Grizzly mini-mill & resonance?

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spro
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Re: Grizzly mini-mill & resonance?

Post by spro » Wed Nov 15, 2017 5:05 am

Wes. I liked that advise about face milling. I found by trial and error that was the way. Dianne, are you using a collet chuck? If so, compare the "stick out" of the end mill compared to holding it by collet inside the spindle. Just a little difference there reduces the rigidity .

pete
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Re: Grizzly mini-mill & resonance?

Post by pete » Thu Nov 16, 2017 8:13 pm

Column rigidity on these mini mills is a massive problem due to the built in design flaws Dianne, I've owned one. Tool extension as Spro mentioned also has a large effect. And as you described taking that last narrow cut can cause vibrations as the material is ripped off. After more time than I'd care to add up learning a few hard lessons then if I'm only trying to get a flat face on the part I'll usualy use a flycutter instead of an endmill. A high porportion of videos on youtube show people taking overly large depths of cut with flycutters. There designed and meant as a light finishing tool. You can hear the tool hammering even with a Bridgeport sized machine if the depth of cut is too much. That beats the spindle splines to death if too much of it is done and it sure isn't good for the spindle bearings either. Yours if it's an X2 sized mini mill doesn't have a movable spindle so there's no splines. But a single tooth cutter in a mill gives an interrupted cut. Idealy you need 2 or more cutting teeth working in the cut at all times to lessen the shock loads. For those reasons I limit a flycutter to .005" -.010" depth per pass in steel as a maximum even with my BP clone. But a flycutter is easily resharpened and dirt cheap compared to any endmill. Yes an endmill will remove material faster because of more cutting teeth, but I'll save my endmills for when there really needed. My time is still cheaper than good endmills are.

Anything you can do to improve that column rigidity will help. Lots of forum posts around about bolting 1/2" or thicker steel plate to the rear of the column but that helps mostly in the X axis so my opinion is that bolting an I beam to the rear would do far more. The single bolt fixing the column in place and allowing the column to tilt was poorly thought out. Redesigning that area so the column is fixed and rigidly bolted to the machines base casting would do even more. Even though the machine has enough power to take a cut you simply have to reduce the material your trying to remove because the machine just isn't rigid enough to do so. Reducing that last cut so there's only .005" - .010" in width to remove on the last pass should help with the vibration issues you mentioned. Depending on your spindle taper then MT 3 or R8 collets as Spro said will work better than the usual ER type collets do that most use with these machines. With the column so flexable it's also easy to get previously cut chips pulled under an endmill so there recut. That causes vibrations and seriously shortens endmill life so keeping those chips constantly vacummed or blown away would also help.

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DianneB
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Re: Grizzly mini-mill & resonance?

Post by DianneB » Thu Nov 16, 2017 8:23 pm

Thanks for the thoughts Pete. Maybe I will look for a heave piece of I beam or square tube to stiffen the column.

pete
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Re: Grizzly mini-mill & resonance?

Post by pete » Fri Nov 17, 2017 2:23 am

There's some very detailed information about using an epoxy and granite mix on various machines and up to building complete machine parts by casting it into forms with it over on the CNCZone Dianne. Lot's of posts about it with mixture ratios by weight so it's correctly done. I think it would take some time searching through everything you can find on that site. But possibly filling the hollow vertical column with it would help a lot also. Mass in any machine helps dampen vibrations and it's just one of the reasons ultra heavy machines work as accurately as they do.I've seen at least one post in the past where a welded A frame structure was fixed to the top of the column and both arms running back and bolted to a concrete wall behind the mill. All depends on your exact shop setup for the best methods to help stiffen up the machine I guess.

During WW II the government put out drawings for home shop people who were building parts in there shops for the war effort. They detailed how adding a thick concrete bench top and bolting the then almost universal South Bend lathes to that made a massive difference to the machines accuracy. The expoxy granite mix used now by a lot of hobbyists is also used because of it's vibration dampening much like cast iron does. But I'm convinced that anything you can do to help stiffen up that rear column will show a large difference in the machines performance. One thing I forgot to add in my previous post is you might double check the bearing preload on your machine. It's not impossible the machines preload wasn't adjusted properly at the factory or got missed before shipment. Maybe unlikely but it still might be worth checking.

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