Machining Polycarbonate

[airnocker]

So, Dave, is the test you did a profile (edge) cut of the full thickness of this material?

It isn't clear to me, and there is no pun intended.

[dave benson]

Yes Nocker, it was a full depth cut along the edge face.
I took quite a few pic's, one of the pic's is looking down through roughly
40 mm, I did not machine the face that is sitting on, it was sawn and dressed,
still a bit opaque though.
The reflection of the crazing from the front and back faces of the material, which was scoured, can be seen
reflected in the face I machined.

Dave
 

[airnocker]

Thanks, nice test Dave.

[Tool-n-Around]

.......I did everything wrong to make an acceptable cut.....Wrong speeds and feeds........Crappy work holding (sitting on parallels in the vice) so no support at all......Wrong type of endmill (it was a well-used one for cutting AL, four flute).......No air blast. Short of spinning the endmill in reverse, I could not think of anything else to do wrong.

The only thing I did was lubricate the cut with straight dish washing liquid at
the start of the cut with my finger......Tooling marks aside, the piece came out optically clear, I can see straight through it
(40 mm) to the woodgrain of the table.

Wow! I must say, I went back and looked at the pics of one of the other polycarb lids I did. Ignorance was bliss. I paid no particular attention to anything and just cut it at an arbitrarily lower speed than I was cutting polystyrene casting patterns foam. The logo had some burrs stuck to it but they came off with a wooden pick. Those were the cut speeds I started with on this filter, however, the cutters may not be the same....cant be sure on that. Attached a picture of the resulting air filter assembly with both aluminum and pc lid.

I’m not suggesting that you use the dish washing liquid, just that if you are still struggling to get an optically clear surface to your satisfaction, after taking all of the above advice (which is good) then it’s well worth considering. Dave

Well if not suggesting it's sure making a strong case for it! I have not gotten back to experimenting with it because I had family visit and they stayed....so will be back at it when they depart.

Best,
Kelly

[Tool-n-Around]

I forgot one important detail. I machined that previous lid with the protective film in place. Not so on the most recent effort. The film adhesive seems to be waxy feeling. I did it to prevent scratching while handling because that is how the professional friend of mine I previously mentioned did it. May it have served as cutting lube? What do you think?

Best,
Kelly

[tau]

Kelly,

May it have served as cutting lube? What do you think?

IMO, it does not make any difference. I did make many successful cuts ands engravings with and without the protective film without any difference. When optimal cutting speeds and feeds are chosen (as well as a sharp and polished tool is used), the  results came out always perfect.

[dave benson]

May it have served as cutting lube? What do you think?

I don’t know, sometimes I have to machine a pre-existing object, so the paper
has been removed, and at other times use virgin stock with it on.
In practice, I haven’t seen much difference, although in the back of my mind
I’m thinking, that paper is stuck on with adhesive so would it stick the light weight
chips to the cutter.
Polycarbonate’s super power is impact resistance, scratch resistance not so much.
There are formulations, where a thin glass hard like layer is applied during manufacture
to help this.
Polycarbonate starts to slump at 140 Deg C and melts at 300 Deg C.
This is one of the reasons that 3D printed “under the bonnet” items are made from
ASA or some other higher melting point plastics are used.
If you want to remove the tooling marks in the bottom of the cut, there are two
common ways to do this, Flame cleaning where you use a very gentle lpg gas flame
and pass around the cut surface, I’ve done this and it works but you have to keep the flame
moving at a consistent pace.

The other method is to use Vapour cleaning, the 3D printer crowd use this method to
cleanup and eliminate Layer lines, I haven't done this as I looked at the materials safety
data sheet, and thought I’d give it a miss.

I don’t know if it’s my imagination or not, but looking at the tool marks at the bottom of the
cuts (on the outer periphery) it looks like the tool is digging in at times, so the tool is porpoising
along as it’s making the cut, It might pay to check the backlash in the Z axis, on your type
of machine the Y axis can contribute to this as well, if you look at the wheels on the Y axis
and they appear to have a white ring around them, then they have worn in a little and need a snug up.

Love to see your work, always puts a smile on my dial.

Dave

[Tool-n-Around]

Polycarbonate’s super power is impact resistance, scratch resistance not so much. There are formulations, where a thin glass hard like layer is applied during manufacture
to help this.

I may need to look at using acrylic. I thought the toughness trumped the scratching but pc scratches easily. Since they're just display pieces, durability may be more a matter of scratch than impact resistance. Not sure it will change the machining issues much though.

I don’t know if it’s my imagination or not, but looking at the tool marks at the bottom of the cuts (on the outer periphery) it looks like the tool is digging in at times, so the tool is porpoising along as it’s making the cut, It might pay to check the backlash in the Z axis, on your type of machine the Y axis can contribute to this as well, if you look at the wheels on the Y axis and they appear to have a white ring around them, then they have worn in a little and need a snug up.

I’ll check both. It may also be the stock getting lifted off the table by up shear. I’d probably need vacuum hold down if so. We’ll see how much pc I do in the future. I need to get through these two peices in the present.

The z-axis is just an acme screw with kind of a hokey backlash anti-device but it’s on linear rails. The wheels on the rest may very well may need tightening. That darn foam has quite a charge when cut and that which escapes vacuum collection clings to everything including the v-grooves for the wheels which compress the fluff with cycles into much harder interference. If I don’t wipe (especially the underside that I can’t see) it can build up enough to cause step loss. I was considering adding some air jets to keep/blow the fluff out of there, but I need to modify the machine for more Z-axis travel and when I do so, I’m giving serious consideration to upgrading everything to linear rails (versus wheels) and ball screws (versus rack and pinion). It would just be imported rails and screws, but I can do the whole machine for $350. The rails have wipers that will probably handle the foam dust fine, not sure about the ball screws. Re-engineering my cnc machine hardware for such would be no big deal for me. While I was at it, I would probably fill the aluminum extrusions for stiffness…….but maybe not the one on the moving gantry to keep it light. Even with the cheap imported linear motion stuff I bet it would be a significant improvement for the machine and probably good value for money.
 

Love to see your work, always puts a smile on my dial.

Me too, I really enjoy it. I’m working on a couple projects now that will be my most complex and best work so far. Right now it’s being paced by my CAD learning curve to model the projects, not cut them, but I’m getting there. Th efoam patterns should be some pretty impressive (at least for me) 3D machining work. When I do, I’'ll stop back and share. Heck, I'll probably be back for help. I still have great appreciation for all the help and patience this forum provided to me when I was just starting out.

Best,
Kelly

[airnocker]

Acrylic is nicer to mill in my experience (and compared to many on this forum I do not do nearly as much).

I’d probably need vacuum hold down if so.

I strongly recommend you look into to using the "Green Frog Tape/Super-Glue" hold down method...it works wonderfully for many materials.  It is described in recent threads (well, er, this year sometime).

[Dragonfly]

I strongly recommend you look into to using the "Green Frog Tape/Super-Glue" hold down method...it works wonderfully for many materials.  It is described in recent threads (well, er, this year sometime).

I am using this method on daily basis for machining aluminum parts. When the workpiece has to be machined on top and all sides without fixtures that will stay on the tool path. Using blue tape - has the strongest glue layer of all available locally I tried. And has a higher temperature level before losing grip. One thing to take care is not to let the material get hot, not above around 40 degrees C.
Haven't worked much with PC/acrylic but my personal experience says shallow DOC per pass + high feed rate, and a razor sharp tool (usually polished single flute for acrylic). The idea is to not let the material melt at point of cut and the tool moves away before it happens. Using constant air blow and when possible -  fine spray of highly diluted cooling liquid.
With deeper profiles and if the project allows using 'cut width' > TD helps chip evacuation and the final cut is done with less than 100% tool engagement (less generated heat).

[Tool-n-Around]

What's a practical/economical waste board material?. When I surface the mdf wb, it's fuzzy and I dont think lacquer finish would be sufficient for bonding tape and using the tape/CA Glue method. Maybe just contact cement thin plastic sheet (like .125" acrylic) to the mdf? I'll need 24" x 16" waste board.

Best,
Kelly

[pixelmaker]

What's a practical/economical waste board material?.

Always the same material as what is cut.
If you use wood or mdf, wood dust is drawn through the cutter into the plastic surface. In addition, there are other substances. For example, if oak is used in the MDF, PC and PMMA will turn brown or yellow in the cut due to the tannic acids.

ralf

[lloydsp]

If you're milling plastics, consider:  Most sheet plastic vendors who deal in pallet quantities will deliver the pallets to customers with a top and bottom 'waste sheet' of 1/16" PVC or acrylic, so as to protect the actual goods from the shipping.

Ask them for a customer's contact info from whom you might 'buy' some single-quantity sheet goods.  I'll bet they'd be happy to dispose of some of those waste sheets for free.

I get two of them with every pallet of PVC foam-board I use for fireworks base plates.  I have to dispose of most of it.

Lloyd

[Dragonfly]

I am using aluminum plate, surfaced to even thickness. There are two layers of paper, about 0.2 mm thick. Enough to go some 0.05-0.06 mm below stock bottom. When using liquid coolant it prevents the tiny paper fibers from sticking to the cutter and the stock.

[Tool-n-Around]

I did some test cuts and used those learnings to cut another polycarbonate lid.

Changes:

.25"D and .031"D End mills were polished O-flute.
I added spiral lead in moves to everything.
Everything was cut at 45 in/min except the logo which was 30 in/min
I used soapy water for the logo cut only chasing the cutter and clearing chips with a wet chip brush, everything else was cut dry.
I reduced DOC on the dish cuts to the filter gland so there was a light finish cut.
I reduced depth of cut on the .25D EM perimeter cuts to .1". I used a rouging cut at .35 width which was the longest of ops, then two full depth finish cuts removing .005" each time.
The stock was secured with six screws. The stock was warped .090" high in the middle over 21" of length.  The screws were able to pull it flat but not very happy with the stock flatness.
I removed the protective film for all machining.


General Impressions:

Significant improvement over previous attempt. Still some areas could be better. I uploaded a high(er) resolution photo.
The logo is crisp and I'm quite satisfied with how it cut.
The ornamental grooves are ok, had some minor burrs here and there on the surface edges that needed to be struck off with a razor, but a fair trade for me to be able to cut it dry.
The dish cut on the filter gland is much better......still some tool marks but much improved.
The perimeter cut is much better but still the most difficult for my machine and leaves greatest room for improvement.

I think I'm bumping into the limits of my machine rigidity greatly aggravated by my having the router fully extended and all things biased for maximum stroke and height under gantry. It's just a hobby machine fellas. It just makes for a lot of cantilever leverage on the Z-axis and the it's not exceptionally rigid to begin with, coupled with the fact that it rides on extruded aluminum gantry this 44" in both X & Y. I think I can stiffen it up a bit and have a few things on my upgrade list. I'm sure cutting these ops with fluid would help. I'm just not set up adequately to do so, but could remedy that.

I can do a little hand work on the perimeter and this one and it will be passable. I think I may order some acrylic and make a couple for comparison. I'll post some completion photos when I get there.

Best,
Kelly