Cutter digging into acrylic?

[Darrell Johnson]

When I'm turning the cast acrylic Pen confusedly cutter sometimes digs in and ruins the piece. I'm using a hunter carbide cutter and should I be cutting with a scraper instead of a sharp carbide cutter? When it digs in in shatters the acrylic and peels it off down to the brass tube. I'm really puzzled.

 

[KBs Pensnmore]

When I'm turning the cast acrylic Pen confusedly cutter sometimes digs in and ruins the piece. I'm using a hunter carbide cutter and should I be cutting with a scraper instead of a sharp carbide cutter? When it digs in in shatters the acrylic and peels it off down to the brass tube. I'm really puzzled.

Hi Darrell, How many acrylic pens have you turned,as it is different to timber IMO. I've used a home made carbide tipped cutter on PR and the only time something like this has happened is if the cutter edge is blunt or damaged, if it is sharp maybe you are a little aggressive on the cuts, I take only very fine cuts. I also use an oval skew with no dramas.
Kryn

 

[soligen]

Does it happen before or after it is round. I find carbide (R2 insert) works well only after the blank is round. Before being round, they are prone to do just what you describe. I use a skew with a planing cut to get an acrylic blank round. Its a bit slow, but much gentler on the blank.

If it happens after rounded, then as noted above, dull tools or you are being too aggressive, or even possibly technique. With a carbide tool you want the tool level with the cutter slightly below center line. Carbide tools (I use R2 insert) work great on PR - with a sharp insert and light final passes, you can start sanding at 600 to 800 grit.

 

[randyrls]

My experience is that the cutters that have acute angles (~45 degrees) are much more "grabby" than cutters with angles closer to 90 degrees.

Acrylics should be cut below the center line. You may have problems with chip out pits if you are too aggressive. You should get nice long ribbons of material peeling off the blank. If you are getting tiny shards, you are pushing too hard, or the cutter is dull.

PS. Flat topped cutters can be easily sharpened with a credit card diamond hone. A drop of water or oil on the hone, place the top surface down against the hone, side in a figure 8 motion, turn 90 degrees, and repeat. Takes 1 minute!

 

[sbell111]

I'm thinking that people are defining 'carbide cutter' differently. I assume that the OP is using an 'easy rougher'-like tool.

 

[ldb2000]

Hi Darrell , the problem you are having is twofold . First you are most likely trying to use the tool at the wrong angle and second , you are being way too aggressive with it .
I use my Hunter tool to round over ALL my blanks from wood to metals . The Hunter tool is very aggressive if held wrong , you should be holding it an aprox 45 degree angle to the work . This will give a shearing cut with the tool and control the aggressiveness of the cup shaped cutter . When held at a 90 degree angle to the work it will dig in and tear out chunks of the blank . The Hunter tool is a wonderful tool for quickly rounding down or shaping the blanks , but it has a learning curve that takes awhile to get proficient with .
Try holding the tool with the cutter edge almost straight up and down then slowly rotate it until it starts to cut . Take slow passes with the tool letting the cutting edge do the work , these cutters are very sharp and will cut with very little pressure . Once you get used to how it cuts you can adjust the angle you hold it at to be more or less aggressive , when you hit the "Sweet Spot" you will get longer streamers that fly off the cutter .
Good luck and if you need any further help , feel free to post here or PM me for further help .

 

[Darrell Johnson]

Hi Darrell , the problem you are having is twofold . First you are most likely trying to use the tool at the wrong angle and second , you are being way too aggressive with it .
I use my Hunter tool to round over ALL my blanks from wood to metals . The Hunter tool is very aggressive if held wrong , you should be holding it an aprox 45 degree angle to the work . This will give a shearing cut with the tool and control the aggressiveness of the cup shaped cutter . When held at a 90 degree angle to the work it will dig in and tear out chunks of the blank . The Hunter tool is a wonderful tool for quickly rounding down or shaping the blanks , but it has a learning curve that takes awhile to get proficient with .
Try holding the tool with the cutter edge almost straight up and down then slowly rotate it until it starts to cut . Take slow passes with the tool letting the cutting edge do the work , these cutters are very sharp and will cut with very little pressure . Once you get used to how it cuts you can adjust the angle you hold it at to be more or less aggressive , when you hit the "Sweet Spot" you will get longer streamers that fly off the cutter .
Good luck and if you need any further help , feel free to post here or PM me for further help .

I will try the hunter at the 45 degree angle but tonight when I was trimming the ends with a barrel trimmer the acrylic cracked off. I just picked up a carbide barrel trimmer and they only have 2 cutters instead of the 4 cutters a none carbide have. Maybe that's the problem. What do you think?

Thanks
Darrell Johnson

 

[ohiococonut]

Being a newbie I've learned that the gap between the piece and the rest is critical for my style of cutting. Without changing anything, if I don't maintain a close gap I have the same problem. The tool grabs and tips down and there goes a chunk.

 

[ldb2000]

Yes , the two cutter barrel trimmer is no good for acrylic , or anything else for that matter IMO . One of the biggest problems with acrylic is that most are to some extent very brittle . The two cutter trimmer only makes contact at two points on the blank so if you push a little too hard they dig in and cause the blank to chip out . Barrel trimmers in general are too aggressive to be used on acrylics and the two cutter ones are the worst .
Take your barrel trimmer and reverse the cutter head then glue a sandpaper disc on the smooth side of the head and use it as a sanding mill . You can make the discs from any grit sandpaper but I would advise using a 100 to 150 grit . Cut them aprox the same size as your cutter head and punch the center hole with a paper punch , the kind you use for adding pages to a photo album or loose leaf book . Use some spray glue on the back of the disc and just stick it to the cutter head .

I will try the hunter at the 45 degree angle but tonight when I was trimming the ends with a barrel trimmer the acrylic cracked off. I just picked up a carbide barrel trimmer and they only have 2 cutters instead of the 4 cutters a none carbide have. Maybe that's the problem. What do you think?

Thanks
Darrell Johnson