Removing 1/2 inch of acrylic

Hello all,
Suppose one needs to remove 1/2 inch of acrylic from a 2x1 inch (in
cross section) block. The goal is to end up with roughly the following:
+----+
| |
| |
| +-----------+
| |
| |
+----------------+
I find that with deep cuts, there is some risk of breaking off material
as the endmill reaches the end of the part. Slowing the feed rate near
the end helps, but does not eliminate it. Lots of light fast passes
seems to work.
What _should_ I be doing? Out of curiosity, would it change if the
material were Aluminum?
The part is a little long to stand on end in my bandsaw. With a
suitable jig, that might work. But if it must be roughed on a mill,
what is best approach?
Bill
Reply to
Bill Schwab
Loading thread data ...
============================ Woodworking practice would be to put a piece of scrap at each end to support the edges. The edges of the scrap will chip rather than the good part.
Uncle George
Reply to
F. George McDuffee
Rough the part from both ends to the middle reorienting as required to avoid climb milling, then finish with light cuts the whole length. The final can be a very light climb, if that gives you the best finish.
Ned Simmons
Reply to
Ned Simmons
Ned,
Dumb question: what happens in the middle? For any given pair of "half passes", won't the first half of thecut leave the second half breaking out of the material? Or is the point that when it happens, there is material under (and in front of) the endmill?
If that is the idea, would you take a large endmill and cut almost full depth (up/down)? How much would you take front to back on each pass?
Bill
Reply to
Bill Schwab
Yes, in order to create a defect in the middle you'd have to pull a divot out of the part, whereas on the end you're chipping off a sharp outside corner.
Depends on how well you can hold onto the part and, especially in this case dealing with acrylic, how much heat is generated. Unless I was in a big hurry to make a number of parts, I'd probably use a dead sharp 5/8" 2-flute endmill taking perhaps 1/8" off both surfaces on each pass, and make adjustments depending on what's happening in the cut. Finish with a pass of .005"-.010" on both surfaces simutaneously.
Ned Simmons
Reply to
Ned Simmons
Maybe a dado blade on a table saw? Ken.
Reply to
Ken Sterling
To use a circular saw in the mill for removing most of the matl.? Then none or two more passes with the mill.
Nick
Reply to
Nick Müller
Ken,
That would work, assuming the plastic can take it?? I'd want one hell of a jig though.
Bill
Reply to
Bill Schwab
Try using a "for aluminum" endmill. These have, in concept at least, a greater cutting angle and a greater relief angle than normal endmills.
Bill Schwab wrote:
Reply to
Hul Tytus
Some alcohol in a squirt bottle will help keep the cutter (and acrylic) cool, Keep your cuts light and perhaps use a roughing cutter for all but the finishing cuts.
JMiller
Reply to
<jamw42
And make a pretty pattern when cracks propagate thru the part. Alcohol will craze acrylic, especially in the presence of stress.
Ned Simmons
Reply to
Ned Simmons
I've never experienced crazing and cracks using alcohol.
Forgot to mention that if the acrylic is 'old' it is more prone to chipping and shattering. Sometimes putting the acrylic in an oven at 150 deg F for a couple of hours or so depending on thickness will anneal it and reduce the tendency to chip and shattter. Best to have 'new' material though.
JMiller
Reply to
<jamw42
I'd go for the circular saw (table saw, of course) approach. Full depth, and very slow feed. Two cuts - done...
-- Jeff R.
Reply to
Jeff R
It's not always a problem, but when it does happen the effect is something you won't forget. This document puts a number on the stress required to start the cracking.
formatting link
The stress is up around 900 psi, which is higher than I would have guessed. My introduction to the problem was when deep drilling a rod using alcohol as a coolant, which I'm sure causes higher stresses in a larger volume than milling with a sharp cutter would. But it seems to me that since a milling cutter is shearing material (i.e., exceeding the strength of the acrylic) the stresses at the cut point are high enough for the cracking to potentially be a problem on a micro scale.
Ned Simmons
Reply to
Ned Simmons
Hello bill-
Woodworking tools usually work better than metalworking toold in plastic. try and source a woodworking router ( approximately 20,000 RPM ) and give this a go. Be careful because the plastic is tough and the workpiece and the tool both have to be well supported to avoid a dangerous fly-out. If the router has a vacuum swarf remover them remove that because it will probably block up regards fredf
Bill Schwab wrote:
Reply to
fred
Rather than using a dado on a table saw, I'd use just a very sharp HSS or carbide rip blade and make one cut that doesn't go all the way thru, then flip the part 90 degrees and repeat. Use a block of wood to push the whole thing all the way thru and past the blade.
Mike
Reply to
The Davenport's
Mik,
That sounds better. I still think it would take a fairly good jig to do it safely. If the rip fence is involved, it should be on the part side to avoid trapping the waste between the blade and the fence and creating a missle. On the up side, the part acts as a guard of sorts, and an auxillary fence would compensate for the uncomfortable situation of reaching down between the fence and the blade.
If I end up making a "large" run of these, I'll make a simple jig and give it a shot.
Thanks!
Bill
Reply to
Bill Schwab
--Agreed; don't use the fence. Rather, position the part on a panel-cutting sled and push that thru..
Reply to
steamer

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.