Requesting help from experienced millers

I did my first milling tests (on a McWire bot) and came up with a few issues maybe some of you experienced millers could help me with.

First of all, my geometry was a little skewed from flex in the mounting bracket and the bit itself. I plan on slowing down the movement speed and upping the rpms to try to remedy this.

The other problem I was having was that the milled material would stick to the bit and when the bit went back down into the acrylic it would dig a hole. It is similar to a countersunk hole, but it is unwanted. Is there any way to prevent this?

Thanks!

Here is a video and some pictures of the test

[www.binaryconstruct.com]

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[www.binaryconstruct.com]

Hi

You are probably melting the plastic. You must take a cut and not rub. This usually means a higher feed than you think. Your machine may not be rigid enough for a high feed so in that case you must lower the dremel speed. To give you an idea at 5000rpm to advance a single flute cutter .001" per turn you need to feed 5" a minute. A 2 flute cutter feed would be 2.5" a minute. The ratio of feed to RPM needs to be maintained. If the plastic still melts at 5000 rpm you need to lower the speed and feed at the same ratio or maybe use a cooling fluid or even compressed air. I don't cut a lot of plastic so the cut may need to be more than .001" per cutting edge. I would run a series of tests advancing the feed rate say 10% each time and observe the chips you are cutting. Then try the same thing at higher spindle speeds. You should then find the best speed and feed for the material on YOUR machine.

Also if the cutter deflects badly at the higher feed rate you then need to reduce the Z depth of each pass.

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Regards

Ian

Thanks for the response. I hadn't though of multiple passes, but that seems like a viable idea.

Contrary to what I though would happen, I actually had more melting at lower RPMs. When I cranked the speed up, the cuts were cleaner and the chips were not melted or sticking to the bit. Can anyone explain this effect? Is it simply from the extra wind velocity coming off the bit?

The rig seems to take the stress better than the bit, so as suggested I am going to try and find a shorter bit (I can't fit a thicker one in the dremel). I've been writing my gcode by hand, and its very simple code but I can probably just copy/paste with a change to the Z to do multiple passes.

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[www.binaryconstruct.com]

Hi Jeff,

last week i milled some 3mm and 5mm thick acrylic (apended an image of the parts) - this was milled with a 1.5mm millhead, 11000 rpm, 1.6mm cutting depth and soap-water with some alcohol (common car-windshield washing fluid).

If i draw faster and not so deep, i could mill without coolant, but sometimes (especially with older/blunt millheads) the bits are melting though ...

Viktor

How do you get the edges clear? Mine always have the coarse look.

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[www.binaryconstruct.com]

... i haven't do anything - it's the cooling and sharp millheads.

When looking close, it's like little stairs where the steps of the motors (halfstepping with 0.0125mm) and some cutting-resonances are visible, but overall it stays clear.

If the material melts, then you have a milky-white cut, with coolant the cut stays clear.

Viktor

I'm successfully milling polypropylene, which has serious melting problems like acrylic does. I'm using a 2 mm single flute carbide end mill and running at 20K rpm with a feed rate of about 6-8 mm/sec and a cut depth of about 0.5 mm.

Previously, I was using a two flute end mill which works well with HDPE but which is no help whatsoever with polypropylene.

[www.3dreplicators.com]

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Forest, or anyone for that matter, where do you get your bits from? I'm in the US, so US suppliers preferred.

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[www.binaryconstruct.com]

I buy from Fred Schultheis at North Bay Technical just up the road from me in Fairfax, California.

[northbaytechnical.com]

Fred's the guy who suggested the one flute end mill. It's let me mill polypropylene, which is dirt cheap, without coolant.

Fred's good people.

I bought his last one flute end mill, but he was reordering and should have more stock soon if not already.

It looks like you are using a cutting bit in your Rotozip instead of a proper end mill. I tried just about ever kind of end mill and cutting device made for Dremels before I found Fred and North Bay and started buying proper cutting tools.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison