CarbideCreate/Nomad cut size slightly wrong

(Timothy Masters) #1

I’ve done a lot with my Nomad, but this is my first project in which dimensions are critical.

I created a square drawing, 7" by 7", with some pattern stuff in the middle, and exported it as 2D SVG. When I imported it into Create, the screen confirmed that it was exactly 7 by 7. I loaded the #102 bit, 0.125" and made sure Create knew that was the bit I was using, and secured my thin plywood with tons of double-sided tape. The final cutting pass was running around the outside of my work (the 'Outside" option) to cut it free from the 8x8 stock. All looked beautiful, but when I measured my piece it was just under 7 1/6 square, almost 1/16 too large. Any thoughts on what happened? Thanks!

(William Adams) #2

There are a couple of things which could be causing this:

(Timothy Masters) #3

Thanks for the reply. Before I answer your questions, I’ll say that the oversizing happens equally in both the X and Y dimensions. And it’s accurately repeatable: I cut four rectangles supposedly 7" long as four separate jobs, and all four were just under 7 1/16, all exactly the same size.

I don’t have even an hour of runtime on my Nomad, so I don’t see how any belts could have stretched.

The endmill is your #102 1/8". That’s the first thing I checked, and I even measured it in case the wrong tool got put in the tube.

I’m cutting 1/4" plywood using ‘hardwood’ specs to be conservative. The mill is well up into the collet, with only about 3/8" exposed. The quality and appearance of the cut is excellent.

I’m not sure what you mean by ‘lost steps’.

I’m measuring with an accurate drafting rule, and I double checked with my carpenter’s tape measure.

When I import the SVG file into Create, it correctly reports the length as 7".

This is driving me crazy, as this project requires accurate dimensions. Thanks for any help!


(William Adams) #4

The Nomad doesn’t have belts — the option to calibrate steps is the same for Acme screws which are a bit off.

Lost steps would be the machine sending in instruction to move, but not moving — since this is so even and you’re not noting any cut quality issues I doubt it’s that.

Since it’s off consistently for this size, I’d just adjust the endmill diameter in your CAM program by half what it’s off and recut thusly if adding geometry and cutting as a pocket with a roughing clearance and finishing pass doesn’t help.

(Timothy Masters) #5

Will - Thank you again. I’m still new at this, so please forgive some possibly dumb questions.

I don’t understand why I’d need to get into geometry things and a finishing pass with such a simple project just to make the cut size equal the specified size. I’m just cutting out a simple strictly 2D shape, not even 2 1/2D. I have a simple SVG file of the outline. I import it into CarbideCreate, make an ‘Outside’ toolpath, and save the Gcode. Then I run Carbide Motion, import the Gcode, and run it. Should that not be sufficient?

As for changing the size of the endmill in Carbide Motion, that seems weird. Why would a kludge like that be necessary? The program is written for the machine. Surely Carbide 3D does not expect users to change the specified endmill size to something that it’s not in order to make it work.

You said something about Acme screws being off. Is this a calibration that I need to do and failed to do? Is it documented somewhere? Isn’t the machine factory calibrated before being shipped? Can jostling during shipping throw off the calibration?

I’m sorry to be such a pain, but I just don’t understand this. It’s not like I’m trying to do something complex. All I’m doing is trying to do what amounts to using the Nomad as a cookie cutter for thin plywood, cutting out a simple shape (a rectangle in this case). Something fishy is going on when CarbideCreate correctly says that my rectangle is 7" long, but I get a cut rectangle 7 1/16" long. Either I’m making a silly mistake, or some calibration in the machine is off. And the fact that the error is equal in both the X and Y directions seems to discount some single adjustment getting out of whack. I’m mystified.


(William Adams) #6

Slotting is hard.

Usually the Acme screws are spot on — some folks calibrate them using the technique used for Shapeoko belt stretch.

The single adjustment which can be adjusted is the endmill diameter.

Please try cutting a slot and measuring that to work out the effective endmill diameter.

(mikep) #7

Consider using a more accurate method of measurement - a caliper would be ideal. A carpenters tape, a drafting rule, not accurate enough. I’d recommend a smaller cut just to get things sorted out - mayeb a 4 inch square.

(Timothy Masters) #8

I have a little more information. I was afraid that perhaps something went wrong with the SVG import, even though Create reported the correct size. Also, I was wondering about the cutter size issue.

So I use CarbideCreate to create three simple line figures. One was a single line 175 mm long with the cutter path set to ‘No offset’. The other two were rectangles 175 long by 5 mm wide. One was set for ‘Inside’ cutter path, and the other for ‘Outside’. In all three cases, the cut length was 176.5 mm (measured from the cutter centers at the ends for the ‘no offset’, and measured appropriately for the inside and outside cuts). So it’s not a cutter diameter issue, because if the program were thinking the cutter were larger than it is then the inside cut would be too short, not too long.

So somehow it’s a scaling issue. I can’t believe my measuring tools are wrong, because I used a professional drafting rule, three metal rulers, and two high quality tape measures. All six of my measuring tools are in perfect agreement with one another. They aren’t all wrong!

Is there some kind of ‘master scale’ involved when translating measurements into Gcode for the machine? Or a firmware or mechanical scaling in the machine? Some scaling factor somewhere must be set wrong.


(Neil Ferreri) #9

@TimM Can you share the gcode?

(Phil Gorsuch) #10

I am going to show my complete inexperience with SVG - perhaps with ‘outside’ it is following the outside of the line thickness not the nodes? I only do parametric so pardon my inexperience.

(William Adams) #11

There are 3 ways to cut out a shape — given a 1 inch square and a 1/4" endmill one gets:

  • outside — this should result in a 1" square — the toolpath is offset by half the endmill diameter
  • follow path / no offset — this should result in a 3/4" square — the toolpath is on the path and the part cut out is reduced by the half the endmill diameter all around
  • inside — this should result in a 1/2" square part inside, and a 1" hole in the stock, since the path is inset inwards by half the router diameter

(Timothy Masters) #12

Will - You are correct (of course). And if I were to do this, I would get that result, except that all numbers would be slightly larger. Your 1" example does not really apply to me, because the error I get is proportional to the size of the cut; 1" is small enough that I could not easily measure the error. But to exaggerate the effect, what I would get with your example is:
Outside cut - A 1 1/16 square
No offset - 13/16 square
Inside cut - 9/16" square part inside, 1 1/16" hole in the stock.

In other words, what I am seeing is ALL dimensions increased by about 0.8 percent. This is not apparent with small parts, but it does show up for large parts.

Here’s an actual example that I did. I made two concentric squares, 7" and 6". I cut the outer with an outside cut and the inner with an inside cut. This should give me a 7" square with a 6" hole in it.

Actually, the outside dimension is almost 7 1/16 and the inside almost 6 1/16

Here are the files:
ErrorDemo.c2d (7.9 KB) (14.9 KB)

Thank you!


(Neil Ferreri) #13

@TimM The error is not in the gcode. It is showing a 7" outside cut with a 6" inside cut (assuming a .125" end mill).
I’d suggest getting some calipers and calibrating your steps/mm for your machine.

(Timothy Masters) #14

I’d suggest getting some calipers and calibrating your steps/mm for your machine. <<

That makes perfect sense, because all of my dimensions are getting multiplied by about 1.008 when cut. Thank you! But now the question is, how do I do that? Is the process officially documented anywhere? Is it able to be done safely by the user?

I also wonder how it got out of calibration. The machine has only a few hours on it. I would have thought it would be properly calibrated at the factory. Can jostling during shipment cause this problem?

Anyhow, this sounds like a likely explanation. Thank you!


(William Adams) #15

As was noted previously:

(Timothy Masters) #16

Will - You said this should be a last resort, so I treated it as such, an unlikely source of the problem and dangerous to mess with. I also didn’t understand “only used for a specific pair of dimensions”. What does that mean?

I read the material in the link, but I have no idea what it means. What is Grbl?

If this is an adjustment that the user is expected to make, then I assume it’s got some official Carbide 3D documentation somewhere. Or maybe I should just call Tech Support and have someone hold my hand as I do it. I sure don’t want to damage my new machine!


(William Adams) #17

If a person was cutting out a part such as you describe and didn’t want to alter the dimension of the part, they could change the scaling in Grbl to get the desired dimension.

Grbl is the firmware which is installed on the Arduino in the controller and which interprets G-Code into machine movement.

We’ve had a couple of folks make this change on their Nomads — it seems to work well for some folks — the concern is throwing off where the machine moves to for the tool length sensor. If that’s an issue, just hit the power, re-connect and undo the change.


Several years ago, I spent a day setting the scaling on my Nomad for the x and y axes as close to true as I could. The machine was in spec, but on the job I was doing at the time (microscope parts that needed to align pretty much dead on) it wasn’t quite there. Easy to do.

(Timothy Masters) #19

Okay, it sounds like we are closing in on the solution. Not much of a scale change should be needed, as the error factor appears to be about 1.008, pretty close to perfect. But how do I do this? I just fired up CarbideMotion and searched in vain for a way to do this. There is a Settings menu selection, but it just lists some parameters, says nothing about scaling, and not a clue as to how to change it. I also searched every document I could find on the Carbide site and could not find any documentation for this. Is it documented?

(William Adams) #20

It’s $100, 101, and 102 as noted at: