PM25 CNC Conversion, cause of chatter?

patrick31 · Sep 22, 2018

I have a Precision Mathews PM25-MV milling machine I converted to CNC. It's been working for a few months now. I'm having a problem with chatter when machining on larger cuts.

Video of said chatter, it's the worst when it gets to left side of this hole in the video: PM25 Chatter for Dave Clements - YouTube

In this cut I was running a 1/2" 4 flute end mill from Kodiak, 2500 RPM, 6in/min feed, .3" DOC, 0.044" WOC with flood coolant. Machining 1018 Steel 1 inch thick. It chatters a lot worse on one side vs the other when milling this hole.

Any idea what is going on with this? Things to check?

Vancbiker · Sep 22, 2018

You should take this to the CNC Zone forum. This forum is not for hobby level machines.

patrick31 · Sep 23, 2018

Thanks. I didn't realize that when I posted this. Any other forums appropriate for my machine?

machtool · Sep 23, 2018

G'day Patrick..

Out of all the Forums, CNC Zone will be the pick of them for what you're doing.

Regards Phil.

metlcutr55 · Sep 23, 2018

another good place for you to read would be the Artsoft forum

with a "flexible" machine like this, I think id reverse your cutting strategy, ie, multiple passes, 2500 rpm x .04 doc x 30 ipm and use at least 1/2 of the mill dia, maybe all of it for woc. if that cuts and sounds good push a little harder until it doesn't, then back off a little.

patrick31 · Sep 23, 2018

metlcutr55 said:
another good place for you to read would be the Artsoft forum

with a "flexible" machine like this, I think id reverse your cutting strategy, ie, multiple passes, 2500 rpm x .04 doc x 30 ipm and use at least 1/2 of the mill dia, maybe all of it for woc. if that cuts and sounds good push a little harder until it doesn't, then back off a little.

Thanks, I will give that a try. If you don't mind, could you explain why reversing the DOC/WOC would improve chatter? Just trying to understand things for myself, not doubting your advice. I will be cutting metal in a couple hours and try this.

I already regret buying a small hobby machine. Won't do that again.

metlcutr55 · Sep 23, 2018

lots of little tiny chips vs less but thicker wide ones, puts less pressure on your machine frame and spindle. im mostly a knee mill cnc guy, no genius cutting strategist, just know from years in the shop that you attack the same problem differently if youre running a 2 hp bridgeport type machine or a 6" spindle boring mill with 40 hp. theres a lot of damnation for the hobby type mills here, but look at the situation. my 7 x 12 hobby size lathe weighs maybe 80 lbs and 1/3 hp direct drive. my 17 x 40 nardini cnc lathe weighs around 2 tons with 8 hp which aint much but theres also a 2 speed motor and a 9 speed gearbox. nardini in mild steel .18 doc x .015/rev rough feed. the hobby machine .025 doc x .005 feed. haven't done the math but pound for pound they may be comparable. think of the hammer/nail parable they speak of. if all you have a small hammer, learn to swing it effectively. your cnc converted PM25 is in my opinion 10x more valuable than a manual PM25. learn to run it effectively as possible. stash some of the cash you make. then buy a 2nd hand Haas or similar much more capable. stash more cash and repeat. maybe in 10 yr youll have a couple nice 5 axis rigs and some support equipment. and some smart young folk to run them for you while you golf.

SteveEx30 · Sep 23, 2018

.300" is a crazy depth of cut for that machine.

patrick31 · Sep 23, 2018

metlcutr55 said:
lots of little tiny chips vs less but thicker wide ones, puts less pressure on your machine frame and spindle. im mostly a knee mill cnc guy, no genius cutting strategist, just know from years in the shop that you attack the same problem differently if youre running a 2 hp bridgeport type machine or a 6" spindle boring mill with 40 hp. theres a lot of damnation for the hobby type mills here, but look at the situation. my 7 x 12 hobby size lathe weighs maybe 80 lbs and 1/3 hp direct drive. my 17 x 40 nardini cnc lathe weighs around 2 tons with 8 hp which aint much but theres also a 2 speed motor and a 9 speed gearbox. nardini in mild steel .18 doc x .015/rev rough feed. the hobby machine .025 doc x .005 feed. haven't done the math but pound for pound they may be comparable. think of the hammer/nail parable they speak of. if all you have a small hammer, learn to swing it effectively. your cnc converted PM25 is in my opinion 10x more valuable than a manual PM25. learn to run it effectively as possible. stash some of the cash you make. then buy a 2nd hand Haas or similar much more capable. stash more cash and repeat. maybe in 10 yr youll have a couple nice 5 axis rigs and some support equipment. and some smart young folk to run them for you while you golf.

Thanks. So more RPM would help, cause then I could do lots of tiny chips and keep the heavy load off the frame. I am going to take your advice, I will save up and get a better machine as soon as I can. For now, I'm going to up the spindle RPMs and use this machine at whatever it can do until I can upgrade to a larger more rigid machine.

SteveEx30 said:
.300" is a crazy depth of cut for that machine.

Thanks, I'm beginning to see that. Looks like I'll just have to make light cuts with this machine from the lack of rigidity. It has a 1HP spindle with a 2500 RPM max, but I can't use all the HP at that RPM without causing severe chatter.

metlcutr55 · Sep 24, 2018

another parable. Dick Burleson, the fastest man on a dirt bike in the woods 40 years ago was asked "how do you go so fast in the woods?" his reply was I turn up the gas a little. then a little more. then a little more, until I crash. then I just back off a little bit. same thing applies here, start conservative, bump things up a little, if still good, repeat, and repeat. when it sounds bad back off little bit. you may get more out of the PM25 than you think. if so, great. if not, you move on to something bigger, faster or better when you can.

SteveEx30 · Sep 25, 2018

metlcutr55 said:
another parable. Dick Burleson, the fastest man on a dirt bike in the woods 40 years ago was asked "how do you go so fast in the woods?" his reply was I turn up the gas a little. then a little more. then a little more, until I crash. then I just back off a little bit. same thing applies here, start conservative, bump things up a little, if still good, repeat, and repeat. when it sounds bad back off little bit. you may get more out of the PM25 than you think. if so, great. if not, you move on to something bigger, faster or better when you can.

Yep what he said.

garyhlucas · Sep 25, 2018

On bigger machines I say that if you aren't breaking tools you have no idea how far you are from optimizing your speed. An operator was milling slots on a Fadal and went out for knee surgery. I had to finish the job and watching him work I felt it could go a lot faster. He had worn out two 1/4" end mills, and the job had a long way to go. I played with feeds and speeds and quickly broke 3 more. The fourth one finished the job, literally in half the time! Lots of slots, saved a whole day.

npolanosky · Sep 25, 2018

I think you're on the money as far as RPMS go, but WAY under feeding it for that width of cut. You're probably rubbing pretty bad. Try around 15-20IPM on the low end and see if it improves, or increase your width of cut.

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