Is anyone else having problem cutting circles?

[knightsofnii]

I have had shape and curve problems from a couple things:

1: the set screws spun out of my pinion gears, causing the x axis to "studder", most notably after switching directions. That took a long time then a eureka moment to find.

2: I imported .stl 3d shape from one cad software into Aspire, and the shape changed a pinch.

3: on shapes with curves, the CAD software I have created different line thicknesses for different curves. Now I always make sure I set my line thicknesses to 1 pixel, and then when in my cam software I convert polylines to splines (which also can skew things a pinch).
this way, the machine doesn't get confused which parts of curves are starts and ends, therefore making it cut on different sides of a line, or if cutting on the outside or inside of lines of different thickness, not producing squiggles.

After "fixing" 1-3, I have really nice shapes.

[Jon.N.CNC]

Calibration does mean a lot on some machines. You cannot just punch in the numbers that should be there after doing the math. You need to actually measure the travel to confirm or adjust the steps to get the target length. Mach 3 has a calibration aid for adjusting the steps per inch or MM and it will let you dial each axis in. Once you know that the machine travels 5.75" when you call for that distance, then you are farther along in troubleshooting the cause. You may find that the issue disappears.
On a plasma, just because the torch isn't inducing any cutting forces into the machine, doesn't mean that all the other axes aren't either. Especially given the speed needed for some materials. A gantry traveling at 150 IPM may suddenly need to go back where it was coming from. It has to stop all that momentum instantly. Any slack at all will show itself right there. Consider how many times the machine may do that during a job. Mach 3 does try and lessen that type of impact with CV.

Ive always found the axis calibration to be less effective than a calculator, mostly because it does not calculate to enough decimal places. To calibrate better I simply work out the steps per mathematically. Place a dti at one extent of axis travel, and run the axis from - extent to the dti, if it's off I simply add or subtract step units incrementally from the end decimal places and repeat until it is within the desired accuracy.

It's the only way to get it spot on imo, re entering the travel into the axis calculation will loose you decimal places of accuracy, you want it to around 9-10 decimal places.

[LeeWay]

Mach 3 gets pretty deep in the decimal point. I often thought that was overkil, so I always rounded them up to 4 points.
It has to be repeated many times in several locations on the axis, but it you get you very close if you stick with t. I also recorded each step change, so I could average them all out across the length of travel..

[Jon.N.CNC]

Mach 3 gets pretty deep in the decimal point. I often thought that was overkil, so I always rounded them up to 4 points.
It has to be repeated many times in several locations on the axis, but it you get you very close if you stick with t. I also recorded each step change, so I could average them all out across the length of travel..

I think Mach 3 goes to 6-7 decimal places.

Just 4? That's either very coincidental that you have accuracy with just 4 decimal places or your not as accurate as you think you are.

Obviously due to steps being exponential, on smaller machines you can calculate to large number of decimal places but will be accurate with less. On an 8x4 machine over 2440mm a change at 9 decimal places does register on a dti.

Wtf Sorry can not does, if I try to edit this forum deletes my last paragraph :s infact it's deleted this paragraph twice now when I have entered the last paragraph.

With mine i am lucky to be within 1mm with over full 8ft of travel with the inbuilt calibration, with a calculator and repeatedly making manual changes I get within 0.05mm which is the mechanical accuracy of the machine.

O and it's deleted it anyway, I have to type again :s

But you kind of already confirmed that by saying you have to repeat multiple times, you really shouldn't need to, sounds like your just retrying until you get lucky.

With a calculator you should be within 0-0.15mm instantly.

[Jon.N.CNC]

I think Mach 3 goes to 6-7 decimal places.

Just 4? That's either very coincidental that you have accuracy with just 4 decimal places or your not as accurate as you think you are.

Obviously due to steps being exponential, on smaller machines you can calculate to large number of decimal places but will be accurate with less. On an 8x4 machine over 2440mm a change at 9 decimal places does register on a dti.

Wtf Sorry can not does, if I try to edit this forum deletes my last paragraph :s infact it's deleted this paragraph twice now when I have entered the last paragraph.

With mine i am lucky to be within 1mm with over full 8ft of travel with the inbuilt calibration, with a calculator and repeatedly making manual changes I get within 0.05mm.

O and it's deleted it anyway, I have to type again :s

But you kind of already confirmed that by saying you have to repeat multiple times, you really shouldn't need to, sounds like your just retrying until you get lucky.

Sorry this forum started to spin me out then, it seems to be buggy in safari.

[LeeWay]

No. I repeat multiple times along the axis in different locations. Then average them out. I was mainly talking about mils with short axis travels generally. Longest being maybe 14". You can log deeper decimal places if you choose to of course. I do not have any equipment that would measure any closer than 4.
For my bigger plasma cutter, I just went with whatever Mach gave me. Only did it a couple times on each axis. It has rack and pinion and X and Y.
I will dial it in closer on my new CNC router parts machine though. It has R&P too, but I will be looking for a little better accuracy across the board than I set up for on the plasma.

[Jon.N.CNC]

No. I repeat multiple times along the axis in different locations. Then average them out. I was mainly talking about mils with short axis travels generally. Longest being maybe 14". You can log deeper decimal places if you choose to of course. I do not have any equipment that would measure any closer than 4.
For my bigger plasma cutter, I just went with whatever Mach gave me. Only did it a couple times on each axis. It has rack and pinion and X and Y.
I will dial it in closer on my new CNC router parts machine though. It has R&P too, but I will be looking for a little better accuracy across the board than I set up for on the plasma.

Try the manual way, I too used to do it a similar way to you but it just never seemed to be right with poor repeatability. One way to test on a smaller machine is to make up a program that starts a minus extent travels to positive extent into dti and back again and repeat 50 times, just monitor the dti for a drift will tell you how accurate you are.

The more decimal places you have the better chance you stand at the test being repeatbly accurate. Preferably all the decimal places of the mathematical calculation or maximum software denomination.

[LeeWay]

I would need better instruments.
Mach 3 is only going by what I have told it that the machine actually moved. I get that number from my dial indicator. It only read s 4 decimal points, but I would not count on that forth one. Unless it is a Starrett Or Mitutoyo or something. Mine aren't.
I bought a Starrett dial indicator once. Next time I went to use it, it was in pieces. If I could have laid my hands on No body, I would have. Apparently No body did it.

[dgage]

Can someone point me to some info on this DTI and multiple axes? I'm not familiar with DTI and the only thing I really found looking up "CNC DTI" was a video on Youtube checking height with a dial indicator over multiple points on the table. Not sure how I'd do something like that for the X or Y axes to get to 1 digit much less 6 or 7 digits of accuracy. Thanks.

[Jon.N.CNC]

I would need better instruments.
Mach 3 is only going by what I have told it that the machine actually moved. I get that number from my dial indicator. It only read s 4 decimal points, but I would not count on that forth one. Unless it is a Starrett Or Mitutoyo or something. Mine aren't.
I bought a Starrett dial indicator once. Next time I went to use it, it was in pieces. If I could have laid my hands on No body, I would have. Apparently No body did it.

.4 decimal points is plenty your machine will only be accurate to two.
A decimal point in steps per is not a decimal point of a mm unless your machine does 1mm per step. In which case the accuracy would be horrendous.

It's a repeatable accuracy your looking for so with a program such as the one one I mentioned, if your accuracy is out the drift will be exponential Ie one travel is out .001mm per step over 100 steps it's out 0.1

So you want to set the dti up at the furthest possible travel and start at the furthest opposite extent, you want to be measuring as many steps as possible.

If it's accurate to 4 decimal places over the entire axis travel then you must have some next generation machine as mechanically the best screws you can get are only accurate to 0.012mm.

And you won't get enough decimal places of measure with any instrument if you are say stepping one mm into a dti and using the measurement. This is again why a mathematical calculation is more accurate.

Sounds as maybe you are getting slightly confused between a backlash test and a steps per calculation.

[ger21]

.4 decimal points is plenty your machine will only be accurate to two.

Maybe for you metric guys, but not here in the US.

[LeeWay]

Yep. Even the backlash on my rolled ball screw is in the low third decimal point. And it's a cheap screw.

I do get what you are saying about the step count being different from distance traveled. They are different.I used the method I mentioned on my home made mill. It has some nice NSK ground ball screws on it. They were preloaded with two different size balls.
That all helps to lessen any backlash.
My parts do not need high tolerance accuracy, but when I did machine stuff in testing, they came out to the exact final dimensions I was shooting for. I would have to take into account the actual end mill size and do a finish and spring pass, but hey, it preformed accurately at least as far as I could measure. Not bad for a home made machine.

[diyengineer]

Another thing you can do with small holes is either choose to drill them, drill and followup with boring them, spot drill them and followup by hand drilling them, drill them under size and followup with a router bit, or endmill to clean up excess with a lighter cut.

[Rathkamp-Drums]

This has been an interesting read. It's even more interesting that backlash is only briefly mentioned. To interpolate a circle, both the X and Y axis change directions.

In a production machining environment, circular interpolation is NEVER used to achieve a finish size in a hole with a tight tolerance.

You can often tune some of this backlash out, but it will never be perfect.