Vibration, Runnout, and Spindle Choice

I’ve embarked on a project to mill large blocks of HDPE, including cutting profiles to a full stock depth of 7". Speaking with a tooling manufacturer, they advised 3/4" carbide. The next question is appropriate spindle. Due to tooling diameter, a spindle supporting an iso 30 tool holder paired with an ER 32 collet. Some have reflected/remarked that the 6.5 KW Hiteco spindled features less vibration than the 3 KW model; I wonder how it compares to the S30C, commonly employed for ATC applications, as far as vibration, runout, and bearing reliability/longevity are concerned. At 7" stick out, I wouldn’t be tapping any where near the power capacity of a 8.7hp spindle.

Any reflection on spindle choices would be most welcome.

Please do post pictures and your thoughts when you do it.

I am always interested in people’s ambitions when they start these projects and where reality lands them.

For me it was aluminum and a 3 inch stickout on a far more rigid machine than anything AvidCNC has ever made or will ever offer.

I would love to see pictures of your work when you get there! :grin:

Experienced pragmatism is useful.

With all due respect, how long have you owned a AcidCNC machine?

And that emoticon was a grin, not a sneer. I would suggest you go grab a cup of coffee and get back in your shop and get that project done. We are all waiting to see the results.

I cast no insults by DM- rather, I didn’t want feed back to be taken for public reproach.

To reiterate: the topic here is spindles that will accommodate 3/4" tooling featuring comparatively minimal vibration, runout, and bearing longevity.

HDPE isn’t hard to machine. You want 3/4" to reduce deflection. The vibration is not going to come from any of the spindles you mentioned but rather your long reach tooling. You will need the 8/12 Z to be able to retract your tooling high enough clear your work piece. Your question about bearing longevity would be something I would get directly from Avid. Inaccuracies will come from several other places before you have to worry about spindle run out.

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As to #1 I sometimes use a 6" .5 EM, I tried a 6" .25 EM and I could not prevent chattering. And to top it off I believe by using long reach tools resulted in the ruination of my Hiteco 4hp. It developed an unacceptable runout when the chatter caused the bit to particiall pulled out the collet. :grin:

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I reckoned all vibration, to an extent, and runout would be cumulative- be it from the tool itself, collet or tool holder, spindle, gantry, or frame -and that the impact of vibration would be greater the longer the tooling. Is that wrong? I have been cautioned the longer the tool, the tighter the necessary tolerances.

I plan on the 8/12" combination, combined with a sub table frame support surface and indexing fixture to allow for stock adjustment and use of extended tool holders.

Speaking to HDPE, the required feed rate to guard against chip melt strikes me as the greatest challenge to such an extreme stick out. I wonder if plunge roughing would be advised.

What material were you milling with the half inch tool? What was the nature of the operation? Was 6" the stick out, or over all length?

The 7" stick out, three quarter carbide tooling proffered by one manufacturer has a ~8.5" OAL, priced North of $700 USD.

I don’t think you need to worry about melting. What kind of tolerance are you trying to hold? HDPE cuts like butter.

Generally, my tolerances are rather gross; large billets of HDPE are to be shaped into matching blocks, with a single relief carved face requiring surface continuity only to a degree easily obscured by hand finishing. Where accuracy is most critical is for fixture placement when roughing the plastic billet- first, developing parallel opposing faces with surface milling, then cutting cylindrical through pockets for a supporting pivot and fixture registration pins. The initial intent is manual indexing.

Post machining, tolerances are more in line with wood joinery- at best.

Is this something you could share a picture of?

There are only models of the press mould for the plastic at this time (the steel is in the laser cutting queue at this moment). I’ll work up a model of the indexer for feedback, as time allows. I’m imagining a pair of fixture plates perpendicular to and bellow the table surface, hosting an array of holes for a through axle and pair of locating pins, with spacers to allow for tooling access; the stock would be located on the axle, oriented with the pins, machined, pins removed, rotated on the axle, locating pins replaced, etc. Indexing would be limited to pin locations on the fixture plate.

The billets produced by the press mould measure ~14.5" x 20.5" x 7.75", weight as much as 80 lbs, and may be rough sawn prior to milling. The axle would pass through the narrow aspect.

Some challenges to utilizing a 4th axis- especially for heavy asymmetric parts -but that possibility is to be explored, too.

Received some historied feed back over at the Practical Machinist forum- worth a read, for anyone with similar needs (milling HDPE with long stick out):

Looks like 3/4" diameter with 7" stick out is doable.

Interesting… I would think that this is doable. Like with any job the toolpathing is going to be key on this one. What are you using to make your toolpaths?

Almost certainly Fusion 360 due to familiarity, the capacity to visualize fixturing and stock development and position for indexing, integration with my native CAD environment, and initial cost.

That’s what I figured you’d say!

I’ve been messing with some of the adaptive clearing stuff lately and it’s been wonderful to work with. I suspect some of those toolpathing strategies could do well for what you’re trying to do where I suspect that precise load on the tool is going to be important.

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Materials were hard and soft woods. 6" OAL, the operations were squaring slabs and sink cut outs etc on really thick pieces. It can be done, super light cuts and plunging then clean up.

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