Precision cut-outs: tool placement and hold downs

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Precision cut-outs: tool placement and hold downs

Post by tchad on Mon Apr 10, 2017 5:56 pm

I'm considering using the D3 to cut out relatively small printed items from normal paper, and I have a couple of questions about how I might be able to make this work.

My first question is: is it possible to position the tool for cutouts that require reasonably precise alignment to an existing print? To what degree of precision, and how would this be performed? I'm thinking somewhere around 0.02" - 0.04".

My second question is: what kind of hold-downs or friction surface is necessary (if any) for cutting? If I wanted to cut plain printer paper into small-ish pieces (say 0.5" - 2" per side), what is necessary for that?

Thank you!

tchad

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Re: Precision cut-outs: tool placement and hold downs

Post by tchad on Thu Apr 13, 2017 9:47 am

No one, huh?

Well... I ordered a D3. I'll test out the above two questions and report back. It seems like these should be solvable problems. Fingers crossed!

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Re: Precision cut-outs: tool placement and hold downs

Post by tchad on Tue Apr 18, 2017 7:53 am

I've received my D3. Based on some preliminary testing, I think I will be able to get within .040 once I refine my process. I've been experimenting with two different ways to set the starting point:

1) Pre-zero X and Y using a different bit that has no offset, then swap in the D3 and zero Z.
2) or, put in the D3, set the blade parallel to the X axis, and zero the Y axis by sighting down the blade. Rotate 90 degrees and repeat to zero the X axis.

Both seem to achieve the same positioning, as accurately as I can get visually. My testing continues and I'll update this when I have more objective results.

One thing worth noting is that aligning the material perfectly square with the table is much more important than the precise starting point. I'm working on creating a jig that will make square placement easier.

The answer to my second question was answered in the Donek video tutorials: a vacuum table. I'm currently building one with a relatively tight hole pattern to test cutting small parts (1/8" holes on a 1/4" on-center grid). I know that in general larger holes are required for good hold, but I only need to hold paper, and I'm planning on cutting relatively small pieces, necessitating the tight pattern. I'll post more results on how this goes as well once it's up and running.

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Re: Precision cut-outs: tool placement and hold downs

Post by tchad on Thu Apr 27, 2017 10:58 am

The quest continues, but I have some lessons learned that I can share:

I've built a vacuum table that has dowels that slot into pre-drilled holes on my CNC bed, so I can get perfectly square with the vacuum table easily now (within about .02 over 11"). I created a sunken bed in the vacuum table that is about .0625 deep (which in hindsight is to deep), and 8.5x11", so I can easily drop a piece of paper in and achieve relatively perfect alignment from the vacuum table to the paper.

One thing I quickly learned is that standard letter paper is not cut to size within .040. Different brands seem to have different levels of precision, but after achieving perfect square, my next problem has been aligning my paper on the X and Y axis.

Another problem has been that my paper is only .04 - .08 thick (standard and premium, respectively) so changes in cutting depth of .02 makes a difference in the quality of the cut, and my removable vacuum table does not have a repeatable Z precision (specifically, perfectly level) within .02. (The bed of my CNC is MDF, and the vacuum table is MDF with a HPD surface).

For now, I've been over-cutting into the vacuum table's surface, but this gives me two new challenges:

1) The swivel height cannot be set properly. Since the table height isn't perfectly flat, the cut depth is either is too shallow and doesn't swivel sometimes, or is too deep and leaves small tears in the paper when it swivels.

2) Since I have relatively large 1/8" vacuum holes, when the blade cuts along these, I get some small horizontal scalloping, where the paper was depressed into the holes, resulting in a cut that is not perfectly straight.

On the positive side, I have learned that 1/8" holes on a 1/4" grid has plenty of holding power for paper. I have to wait for the vacuum (condition) to fully dissipate after turning off the vacuum (machine) to remove the paper without damaging it.

I'm going to build a new vacuum table sometime soon that has even smaller holes on an even tighter pattern. Before I do that though, I'm trying to figure out how to improve my repeatable flatness (level) of the vacuum table. If my CNC bed and vacuum table were both aluminum, this would be much easier, but the aluminum bed for my table is currently too much for my budget.

Another challenge that is somewhat unrelated, but worth mentioning, is that knife-pathing for small parts with complex, organic shapes is non-trivial. The Excel spreadsheet didn't work at all, and SheetCam produces paths that are incredibly complex, with many swivel actions on sections of the vector that probably don't need swivel actions. It doesn't seem that SheetCam allows for setting a turn radius filter for swivel actions, instead it provides an "accuracy" slider (probably because of the knife path offset compensation algorithm), which means I cannot easily reduce the number of swivels. So many swivels on relatively straight lines leads to some artifacts on the cut, especially in such thin paper, when combined with my non-level vacuum table surface.

As I said above... the quest continues. Smile


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