What did I do wrong?

[araugh]

Hey there folks, here's my first project on my PRSAlpha. Looking for a bit of constructive criticism if you don't mind.

https://lh6.googleusercontent.com/-1HZy4DAcdew/UCUff06XZUI/AAAAAAAACQk/XXkxmF8rzu8/s1032/2012-08-10_10-49-11_21.jpg

Ok, the obvious stuff first, my depth on the V-Carve was set too deep and as a result the word "LASER" is a bit butchered.

I also had a good deal of bit scream during the pocketing operation and a ton of it during the profiling. I used the 52-910 that came with the starter bit set. Am I correct in thinking that the majority of the screaming was from using a bit that was too long for the job and having my cut rate set too high?

Then there's the part that I'm a bit lost on. You'll see that in the pockets above the R, underneath the E and below and to the left of the L the bit went a bit too far into the material and I end up with a little divot of missing material. What did I do to cause this? Is this related to the start point of the operation and if so can I move the start point to the middle of the pocket?

Please speak up if you see anything else as well, looking to learn from this. Thanks for any guidance you're willing to provide.

Also I was a nerd and recorded the whole thing if you're looking to hear the noises and see the setup.

http://www.youtube.com/watch?v=XTaCFoHHz9Q

[harold_weber]

First, congratulations! Not everyone is successful doing their first project.....

As for the bit screaming, you did not have a really bad case of bit screaming from what I heard. Really bad screaming feels like your teeth are vibrating. I have never used that bit, and it looks like your material is MDF. Onsrud does not recommend that bit for cutting MDF. You can download Onsrud's Production Routing Guide from their web site, and it has recommendations for cutting different materials.

As for the "divots" in the lettering, that looks like it is a result of creating a V-carve toolpath for that particular font. The Vcarve toolpath is created by sending the V bit deep enough so that it touches the boundary lines of the letter outline. You should have seen those divots if you ran the cut in Preview mode. Suggest you use a different font. You could also set a limit on the cutting depth by specifying a "flat depth", I think thats available in V-Carve Pro, I use Aspire. Experiment with this and check the preview to see if its acceptable to you.

[steve_g]

Adam

http://www.talkshopbot.com/forum/showthread.php?t=15176 (http://www.talkshopbot.com/forum/showthread.php?t=15176)

This might help you in selecting bits and speeds

SG

[araugh]

Thanks for the reply Harold! I did run the preview before cutting anything (about a dozen times, I've been really paranoid about blowing this machine up) and it appeared fine in the preview. I have reset it to be flat depth of 0.2" and will try to cut this again.

So I shouldn't be concerned about the noises I was getting? I got the same noises today out of some aluminum I was drilling on the bot, but I sortof expected them with metal.

Thanks for the link Steve!

[harold_weber]

I don't think the noise is anything to worry about. If you plan to cut much of that material and don't like the noise, try a different bit geometry.

For the 52-900 series, Onsrud recommends a chip load of between 0.006 and 0.008 inches per flute to start with, are you in that range?

[Bob Eustace]

Could be a bit of resonance as well Adam. Try 4 screws intead of 2,

[myxpykalix]

For a first cut it looks pretty good. My first cut resulted in a hole in my table:eek: so don't sweat it. You will learn what different bits to use in combination with various feeds and speeds with varying material. Experience will teach you that.
I want you to let us know when you break your first bit. We have a pool going on here where we bet on newbie bit breakage after purchase. My money is on the first week after you start cutting:D

[MogulTx]

"Screaming" might be due to excessive side pressure on the bit ( too fast a travel speed), inappropriate bit for the job, too high a speed on your router/ spindle ( WHich are you using?), Bad bearing in the router,..... lots of reasons.... The GOOD parts is that you are LISTENING to your system, and you are concerned. That is excellent. Do that, and you will develop a keen sense of when something is out of normal orperating parameters.

Did you use ramping into this cut? What depth of plunge per pass? What overlap for the pocketing? what speed IPS? What speed plunge?

There are no perfect scenarios, IMO. I think you will want to operate within a tolerable level for you on time and noise and bit replacement, etc. You want to work within reasonable parameters for machinery capability, longevity, and quality of output, etc...

Congrats on a successful first project. I snapped a bit within about 5 minutes, I think.

[pkirby]

I'm pretty sure your "divots" are from the material being "torn out" instead of cut out near the end of the toolpath. There would be 2 possible solutions that I'm aware of.
1) Reduce your stepover so that less material is being cut on each pass.
2) Change the toolpath so that the tool starts in the center of the pocket and then cuts its way to the outside.

I've had this problem before and these 2 solutions have helped reduce or eliminate the problem.

[adrianm]

With MDF on my PRS Alpha I cut at 6.5ips at 14k rpm. Should be no screaming at those speeds other than when the bit is slowing for the corners. I always use a spiral ramp as well.

I prefer a down cut bit with MDF as it doesn't mess up the bottom edge as badly as an up cut messes up the top edge.

[araugh]

I want you to let us know when you break your first bit. We have a pool going on here where we bet on newbie bit breakage after purchase. My money is on the first week after you start cutting:D

That's good money because I'm moving straight into brass and aluminum this week as soon as a get a few new bits.

[araugh]

Thanks for all the feedback guys, I'm glad to be learning from this.



For the 52-900 series, Onsrud recommends a chip load of between 0.006 and 0.008 inches per flute to start with, are you in that range?

0.006 according to the feed rate and RPM I was going with.



Could be a bit of resonance as well Adam. Try 4 screws intead of 2,

Will do, thanks!




Did you use ramping into this cut? What depth of plunge per pass? What overlap for the pocketing? what speed IPS? What speed plunge?


I'm using a 5.5hp Colombo spindle. I did ramp it, used a .125" plunge depth which was probably too timid and used a 7% overlap. I don't remember the speeds off the top of my head, I'll look those up.



I'm pretty sure your "divots" are from the material being "torn out" instead of cut out near the end of the toolpath. There would be 2 possible solutions that I'm aware of.
1) Reduce your stepover so that less material is being cut on each pass.
2) Change the toolpath so that the tool starts in the center of the pocket and then cuts its way to the outside.

I've had this problem before and these 2 solutions have helped reduce or eliminate the problem.

Got it, thanks!

[adrianm]

0.006 according to the feed rate and RPM I was going with.

In my experience a chip load of about 0.013 works better with MDF and that bit.

When I first got my Alpha and tried it with a 0.006 to 0.008 load as per the chart it screamed like a banshee.

Also all I got was very fine and warm dust. With settings I have now I get cool chips.

[Joe Porter]

How did you hold the cut piece down? I know you had screws in the outer piece, but I saw you shake the finished piece out with no tabs or onion skin. Maybe your finished piece moved a little bit and caused the divots. But, great job over all. Also, when you v-carve fonts, be sure and check the vectors well and look for little surprises. Sometimes these need cleaning up with your scissors tool. joe

[araugh]

Joe, I had it held by both tabs and a skin, if you see in the video I pop the piece out, but then there's some heavy utility knife trimming done before the piece is presented to camera. I'll try a more aggressive hold down next time.


Thanks again for all the tips guys, I'll try cutting that piece again today with this advice.

Later today it's my first attempt with brass...

[araugh]

Well, here's part 2 of what did I do wrong.

In this episode I'm going to try to figure it out and please tell me if I'm in the ballpark.

The good news: I successfully pocketed out an awful lot of aluminum, used a drill to mark centers for drilling on a drill press later, and cut out a few holes using a peck drill toolpath.

Here's what I did wrong with those operations:

My overlap was set too high for the pocketing operation
My stepdown was set to half of what it should have been for the pocketing operation
I realized my errors into cutting, but didn't know how to start the file again from the middle after fixing the mistakes so instead of aircutting the first half of the pocketing operation I just let it go.

That's the minor stuff. The major part is that I owe myxpykalix a few bucks because I broke my first bit on the profile cut. I was amazed at how well the pocketing went that I went a bit heavyhanded with the profile cut, telling it to step at 0.050", cutting at 14000RPM with a 1IPS feed rate and .5IPS plunge rate. I went with a conventional cut when I should have gone with a climb cut. I should have told it to ramp as well. Here are the results:

https://lh3.googleusercontent.com/-APSreJ3DE00/UCquD9I_2tI/AAAAAAAACWc/ApqPNQ0UZiU/s1032/2012-08-14_15-58-54_658.jpg
https://lh3.googleusercontent.com/-BYzaTwXusi4/UCquG0jviNI/AAAAAAAACWk/lg9R06dsECo/s1032/2012-08-14_15-59-10_264.jpg
https://lh3.googleusercontent.com/-bHy7_sbjnos/UCquRBiwK1I/AAAAAAAACXE/qe7_Cgl__iU/s1032/2012-08-14_15-59-57_271.jpg

So am I relatively on point about what I need to do to prevent this from happening again? I have nothing against paying "tuition" I just want to make sure I learn from it. The folks at my local bit reselling and sharpening company are coming by in a week to check things out and chat for a bit, but in the short term he suggested a dishsoap and water mist to help out too if I find myself breaking more bits.