Spindle noise, poor edge finish on HDPE

[lto]

I was just testing cutters and feed speeds to determine what works best in 2d profiling 1" HDPE parts. I was using #6300 series upcut O-Flute Onsrud bits in 1/2" and 3/8". The 1/2" was bought for another job and has a cutting length of 1 5/8",when first starting the spindle to 18000 rpm I was greeted by a sick sound and vibration. I remounted the bit making sure everything was clean and still got the same thing untill I adjusted back to 10000 rpm. The cut at .8 per sec and .5 stepdown was not really impressive. The 3/8" bit did not cause nearly the vibration on the spindle and actually exceeded the 1/2" in edge quality. Whats up? Are O-Flute bits off balance? Is the Columbo 3HP very sensitive to imbalance? Should I expect to get a good edge quality on HDPE on a Shopbot? Will using a cleanup pass removing .020 full depth be a solution?

Thanks everyone,
Louis

[Brady Watson]

Louis,
My Colombo shows some harmonic vibration between 16,400 and 18,600 RPM...but it's the cooling fan housing vibrating. It does this with or without a bit.

You should be able to cut HDPE like butter...nice soap shavings coming off of it. I have cut it with straight bits without any problems. I would try cutting it with a straight or upcut spiral. HDPE is self-lubricating and very easy to machine, so chip extraction and welding issues are really not as critical as acrylic for example.

If you want a perfect edge with no lines, then do a few passes leaving a skin, and then do a full depth pass to clean it up. HDPE will show your stepdown passes if you don't do a cleanup pass.

Try the straight bit...I think you will see a difference right away...

HTH,
-Brady

[fleinbach]

I Don't have any experiance with HDPE but I too had the high pitch noise you experianced with the Onsurd bits. There is another thread you can see about it here (http://www.talkshopbot.com/forum/messages/29/4854.html?1104779347)

[robert_starkes]

Louis,

I cut 1" HDPE (pipe grade) all the time using the PC router with a 1/4" spiral upcut. The settings I use are 2"/sec with 0.5" step down at 21,000rpm. The edge quality is very good.

Brady is right about the step down edges showing, so a full depth clean up pass may be required.

[lto]

Thanks everyone,

I've ordered straight O Flute bits. I think I may have another problem, the carriage was visibly jerking while cutting. I was cutting 3" x 5" ovals drawn in parts wizard. The machine is new so I am not yet sure what is "normal", but right now I'm looking at it and wishing everything were biult of cast iron.

[elcruisr]

Sounds like a software tuning issue. You might want to contact the support folks at Shopbot and see if they can't get you some different VR values to solve the "jerking". Software tuning is an issue any CNC machine has to deal with. The default values are a good average but for some shapes need to be tweeked.

Eric

[lto]

I've been working on improving my edge quality on 1" HDPE. I had some improvement by making sure the work piece is Very well secured and tightening the Y car bearings. The Z can be rattled up and down. I hope this isn't normal, but how do I get to the bolts to adjust the Z motor tighter to the cog track? My test cuts in rectangle shape are looking fairly good now, I just wish that were the case with the ovals. Is it possible to get the curved edges nearly as chatter free as the straight lines? I sure hope so because I am needing to cut 90% curved shapes. I am pleased that there is virtually no evidence showing of having used 2 passes,or stepdown.

[olober@ptd.net]

Save your $, don't buy single edge o-flute tools larger than 3/8" diameter for use in the 3HP Columbo spindle. I wish I had thought about balance before I ordered several different 1/2" diameter, long CEL solid carbide bits. The sound told me immediatly that something is wrong, I just didn't realise it wasn't possible to run this geometry on a small spindle. After all, if it says Onsrud it should be balanced, right?