Cracked Acrylic

[cpd5899]

I've been having problems turning very hard acrylic blanks. They kept exploding. Finally got a negative rake carbide cutter.

I've been able to turn them down, but the last two have ended up cracked right at the bushing.

I know I didn't tighten down the 2nd one too hard because because I thought that was the problem with the first one.


PLEASE help!

 

[sorcerertd]

The first one (brownish) doesn't look like the blank was flush with the end of the brass tube. I can't tell if that is the case with the second one from that angle, though. In any case, that would definitely put all of the pressure on the blank, no matter how much you use.

 

[KMCloonan]

The brown-ish one appears to have some glue residue on the inside of the tube. If there is enough residue, it can make the bushings fit pretty tight, which could possibly crack the acrylic blank.

 

[randyrls]

David; With synthetic blanks the blank may crack as the drill bit exits. So cut the blank(s) about 1/4" long, mark the length of the tube on the side of the drill bit and drill only to that depth. Then trim off the uncut end.

Use a power sanding disk to round the lengthwise corners of the blank. Use a sanding rig to trim the blanks to final length.

When turning you should get ribbons coming off the tool. Use carbide tool just below center. this puts the force into the blank.

PS. Paint the inside of the drilled hole with spray paint to hide the brass tube and glue.

 

[RobS]

Hello, please read below for tips for dealing with polyester.

1) this is not acrylic it is polyester the 2 materials act very differently
2) please continue using the negative rake scrapper (that will help a lot)
3) do not use a pen mill on polyester, it is better to use a sanding disk, it is less stress on the part, see @rherrell for the custom sanding jig he makes
4) as noted earlier, do not drill all the way through, cut the blank so you can drill just short of breaking through. Then trim off with a band saw or saw, then glue in the tube, sand the ends square with the sanding jig.

Yes it sucks to need a sanding jig, but this material works best of sanded square.

5) use a punch, to make sure there is no glue debris in the tube prior to mounting it on the lathe.

Good luck