An interesting material - need advice

[DAB_Design]

I got a call from the company I was supposed to start working for tomorrow(monday), and they wanted me to start early.

Went straight to the CNC where one of the engineers was running a part. Right now, they don't have any machinists so the engineers are running the machines.

Anyway, he was cutting this green plastic looking stuff. I asked what it was, and was told it was wax with some extra stuff added. After a few minutes, he told me that the endmills don't last too long because of an abrasive they add to the mixture. Since I'm not sure if this is a proprietary thing, I'm a bit hesitant to say exactly what is added.

But I'm looking for ideas on speed and feeds. They're using 2 flute carbide endmills. Running around 400 SFM. 1/2 endmill, and I think it was 150ipm. Radial and axial depth of cut varies greatly. Going from full diameter and about 1" deep to .1" step over and .1" deep.

To me, both speed and feed seemed slow (RPM espcially). Buildup is also a problem.

Anyone have any ideas? I know it may be a bit hard to suggest without knowing the exact details on the material.

[lapuser]

PVD coated carbide is recommended for materials with abrasive content-like aluminum castings with high silica content that would destroy in short order other coatings and substrates. I do not know if these can be had for your application outside of a special order. Have you called a good tooling rep on this?

[miljnor]

you got the double wammy going there man! An easily melted substance (wax) coupled with and highly abrasive substance (can you say heat).

when machining plastics (wax fits this catagory IMO) you generaly use slow sfm to reduce melting and higher feeds (because you can) 400sfm for wax actually is an ok 450sfm is what i have run before (although no abrasive) build up at the IPM is probably a given. Because I dont know if a 1/2" 2 flute End mill is going to give enough chip clearance at ruffly .024ipt (that a large chip that is probably going to get jammed in a tiny flute and the fact that the abrasive is generating heat and probably no coolant, do to the material (you didn't say so I assumed)) If this material didn' melt so easily i would say try a faster sfm (to reduce chip build up, but since this isn't the case maybe a Cool air nosle or just straight compressed air as your coolant might be to trick (for the buildup at least).

without knowing more info cant realy say much but it looks like the compressed air idea is the way to start, then if that is looking like the right direction maybe a cool air nosle for prolonged cutting. Unless of course you can use a liquid coolant.

sorry I couldn't be of more help.

[DAB_Design]

Well I got most of the problems fix with this. They are using standard 2 flute endmills. So I took them to the grinder one by one, and worked a little magic on the back side of the flutes (really wish I knew the correct term). I also doubled the feed and speed. The 'chips' now fly out. I even brushed on some wd-40. Not sure it was really needed, but figured it couldn't hurt.

This worked on .500, and .250 endmills. But with the .125, I just couldn't get where I needed to with grinder. Of course the wheel on the grinder wasn't really fit for what I was doing. I think if I had a diamond wheel, it would have worked.

So does anyone have any ideas on what could be done with the .125? There just isn't enough clearance for the chips to come out.

[JPMach]

Onsrud makes some nice 0-flute bits that might do the trick, have to go slower since one less tooth but it just might work I have used one of their 0- flute bits in a .062 for finishing small corners in large plastic signs. Chips were lifted out quite nicely.

JP

[Mcgyver]

Dab, now that you are the hero and restored the virtue of machinsist everywhere, can you elaborate on the magic?? some characteristic of the wax? this was freehand endmill grinding?

[DAB_Design]

Mcgyver, sorry about the long delay in response. With the new job, came new and longer hours.

The wax that was originally used seemed to be hard, and not too brittle. I'm not sure what kind of descriptions you are looking for, as I have never worked with wax before. I know some stuff is added to it when melting it down to preform into the slabs. But I don't want to get into specifics, as I'm not sure if it would be considered 'proprietary'.

We have tried a new wax. It looks as though it may be the same as sold here . It seems to be a lot more 'elastic' than the previous stuff.

And yes, I was grinding on the cutters by hand. With a standard 'I have one in my shop to grind off welding slag' grinder. Definately not the right tool for the job, but it got things done. I'm going to use the terminology I'm used to to describe where I did the grinding. If I'm not clear, let me know, and I will try and find a close up picture of an endmill and edit it to show where I did the grinding.

First, I relieved the 'heel' behind the radius of each cutter. Then behind the cutting edge there is a small area at a slightly different angle than the cutting edge, behind that it 'fades' back to the main clearance. I ground the 'clearance' area down to the area just behind the cutting edge. After trying this, the chips were still sticking slightly. So I went back and ground the small area just behind the cutting edge. I did this right up to the cutting edge, or as close as I could get without distrubing the edge. This seemed to do the trick for the most part.

I'm still not completely happy with the results, but it's definately better. And just for reference, I am running the 1/2" endmills at 6000RMP and around 150ipm. Radial depth of cut ranges from .05 to full diameter. Axial depth of cuts from .05 to approx. 1.0". And a full range of both at any given time (ie. full diameter radial at 1.0" axial all the way to .05 radial at .05 axial).

Confused? I am.

[Geof]

Have you thought about doing your initial roughing with a 1/2 dia. square end cutter? Just pull it up by slightly more than the radius and then your ball nose will never experience a depth of cut greater than this. I have used a carbide two flute 45 degree helix on the blue machinable wax that I think comes from the link you gave, running at 10000rpm and 400 ipm, full cutter dia., about 7/8" deep no coolant and the chips cleared perfectly. The only problem was the top edge of the cut chipping out a little because of the strong upcut.

[DAB_Design]

I have thought about it. But I haven't recommended it to anyone, as I know it would fall on deaf ears. Everything they do, goes straight to finish. I'm sure the responce would be 'why?'.

And I almost forgot about the chipping. We experienced this also with the blue wax. But not with the previous (green) stuff.

[Geof]

Originally Posted by DAB_Design ... bout the chipping. We experienced this also with the blue wax. But not with the previous (green) stuff.

No chipping, something added and abrasive nature: this all adds up to adding glass microfibres or beads. An additive like this would also give you a stiffer material.

Sounds like you are dealing with engineers; you must have heard the saying gee last we I couldn't even spell enguneere and now I are one.