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Author Topic: Cut parts are wrong size, CNC knee mill problem  (Read 21412 times)

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Cut parts are wrong size, CNC knee mill problem
« on: July 25, 2012, 10:34:34 AM »
Hi Guys,

I'm having a similar problem to the user in this thread: http://www.machsupport.com/forum/index.php/topic,21962.10.html

My setup:

Acer 5hp knee mill
Brushed servos
Granite Devices Servo Drives and interface card

This is my second cnc conversion but my first servo setup. I have brand new ballscrews in the machine, Hiwin ground screws with preloaded nuts.

I've set my travels with a mitutoyo digital dial indicator with accuracy of 0.001mm. My travels are accurate to within 0.002mm and it doesn't matter if it's 1mm or 100mm of travel. I have around 0.028-0.032mm of backlash on both axis that is 100% consistent.

The problem that I have is that my parts after being programmed in Solidcam were cutting outside diameters perfectly, but inside diameters were always 0.2mm too small. It didn't matter if it was a hollow circle, square, rectagle etc.. always 0.2mm. I adjusted feeds to super fast and super slow and always the same result. I rechecked the travels with the set steps function and it was always the same as mentioned above. I also tried cutting parts that were 2 inches thick, in 5mm step downs to see if the machine was missing steps as there woudl be ridge marks but it always cut perfectly. It doesn't make a difference if I cut 5mm deep, 12mm deep, 20mm deep, at 100mm a minute or 700mm a minute, the results are always the same. The bit has always been a new 3 flute, 0.5" diameter (12.7mm) carbide that just eats aluminum like butter.

Last night my dad suggested that I use the onboard wizards to cut a circle as I've used them on my smaller mill and it always generates code that cuts to size. The same problem occured, when I cut a circle or a circular pocket the diameter is always 0.2mm to small. So this has eliminated the cad/postprocessor and it has something to do with Mach3 but for the life of me I have no idea what could be wrong. There is no play in the table, no play in the angular bearings that support the ballscrews, the ballscrews are mint and 100% repeatable with 0.03mm of backlash, the servo drives are set to stop the machine if there is more than 30 encoder errors from target to destination (4000 encoder inputs per revolution, that's around 0.75% error) and based on my observation it never exceeds 15 errors.

Any ideas? I'm thinking of going back to the same version of Mach3 that my benchtop mill has but I don't know if I will be able to find it as it was installed 7 years ago.

Cheers,
Sebastian

Offline Hood

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Re: Cut parts are wrong size, CNC knee mill problem
« Reply #1 on: July 25, 2012, 10:49:02 AM »
I presume you are using the parallel port and if so then I would also assume you are using electronic gearing in the servo drives? If you are try with it off and see if it make a difference.
Is there any play in the spindle bearings or quill that could be deflecting the tool?
Hood
Re: Cut parts are wrong size, CNC knee mill problem
« Reply #2 on: July 25, 2012, 10:59:58 AM »
Hi Sebastian,
  Have you compared Climb to Conv. milling the same file ?
Could be a spindle issue ?

Russ
Re: Cut parts are wrong size, CNC knee mill problem
« Reply #3 on: July 25, 2012, 11:05:53 AM »
Parallel port yes. Electronic gearing: I don't think so but I'm not familiar with the question. I have a belt setup from the servos to the screws, and the bodies are heavy duty cast pieces that do not flex at all even under the heaviest of cuts (checked with my digital dial indicator). The entire head was rebuilt before the new screws were put into place with top of the line NSK bearings, no play there. If there was play I wouldn't be getting a near mirror finish on the edges. That's the strange thing that no matter the size of the part, the internal diamter is always 0.2mm to small.

Climb/conventional doesn't make a difference. Surface is near mirror finish so there is no chatter.

Sebastian
Re: Cut parts are wrong size, CNC knee mill problem
« Reply #4 on: July 25, 2012, 11:10:29 AM »
I've also tried Fanuc, Fanuc 5A, Mach3 post processor files and no difference. I've eliminated the cam side as the problem because the internal wizards in Mach3 reproduce the same problem, internal diamter circles are 0.2mm to small. But when setting the travels with the step config function, the table moves exactly how much I asked. This is the part that has me stumped because I'd understand if it was losing steps, or there would be ridges in the finish but nothing of that sort.

Sebastian
Re: Cut parts are wrong size, CNC knee mill problem
« Reply #5 on: July 25, 2012, 11:32:32 AM »
.... and of course you have verified that the code produced is EXACTLY as you are expecting for the given geometry and tool dimensions ?
sorry if this is too obvious, but I did not see it mentioned.  :)
« Last Edit: July 25, 2012, 11:36:47 AM by Overloaded »

Offline BR549

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Re: Cut parts are wrong size, CNC knee mill problem
« Reply #6 on: July 25, 2012, 11:41:44 AM »
Are your cuts WITH tool radius comp or without?  Is the difference in cut symetrical around the part?

(;-) TP

Re: Cut parts are wrong size, CNC knee mill problem
« Reply #7 on: July 25, 2012, 11:48:43 AM »
Yes, that is where the idea of eliminating Solidcam/Postprocessor and using the onboard cam wizards in Mach3 was mentioned. My dad suggested this to pinpoint where the problem was, Mach3 or Solidcam. Using the onboard cut circle or circular pocket wizard, I entered a circle diameter (tried several sizes progressively getting bigger) and 12.7 for the tool diameter and each time it was 0.2mm too small all around the circle.

Now I know that it seems that the grind on the bit is smaller than the 0.5" diameter of the shaft, but I've verified with my distributor that they are in fact 0.5" cut diameter with a tolerance of 0 -> -0.05mm of diameter just like the part number listed.

With the Mach3 circular pocket cut, I have no idea. Is there a tool comp setting in Mach3? I've never used this feature in the past.

Sebastian

Offline Hood

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Re: Cut parts are wrong size, CNC knee mill problem
« Reply #8 on: July 25, 2012, 12:37:51 PM »
I was just thinking if you have 4000line encoders and are using the parallel port then you would really need to have some electronic earing in the drives or your Velocity would be rather low.
Or were you meaning your steps per unit were set to 4000?
Hood
Re: Cut parts are wrong size, CNC knee mill problem
« Reply #9 on: July 25, 2012, 01:21:47 PM »
I don't understand what you mean by electronic earing? The drives have a gui interface that allows you to set it up based on your machine, you enter the type of servo, encoder count per rev, max current, voltage being entered into the drives min/max, you tune the PID and it shows you a digital oscilloscope, etc... The encoders go direct into the drives, how would using a parallel port affect the way they run?

 I also don't have them setup to run very fast, about 900mm a min max but I cut usually 12mm down and 10mm step over at 800mm/min so that is more than fast enough for me.

Sebastian