How bad is an eccentric load/heavily interrupted cut for lathe spindle bearings

[Wren]

I was swinging a part this weekend that received it's first op in the mill and had it's weight offset to one side and was producing a heavily interrupted cut.

The vibration coming off of this part was pretty bad, shaking through the foundation of the building.

My question is whether or not this is bad for the machine.

There are two different parts, one about 8 pounds and one about 14 pounds I was cutting on. Material is aluminum and magnesium, cut parameters are .105 DOC .02 ipr. Inserts are aluminum wnmg. RPM range is up to 1500 RPMs.

Both parts look about like this:

 

[Tonytn36]

Shouldn't be an issue unless you are doing this 24/7. The interrupted cut won't matter much, we do that 24/7, the out of balance can be an issue though. I would suggest some balance rings for the chuck if you are going to be turning off-balance parts continuously.

 

[Wren]

these should be the last ones for a while. It should only amount to about 6-8 hours of spindle time.

I didn't really expect this small of a task to be a problem, so I was happy to run them off, I was just curious mostly if this was considered bad for the machine at all.

I'd like to hear more about balance rings though. I'm still new to this and would like to learn.

 

[Joe788]

Neat looking parts. What kind of lathe is that?

Like Tony said, the lathe will handle the interrupted cut like a champ. The problem is the out of balance condition, but 8 hours a year shouldn't hurt anything.

 

[Gary E]

Looks to me as a very light duty lathe.... so be very carefull...

If on the other hand that is a big beefy machine... no problem...

Forgit balance rings... more trouble than they are worth unless you plan on spinning it very fast... Figger out how much time you "might save"...

After re reading,,, it's Alum... just be carefull and you'l be ok...

 

[Wren]

the machine is a Sharp SLT-80, about 7000 pounds.

Box Ways, 10/12.5HP, Mitsubishi control, motor, and drives, A2-6 spindle, hydraulic chuck.

I'm not sure what qualifies as light duty, but the cut parameters are in the original post.

That part is about 10.5 inches tall, it may be throwing off the perspective some.

 

[Wren]

Neat looking parts.

These are uprights for an open wheel formula car. Another member of this board and I are working on these cars.

Here is the back of the part as it comes out of the mill

here is the part in it's assembly

the next step after what was shown above is to cut two bearing bores, a front that is done with a boring bar and a back bore that is done with a notch/groove tool, it is all done in the same setup.

These are the two parts completely finished and anodized:

 

[adama]

just swing it a little slower if u can. for a few off it saves the hassel of trying to approximate a balance. 1500 rpm seams fast for a out of balance part in that size in my book. ignoring machine size. a small out of balance at a thousand rpm is a lot of force!! very nice bit though!!

 

[Wren]

I wasn't sure if it was fast or not, I was trying to keep surface speed up to get a good surface finish since the parts need to be pretty as well.


The part on the left was spun at 1500 rpm, 2500 sfm, this did vibrate the foundation, I couldn't really hear much over the iterrupted cut though.

the part on the right I maxed out at 1000 rpm, I think it is balanced much better though and I didn't really feel any vibration in the floor.

 

[adama]

yep thats the trade off u have to make. but personaly if ur using the nice polished alu inserts they will go low speed and do ok if used with colant before u get into BUE speed range.

 

[gar]

Would a milled circular pocket be adequate for the bearing bore.

See my photo P8 at http://www.beta-a2.com/misc_photos.html

.

 

[Tonytn36]

I'd like to hear more about balance rings though. I'm still new to this and would like to learn.

Basically, balance rings are 2 eccentric rings that clamp around the chuck body. You move them toward/away from each other until you get the balance right. You normally only do this for higher volume work though, as it can be a PITA to get them right.
I notice in a later post you have a model. With that it should be not too terribly difficult to get the CG and vectors for it, and create a single ring that would offset the balance.

 

[Brandon Dixon]

I'm doing the milling work on these parts, and I've got a question about the magnesium versions of these parts.

What is the best way to "store" the bare machined mag parts, say between operations. It's going to be a couple of days between each operation and it's going to be 3 operations + the drilling and tapping which on thier own will take 4 repositionings (4th axis would be sweet).

Should I coat them with WD-40? Boeshield? other ideas? The volume of the parts makes submersing them in oil not an option.

We're figuring out which of the Dow processes that we will use once they are finished, and they are probably going to have to be shipped to the finisher as well.

Thanks for your thoughts!

Brandon

 

[Joe788]

Brandon, you guys are doing some killer work on those parts! What type of machining center are you using for the milling work? What is the overall cycle time with turning and milling?

I love it when people post pictures of their work on here. I always enjoy seeing what other people are making.

 

[cnctoolcat]

I agree with lesser max rpms. 1000 is plenty. Polish 'em a bit if necessary.

 

[Wren]

does anyone have a suggestion for between process magnesium coatings?

 

[Brandon Dixon]

Brandon, you guys are doing some killer work on those parts! What type of machining center are you using for the milling work? What is the overall cycle time with turning and milling?

I love it when people post pictures of their work on here. I always enjoy seeing what other people are making.

Flattery will get you a long way with me. I just started a new thread with some video of this part in the mill. I'll try to catch some video of the part in the lathe.

Here is a link to the new thread:
http://www.practicalmachinist.com/vb/showthread.php?t=148998

Here is a direct link to the video on YouTube:
http://www.youtube.com/watch?v=hzCUWi5Lxxo

 

[SteveinAZ]

Brandon...nice work.! We use WD-40 for inprocess protection for short term and LOW volume of parts because it's easy to use and readily available. The down side is not so much removing the WD (we use Zep ID Red as it seems to leave no residue and evaporates very fast) but what to do with the WD after it washes off the parts. We usually collect it on shop towels that go the linen supply, and they remove and dispose of the oil properly - remember LOW volume.

Skip the Magnesium - that's so '60's! Go to Titanium - we want to see that SV2412 cutting those uprights out of 6Al-4V! I visitied a shop a couple of weeks ago where their SV2412 had a slab of Ti about the same size as yours on it...I bet you'd love to have a few of those slabs!

I know many of the sport racers' uprights have always been mag, but what about 7075-T6? As I recall, it does weigh about 35% more than mag, but also has 3-4x the yield and tensile of the mag. I remember the mag also dropping off pretty quickly as temps go up - you have quite a bit of heat generators - brakes, bearings & CV's. You may want to check running temps, and compare strengths at those temps.

OT - Out of curiosity, what CV's do you run and engine (hp) / trans combo? I have not been around the sports car stuff for a very long time, and am very tainted as I have been into the higher end off road racing for the past 20 years (Baja 1000 type desert racing, no sand dune stuff)...where DG300's and 930 cv's don't hold up to 200hp. The off road car we are building has an "aftermarket" DG-300 based trans with wide gears (4-spd) and a 10" R&P, 934.5 CVs with 300M cages...since we are limiting this one to <500 hp, we can use a double disc clutch...any more than that you need to run a torque converter to make everything south of the motor live - yes...converter with a crash box!

Anyway, great job on your parts!!!! Steve

 

[Wren]

Unfortunately, titanium is not allowed, or maybe that is fortunate, b/c if it was allowed, we would end up having to use it. The mag is an experiment, we got a very good price on some good stuff, so it should be interesting.

As for 7075, I agree, that would be a good thing to make some uprights out of;-)

 

[Michael Moore]

The new Moto Tech magazine has an article on the Ilmor MotoGP project. I thought it was interesting to see them say that Mario Ilien "has a longstanding antipathy towards magnesium" and mostly used aluminum (other than the steel/ti parts) in the engine.

I've never really liked magnesium as a material for engines. It's expensive and its stiffness is not so great so that you can end up making parts thicker and heavier which cancels out any weight reduction. If your aim is to make something structural and light I think it can be achieved more easily and to a better final result with aluminum. A properly designed and engineered aluminum component will be almost as light as one in magnesium.

A friend did successfully make and run a magnesium piston in one of his race bikes.

cheers,
Michael