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Cam
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Post by Cam » Wed Aug 06, 2014 7:59 pm

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Last edited by Cam on Sat May 14, 2016 10:16 am, edited 1 time in total.

secretspy711
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Re: DW 660 and 30000 RPM issues

Post by secretspy711 » Wed Aug 06, 2014 8:40 pm

I've only got a little experience cutting acrylic and am still pretty new myself, but I've learned that thermoplastics such as acrylic are very finicky in terms of what speeds and feeds they need. Sounds like you are melting the swarf, causing the bit to seize.

By your numbers it looks like you are using the feed and plunge rates from here?
http://www.shapeoko.com/wiki/index.php/ ... ls#Acrylic

But your spindle speed is way off for those feed and plunge rates.
Also, you didn't say what bit you were using (how many flutes?) and what your cut depth was.

Generally you'll want a 1 or 2-flute bit, and probably want to slow down the spindle as well with a speed control. Running at full tilt, the correct corresponding feed is probably too high for the stiffness of the machine (or lack of it). You can sort-of get around this by cutting shallower passes. You can also try a speed control like this:
http://www.amazon.com/MLCS-9400-Standar ... ed+control
I have one of those, and although it seems to be working fine at the moment, further reading after I bought it suggests that running the DW660 at slower speeds might lead to overheating, then bearing failure and excessive runout.

Give this a read, and maybe run a few tests to find your own sweet spot:
http://www.precisebits.com/tutorials/ca ... speeds.htm

In my own experience cutting acrylic before I got my DW660 (using the stock rotary tool), changing the feed even slightly made a huge difference in cut quality.
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cvoinescu
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Re: DW 660 and 30000 RPM issues

Post by cvoinescu » Wed Aug 06, 2014 9:04 pm

What secretspy711 said.

Basically, acrylic wants a high chip load -- it wants you to bite deeply each time a flute cuts into the material. You can do that by (all other things being equal):
  • moving faster (higher feed rate);
  • turning the spindle slower;
  • using a cutter with fewer flutes.
You can't slow down the DW660 much without destroying it, and open-loop control results in much reduced torque, so it's less than ideal. Worth a shot, though, even if you reduce it only from 30,000 to 20,000 RPM.

You can only reduce the number of flutes to one: but moving from two flutes to one is like halving the spindle speed, or doubling the feed rate, so definitely worth doing.

You can't move a lot faster with a Shapeoko, because it's not rigid enough to deal with the increased force. But you can move faster if you reduce the depth of each pass (to a point). That's because it matters more how wide the chip is than how tall it is. Try to increase the "stepover", increase the feed rate, and decrease the depth of each pass to compensate.

Don't feel bad about it: acrylic is an unfriendly material. This brings me to the last piece of advice: acrylic is not one material, but two: cast acrylic and extruded acrylic. Extruded acrylic machines badly, cast acrylic is much better.
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Improbable Construct
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Re: DW 660 and 30000 RPM issues

Post by Improbable Construct » Wed Aug 06, 2014 10:38 pm

I cut acrylic at 25-30k rpm all the time. I usually run 600-1000 mm/min depending on depth of cut.
Are you using cast or extruded acrylic? Cast usually has paper backing. Extruded usually has plastic backing.
Extruded is not usable for milling or laser cutting.
Make sure you have a sharp, quality end mill and cast acrylic should mill no problem at those speeds.
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Woodworker
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Re: DW 660 and 30000 RPM issues

Post by Woodworker » Wed Aug 06, 2014 11:22 pm

If you had a fan blowing down through the 660, would it help or would the internal fan throttle the the air flow anyway?
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secretspy711
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Re: DW 660 and 30000 RPM issues

Post by secretspy711 » Wed Aug 06, 2014 11:39 pm

Woodworker wrote:If you had a fan blowing down through the 660, would it help or would the internal fan throttle the the air flow anyway?
The 660's fan is directly connected to the shaft so I don't think another fan blowing down through it would help much. I think the overheating problem stems from the plastic housing not being a good heat sink. I think the best you could do is make a custom part to replace the stock fan piece that pushes more air. That's probably more trouble than it's worth, but maybe I'll look into it. It's not sold as an individual part (it's part of the whole rotor assembly) so removing the stock one may be difficult.
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Cam
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Post by Cam » Mon Aug 25, 2014 2:55 pm

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WillAdams
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Re: DW 660 and 30000 RPM issues

Post by WillAdams » Mon Aug 25, 2014 3:09 pm

Cam wrote: I'm still learning about the formulas, is there a good source that explains it in depth but simple enough to understand if I'm new?

EDIT: Also where does pass depth fall in with the rest of the variables? Does increasing or decreasing pass depth affect feed rate?
What I've been able to find I've put here: http://www.shapeoko.com/wiki/index.php/Materials --- esp. note the discussion: http://www.shapeoko.com/wiki/index.php/ ... ng_Formula

If there's anything which you find confusing or hard to understand, please ask about it here (or research it and post what you find, either here or to the wiki) --- you may want to read all of the links as well.

Basically, one needs to cut a piece off w/ each revolution of each flute --- the size and shape of it will be determined by the # of flutes and the shape of the endmill, its r.p.m., the direction of cut (climb vs. conventional) and how fast it's moving. Cut depth determines the height of the chip which one will cut off, and is the one value which can be tweaked w/o affecting the others. Reducing it allows a more delicate cut which requires less torque and which stresses the machine less (but takes more passes / time). Increasing it results in a heavier cut which requires more torque which will be more likely to cause deflection, lost steps, &c.
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cvoinescu
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Re: DW 660 and 30000 RPM issues

Post by cvoinescu » Mon Aug 25, 2014 3:29 pm

Can you clarify something for me, essentially if one variable in the CNC formula is off, I have to adjust it so that it meets the materials preferred chip rate correct? So in this case I had to either reduce RPM by reducing flute size or increase feed rate?

I'm still learning about the formulas, is there a good source that explains it in depth but simple enough to understand if I'm new?

EDIT: Also where does pass depth fall in with the rest of the variables? Does increasing or decreasing pass depth affect feed rate?
Basically, you want each flute to bite to a certain depth into the stock each time it contacts the stock. You can get it to chip more by turning the spindle slower (so it bites less often), by using fewer flutes (so a flute bites less often), or by feeding faster (so that each time a flute bites, it needs to take off more material). This matters for three reasons: (1) each bite wears the flute, so, to prolong tool life, you want to bite fewer times total; (2) if the chip is large, it cleaves off neatly; most heat is caused by the chip deforming, and it's carried away in the chip, while the tool and the stock remain relatively cool; (3) if the tool doesn't bite deeply enough, instead of cleaving a nice chip, it deforms the stock and rubs against it, at least during some of its travel. This heats up the stock and tool, both from deformation and from friction, and the chip does not carry this heat away. A hotter tool has a shorter life, and a hot stock can burn or melt. Hot material sticks to the tool, which usually breaks it.

Of course, taking larger chips increases the force on the machine, and pretty much the only thing we can do to make up for that is to reduce the depth of pass. It's a bit counter-intuitive, but in delicate materials such as acrylic, you want to use single-flute endmills, run the spindle slower (which may not be possible), increase the stepover, increase the feed speed, and decrease the depth of cut.
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WillAdams
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Re: DW 660 and 30000 RPM issues

Post by WillAdams » Mon Aug 25, 2014 3:54 pm

cvoinescu wrote:Basically, you want each flute to bite to a certain depth into the stock each time it contacts the stock.
I believe what you wrote would be clearer if instead you said, ``to a certain thickness into the stock'', that way it's not conflated w/ depth of cut / plunge.
Shapeoko 3XL #0006 w/Makita RT0701 Router w/0.125″ and ¼″ Elaire precision collets
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