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Scaling troubles
« on: April 19, 2017, 08:32:10 AM »
Hi all. I am having trouble milling a hole. I have my cad gcode cutting a 1 inch diameter hole but it keeps turning out under size. I mic Ed my bit and put that in my tool size and my axis are all set to .Ooo5 accuracy. What is my problem. Or why.

Offline Tweakie.CNC

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Re: Scaling troubles
« Reply #1 on: April 19, 2017, 10:39:57 AM »
It could be tool flex or machine flex - are you using G41 / G42 ?
Compare the results from one hole cut clockwise and one hole cut counterclockwise - is there a difference in dimension ?

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Scaling troubles
« Reply #2 on: April 19, 2017, 05:49:44 PM »
No I am not using G41/42

Offline RICH

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Re: Scaling troubles
« Reply #3 on: April 19, 2017, 05:55:55 PM »
There are number things that affect the accuracy.
How accurate do want want the 1" hole to come out?

1. What are you steps per unit for the axes?
   Say it's 20000, then best possible resolution would
   be  0.0005". Also the axis needs to calibrated correctly.

2. How accurate is your machine in both axis when actualy
   moved at the location where the 1" hole is done?
  
   ? Any backlash, don't say none, it's just a matter of    degree!

3. How did you measure the diameter of the tool?
   I use V anvil mic's as they are accurate and made for       that purpose.

4. How much run out do you have at the end of the mill?

5. How is the coding done? ie; mltiple cuts, spring passes,
   etc.

If everything was perfect and the code was correct then the hole would be correct, BUT , that is not the case in the real world.

BTW,
If the hole comes out under then in cam you can just change / adjust the tool diameter ( same tool ) as the machine knows no difference!


RICH
Re: Scaling troubles
« Reply #4 on: April 19, 2017, 06:48:36 PM »
As far as I know (a new by) my axis are set to at least 2000 steps per. I have my backlash is close about .001 or a bit less.
I measured the cutter with a mic (1/4 inch tool) measured .238 dia.
Re: Scaling troubles
« Reply #5 on: April 19, 2017, 06:53:42 PM »
I have no idea how to measure the run-out

Offline RICH

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Re: Scaling troubles
« Reply #6 on: April 20, 2017, 06:01:04 AM »
Use an indicator , preferably one that can measure to 0.0001", to measure the total run out.
Put a ground rod, drill blank, etc into the collet / chuck, rotate the chuck manually,
Measure both very near the collet / chuck and also at a distance approx. where the cutting edge of the end mill would be.

Do a few tests with your machine.

Mill a hole through some scrap for say a 1" dia hole doing a rough cut, a finish cut, and a spring cut in the kind of material you want an accurate hole to be milled. Measure the hole. If it's under than change  diameter of the end mill to something smaller in the wizard / cam and do it again. Say your under by .004" then change the end mill diameter  down by .002" or half what it is under.

That gives you a practical idea of how good the system is how to adjust for YOUR machine.

Easier would be to do the hole with code based on the center line of the hole and start point of the first move from the same location.

That takes into account everything. You have what you have, just need to consider what you can do with it and how to adjust to get what you want.

RICH
Re: Scaling troubles
« Reply #7 on: April 20, 2017, 06:12:06 AM »
Thanks for the help. I will try what you said and let you know how I make out.
Re: Scaling troubles
« Reply #8 on: April 20, 2017, 06:20:08 AM »
What I found with some of the holes is that they were slightly oblong. The x&y was about.012 undersized but at the 2-8 o'clock measurement was perfect. But I will try what you suggested I know I have run-out as I have a router for a spindle.
Re: Scaling troubles
« Reply #9 on: April 21, 2017, 05:01:13 PM »
I haven't been able to measure for run-out as of yet but should it not cut undersize on the complete hole? I can understand that if the run-out is say .010 than the hole should turn out to be .020 under. Is this correct?