Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: billcameron on July 13, 2013, 04:40:01 PM

Title: 4th axis
Post by: billcameron on July 13, 2013, 04:40:01 PM
Hi newbie to forum. have just built a 4th axis for my router and now need to calibrate same. It is geared 1:3 i.e. the motor turns 1 rev and the axis turns 1/3rd rev.having a bit of brain fade, can anyone help me with this. many thanks
Title: Re: 4th axis
Post by: Hood on July 13, 2013, 06:04:56 PM
Will depend on what you are driving it with. For example if its a stepper that is 200 steps per rev and you are using a Gecko with 10 microsteps then that would mean your motor needs 200 x 10 x 3 = 6000 steps to turn the axis 360 degrees and as such your steps per unit for the axis would be 6000/360 which would equal 16.66666666 steps per unit (degree)

Title: Re: 4th axis
Post by: billcameron on July 13, 2013, 07:31:20 PM
Thanks for replying to my post. My drives are set to half step so 400 half steps per rev. my linear axis are set to 80 half steps which with 5mm pitch ballscrew gives me 1mm movement With the same setting on my a axis this equates to 1200 half steps per rev or as you say 1200/360 = 3.333 steps per degree. it appears i may have made an error gearing my drive 1:3 may have been better 3:1. my motor is never going to get up to speed. I do have the option of going to 1/8 step could this help? not sure I have a complete grasp of this yet
Title: Re: 4th axis
Post by: Hood on July 14, 2013, 03:38:27 AM
Gearing the opposite way would greatly reduce torque so it is probably not a good idea.
Why you think  your motor would never get to speed I do not know, you should be able to command any velocity up to what you have set in motor tuning for that axis.

Title: Re: 4th axis
Post by: billcameron on July 17, 2013, 05:52:03 AM
You are absolutely correct of course. I have changed my stepper driver to 1/8th step. therefore have now 1600 steps per rev on my motor and 4800 steps per rev on my axis which gives me 13.33 steps per degree rotation.However I have set my a axis to linear as opposed to angular for carving in the round.

What I had failed to take into consideration is that I am using a timing belt to drive my axis which has a 5mm pitch so the result of one rotation of my axis is a linear move of 300 mm.(20 tooth pulley on my motor and 60 tooth pulley on my axis). so as I understand it I have to set my steps to 16 steps per mm i.e. 4800/300=16.

This dosen't seem very practical so i may have to rethink my gearing arrangement and use a timing belt with a smaller pitch. I have tried the 16 steps per mm but this results in lots of lost steps. I have not much experience in what might be the best velocity and accelleration settings should be, so it is trial and error for me.

Do you have any suggestions? By the number of posts I see youv'e made you must have lots of experience. again thanks for your interest in my post and putting me on the right track.

I'm getting a bit long in the tooth now, not as quick as i used to be. However will get there in the end.
Bill Cameron
Title: Re: 4th axis
Post by: Hood on July 17, 2013, 06:04:57 AM
Bill, it will depend on your G Code which is the best method for your axis (angular or linear) however I would think angular will likely be the way to go for 4th axis carving work, especially if you are using CAM.
One thing you may be overlooking is the rotational radius setting (on Settings page), this can be set so that Mach will alter the velocity of the axis depending on the diameter of your rotational axis in your G Code.
 I am sure there will be some info in the manual but more likely you will find info if you do a search on the forum for rotational radius or rotational diameter.
 I have never used a 4th axis (except on a coil winder and I treat that as a linear) but quite a few here have and I am sure Ger21 often posts with advice, so if you do an advanced search for one of the afore mentioned phrases and have Ger21 as the poster I am sure you will find some useful info.
Title: Re: 4th axis
Post by: billcameron on July 17, 2013, 10:01:13 AM
You are right i haven't considered that but I will have a look at Ger21 posts if I can find them. My problem stems from with one rotation of my motor I am moving the "a" axis 100mm as opposed to my "xyz" of 5mm. However I have seen many similiar 4th axis and they seem to work ok so I am definately missing something. thanks again I will stick at it.