tumbling to remove machining marks?

[North Arm Knives]

Anyone have experience removing cnc milling marks using a tumbler? My research so far hasn't turned up a whole lot; so I thought I'd consult some of the knowledgeable people on this forum!

When our blades come off the mill we have to do a lot of buffing (starting with fairly aggressive compounds) to remove the machining marks and it is quite tricky to do it evenly without taking the lateral CNC lines out (we leave the lines as a signature look). I've been doing a lot of research on tumblers and have found that most people finish up to 400 grit or so before tumbling; because we aren't grinding the knives that is not really a possibility. What I'm looking for is a way to remove or at least blend in the machining marks and get a consistent finish. After buffing there are still some light marks but they aren't very noticeable. My questions are as follows:

1. Is there a type of media that is aggressive enough to remove these type of milling marks and small enough get in to all the nooks (the corners of each step where there is a bit of a sheltered area)?
2. If such a media does exist, would it abrade fairly evenly or would it affect the high points at a higher rate than the nooks and corners?
3. Are the lateral milling lines (the ones we want to keep) likely to be eroded away in the process?


Below left is a photo of one of our fillet knives as it comes off the mill and on the right is a blade after buffing. Sorry for the bad photos.

Any insight from all you makers would be greatly appreciated!

thanks,
mike

 

[Karl B. Andersen]

Ummmmmm.....no.

 

[Huntsman Knife Co. LLC.]

Tumbling wont even remove 120 grit marks . Hit them with a coarse scotch brite belt. Busse leaves mill lines on most of their blades with great success i believe you can as well.

 

[FTO_dude]

He's not talking about the steps, but the machining marks within the steps.

Nathans the guy to talk to but yes you should be able to get machining marks out with tumbling

 

[Mossyhorn]

I'm very familiar with industrial tumbling and even with aggressive media, I don't think it would work. The steps will be very tough to remove. You'd have to use a media small enough to get in between the grooves and that would affect your makers mark as well.
I'd use 3 scotch brite course/brown wheels stacked on a buffing spindle. You will have to use large washers at the ends to add support. A belt would just hit the surface and not get in between the grooves. Check out MSC Industrial Supply.

 

[S.Alexander]

I know that you leave the step marks on as a "signature," but I humbly believe that you should grind them off. You have some really good designs, and I believe that you could benefit from a cleaner looking finish. You would find a wider range of customers. Maybe just try it for a few knives and see how you like it.

I know that this doesn't answer your question, but I have thought this for a while and felt I should share it.

Shane

 

[JG Custom Metal Works]

I wonder if sand/powder blasting would be an option? I'm sure finer material would get into the nooks an crannies. You could then maybe tumble with some ceramic media.

To be honest, I know you're wanting to keep the "lateral milling lines," but you might consider grinding them off. Functionally, what do they accomplish? In cutting, creating the path of least resistance for blade surfaces usually the goal(except for in some kitchen cutlery to reduce sticking). Please don't take this the wrong way as there's a good chance I'm ignorant of the design and it's purpose. It wouldn't be the first time . Good luck.

 

[slambo32]

Just a question. Why so many passes? Can you use a larger endmill? Or maybe diffrent fixturing, holding the blade perpendicular to the z axis

 

[Nathan the Machinist]

A good milled finish is easily as good as 400 grit ground and will usually tumble out okay. An aggressive media and doping it with silicon carbide grit works fast enough to clear everything before washing your lines out.

Your problem is your milled finish. You have burrs, and built up edge. Before you can tumble clean you need a clean finish to start with. You need to rough with one cutter and finish with a fresh cutter and your speeds need to be high enough to avoid BUE and your feeds need to be hard enough to maintain a good chip without smearing. You should focus on your cutter selection and machining parameters first and get a good milled finish. Then tumbling or buffing will become quite simple.

When milling, it's just like grinding, in order to get a good finish you have to use the cutters like they're free.

I buy quality cutters in bulk from Lakeshore Carbide. They work well, last longer, and are consistent from one cutter to the next.

 

[North Arm Knives]

Thanks for all the suggestions and feedback guys, lots of food for thought.

@curtis: we've tried a few different scotchbrite wheels and found it doesn't get down in to the valleys at the same rate as is gets the ridges so we end up with unevenness in the finish. Maybe its time to give it another shot.

@shane : it is something we'll have to try, if it helps us appeal to a wider range of people that is always positive

@JG: we've tried blasting with a few different types of media and have yet to find one that does exactly what we need. As for the lines, they don't have a functional purpose, but we also haven't found them to affect cutting performance in our testing.

@nathan: I appreciate the detailed response. Advice from an experienced machinist is just what I need!

I've wondered for a while whether our milled finish could be further improved. We have done quite a lot of experimenting to try to optimize feeds and speeds, (starting with feedrates from g-wizard). We do use separate roughing and finishing cutters. The finishing one is replaced after each sheet of knives. Depending on the model this ranges from 4 to 10 knives per 7x17" sheet. The machining time is similar for each model though. We also get our cutters from lakeshore.

I'll have to do some further tests. Maybe we need to replace the cutter more frequently, or feed faster. Is it possible that the coolant type/concentration could affect the finish?


-mike

 

[walterpancakes]

Agreed on the milling finish.
I am just now working with mill parts and to do what you want you should be looking at your setup.
Roughing tool then fresh tool finish
How rigid is your setup?
Do you have a "tooling guy" to talk to about the cutters?
Have you chased feeds and speeds to a sweet spot?
Is your coolant clean?

I myself am not looking for that style of milling markings but I am hyper focused on getting the best Finnish I can off the mill. The whole grinding thing is not the fun part for me.

 

[North Arm Knives]

Well we did some pondering and ended up removing the lines on a knife and trying a mirror finish, in large part due to some of the suggestions from you guys. We're quite happy with the result. What do you think?

 

[North Arm Knives]

With an improved milled finish as Nathan mentioned above or the milling lines completely removed we're now looking at tumbling to finish at least some of our blades. What we require is both material removal to take out fine milling marks or 400 grit sanding scratches (in the case of the knives that we have removed the cnc lines from) as well as an attractive non directional finish. After reading through numerous threads on BF and elsewhere I'm still not sure whether I can use a rotary tumbler to achieve these goals.

Do any of you have experience with how rotary tumblers compare with vibratory ones? It seems like most makers who tumble or stone wash are using a vibratory. I'm wondering whether the same results are possible with a rotary in terms of both material removal and finish quality.

It seems like it would be much easier and cheaper to build a rotary tumbler large enough for say 14" blades than it would to build or buy a vibratory one of that size. Any input is much appreciated!

thanks,
-mike

 

[North Arm Knives]

anyone?

 

[Salolan]

I have no experience with a vibratory tumbler to compare with, but I converted my old wood lathe in to a rotary tumbler. Using it, I could tumble items up to almost 48" long if I needed to. I'm struggling to find photos with enough clarity to show the finish, but I've been please with it.

Sorry, I can't be of more help.
Chris

 

[Nathan the Machinist]

anyone?

I think there might be a good reason most use a vibratory, despite the cost.

I don't really know, because I haven't played with a rotary tumbler since tumbling rocks as a kid, but I don't see the cement mixer kind of churning and dumping giving an even frosted finish like a swirling pool of high amplitude vibrating media. I foresee more tiny dents and less tiny rubs. I don't know.

 

[P.Brewster]

I use a self-built rotary tumbler. I did have a problem with dings before I figured out the right ratio of media to parts, etc. A rotary tumbler erodes the corners more aggressively than a vibe tumbler, which for me is a good thing.

When the steel is annealed, I can completely erase 120 grit grind marks in a 4 hour tumbling session. The spikes shown here are annealed O1, ground to 80 grit (not 120!) and tumbled for 4 hours. Next step is heat-treat.

 

[North Arm Knives]

Ok thanks guys that helps a bit! I may just have to try it to find out. If I do I'll report back.

@nathan: do you have a good photo of your vibratory finish? Would be awesome to have a solid reference while playing around with rotary.

@P.Brewster: thanks for the photo, 80 grit is pretty coarse! That's impressive.

 

[stezann]

I wish you could work out the parameters for a nice finish of your "stepped" milled blades. I like the idea

 

[North Arm Knives]

I wish you could work out the parameters for a nice finish of your "stepped" milled blades. I like the idea

Thanks, we're working on it! We're already pretty happy with how they turn out, we're just looking for a more consistent way to remove the machining marks without eroding the steps.