Tips for getting the best cnc milling surface finish

CNCCookbook Software and Information for Machinists

CNC Milling Feeds and SpeedsContents

Tips for Getting the Best CNC Milling Surface Finish

CNC Milling Feeds and Speeds Cookbook

Introduction

We've given tons of tips and information for CNC feeds and speeds throughout this Cookbook. This article is about the little tricks focused expressly on creating a better surface finish. It assumes you're following all the other tips but want to do something even more for surface finish.

Use the Right Feeds and Speeds

It's absolutely critical to have the right feeds and speeds for finishing work--no guessing!

Use a proper feeds and speeds calculator like G-Wizard--most finish work needs to take radial chip thinning, ballnose compensation, and a host of other sophisticated factors into account.

Keep the rpms up where surface speed says they should be and back off the feedrate for a finer finish. G-Wizard has its "Tortoise and Hare" slider to do this adjustment. Move it all the way left.

Tortoise Hare Lever

Move the Tortoise and Hare lever all the way left when finishing...

Don't allow the chipload to go too low or you'll be rubbing the tooling instead of taking clean cuts.

Clear the Chips!

Chips down in the work will scratch it up as your cutter slides them around in the hole. With work hardening materials like stainless, you may as well scatter a handful of hardened steel chips to scratch up your work. Use your coolant or air blast to get the chips completely out of the way of the hole.

Use Different Tools for Roughing and Finishing

Don't finish with the same tool you used for roughing. Keep brand new sharp tools for finish passes and rotate them to roughing work after they've done a brief tour finishing. If you're using an indexable tool, like a facemill, use two different sets of inserts.

Reduce Depth of Cut for Finish Pass

For the finish pass on a feature with a floor, reduce your depth of cut versus the roughing pass by a few thousandths. This means you'll only be using the side and not the bottom of the cutter, which results in a better surface finish.

Minimize Deflection and Chatter, Maximize Rigidity

You can't get good surface finish if your tool is deflecting excessively or chattering. You'll see every bit of that in the finish on the cut sidewalls. Use G-Wizard's Cut Optimizer to minimize deflection, avoid chatter on finish passes, and do what you can to increase rigidity and minimize vibration of any kind.

Use a Flycutter or a Facemill with Individually Adjustable Inserts

One of the challenges for surface finish when face milling is that each insert differs in cut depth by just a little bit--they're not all cutting at exactly the same depth. More expensive facemills let you adjust each insert individually. If you can get them adjusted to a couple tenths or less, finish will be greatly improved.

Failing that, try a flycutter. A big adjustable flycutter can cover the most territory in the fewest passes, and will often leave the most pleasing finish. However, you can convert even a regular facemill into a flycutter simply by removing all but one insert.

Use a PCD Flycutter for Aluminum

PCD inserts are supposed to leave the most amazing finish possible on aluminum. Some machinists keep a PCD-equipped flycutter in their tooling arsenal just for area finishing aluminum.

Use a Radius

A bullnose or endmill with a radius will leave a finer finish than a flat endmill. If we're talking insertable tooling, use a radius.

Use Sharp Inserts, Lead Angle, and Positive Rake

All things considered, sharper inserts, more lead angle, and positive rake all lead to better surface finish. A 45 degree face mill produces a much nicer finish than a 90 degree square shoulder face mill.

Run a Cutter Backwards to Burnish the Material

Be careful with this technique--it's the fastest way to dull a cutter and if you move the tool into the material much you'll break the tool. However, if you can get a tool you don't care about dulling (just endmills, no indexable tooling!), and you control the tool well enough, it leaves a burnished finish on the workpiece that may be just the ticket for your job.

Climb vs Conventional Milling: Assume Nothing About Which is Best for Finish

This is a complex topic. Climb milling reduces cutting forces and ordinarily that would be the best answer for finish. But, climb deflects into the wall and conventional does not. Read our Climb vs Conventional Milling Page to decide which will be best for your needs.

Entering and Exiting the Cut

Roll into and out of your cuts for better surface finish. See our Toolpath page for more info on this technique.

Don't Dwell

Every time the tool stops while in contact with the workpiece it'll leave a mark. Never dwell (stop the tool against the workpiece) when surface finish matters.

Avoid Cutting Down the Centerline

Cutting down the centerline will slap an insert or flute flat against the edge of the material, which creates burrs and generally makes it hard to get great surface finish. Instead of a 50/50 cut down the centerline, choose about 70/30 for a better result.

Tram Your Mill Carefully, or Put it Out of Tram Intentionally

We're talking about facemilling and fly cutting here, or area finishing. When it comes to area finishing, there are two schools of thought on tram and surface finish--either get your mill trammed near perfectly or put it out of tram just a little bit. The considerations are largely aesthetic. If your mill is well trammed, you'll see equal tooling marks on both sides. If the spindle "leans" into the cut a bit, you'll only see one tooling mark. Some folks prefer the symmetry of two marks and some prefer the one. Either way, if finish matters and your mill is trammable, you'll want to retram just before critical finish work is done.

The Truth About "Mirror" Finishes

Anyone who frequents the machining boards online has seen countless threads where someone is asking after how to get a "mirror" finish from their mill or lathe. While it is easier to come close on a lathe, the truth about mirror finishes is that they largely do not exist. Yes, there are machine tools capable of doing optical quality finishes. These are extremely rigid machines that largely use diamond tooling. But, those are not the machines most of us will have access to, and they are not the machines most people are talking about as they ask after a mirror finish.

Have you ever noticed how most photographs of a so-called mirror finish have the workpiece tilted at an extreme angle? Have you ever seen the highway mirage where it looks like there is a pool of water on the road far ahead of you that vanishes as you get closer? Same principle. Even asphalt will apparently have a "mirror" finish (that water you saw in the mirage was a reflection of the sky) if the angle of incidence is right. Ask to see the mirror finish looking straight down on the part and you'll see more clearly what these finishes are all about.

This is not to say that a fine milled finish cannot be extremely attractive, its just that it doesn't really look like a mirror when viewed straight on. The tooling marks will still be visible, but they will be extremely faint grooves. In the best case, you'll see rainbows created by diffraction effects acting on the grooves. It's a look that has been called "prismatic". The closest thing to it is the back of a CD where all the rainbows are, but the milled finish will be a more subtle effect.

That's what you're looking for to decide you have achieved a great finish. If you need to do better, it's time to go to some abrasives of some kind and either user vibratory polish or a buffing wheel.

Next Article: Feeds and Speeds for Fly Cutters and Manual Mills

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