Tool Marks in signfoam

[Bagan]

I am a newbie at the shopbot and sign making. The attached photo shows irregularities in the machined surfaces around the letters.

I am using a 1/8 flat bit cutting at 15000 rpm and 5ipm cut in 2 passes to a depth of .200

Shopbot is new.

This forum has so much great information and any help would be great.

Thanks Dan

[knight_toolworks]

it's hard not to get any toolmarks but you really want a 2f upcut bit for clean bottoms.

[Bagan]

Steve,

Thanks for you reply. I was not sure if I should expect the bottoms to be tool mark free and also didn't know if what I have is typical/expected?

I will get a 2f up bit also.

I have read about filling primers is that the solution?

Is this the reason that many signs of this type have a carved / textured bottoms so they wont need to deal with tool marks?

Thanks Again

Dan

[joe]

Dan,

All of us have this kind of problem from time to time especially when machining HDU. There are lots of reasons. First up, I'd suggest getting your Z Zero from the table top when changing bits.

The main reason I use background textures, is to add interest. A flat background with raised letters looks like applied, cut out letters, glued on the surface. Since you're armed with a CNC why not put it to full use and try some texturing?

As you progress along. I think you will find higher density HDU will give much less trouble in finishing and maintained much sharper edges.

Joe
www.normansignco.com

[joewino]

As Joe said, some of the lower density HDUs may tend to flex some while cutting, so it is important to have the panels secured well. We just screw them into the spoil board, but if fine detail is needed we also put some double stick carpet tape on the back. We normally use 20# Corafoam (Duna, USA).

A little bit of an uneven background will usually occur and can be easily sanded. Yours seems to be a bit deeper, but could be sanded out with not a lot of trouble. What is the size of the sign?

Normally, we add some type of texture and pocket the letters, which just adds more dimension.

Play with bits, speeds and RPM's and you will probably eliminate a lot of your problems. No one said it was easy.

[Bagan]

The material was screwed to table.

Sign is 12x18 and is out of 15lb signfoam. Is this a good general purpose sign material choice? Most signs will be 24x24 or less in diameter.

Sounds like some flex in material could be cause or is it possible that the material is not flat enough?

Regarding a bit would the Onsrude 52-554 foam cutting bit be the best choice to get the flat bottom desired?

Maybe I just need to go with textured backgrounds? Any bit suggestions?

Thanks for your input.

[joe]

Dan,

In the future, if you are concerned about bit flex, or your materials moving up and down, you can run the file once more with all the same coordinates. Try getting your Z from the router bed not from the material surface. Give that a try.

SignFoam is a second best option. It gets a B+. Their 18lb is much better. Duna gets an A while Precision Board gets a D-. We won't let that material in the shop.

Texturing the backgoruns:
If you add a mask like Gerber Mask, it's possible to add several textures by hand. Those could be sand, smalts, or spray on textures. One of my favorites is Rocker Panel Texture from your favorite auto paint store. It comes in a rattle can.

A flat painted background is asking for trouble. It will show all defects.

Joe Crumley
www.normansignco.com

[GlenP]

Dan, do you get these marks when you pocket say mdf material? I used to get alot of marks like that when I got my PRS Standard and thought it was only due to material movement. Shopbot ended up replacing my controller due to another issue and the problem you are having is gone completely. If you haven't can you cut the file or try a pocket in mdf to see if is the same. Not saying you have a issue but thought I would pass my experience with that very same result along. I do use textured backgrounds as much as I can as well.

[marysvillesign]

Dan,
I get this occasionally too.
I have seen my Z stutter from HDU dust on the bearing wheels. It shows in the control software as not reaching the intended depth as well. I have saved a couple of backgrounds with a cleanup pocket pass without the letters. That keeps the bit at the final Z level preventing any issues that won't allow it to return to the same depth. I know my machine is a bit finicky about that and I hate sanding.