Cutting differently

[foamcarver]

I was cutting a circle with 1/2 inch wood the first time I cut it it cut fine, the second time it drug across my board, the third time it seemed to cut fine, but then when I took the board off, it cut into the spoilboard about an 1/8 inch. I didn't change anything in my cut file. Any one have any ideas as to what is happening?

Thanks
Nikki
nicole@webstudiousa.com (mailto:nicole@webstudiousa.com)

[richards]

I need more information. Did the circle cut all three times? Was the problem with the Z-axis changing height? Was the problem erratic cutting that didn't follow the sbp file?

[bill.young]

Also did you cut the circle in the same place on the ShopBot table each time? If not you might have high and low spots in your table top and need to surface the table top to flatten it.

And how did you hold the material down to the table top...could the material have been bowed up in the second cut?

[foamcarver]

we cut it in the same spot. the second and third time the z-axis went down to drag across to the start point. I never let it cut the second time. The third time I stopped it again and then tried it again and it worked fine with the exception that it cut into the spoil board bad. We were using the same amount of clamps each time.

[richards]

Now you've narrowed it down to two potential problem areas - software or hardware.

Does your sbp file raise the Z-axis to a safe height at the end of the cut? Does the sbp file contain commands to re-zero the z-axis while cutting (the VA command)? Have you verified that the z-axis always starts at the same height above the material before the part is cut?

Assuming that the software checks out and normal procedures are followed when zeroing the z-axis, and you still have problems leaves mechanical/electrical problems to check. Is the spur gear on the z-axis securely screwed to the stepper motor's shaft? Is the router/spindle mount loose? Are the BW v-bearings snug against the rails? Is anything in the Z-axis assembly loose or wobbly? When you give the MZ command, does the z-axis move the expected distance (use a small ruler or height block to verify that a one-inch move is really one-inch)?

Assuming that everything mechanical checks out leaves the possibility of electrical problems. Have you attached grounding straps to all three axes of the machine? Do the grounding straps connect to one common ground point? Does the common ground point connect directly to the ground wire from your power panel?

[gus]

I have had some problems cutting circles using the CP command. Would not cut CCW in our case. There may be other issues with this command. We are running Version 3.4.7 just something eles to think about.

[jsfrost]

Nikki,

What kind of bit are you cutting with? How deep are you cutting each pass?
The problem could be the bit slipping in the collet as the cut progresses. This would be most likely if the cut is being made in one pass using a smaller bit, or it the spindle collet is slightly loose.

In PW, Insignia, or Pro, the bit definitions will automatically determine the depth of cut on each pass. Since you will be cutting foam frequently you may want to define "foam" and "wood" bits separately (same bit, different definitions). Foam can be cut in fewer passes than wood.

Jim

[gerald_d]

See this thread. (http://www.talkshopbot.com/forum/messages/26/3736.html)

Here is a repeat of something I said there:

Basically you have to narrow your problem down to one of three areas:

1. Is something moving/slipping mechanically during the cutting process?
- Bit slipping in the collet
- Router slipping in its bracket
- Material lifting from the table
- Etc.

2. Is the stepper motor losing steps? ie. is it being fed good signals, but because it is overloaded, it doesn't do every step. The overloading can be caused by:
- Using a bit that won't plunge vertically down
- Dirt on ball-screws
- binding rollers on V-slide
- wrong tension in springs
- etc.

3. Is the stepper motor being fed with wrong information? ie. it does what it is told to do, but the control box is sending false info. This false info is caused by electrical interference getting to the control box. There are three directions to check under this category:
- Check emitters: The router is probably the biggest source of emission, especially when the brushes are bad - run the router off a seperate circuit and keep its cable at least an inch away from other cables. The static on a ShopVac hose is a huge emitter. The stepper motor cables can also emit.
- Drain emited signals to ground before they reach the control box. The metal structure of the SB can act as a receiving antenna and it should be grounded electrically. The ShopVac hose needs a ground wire. In some cases even the stepper motor cables need to be shielded and grounded.
- Protect receivers from the emitters: Again the wires of the z-zero plate, the E-stop, etc. can behave as antenna and lead interference to the control board. Keep them away from other cables, and if necessary, shield and ground those wires as well.

[foamcarver]

Thank-you all for your help. I'll have to remember those tips next time it happens, but I had already changed my bit before I had written in the forum. I also have another question for you guys. I was cutting my first 3d cutting. The z level roughing worked fine and looked how it did in artcam, and then I did my second cut the machine relief, It seems that it was offset. I didn't change the home positions, the z level was the same. I didn't move my piece of foam. Any ideas as to what might of happened??

Thanks for all your help
Nikki
nicole@webstudiousa.com (mailto:nicole@webstudiousa.com)