Best approach for milling bearing pockets

[QuinnSjoblom]

I'm milling small parts out of 7075 on a 3 axis cnc with a high speed spindle (6k to 24k). Bearing pocket sizes range from 8mm to 25mm, all have a square bottom lip. Rpm range means no boring. Up to 10mm or so I use a 1/8 3 flute carbide zrn, for over 10mm I use a 1/4, same type. Here's where I have issues, I like to get a nice perfect press fit so I prefer to start a bit tight and sneak up on it. Once I'm taking very small bites for final sizing, it doesn't like to bite. Seems like I have to overshoot by quite a ways to get to proper size. If I go for dead on size right away when it has a decent step over to cut, sometimes it's perfect but there's possibility of over shooting and scrapping a part. I understand that cutters like to take an actual chip or they tend to rub instead of cut. My current method of sneaking up does work eventually, just seems to take forever since it doesn't like to cut with such a small step over and sometimes I end up with a slight taper since the tool is deflecting as it resists cutting.

What would be the best approach for handling this? Would a different type of endmill be better at taking very light cuts? I've heard hss can have a sharper edge than carbide, but less rigid so maybe no help there. I could probably be close enough 90 percent of the time if I measure tool run out after loading it and then go for dead on first try but some of these parts are very complex with multiple setups and holding positions so I prefer to slowly sneak up to make sure I don't overshoot.

Is there a better way to do this or a better tool than what I'm using? Any suggestions are much appreciated

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[ianagos]

Strange to me it seems like your machine might be deflecting? I take a shallow skim cut sometimes and it works fine.

But if you can’t do the shallow cuts just do some test parts until you hit it dead on the first time. I have a product I do that have bronze bushings presses in and I interpolate the hole then give one finish cut and it’s done light press fit no fiddleing.

[QuinnSjoblom]

Strange to me it seems like your machine might be deflecting? I take a shallow skim cut sometimes and it works fine.

But if you can’t do the shallow cuts just do some test parts until you hit it dead on the first time. I have a product I do that have bronze bushings presses in and I interpolate the hole then give one finish cut and it’s done light press fit no fiddleing.

Normally when I mill parts I rough leaving .1mm, then do a finish pass and end up with a tolerance within .01mm. It's when I do a cut leaving .02mm or so and try to sneak up .01mm at a time that I have issues. Still cuts, just resists a bit with such a small cut and cutting into a concave surface. It's mostly paranoia lol. If I just went right for it, it would be fine probably more often then 9 out of 10 times. Sometimes it's just parts that I have a ton of work into and don't want to risk a loose bearing fit. I have a friend with a vf2 and he says he runs into the same issuesometimes when taking such small wisper cuts. I think it's more of a tool deflection issue than machine deflection. One thing I could do that I'm sure would help is get some stub mills that are as short as possible. I use 3/4 inch flute length when milling these because the length is needed for the rest of the part. Usually bearing pockets are no more than 6mm deep so surely a shorter endmill would help, just more work doing tool changes

[ianagos]

I see yea those long flute lengths could really be hurting you especially with 1/8 cutters. My 1/4 cutters have .75 flute length.

[ger21]

Are you climb cutting? If not, you should be.

[ianagos]

Are you climb cutting? If not, you should be.

Good point I just assumed he was because I almost exclusively climb cut.

[QuinnSjoblom]

Yes, always climb cut

[deadlykitten]

hi quinn, what do you think about this : Vulcanaire® Grinding System

or about helix with high-rpms-coolant-heads + a grinding stone ?

in both cases you will be controling microns, if not tens of microns ( especially with the 1st equipment )

" steady &straight forward " is faster then " fast & sudden stop "

[QuinnSjoblom]

hi quinn, what do you think about this : Vulcanaire® Grinding System


*steady and straight forward is better than fast and sudden stop

I could probably already grind with 24k rpm, but didn't think aluminum was great for grinding?

[deadlykitten]

i am sorry, i am a bit tired, and i did not fully read your post, but only the title of your thread

let me read it a bit ... hmm : stabiliy for tight tolerances ? yup ...

it is the tool ... obviosuly, as you said, a small corection may lead to sudden friction and cutting edge wear

i would recomand talking your applications directly with a local tool manufacturer : maybe there is someone near you; recently i configured some endmills for tough materials : carbide type, number of flutes + angle, tool dimensions, coating, etc ... maybe such a person may help you with more specifics / kindly

[deadlykitten]

also, there is another problem with aluminium : at least arroud here, some materials comes with great internal tensions, and only if you mill it a bit, it will wrap

internal material tension is a thing, especially for soft stuff like aluminium

when you go for tight tolerances you may encounter this at a smaller scale, hard to measure, but enough to make your process unstable; just saying