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Author Topic: X and Y Axis Ball Screw pitch  (Read 4602 times)

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X and Y Axis Ball Screw pitch
« on: June 29, 2008, 09:49:45 PM »
Hello to all;

My name is Ferrall and I am an CNC programmer of 20 some years but new to building CNC from the ground up. I have my own machine, welding, and fabrication shop from which I build lots of things including Three Wheeled Motorcycles (Trikes). 

I am building a new CNC Plasma machine.  I already have the XY table.  It was a former CNC 3 axis plasma spray machine built by Miller Welder Co.  For a number of reasons it did not function as expected so it was abandon.  The drive table was put out as junk and I got it.  It has been robbed of the motors and controllers. The drive table is built on Thompson rod ways with ball screw drives.  The Y axis is what carries the X and Z axis so it is the heaviest.  It is on 1" Thompson rods with a 1" ball screw with a pitch of 1" per revolution.  The X axis is 3/4" with a pitch of .200" per revolution.  My very first question is can the software take into account these different pitches so I can drive the lead screws directly off the motors.  This would be the best and easiest way.  As I said "this is my first question".  I am sure there will be more as I get into the 'nitty gritty' of such a exciting project.
Thanks Ferrall

Offline Chip

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Re: X and Y Axis Ball Screw pitch
« Reply #1 on: June 30, 2008, 12:58:58 AM »
Hi, Ferrall

Mach uses a "Steps Per" value in Motor tuning to calibrate each axis, So "Yes" thy can be different values.

One thing to keep in mind, To get the proper cut resolution you need to have adequate number of Steps Per "Inch".

With your X axis 1" in 1 turn ball-screw, It needs 10,000 steps per turn  for .0001  Accuracy or 1000 steps per turn for .001 Accuracy.

Ask Away, Chip

Offline jimpinder

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Re: X and Y Axis Ball Screw pitch
« Reply #2 on: June 30, 2008, 04:05:23 AM »
I would agree with Chip - every axis can be individually set, but  a 1 inch pitch is very high.

1. You will need a very big motor (out of proportion with the others) to drive the axis.
2. Stepper motors normally have 200 steps per rev, the drives contribute 10 microsteps so if you fastened this up directly, you only have 2000 steps per inch, so your accuracy can only be 1/2 a thou - and even that cannot be guaranteed, becasue you shouod not rely on microsteps for accuracy.
3. My steppers are 60,000 per inch, and I can get 40 ins per minute so I cannot see that speed is going to be a factor, so I would seriously think of gearing that axis down, so you can use a smaller motor, and gain accuracy.

I appreciate that you may be using servo motors - but the arguments are the same.
Not me driving the engine - I'm better looking.
Re: X and Y Axis Ball Screw pitch
« Reply #3 on: June 30, 2008, 10:03:27 AM »
Thanks Guys
I will go ahead and get the drive pulleys and belt to do a 5 to 1 ratio.  That will give me resolution of .001 per step at 200 steps per rev.  That will be enough for a plasma cutter.  I appreciate the info and I will be using steppers.  My next item to solve is how big a motors I will need.  I will do a weight calculation on the mass that has to be moved and start working out motor size.  I will past some pictures as soon as I can. 
Thanks again, Ferrall

Offline Hood

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Re: X and Y Axis Ball Screw pitch
« Reply #4 on: June 30, 2008, 11:08:59 AM »
The way I worked out for my lathe is  to get a 2ft long flat  bar, bore a hole in the centre and fit this to the ballscrew end. Position it so its horizontal and start adding weights to the end until the axis moves. The weights is Lbs is the amount of force in Lbs/Ft that you will need as a minimum. You will probably find that the actual cutting force will add little to what is needed as actually getting the axis moving from a standstill is the greatest force.

Hood