There are several things we need to keep in mind in order to get a perfect CNC machined part. Some of these major pointers come before we begin manufacturing them, like correct dimensions and tolerances, shape, quality of the raw materials used etc. But there is something that needs to be worked on after the machined part is produced.

Surface Finish: A process that helps define and refine the overall texture (lay, roughness, and waviness) of the machined part. We just cannot ignore the importance of an impeccable surface finish.

How to ensure the steps we are adopting will get a better surface finish?
We understand your dilemma. Hence, we have created a checklist of major surface finish dos and don’ts that will help you make your CNC machined part better. Check them out!

Do’s

1. Understand How Surface Finish Will Be Measured
There is nothing better than knowing the surface you are supposed to work upon.There are several techniques and characteristics that we measure with respect to surface finish, including profiling, area, and microscopy that majorly focus on roughness peaks (Ra) and their separation (D). We need to understand which technique fits in the best and will give us desired results without taking much of our efforts and time.

2. Stick to Right Feeds and Speeds (Increase the speed & reduce the feeds)
Guesswork is fine but an absolute no-no when it comes to choosing the speed and feed for a finishing work.
Make sure you always stick to the pre-defined and proper feeds and speed while working with your valuable machined parts. We don’t want things to go wrong, right?

The correct way handle your finishing machine is to increase the Surface Feet per Minute (SFM) and reduce the Inch per Revolution (IPR). The former helps reduce the Built-up Edge (BUE) that aids to a prolonged tool life, whereas the latter is to prolong insert life.

3. Use Chip Breakers
Controlling the chips is the key to get a good surface finish.The chips produced are very much responsible for hampering the entire machining process. They should be controlled before they make contact with the workpiece.

We recommend you use good chip breakers that may ease your efforts in cutting and re-cutting these troublesome chips. If you have been using one, it’s better you change their style as it’s, of course, not effective enough. Chip breakers reduce cutting pressures and assist easier evacuation of even the longer, string-like chips by reducing them into bits that are easier to fall in the cutting zone.

4. Minimize Deflection and Chatter, Keeping Tool Rigidity Up
For a good finish, you need to make sure your tool doesn’t deflect or chatter. This will create a very messy and chippy finish that we generally discard. Why waste your resources, money and time on something that won’t yield us anything?This brings us to the most important point- Make your tool effective and rigid!

5. Use Balanced Tooling to Reduce Vibration
It’s important to use balanced tooling technique to reduce the apparent vibration during the finishing process. This step gets even more essential if your RPM is higher.

6. Use Sharp Inserts, Lead Angle, and a Positive Rake
It’s a no-brainer that we need sharper inserts, more lead angle, and a positive rake for a better surface finish.

7. Minimize Runout
A higher runout also paves way for a bad surface finish. It adds on to the improper functioning of the tool, making it spin in an eccentric path instead of the desired circle. You may switch tools with a better and lower run out, like an ER collet chuck for setscrew-style end mill holder which is noticed to produce lower runout than the latter.

DON’T’S

8. Don’t Use the Same Tool For Roughing and Finishing
Learn to keep your roughing tool for roughing and finishing tool for finishing purposes.
We might want to cut costs and use them for different processes but that might not deliver top-quality finishing.

Also, keep your tools sharp and up to the mark. Use two different sets of inserts, if you are working with an indexable tool like a face mill.

9. Avoid Dwells & Pauses
Unnecessary dwells and pauses are also a hindrance to a proper finishing work. Always remember, each time your tool stops while in contact with the lathe or workpiece, it’s going to leave a mark. Something we definitely don’t want.

If that’s happening too often, we would suggest you change the process altogether! Just do your best to ensure your tool doesn’t stop or hesitate during the entire cutting process.

10. Avoid Cutting down the Centerline
The best way to ensure a proper cutting process is to follow a 70:30 ratio rather than going for a full on 50:50 method. Cutting down the middle is probably going to slap the insert along the edge of the material which may cause burns. This may lead to an improper surface finish.

All these tips and tricks are from our 50 years of experience in the field. It helped us, it might help you too. So, go ahead and adopt these practices so you get a perfect surface finish.

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