How can I fix this?

[r_jones]

I am cutting Lexan with a bullnose bit and for the most part the cut quality is pretty good except for where it starts and ends. Any idea on how I can prevent this divit?

http://www.rjoneswoodworks.com/downloads/IMG_6683 (Large).JPG

[knight_toolworks]

ramp the cut longer the better. well within reason of course.

[r_jones]

Good thought but I can't ramp because it's a bullnose it needs to come in from the side. Could this be a stepover issue? I was thinking maybe I could run a tool path that comes in slowly? Something like in the sketch below. I am not sure how to generate a toolpath like this though?

http://www.rjoneswoodworks.com/downloads/ScreenShot001.jpg

[scottbot]

Ron,
If you are using Aspire you could use the Leads feature.

Scott

[r_jones]

Scott,
Nice catch!! I actually see I have that option in the newer version of part works:) I will have to give that try and see how it goes but I think it's what I am looking to do:)

Thanks again,
I will post my results!

[Brady Watson]

That bullnose is really moving a LOT of material when it is cutting. You may want to try offsetting the desired shape to the outside a few times in .04" increments and try cutting from the outside in. Acrylic needs at least .03" to shave off when you use allowance in order to get a nice cut. Any less than this and chips want to re-weld to the part.

Just going in and cutting in one shot really puts a lot of pressure on the bit, and it will have a tendency to deflect and put a divot at your start point. Doing multiple passes will reduce deflection & chatter. Lead-ins rarely help in acrylic.

-B

[r_jones]

Brady,
That's actually exactly how I have been cutting the project. Except I have been taking smaller steps, more like .02 per pass and about 6 ips. That seems to work pretty well however taking a slightly larger pass will mean getting the job done quicker:)

[tracys]

Not sure I follow the cutting allowance, but is it that you cut the part oversize my the .02 or .04, them the final cut is just trimming a bit without all chips heating up.

[r_jones]

Tracy,
Yes, that is pretty much the idea. Rough cut oversize, then trim .03-.04 to final dimension.