Cutting a part and scratching my head

Tem · April 21, 2017, 12:32am

Hi All,

Here goes. I’m creating pieces for a mold. But the mold cavity is a bit deep, so I have sliced the file into layers.

I used Fusion 360 to create the tool paths. The material is 3/4" MDF.

The part was screwed down from underneath the spoil board, the spoil board was then screwed down to the waste board, its solid. although a bit counter intuitive.

The router bit is about 3.11mm in diameter, Carbide type.

Adaptive clearing was working really well, with a ramp on the side.

All was good.

Then the parallel cutting started offset and that was it. All stop.

I measured the holes, because they aren’t round.
The CAD file: both holes are 6.35mm in diameter.
Hole on right is 7.3 in X and 5.2 in Y.
Hole on left is 7.2 in X and 5.35 in Y.
IIRC, they should be off about .01"= 0.254mm
The holes kind of fall outside of that tolerance.

The length of the long edge in the computer measures 143.65mm.
Measured with digital calipers the length on the part measures about 143.58.

The width of the part at one location also shows an in-discrepancy, it should be close to 64.281 in the Y, but it measures about 61.51 in the Y.

About my Shapeoko 3 XXL:
Sometimes, when jogging in the X in 1mm increments, it seems to jump more than 1mm a few times. Then it seems to return to nearly 1mm increments, but larger. This is in the air so I can’t quite measure it, and because it is intermittent it is hard to catch.
I haven’t noticed that behavior in the other axis.

All help on this matter will be greatly appreciated.

So far, I have really enjoyed getting up to speed with SO3 and Carbie Motion.

I have suggestion:
One feature I would like to see on screen is the offset values, these don’t have to be editable. But it would be very helpful for comparing dimensions and making decisions.

Thank you all!

Tem · April 21, 2017, 12:37am

tchad · April 23, 2017, 6:15pm

Hi Tem.

A few thoughts:

First, jogging sometimes going 1mm and sometimes going 2mm is normal. It has to do with whether the software registers your click twice rather than once. That is almost certainly unrelated to your cutting problem.

Your cuts seem way off, especially the holes, that is very abnormal. What feed and speed rates are you using? You might want to check your depth per cut, and step over rates and possibly adjust them to be more conservative.

Finally, if you have appropriate feed rates and speed, verify that all your belts are tight. If you’re cutting aggressively and the belts are a little loose, you could jump a tooth, and then all accuracy is out the window.

While you’re at it, double check the adjustment on your v-rail wheels. Those being off won’t cause as much play as you are seeing, but its worth being sure. Your carriages shouldn’t wiggle when you grab them and try to twist them slightly in parallel to the rails (e.g., the wheels shouldn’t be able to come off the surface of the rail).

Tem · April 23, 2017, 7:34pm

Hi Tchad,

Thanks for the advice.

I am using a laptop, so the trackpad and my fingers might be inadvertently causing the jog issue.

Yes the holes are off a lot.
I freed the part from the stock and remeasured. The overall form is very close to being with the tolerance claimed. So, without extensively tramming the SO3 XXL this is good. I can make it a bit more exact for my needs later. I like Winston’s youtube videos, I find them very helpful.

Feeds and speeds, I think I might be close to what they should be. 10k RPM spindle, 1/8" diameter ball end cutter, surface speed about 66.2876 M/min, plunge 333.333 mm/min, roughing step down 2 mm. I think it might be too conservative though. I’m still learning a lot.
The material is MDF, the higher density type.

I’ll check the v-wheels, and belts and so on this week.
When manually sliding the x carriage (power is off), there seems to be a slight racking. It could be related to the fact that the sheet metal parts are not the truest. The y-rail on the left has an offset of more than 1’8", the x carriage doesn’t butt up against the sheet metal like it does on the right. There are a few other issues too. I have tried to square it the best I can, but it seems to pop out of square slightly after tightening the machine screws. It was a hassle trying to get the waste board down, the screws wouldn’t locate on the hole in one corner (left y carriage/rail), so material had to be ground away to move the hole. I know this is a kit, but some of the parts I received are just off enough to have caused me some mild frustrations during assembly. Aside from that, I do really like the SO3. I hope that in time I will be able to have a local shop correct some of the sheetmetal issues, or create a few parts using a different method and material. But that will be a bit costly.

Time to inspect the v-rails.

Lastly, when the machine is off, can push the z-carriage down so far that it goes off the rails and begins to swing under and backwards underneath the main plate that it slides along? Does that make any sense? It seems like there should be a mechanical stop feature to control for that kind of travel? I can jog to that point as well. When jogging back up it slides back into the channels/slots. Just seems odd…

mikep · April 24, 2017, 12:56am

"Lastly, when the machine is off, can push the z-carriage down so far that it goes off the rails…"
Normal. There isn’t a stop on the bottom.

That looks to me like you’re getting missed steps - the distances are short, not long.
Non-round holes can be due to a few things
a) Machine not square or
b) machine not calibrated (x and y)

If you haven’t calibrated the machine, stop, and do that first. Then you can prove to yourself that the machine can produce accurate geometry. After you do that, then its easier to check things off the list that might be going on here.

tchad · April 24, 2017, 1:48pm

Hmm, I think your feed rate is high, but I could be misreading the chart.

I generally consult the Shapeoko feed and speed chart. It lists the MDF feed rate at “80”. I’ve always assumed that the units are in feet / minute, so 66 M/min (or about 216 ft/min) is pretty fast. If I’m reading the chart correctly, I think your feed rate is high. [Edit: I’ve successfully cut “normal” MDF at 80 ft/min, as deep as 1/4" per pass (with a 1/4" square end mill), at 17,000 RPM, or 1 on the Dewalt dial, so even if I’m misreading the chart and that’s not the right feed speed, it might be worth trying your part at the much slower feed, simply because I know it works.]

Assuming that I understand the units correctly (always bad to assume… apologies!), MDF has a plunge of 30 ft/min, so your plunge rate on the other hand is slow, you could speed it up (depending on your bit of course).

Those feeds are for a 1/4" bit, you may need to slow them down a bit if you get too much deflection or any chatter - especially if as you said, your MDF is a particularly hard variety.

I agree with everything that @mikep said. You might also want to cut a square circle diamond test pattern to confirm your machine’s accuracy, but make sure you sort out the feed rate, first.