5in facemill leaving ugly patches

[rk9268vc]

Ive got a SEHT insert 5in diameter facemill that leaves a beautiful finish, except randomly it will leave an ugly patch. It doesnt happen every pass or every part.
You cant feel it at all, but it looks bad. It seems to always be on the side of the part where the inserts enter the material.
It doesnt make any chatter sounds or anything, and i have the part grabbed securely. I cant figure out why it is doing that.
any thoughts?

here is a picture of what it looks like in aluminum and UHMW

 

[mhajicek]

Looks like a tram issue at first glance. What we're seeing is a difference between leading-edge contact and trailing-edge contact. Any chance the part is moving just a hair during the cut?

 

[rk9268vc]

Looks like a tram issue at first glance. What we're seeing is a difference between leading-edge contact and trailing-edge contact. Any chance the part is moving just a hair during the cut?

i dont think the part is moving, and have had it happen on multiple different parts, but i guess i cant guarantee it

I would think tram, but what i dont understand is why it wouldnt be the full length of the part and not just a patch?
I would think since the head isnt moving that the angle wouldnt change throughout the travel.
also last i checked i was trammed in within a few tenths across 34 inches and i have no reason to think it has changed since i checked, but i will check again tonight.

do you think maybe the bed is moving/bending a tenth or so during the travel? I do have a 18 in long part ive got to machine this week, i wonder if when i do both sides if the patch is in the same spot on the bed both times. it wouldnt take much since the cutter is pretty well trammed

 

[mhajicek]

do you think maybe the bed is moving/bending a tenth or so during the travel? I do have a 18 in long part ive got to machine this week, i wonder if when i do both sides if the patch is in the same spot on the bed both times. it wouldnt take much since the cutter is pretty well trammed

That's what I'm wondering. If you have a worn spot on your X ways it might show up like that. Maybe try putting the vise in a different spot along the X travel?