3.17mm downcut endmill max pass?

3.17mm downcut endmill max pass?

Postby marbles » Wed Oct 10, 2018 10:12 am

Hi,

I have a complex job to cut out irregular and complex map contours in 18mm birch ply. I need to use a 3/17m (1/8") end mill as it give significantly better resolution over 4-5mm endmills. I ran tests yesterday on my 4x4 R&P cnc router, 2.2kw spindle table and lefthand downcut was the best, marginally better than a compression cutter.

The best result was with a brand new 3.17mm downcutter. 3 passes @ 6mm each, 10mm/s feed, 16,000rpm. No burrs and hardly any post sanding. Pretty happy with the results so far just want to tweak to get it right before I start on the expencive veneer faced ply!

My question is about pass depth due to the direction changes and the hours the machine and cutter will take to cut this job out.

Q: Should I reduce the pass depth, bearing in mind the cutter getting blunt over time?
Q: I also noticed some squeal from the cutter, should I reduce the rpm?

Thanks
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Re: 3.17mm downcut endmill max pass?

Postby Adrian » Wed Oct 10, 2018 3:57 pm

If the cutter is squealing when it's at the maximum programmed feed rate then you need to lower the RPM or increase the feed rate.

My personal rule of thumb with bits smaller in diameter than 6mm is not to exceed twice the diameter in pass depth so with a 1/8" bit I wouldn't cut more than 1/4" at a time.

I adjust feed rates and rpm over time as the bit blunts rather than reducing it to start with in anticipation of the bit wear.

With my Shopbot and a 2.2 spindle I cut at a feedrate of 50mm/s at 11,000 rpm in softwood ply with a down spiral 1/8" bit. There is a reasonable amount of bit deflection at that speed but for my purposes that doesn't matter.
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Re: 3.17mm downcut endmill max pass?

Postby Tailmaker » Wed Oct 10, 2018 8:01 pm

My concern with a downcut bit making a slot twice as deep as the bit diameter is the slot clogging up, especially if you have to do several steps to get through the material thickness.
I usually go only one diameter deep with the downcut bit and then switch to an upcut for the rest of the job. On the other hand, 1/8" bits are reasonably cheap and you can just experiment how far to push the envelope.
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Re: 3.17mm downcut endmill max pass?

Postby adze_cnc » Wed Oct 10, 2018 9:22 pm

On a tangent I find it funny that "metric" people seem have an aversion to refer to an 1/8" bit as that. Instead they use the imprecise 3.17mm (rather than 3.175mm).

Despite predominantly using "Imperial" cutters, if I need to refer to a 4mm bit I don't call it a 0.157" bit but rather a 4mm bit. :D
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Re: 3.17mm downcut endmill max pass?

Postby Tailmaker » Wed Oct 10, 2018 10:42 pm

adze_cnc wrote:On a tangent I find it funny that "metric" people seem have an aversion to refer to an 1/8" bit as that....


Not surprising...quite often such bits in Europe are sold only with the metric dimension spec, which sometimes could be 3.17 or 3.2 mm or even 3mm. Some people may know this is supposed to be exactly 1/8" and some may not.
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Re: 3.17mm downcut endmill max pass?

Postby marbles » Wed Oct 10, 2018 11:18 pm

I've reduced my pass depth to 4mm so now within the 6.35mm (1/4") pass max. I'll play about with the feeds and rpm. Its a brand new self build so just getting used to it.

Only Liberia, Myanmar & USA measure in imperial all the rest of us use metric!!
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