Failure Diagnosis

[profexorgeek (Justin)]

Hi all, I’m hoping someone can help me diagnose the root cause of an error. This is the failed cut:

Some things to point out:

• These are two-piece boxes with holes for magnets
• I have cut tons of these individually and just moved to cutting 3 up
• I had just finished a cut of this exact piece in walnut and it worked perfectly. Granted bloodwood is way harder but the cuts were coming out great.
• Each cut for each pass mostly starts in the correct place. If the X axis was loose or had some other registration problem, the machine would not correctly align the start of the cut (I think?)
• The Y travel and radius seem correct, only the X cut distance is messed up
• There were no G Code changes between cuts, I just re-zeroed to make sure everything was lined up with the new piece of wood and started the job
• The machine had been on for a long time and run a lot of cuts (without issue) earlier in the day

The file looks like this:

There are two quick cuts with 1/8 end mill, then the bulk of the work is with a 1/4 downcut. It failed on the final toolpath:

Everything looked good with the simulation:

Any ideas are appreciated. All I can think of is some sort of software error. Nothing hardware related seems to make sense.

[Ed.E (Ed)]

Possible loose stepper motor coupler on X?

[WillAdams (William Adams (Carbide 3D))]

Where possible avoid slotting and add geometry and cut as a pocket

Making vacuum hose adapters

While cutting up vacuum extension wands for this is expedient, it’s a bit problematic given that Shop Vac recently filed for bankruptcy, was bought at the last minute, and production hasn’t caught up. I need a receptacle for the Sweepy 2.0 dust fitting — one option would be to purchase one from Woodcraft, but Carbide 3D sells blocks of HDPE: which looks to be just barely big enough for things to fit. Measuring the hose fitting I get a diameter of ~63.5mm — offsetting that twice we arrive at…

and/or

Adding geometry to cut as a pocket with a finishing pass

One technique which is often suggested to avoid slotting is to add geometry around a part which one wishes to cut out and cut as a pocket down to tab depth — here’s one technique for that. In this case, the project is a bevel gauge which will be cut out of 0.0625" (~1.5mm) thick aluminum: [bevelgauge] Due to the narrowness of the angles, an 0.03125" endmill has to be used, so after importing and scaling the file (we will be cutting out one which is 3") we select the perimeter and offset it tw…

and consider leaving a roughing clearance and taking a finishing pass.

Best Bit for full cut through

One which has a cutting flute length equal to or greater than the thickness of the stock — pretty much any tool should work. Big thing is the toolpaths — if cutting out, rather than just cutting a slot: [image] Offset to the outside by endmill diameter plus 10% or so: [image] [image] [image] Then cut as a pocket: [image] down to tab height or the penultimate pass: [image] then move the contour down to below the pocket and start cutting at the bottom of the pocket: [image] and…

[profexorgeek (Justin)]

I have thought about this for the past couple of days and I think I understand what happened and your comments about slotting @WillAdams.

@Ed.E my first thought was something loose on the X axis but if that was the case it shouldn’t have started the next cut in the correct place, which it did.

But, as I thought about this more - I have noticed that really hard woods like Bloodwood cut with tighter tolerances. When I cut something like alder, my box lids fit easily on over the lip on the base piece. When I cut a really hard piece of wood, they fit so tight I have to do a little sanding so they go together and come apart easily.

I pinch my work pieces very tightly on the spoil board. I generally get the clamps partially tight and then tap them with a mallet as I tighten them. I had a piece come loose and destroy a bit early on and so I make them snug as possible.

On a short length of board, the wood remaining after the contour cut to slot out the piece is short and likely doesn’t flex much. On a long length of board (such as these 3x cuts) there’s a much longer surface of thin wood. Combine tight clamps along a long thin surface and tighter tolerances of a hard wood and that mill bit is really pinched in there for a long cut.

So why did it start the next cut so precisely? Typically once the registration is off, it’s off forever (until you re-init the machine). The only thing I can figure out is that the drag going one way was roughly equalized by the drag going the opposite way and therefore the machine was going out and back into registration as it made a cut and then the reverse cut?

For now I’m going to make pocket cutouts that are slightly wider than the bit as Will suggested.

Thanks for the responses.