CNC tolerances

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whateg0
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CNC tolerances

Post by whateg0 » Sat Feb 15, 2020 9:48 am

I'm reading thru a FB post about ABL and why it's a bad idea and someone says, well, maybe a 3d printer isn't capable of accurate parts. So the op replies with this:

"Yes, I did a lot of homework on these guys and I can maintain tolerances within a couple thousands of an inch. Spend some time dialing everything in and the 3 pro is quite a little workhorse. Poor craftsmen blame their tools. I can operate around 10 different types of cnc machines and hold a 30thou tolerance. Let me loose on a laser and I can dial it in to be 1/2 of a thou off the required spec. The printer is like a mill, but in reverse."

I've only operated small, hobby size CNC machine and my plasma table. Are there tolerances really that much worse on industrial machines? If anything, I'd expect far better with backlash comp, etc.

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Bill Shields
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Re: CNC tolerances

Post by Bill Shields » Sat Feb 15, 2020 10:09 am

you get out what you put in.

rigidity, accuracy of position measurement, tolerance in corners, 'look ahead' at high speeds...everything adds up

machines that cut metal with a tool have a very different personality than one the cuts with a laser or wire (WEDM) since they don't have to 'push' on the metal to remove it.

all have to worry about foundation rigidity, temperature (water with the WEDM) to maintain accuracy

3D printing is in the early stages of learning to attain that level of accuracy with its own sub-set of worries. Putting the deposition tip or laser where you want it is only a part of the puzzle.

Knowing exactly how much is 'deposited' at the time is the major area of attention...and in many cases makes the difference between an average machine and a really good machine..both of which may have seemingly identical positioning kinematics.
Too many things going on to bother listing them.

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NP317
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Re: CNC tolerances

Post by NP317 » Sat Feb 15, 2020 12:06 pm

What does "ABL" refer to?
RussN

whateg0
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Re: CNC tolerances

Post by whateg0 » Sat Feb 15, 2020 7:17 pm

NP317 wrote:
Sat Feb 15, 2020 12:06 pm
What does "ABL" refer to?
RussN
ABL=Automatic Bed Leveling

Which isn't really bed leveling at all. It's just mapping the z position of the bed and then calling that zero. I find that a lot of people print stuff where tolerances aren't even a thing. But if a person has a print bed that has a tilt to it, the part will still print upright. Then when it's set on a level surface, it'll tilt. Again, some people don't mind that. I do.

Dave

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NP317
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Re: CNC tolerances

Post by NP317 » Sun Feb 16, 2020 1:46 am

Thanks for the explanation, Dave.
Learn something every day...
RussN

RMinMN
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Re: CNC tolerances

Post by RMinMN » Sun Feb 16, 2020 6:13 am

The bed on my 3D printer isn't flat. I could do a "mesh bed leveling" and map out that curvature so the printer would "know" what that curvature was to some degree but instead I threw a bunch of money ($20) at it by installing a glass bed. Note that the first thing to do after installing the glass bed is to re-level the bed. The second thing is to wash it with 50% rubbing alcohol. I missed that second step since the glass bed came without instructions and my prints stuck very tight to the point that I had to chisel them off. After the alcohol wash the first print didn't want to stick but the second print was great. After about 10 prints it needed another wash.

whateg0
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Re: CNC tolerances

Post by whateg0 » Sun Feb 16, 2020 10:28 am

Despite the backlash I get for it when I say so, I still use hairspray on my glass bed. Almost never have a failed print due to bed adhesion and they pop right off when cool still. Stupid idea but it works, so...

Dave

pete
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Re: CNC tolerances

Post by pete » Sun Feb 16, 2020 3:01 pm

Would waxing and then buffing the glass with something like automotive paste wax work? If it does it might be a lot more long term than the hair spray. I keep a can of it in the shop and it's got a lot more uses than one might think. A clear micro thin rust preventative is just one of them.

whateg0
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Re: CNC tolerances

Post by whateg0 » Sun Feb 16, 2020 3:18 pm

The pva in it is a release agent, so yeah it gets used for all kinds of things. You have to have something the print will stick to when it's hot or you'll get nowhere, right? But it needs to let go after or you risk damaging the print getting it off. Using blue tape in the beginning, we had prints that we just could not get loose from the tape. Fortunately the tape peels off the build surface but it's not really any better for it.

Dave

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