Area clear troubles

[foamcarver]

I know we've posted on here before about the shopbot taking weird dives into the piece it was cutting, we got a lot of great tips. The problem that fixed it was tightening the springs on the x y and z. We are having the same problems again, but this time our machine is tight. Could it be too tight? It only seems to happen when we use the area clear. The cuts are good if you just do a profile around something. Any ideas as to what we can do to stop this from happening in the future? I have attached some photos of the cuts I made today

Thanks in advance
Nikki
nicole@foamcarver.com (mailto:nicole@foamcarver.com)
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[foamcarver]

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[foamcarver]

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[mikejohn]

Nikki
I assume this is foam.
If you are leaving a relatively small amount of foam after the cut, the 'thin' foam maybe lifting.
What is your hold down method?
Edited to add "On closer inspection, it looks like MDF"

.................Mike

[foamcarver]

yes it is MDF. The first time it messed up I was doing an area clear on some trupan. I double checked to make sure the bit was tight, everything seems to be tight. I'm not sure what the problem is.

Thanks
Nikki

[jsfrost]

Niki,

My PC will be different than your spindle, but I have seen similar sloppiness when I push for more agressive cuts.

As Mike says, holddown method is a possibility. In addition, the issue could be too large stepdown or stepover settings. What bit size and settings are you using? A dull bit could contribute to the problem too.

If you have the SB operating (spindle off!) but not moving and push in X, Y and Z manually, does the system resist with significant force? It is usefull to feel the resistance when the BOT is running well, so you can compare when there is a problem.

Jim

[foamcarver]

Jim

The bit size I used is a 1/4" end mill. The stepover is at 17% and the step down is at a quarter inch. Feed rate is at 4.5"/sec. I wouldn't think the bit could be too dull, we don't use the 1/4 end mill that often. I have 14 clamps holding down a 48x96, it's what I've always used, and have only had this problem one other time.

Thanks
Nikki

[kerrazy]

Nikki,Where is this happening in the 4 X 8 material, I suspect towards the centre of the piece. As it is cupped a bit and not lying flat. With that cutter, your Spindle RPM should be about 6000 rpm and a feed rate of 5 in/sec should yield some beuatiful chips, rather than dust.

[frank_hav]

Try checking the Z rollers for slop. Also try looking at the ramp values. Both of these caused me problems in the beginning. I don't remeber changing the ramp values but some how they were not set at the defualt values. When I reset them to default alot of my problem went away. I also have a 12" Z travel and noticed that the rollers were sloppy where I did most of my cutting.

Frank

[paco]

What bit type is used?... spiral, straight, else?

What is the hold down on this project? Tell us more on the general project conditions.
Vacuum is almost a must for surface (depth sensitive) work...

Check for accurate positioning on air cut only... know how?

Check out for play at the Z motor pinion and at the router/spindle chuck.

[jhicks]

Mark & Nicloe, I would almost guarantee that Dale is on point here. The raster pattern clearly indicates the material is not perfectly flat. Of course someone will say it could be another cause and indeed it was springs earlier but if everything is tight,including the bit in the collet and roller bearings on the Z, then the most likely cause is the material cupping as stress is relieved. When we cut letters like this we prefer an offset cut starting on the outside. It generally takes less time, creates any machine marks following the shape of the letters, and stays on the material all the time so the entry and exit differential is very minimal if any.
Another thing available and especially helpful in plastics is an end mill with slight radius on the corners .002 and .020 R available from Onsrud. It eliminates the sharp corner cuts between stepovers created with a sharp corner end mill.
As much as a vacuum would indeed be the best solution, we screw down the MDF to the table and it seems to work fine. This is a color core sign but the strategy and results are the same using a .125 r emill offset strategy. Generally 35 to 45% stepover

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[foamcarver]

Our material is on the far end. We're going to try and cut a piece out closer to the lower left hand corner and see if that has anything to do with it.How do you check for Z roller slop? And how do you change the ramp values? We are using an onsrud straight end mill, about 14 clamps.(we have pucks for the vacuum but just haven't had a chance to get them going. How do you check for accurate positioning? We'll have to give the offset cut a try today too.

Thanks for all your help
Nikki

[kerrazy]

Save yourself some trouble and postion and zero your cutter on the centre of your work. Then start your router and move to the 0 postion on your z axis and then use your slash key (with the K (keyboard UI invoked) to move your router/spindle across the surface to see if your material is flat. I suspect it is cupped and this is causing the hassle. to prevent this. in your software design of the part insert some circles that are roughly a quarter inch in diameter in between your parts and then crete a area clear/Hog or fill tool path, or drilling tool path and only go down a 1/16th or 1/8th of an inch to mark locations were you could screw your material flat to the surface of your table. By creating these in your software your will see that your cutter will not come in contact with them when it is machining your parts.

Dale

[kerrazy]

Z roller slop is not a concern for me with this post at this point. The 14 clamps holding material indicates a poor substrate. typically I can hold a 4 X 8 pc of material with 2 to a max of 4 clamps, as the side to side pressure is really not that great for MDF.
Dale

[cnc_works]

I commonly attach a dial indicator to my Z axis and traverse it across the stock to check for flatness. Forewarned is forearmed. Also good for just checking table flatness between flattening with a bit.

Harbor Freight has cheap digital dial indicators ($30) that are much easier to see when at the far end of the table.

Donn