View Full Version : Poor cut quality in plex

05-23-2005, 07:57 PM
I've been cutting plex (cast 3/8") for about six months now. Edqe quality hasn't been perfect, but good enough to please my client. I use a 0 flute Onsrud 1/4" cutter (63-726) running about 23K on my PC890 router at about 90ips.

Recently, I took a break from the plex to cut about 10 sheets of MDF basically into strips, so quite a few feet in cuts.

Went back to cutting plex and my edge quality dropped considerably. Now, instead of pretty clean verticle shear cuts, I have angled cuts with a lot of very small tear outs. If you have the patience, check the URL, I didn't think there would be enough quality to really see the picture with the limitations on posting photos here.

www.cncworks.biz/images/plex.jpg (http://www.cncworks.biz/images/plex.jpg)

I've changed routers, used a new bit, cleaned and lubed my ways and rack and pinion. Wiggled, pushed and prodded all to no avail.

I'm posting here in hopes that someone will have experienced this and point me in a good direction to get back to where I was.

Thanks up front for any considerations.


05-24-2005, 07:53 AM

I have no experiance in cutting plexi yet, however I have experianced the same "angled cuts" in HDPE. The tool is obviously vibrating.

Things I would question; Is the plexi from the same batch as before? Is the tool properly colleted? Did the bearings suffer from cutting the MDF? Why use an 1 1/2" CEL tool to cut 3/8" material? (All the above potential problems are multiplied by the excessive tool length.

Good Luck!

Brady Watson
05-24-2005, 10:39 AM
Any changes in design software (like Corel vs. PartWizard) or CAM posts (Shopbot_Inch vs. Shopbot_Arcs_Inch)??? Any changes in ramping settings?

I have to agree with LTO on the tool length...shorter is better. Have you tried cutting in 2 passes? If you cut 3/8" material, then do 2 profile passes at .15 stepdown and .03 offset/allowance, leaving a .07" skin on the material. Then go back and run @ full depth with zero offset/allowance. This will increase your quality tremendously, as well as reducing the load on the tool. Additionally, you may just be moving too fast through the material. Dial your speed down to 60 IPM/ 1 IPS and reduce your router speed to 12,000. Is this an Alpha or PRT?


05-24-2005, 11:20 AM
Hi Donn!

Some acrylic brand I've tested so far, I've never been able to avoid thoses edges chipping (tearout), only reducing to a minimum that was still quite bad...

...and I'll second LTO's and Brady's post.

05-24-2005, 11:34 AM
LTO, plex is from same batch. Even went back and cut from a scrap of identical material. Exchanged routers, exchanged collets, put in new bit.

While I agree with the potential problems with router bit length, the set-up is identical to my previously successful cut process.

Brady, no changes at all in SW. I resist making multiple passes because I know I was getting adequate quality with one pass previously, and because it would triple my cut time. I did slow to 60ipm and only achieved a closer pattern of chatter and a little less tear-out.

I does seem like chatter/vibration of the bit, all right. It seems to me that it would be moderately unlikely that play in the machine itself would contribute heavily to an apparently high frequency issue like this.

What confuses me is that, to the best of my ability, I've set up identically to my previously successful cut process.

Thanks for the responses.


05-24-2005, 01:33 PM
I'm assuming you've double checked all the bearings for proper tightness? Plastic cutting likes everything snugged up tight. Also as mdf creates piles of very fine dust have you made sure that there isn't any dust packed into the v-grooves on the bearings? That one happened to me once.

A 3/8 cutter will also give you much better results in plasics because of the better rigidity. On plexi I've been getting better results with a 52-638.


Just keep telling yourself it this was really easy, everyone would be doing it......

Brady Watson
05-24-2005, 03:59 PM
I forgot to ask....what method of material hold-down are you using? I have found that this, aside from using the wrong bit, causes more problems than anything.


05-24-2005, 06:13 PM
One more thing to think about:

I see you are using the (little) PC890 router. Was it bogging down? Sound rough while cutting?
Could your line voltage have changed enough that it changed your marginal cut to bad?

The pits in the bottom part on the picture look like material was tore off rather than cut off.

I cut 3/16 Polycarbonate at 16000 RPM at 1.5 IPS with a 3/8 bit and get a fair edge. When I tried to cut 3/8 Polycarb at the same settings it bogged my PC7518 down so much I was getting huge chips re-welded into the full cut path. I suppose I could have changed the RPM or feed but I switched to 2 passes and everything was fine.

I get the same bog / re-weld problem when I have a dull bit.

Just a thought,

Edit: I was thinking PC690 (1 ĺ hp), donít know what a PC890 is.

05-25-2005, 02:15 AM
Scott, the PC890 is a 2.25hp (yeah, right!). It is actually a pretty nice router, powerful enough for most of what I do, including single pass cuts in 3/8" acrylic without bogging. Single wrench bit changes and a lot quieter than the PC7518, that's for sure.

The other obvious symptom is more of a fine dust/chip output as opposed to previous peeled sliced chips.

I think polycarb is quite a bit softer and more likely to melt than acrylic, especially cast acrylic.

Because of the many internal cut-outs less than 3/8", I'm forced to use 1/4" bits, which, again, have worked just fine for months.

So, thanks again for the suggestions...I'm going to go scratch my head some more tomorrow.


10-04-2005, 07:49 PM
Hi everyone.
I was watching my bot cut 10mm acrylic just now, and I couldnt help wondering whether that plastic sleeve between the clamp and the Porter Cable router could be flexing somewhat - could it be responsible for a less than perfect cut quality? I dont think anyone has mentioned this in the forum before.
I put my hand on the router body and on the clamp, and I thought I could detect a greater vibration in the router than the clamp. I cannot be sure though.
Has anyone tried an aluminium or other metal insert to replace the plastic sleeve? and if so did it make any difference?