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Discussion Starter · #1 ·
I'm in the process of building some molds out of 1/2" acrylic sheet and I'm using pattern pieces and a straight cut bit on my router to cut the acrylic to size (first time I've done this btw). Although I know my pattern pieces are good and straight (come from a pro cabinet maker) the acrylic ends up having little ripples on the cut face no matter how little I try to take off at a time. Since the patterns are good I guess the problems got to be with my setup.

I'm running a Dewalt DW616 1.75? HP router on a flimsy Skill RAS900 router table that weights maybe 15 lbs. and not held down. The bit set was cheap off amazon and unfortunately has a 1/4" shank.

I'm guessing I need to at least screw the router table down and use a 1/2" shank bit. Any other suggestions?
 

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Acrylic is a PITA to machine, youll have one hell of a time getting a perfect edge on it straight off the tool pretty much no matter what. Its grabby, melts easy, sticks to everything, its just a nightmare to work with. Successful cuts need an extremely sharp tool and the proper speeds and feeds. You want the bit spinning just fast enough to cut, too fast and you risk melting the plastic. Movements speed across the bit should be fast enough to keep the bit cutting, but not so fast that youre forcing material into the cutter.

If the nebulous wording didnt tip it off, its not an exact science. Be prepared to make a lot of scrap
 

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I tried cutting it with a scroll saw.. The blade melts it back together almost instantly. In fact if you stop the blade along the cut you might as well throw the piece out along with the blade..
Didn't take long to figure that one out.
 

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Discussion Starter · #4 ·
Epic yeah I'm glad you mentioned that it's grabby because I was wondering if it was me or what. I had a lot of that grabbing if I took off too much at once. Sounds like I was lucky I didn't melt it.

I precut my pieces with a band saw and that seemed to work ok without any melting.
 

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yacht woodworker
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Acrylic is a different beast. If you're working with square (or straight edged parts), I would normally cut them slightly oversized on the table saw and then clean up the edges on a sharp jointer taking as light of a pass as possible. Since it's a light pass, I'd normally plan for 2-3 passes. The larger diameter head of the jointer will leave a smoother surface with less ripples than the smaller diameter bit in a router. However, if you're cutting shapes and replicating them using a pattern bit, I'd cut out the shape slightly oversized (+1/16 to 1/8") and then flush trim them with a pattern bit in a router. I{'d use a 1/2" shank and the largest diameter bit I could find. Assuming your router is adjustable speed, turn the speed control all the way down and feed with a slow but steady feed rate.

If you need perfect edges, it takes work. For polished edges, I block sand the edges with hard blocks from 400 grit up to 1500 grit and then hit them with a polishing wheel. Careful with spending too much time in once place with the wheel, it can melt the acrylic. So keep it moving. Depending on the size of the piece, I'd either use a die grinder with a buffing wheel for smaller pieces or if the piece is larger, I'd use a stand mounted buffer (like a bench grinder). If you're doing smaller pieces on a full size buffer, careful, I've had some catch and get thrown across the shop.
 

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Bradley - could you post some drawings of the templates you are working with ?
there may be other options that might be better for you.
 

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Have you tried making multiple passes? I think I've done this. Straight bits more pone to chatter. Need adequate pressure against bit bearing. Light and/or mulitple passes.

On a table I would use an upcut spiral bit with pattern on top.
 

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+1 on using a different bit, the spiral is worth a try. in the cnc world, they use o-flute spiral bits for acrylics. i suspect that it would provide the same results on a router table...
 

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A larger bit likely helps prevent burning due to a larger mass of steel to heat up

Spiral bits likely helps reduce ridges

A three cutter bit likely helps the scalping by increasing the cuts per inch

Faster speed helps the same

Stable fence and table prevents vibrations that can transfer into the cut...

I will say that even laser cut acrylic has some cut marks or ridges
 

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A larger bit likely helps prevent burning due to a larger mass of steel to heat up

Spiral bits likely helps reduce ridges

A three cutter bit likely helps the scalping by increasing the cuts per inch

Faster speed helps the same

Stable fence and table prevents vibrations that can transfer into the cut...

I will say that even laser cut acrylic has some cut marks or ridges
Hope you dont mind me addressing a few of those likelies.

A larger bit wont heat up as fast as a smaller bit, but larger cutters are usually used for more rigidity, not heat concerns. Larger diameter bits deflect less, which translates to a smoother cut

Spiral bits do reduce the scalloped edge effect, they also cut smoother by creating a shearing effect, instead of the more chopping action of a straight flute

Number of flutes on a bit can affect a lot of things, but the big ones to be worried about are chip control and feed rate. Chip control isnt hugely important in woodworking application, but still worth knowing about. When you make a cut with a rotating (or any, really) cutter, you need somewhere for the cut material to go. In the cast of a router bit, the chips end up in the flutes. More flutes means less space for the chips to go, which can be a problem if you were doing something like cutting a mortise that could pack the chips around the cutter and bind it in place. Generally though, not worth worrying about for woodworking applications, but it is important for working with plastics

Jumping back to feed rate, theres actually a bit of math that tells you how fast you should be pushing a workpiece across a cutter, (IPT*T)*RPM=IPM. IPT is how much each tooth of a cutter can take off of the material, measured in thousandths of an inch. For woodworking, lets say that number is .015, T is how many teeth are on the cutter, RPM is the rotational speed of the cutter, bout 20,000rpm for most routers. So, plug all that in for a 2 flute cutter (.015*2)*20000=600, you woud be feeding the workpiece across at 600 inches per minute. Change that to a 3 flute cutter and you get (.015*3)*20000=900 inches per minute. This is all a very wordy way of saying that the more cutting edges you have on a cutter, the faster you can feed the workpiece across the cutter and still get the same results.

Faster spindle speeds wont always get you a better cut edge. Sometimes, yes, but what really determines the quality of the cut edge is the feed per tooth on the cutter, and thats affected by both spindle speed and feed speed. Faster spindle speeds let you feed faster, but you still need to find the sweet spot to get good results. Sometimes you can actually get a better cut with fewer scallops by decreasing spindle speed but increasing feed speed
 

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Re: feed rate, it seems to me intuitively a slower feed rate at a given RPM is going to yield a smoother cut.

But, my planer manual (Grizzly G1033) says slow feed = dimensional cuts, fast feed = finished cuts.

Seems counterintuitive to me, what say you all?
 

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Re: feed rate, it seems to me intuitively a slower feed rate at a given RPM is going to yield a smoother cut.

But, my planer manual (Grizzly G1033) says slow feed = dimensional cuts, fast feed = finished cuts.

Seems counterintuitive to me, what say you all?
I agree. I think the Grizzly manual has it reversed.

My DeWalt DW735 planer has two speeds. The manual says:

"To remove material thickness more quickly, set the unit at speed “2”. This setting delivers 96 cuts per inch to the material. For finishing, set the unit to speed “1”. Speed “1” is ideal for ensuring the finest finish on the last pass before your final thickness is achieved.
NOTE: When planing particularly hard or figured species of wood, speed “1” is recommended. The slower feed rate will reduce knife wear and tear-out by delivering 179 cuts per inch to the material."

The slower speed 1 is labeled "FINISHING / 179 CUTS PER INCH". The faster speed 2 is labeled "DIMENSIONING / 96 CUTS PER INCH". When you switch speeds, the blade rotation sound does not change.
 

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It is possible the motor HP is the limiting factor as well, so an underpowered motor might not be able to keep up when doing a deeper cut at high speed.
 

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For what it's worth, when I cut acrylic on my CNC, my feed rate and spindle speed are nearly half of what they are for cutting wood. If I use the same feeds/speeds as wood, it turns into a melted mess of acrylic on the table.

There are polishing bits for a CNC that will leave an absolutely perfect edge. The bits cost $2k or so. The machine has to be very rigid and the feeds and speeds have to be on point. Anything other than that setup, such as template routing acrylic by hand, you're going to get some degree of chatter marks.
 

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Discussion Starter · #15 · (Edited)
J_L Thank you that was a great summation. I'll be following your directions of turning the bit speed down. This router doesn't have speed adjustment but I've got an inline switch that adjusts speed all the way from 0 to full.

John my templates are just 3/4" birch plywood. Them seem fairly sold as pattern pieces. My table saw won't produce good straight cuts so my pro cabinet maker buddy made the patterns for me.

epicfail48 thanks you for the textbook writeup. I was reminded of the perfect gas law, PV=nRT when I first saw that equation. It's funny though, like you suggest, I've had suggestions to both slow down and ramp up cutter speed. I guess the take home less is be ready to experiment a little and see what works for your setup.

Thanks too, DrRobert, Tool Agnostic and BigCountry79.

After a lengthy search I haven't been able to find a O flute upcut trim bit with a guide bearing. I think they're all made for CNC. Closest I can find is something like a Whiteside RFT5200 1/2" shank 1/2" bearing 2" cutting length spiral upcut. Would something like this do?...

428401
 

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That bit should do just fine. Good luck with it. Let us know how it turns out. Share some pics of the edges of the acrylic as well
 

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get more appropriate bits


 

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After a lengthy search I haven't been able to find a O flute upcut trim bit with a guide bearing. I think they're all made for CNC. Closest I can find is something like a Whiteside RFT5200 1/2" shank 1/2" bearing 2" cutting length spiral upcut. Would something like this do?...

View attachment 428401
I bought this bit back in November along with a smaller one that has a 1 1/4" cut length. I went with the downcut versions of these Whiteside bits based on the recommendation of another wood worker as he thought they are better for through cuts. If you're only trimming off 1/8" to 1/4" of material it probably doesn't matter which cut you choose. The Whiteside flush trim bits were both big improvements over my Frued and Yonico straight blade flush trim bits.

I haven't routed any acrylic but I have routed some epoxy along with quite a bit of mahogany, purple heart, blood wood, paddok, white oak, cherry, poplar and black walnut with these bits over the last 8 months. The cut quality has been very clean and the bits have held up well even with some of the dense boards I've been routing.

Even though these bits can easily take a full pass through boards I always end up trimming pieces first on the bandsaw and only trimming an 1/8" or so with this bit.

There is a company that takes these Whiteside bits and adds a ceramic coating on them so they run cooler. I haven't purchased any of those bits yet as heat hasn't been a problem for me so far.

I've been slowly replacing my Yonico bits with Whiteside bits as they wear out (which happens quickly). I think Whiteside bits are really high quality for the price.
 

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to the OP: BTW - if you want advice on better cuts in plastic, please say so in your subject (title)
 
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