 0
Dir Pulse = 4

Are you using metric or inchl?
Hood

Inch

Well I am surprised you are even getting that close to where you command.
What I am meaning is you have 714 steps per inch, the smallest movement Mach can make is 1 step so that is 1/716 =0.0014approx.
If you zero the dro and move off and then return to zero it will likely show that distance but if you enter a dimension into the DRO and then try to go to zero Mach can only get as close to zero as the steps will allow as moving another step would take it past the zero position.
I am probably not explaining it very well so if you still dont know what I am meaning just post again and I will try and explain a bit better.
Hood

I think I understand. I'm not sure why I have 714 selected.
I guess if you zero the DRO's and move off and then push XY0, Mach is able to get you back to 0 but if you change the DRO and your steps per are out of a normal range, then Mach can only get you close. Is this right?
I guess what I need to know is what the steps per should be set to in inches.
Maybe even more important, will Mach be able to run GCode correctly if you start at a position that is not at x and Y zero?
Thanks Again!
Bill

Your steps per unit are determined by your motors, any reduction you have beteen the motors and ballscrew/leadscrew, the pitch of the ballscrew/leadscrew and finally if your drives are microstepping.
For example, say you had 0.2 inch pitch screws, normal 1.8 degree (200 steps per rev) stepper motors, 2:1 reduction and 10 microstep drives it would be as follows.
Amount of steps to turn the motor 1 rev is 200
Amount of microsteps is 10 so that means the drive needs 2000 steps to rotate the motor one revolution.
You have 2:1 gearing so it takes two revs of the motor to turn the screw one revolution so that means 2000 steps x 2 =4000 steps to turn the screw one rev.
Finally your screw is 0.2 pitch or 5tpi so it takes 5 revolutions of the screw to move 1 inch (which is one Unit to Mach) so 5 x 4000 = 20,000 steps per unit.
If your machine is moving one inch when you tell it to move one inch then the steps per unit you have set will be correct.
Hood

Yes my machine does move exactly 1 inch when requested. I simply downloaded a file that automatically took care of the machine setup so I really do not know much about that as you can tell I'm sure.
The only question I have now, is after I touch off and make my DRO corrections for X and Y and Z. Should I go to X and Y zero before clicking Cycle Start or does it matter if I Cycle Start at .0005 or .005 or 5.0 or some other number off of X and Y Zero.
I guess I was concerend that the machine was not moving back to exactly Zero and that made me wonder if I would have an accuracy problem.
Thanks,
Bill

You will only have accuracy probl;ems if you are needing accuracy of better than 0.0014inch and the only way to improve that would be to alter the gearing etc.
As it is a router then being within 0.0014 will likely be more than you will ever need so no worries :)
If it was a metal working machine then you would want better resolution.
To answer your other question, no you dont need to go to zero as you have told Mach where the tool is at that point in time so when you start Mach will just take it to the first point in your code (or as near as it can) and then carry on withthe rest of the code.
Hood

Thanks Hood for working through my concerns. I was kind of guessing that since I had told the machine where it was after each touch off that it would know where to go but I wanted to confirm this. I was also wondering if I had made a mistake in techinque for touch off but I feel confident now. I will use the machine for wood working and from what you explained it sounds like I will have excellent accuracy. I did cut one precise part using machinable wax and it came out right. My machine is a Romaxx HS1.
Thank You,
Bill

No probs Bill and wood probably moves ten times the ammount of your resolution just by heating it with your breath so dont worry ;D
Hood