Removing 1/2 inch of acrylic

============================ 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

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

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

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

Maybe a dado blade on a table saw? Ken.

To use a circular saw in the mill for removing most of the matl.? Then none or two more passes with the mill.

Nick

Ken,

That would work, assuming the plastic can take it?? I'd want one hell of a jig though.

Bill

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.

And make a pretty pattern when cracks propagate thru the part. Alcohol will craze acrylic, especially in the presence of stress.

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.

I'd go for the circular saw (table saw, of course) approach. Full depth, and very slow feed. Two cuts - done...

-- 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.

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

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:

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

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

--Agreed; don't use the fence. Rather, position the part on a panel-cutting sled and push that thru..