Sandblasting is intended to clean the surface and is not guaranteed to remove tool marks. ... unless you tell the operator to spend a lot of time on it, in which case (s)he can blast right on through if you like.

Why were you CNC machining the surface of a plate that would normally be assumed flat, and left as is?
OR
it might be the light, but the photo suggests a little curvature in that area, which might have been written in the CNC code as an approximation to the desired radius with an array of angled flats.

Mike Halloran
Pembroke Pines, FL, USA

Is the plate too large for Blanchard grinding?

Dear Mike,

Thank you for your explanation.
We CNC the surface because we need to mill down the plate to specified thickness.

Dear Ron,

The plate is not too large. just 3146mmL x 1290mmW. but we haven't experience before to use Blanchard grinding.

Many vertical mills have some adjustment for the angle between the spindle axis and the table. Sometimes mills are used with the head angled away from its usual position, say for an undocumented job, and then imperfectly restored to their intended orientation.
These things happen.

Tool marks on a flat milled plate are pretty sensitive to head alignment, but without physically measuring the depth of the step, it's difficult to evaluate the actual angle. You might want to get the head/table perpendicularity checked, and if it's okay, consider rewriting the CNC program to pseudo-randomize the tool path, at some expense in floor to floor time.

Mike Halloran
Pembroke Pines, FL, USA

Sand blasting will not help you to remove these marks. I can't understand the clear shape of the mark. You may use some compatible epoxy fillers and make a uniform surface finish

Its possible you have an issue with the "tool", which is not as likely, or the programming, or as others have mentioned the perpendicluarity of the surface to the "tool" being used to mill this. Normally if there are larger tool marks they aren't noticeable as this one seems. What are you using to blast this? What media? And what is the material of the part made of? Both of these will determine if you can remove tool marks that big with just blasting.

180994....if the size you gave is correct, that's a large plate for any type of machining.

Blanchard grinding is a precision, rotary surface grinding method. Many machine shops can do the process; however, most are limited in size to about 300mm x 300mm. There are specialty machine shop that can do larger, but I don't know if your plate could be accommodated!

OK, so you need to (want to) make a large plate "thinner" by machining.
Does your actual application really, really, really require a perfectly flat surface? That will be very expensive to do that may not matter.