[cryptodecals]

Hi Everyone,

I am cutting 8" circles out of 1" acrylic on my CNC router. I am using a porter cable router with precision collet and an onsrud 1/4" 0 flute bit. The router is running at 10000 RPM at 25 IPM with 5 passes at .2" each.

The 8" circles were coming out strange so I observed the cut closer. Each pass seems to look fine however there was a misalignment between them. If I run my nail along the edge of the acrylic I could feel the passes on certain sides protrude. This also resulted in previous passes being over cut by the latest pass on the opposite side of the protrusion. The minor miss alignments on each cut cause the circles to be misshaped. Some time the miss alignment was as much as .02" - .03"

Just to check my insanity I proceed to do the same cut with a single pass on 3/16" acrylic and I was able to produce a perfect circle.

I will likely look at roughing the shape and doing a finish pass in the future. My concern or questions at this point are:

1. Is it normal for each pass to be slightly miss aligned when using multiple passes?
2. Should I always rough out the shape and perform a finish pass?
3. Could this be a configuration issue?

[bhurts]

If you have misalignment on two sides and are good on the other two then your z is most likely not moving straight up and down. Check the edges with a square from the face.

Ben

[ger21]

Your spindle is not perpendicular to the table.

[cryptodecals]

Thanks for the quick reply. I will check if the spindle is square to the table later tonight.

I have one of the pieces with me and when I check the edges with a square, each pass is square and straight. They're just don't line up to make a perfect edge.

I don't have a camera with me but I did a quick drawing in paint. The left side is straight but the right side as you can see isn't.

[ger21]

It's impossible for the edge to look like you've drawn it. Each successive layer should be further to the right. It doesn't take much of an angle for it to look square and still leave a stepped finish.

It also could be losing steps in one axis only, shifting to the right with each pass. But I wouldn't expect it top be that consistent.

[freejep]

I don't have a camera with me but I did a quick drawing in paint. The left side is straight but the right side as you can see isn't.

Attachment 225490

The edge you've drawn is not impossible.....but there is only one way to get the edge undercut like how you've drawn it...your onsrud 1/4" is acting like a woodruff cutter....
Have you had the shank spun down to stop rubbing...and accidentally only left 0.2" of cutting flute?
As for mismatch....going clockwise for one circle then counter clockwise for the next will create differing amounts of pushoff....
You can always try a spiral cut if your router will do 3 axis simultaneous movement...
To cut an 8" circle with a 1/4" cutter and not using cutter comp(G41)...
G00X0.Y3.875
Z0.1
G01Z0.
G03 J-3.875Z-0.2
J-3.875Z-0.4
J-3.875Z-0.6
J-3.875Z-0.8
J-3.875Z-1.
J-3.875
G00Z1.
etc....

good luck...

Jep

[cryptodecals]

Hey guys. Sorry the image was incorrect. I've attached the proper one for reference. Gerry you are correct it looks like each cut is slightly further to one side its just not consistent pass after pass. I am wondering now if it could be black lash?

[bhurts]

If you have a indicator you should check that your spindle is moving straight up and down. It only takes a few minutes to check and will remove a possible issue from the list. If things look good in one pass this is most likely the problem. It could also be missed steps, backlash or a flimsy z axis that pulls in on one side of the cut in each direction. You can check for missed steps/ backlash by cutting another circle in one pass with the 3/16 acrylic. Then measure the pc while leaving it on the table and run the program again on that same part without moving it. If it doesn't change then its most likely not the issue.

Ben

[cryptodecals]

Hey Bhurt, What is the best way to check if the spindle is moving straight up and down? I have a dial indicator.

[bhurts]

Put a ground pin or carbide blank in the router instead of a tool. Then put your indicater on the right or left side of the blank at the tip furthest from the router. Jog down along the length of the pin and look for movement on the dial. Repeat with the tip on the front of the blank. This will check straight in both directions. Try to keep the dial from moving more then .001 over 1.5-2 inches of length.

Ben

[ger21]

If you have a indicator you should check that your spindle is moving straight up and down. It only takes a few minutes to check and will remove a possible issue from the list. If things look good in one pass this is most likely the problem.

If the spindle is parallel to the Z axis, but the Z axis is at an angle, you won't get stepped cuts, as the tool will be in the same position each time. Imagine tilting the tool 5°, then move the tool down along the center axis of the tool. Each pass cuts in the same place as the previous pass.

You get steps when the spindle is not parallel with the Z axis. Now tilt the tool 5°, and instead of moving it down along it's axis, you move it down perpendicular to the table. With each pass, the tip is in the same X-Y location, but the tool is on an angle.

Backlash won't cause steps, because it's not cumulative. The tool will still be in the same location each pass.

If the steps are as large as .03, then it's possible that you're losing steps in one axis. Does it happen at the exact quadrants of either the X or Y axis?

[bhurts]

You are correct Gerry parallel to the z is what I was trying to convey and what the test will show. I can see how saying straight up and down can be misleading.

Ben

[Michaelofburien]

Hi Everyone,

I am cutting 8" circles out of 1" acrylic on my CNC router. I am using a porter cable router with precision collet and an onsrud 1/4" 0 flute bit. The router is running at 10000 RPM at 25 IPM with 5 passes at .2" each.

The 8" circles were coming out strange so I observed the cut closer. Each pass seems to look fine however there was a misalignment between them. If I run my nail along the edge of the acrylic I could feel the passes on certain sides protrude. This also resulted in previous passes being over cut by the latest pass on the opposite side of the protrusion. The minor miss alignments on each cut cause the circles to be misshaped. Some time the miss alignment was as much as .02" - .03"

Just to check my insanity I proceed to do the same cut with a single pass on 3/16" acrylic and I was able to produce a perfect circle.

I will likely look at roughing the shape and doing a finish pass in the future. My concern or questions at this point are:

1. Is it normal for each pass to be slightly miss aligned when using multiple passes?
2. Should I always rough out the shape and perform a finish pass?
3. Could this be a configuration issue?

Sounds like your Z axis is not quite square to X and Y.

[fixtureman1]

You can check if it plumb by bendng a wire in a z and chucking it. bring it down to just about touching your table and use a feeler gauge to check t as you turn the chuck by hand.

[cimitoka]

how to cut acrylic man... when I try to cut the edge blows!!! and I broke up 4 tools andmill...


Cimitoka

[cimitoka]

some advice please!!!!


Cimitoka

[Michaelofburien]

I have heard, not yet tested, that to cut acrylic with a decent edge you need to have brand new, sharp tooling. And use coolant. I'll be trying that on some 1" clear in the next couple of weeks.

[fixtureman1]

I use a low helex cutter from vortex

[cimitoka]

Hi Michaelofburien

I hope that you find a way that cuts well..


Cimitoka

[cryptodecals]

cimitoka, what tool are you using? RPM? IPM? Material specs?