Surface finish opinion

[CNCWNTTOBE]

Hi,

I am interested in getting a general opinion on a surface finish specification for a cnc job that will provide an end piece with a smooth enought finish that no scratches are too deep that a bead blast wouldn't hide them, but at the same time produced the part cost effectively. The better the cnc finish the more expensive, right?

[JWB_Machining]

Yeah Generally the finer and smoother the finish the more you'll be charged for it.

[mc-motorsports]

What material and manufacturing process?

For instance, it would be a lot easier to make aluminum parts that were turned on a lathe look pretty after a quick blast of glass bead. Milling A36 is another story. Depending on material and process, it may cost a little more for the sand blasting, or a lot more for having to adjust feedrates and tooling and possibly operations to get the required finish before sandblasting.

[ALLtra Mach]

For a good answer, more information would be great, material type, turned or milled?
Up to about a 63 finish on a mill should not cost more.
Any thing 32 or better probably will. Your finish should also match your size tolerance.
If your calling out a .0001 size or location, a 125 finish won't get you there.

Also what a bead blast will or won't hide is dependent on the media and condition of mtl.
I have had 16 finish parts sent to blast and ruined the finish.

[CNCWNTTOBE]

The material is Aluminum 6061 T6. Milled. Is there a shop default or will a shop typically even start a job without a surface finish specified?

I had some parts done recently and got them back with deep grooves in them. I think the grooves are from being held down with a clamp that dug into the metal. These grooves are still showing after bead blasting and I don't think they can be removed without siginificantly altering the part. Not sure how to measure in microns the depth of the grooves. I did not specify a finish initially when the parts were being machined, got them back and tried to get them fixed after I saw the end product. I had no idea that the parts would be so bad. Having not specified a finish do I have any legal rights here to go back to the shop and have them redone?

[ALLtra Mach]

I really couldn't advise you on that without seeing the print and the parts.
If the marks are merely cosmetic and do not alter geometry and you did not call out a finish tolerance then your probably out of luck. However a certain amount of care is implied. If they just flat ruined the part they should replace them.

Have you called the shop and told them you are not happy? Alot of shops, especially in these times where work is getting harder to find, will work with you. If they won't budge on it, seek an experts opinion, and take the parts and prints to a local shop. They will most likely be able to tell you if you got what you asked for. Be careful with this that you take the parts to a similar shop. If you sent these parts to a production job shop, they will probably not look very nice to a custom performancfe shop.

[mc-motorsports]

Surface finish is measured with a Profilometer, pic attached. You can get yourself a cheap comparison chart/gauge.

Honestly, if surface finish is not noted on the print, usually it's 125 micro or better, which is pretty rough. I personally think the minimum standard for milling and turning should be 63 micro.

But if you have clamp marks on your parts, that's just hack work. I would take the parts back to the machine shop and ask them if that is normal work for them. It's possible that someone slipped it through because they screwed up and they take no pride in what they do, maybe management was not aware that crap parts are going out the door.

As far as legal rights? You'll probably be ahead of the game by just finding a decent shop to work with. You'll end up paying thousands and spending a lot of time with lawyers and courts just to have a chance of recovering your losses.

And yes, you should always specify surface finish just the same as you specify tolerances for hole diameters. But for instance, a 125 micro on a hole means a drilled finish is fine. If you needed a better finish for an O-ring seal and required say 32 or 16 micro, the hole may need to be bored, reamed, or possibly even honed depending on the material. Price will reflect accordingly.

Any decent shop wouldn't put parts out the door with clamping marks and excessivly rough surfaces per the manufacturing method. And as much as I would argue that you shouldn't have to specify for a decent surface finish, 125 seems to be the "defualt".

[CNCWNTTOBE]

Ya, I assumed that there would be minor scratches, not grooves. The work was definately shotty. For instance I also I had chamfer that got chunked in a certain area of the part. Like the cnc machine went over a little to far and took out a small piece of chamfer because it was either programmed incorrectly or the machine was cutting to fast. Here are some pictures of the work.

[ALLtra Mach]

Are these castings?
Some of those look like casting voids.
The chamfer however is just straight screw up, send that part back!

[mc-motorsports]

Those pictures are HUGE, I had to save them just to see the picture as a whole, but that's ok.

Definatly look like clamping marks. Basically the same place on both parts... That makes it obvious, and that's just shotty workmanship.

As far as the surface finish, it doesn't look horrible. Could be better, but it looks to be around 63 micro, judging from what I see in the pictures from my computer.

Another thing is did you specify a material? If you simply specified steel, or mild steel, they probably made them out of A36, it has it's place, but it cuts like crap compared to other grades.

BUT, if I was you, I would email those pics to the manufacture, the clamping marks are not acceptable. Let us know if they take any pride in what they do.

Are you the same guy that was argueing with e-machineshop or that online machining website in another thread? I remember reading that a while back, just wondering.

[ALLtra Mach]

My bad, you already told us the material is 6061.
I have to agree with MC, those parts are unacceptable. But they are typical of what you will get from the "lowest bid" shop.You really should talk to the shop management, then find a better shop.
Also, 95% of what I machine is A36, and it is the crap of material.

[CNCWNTTOBE]

You are correct - emachineshop did do these. I sent them these picts so they could see the issue up close and they responded with saying that the part was within spec since I specified no surface finish. I paid them to fix the chamfer and bead blast the part, thinking that the grooves would be less noticable. The more I look at these parts the more I get pissed. The chamfer is fixed, but those grooves are still obvious. These are for resale and I am not happy with them at all. To top it off I received them back after the bead blast and I could see scratches on the parts as if they had rubbed up against each other at one time or during transit.