Help running long stainless parts in lathe. D/L ratio = 4.5
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    Default Help running long stainless parts in lathe. D/L ratio = 4.5

    We might have to pull back on this job. Took it to help a guy out, only 100 parts, loose tolerances, and figured we could find a way to make it work. Material was provided as unknown stainless but is certainly hardened. Bar is .875 and we have 4" stick out. Running a 3 jaw chuck in a 6" 2 axis lathe. We have a series of spindle liners stacked together because we don't have a .875 liner right now.

    Machine has an obvious vibration and when manually running it, I didn't want to push it beyond 3k rpm but programmed the part to go to 4K. For quick testing, I tried everything from 500-3000, manually jogging in the material. WNMG 432 and 431 inserts.

    The machine does NOT have any issues so we can exclude it as such. Without this material in it, it is quiet as a church mouse and holds tenths. The chuck however does have some wear and does not repeat very well and the bar has some runout. I will get an indicator on it but around .010" or more. We are obviously inducing enough runout that we get some vibration.

    When I run the insert to the material, chatter is very bad. I did run a quick facing cut and it was like a mirror. Just unable to cut the OD at that much stickout. I ran the bar back to about 2" stickout and it cut just fine.

    I am trying to figure out if getting the runout and vibration down can solve this or if we need to switch tactics. We don't have a collet chuck for that machine but want one. Only way I can see this working is have less stickout, cut the front features, then feed out more bar. It is an obvious swiss part. We also have a programmable center in that machine but that will just blow us out of the water with complexity and time for 100 parts.

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    Have you tried a tailstock?
    If you can't have a center on the end turn the front long and remove the center in a 2nd opertation.

    If your part it not supported with a tailstock using a negative insert is an exercise in futility with that amount of stickout.
    Use a positive, low angle tool like a DCMT with as small a nose radius as practical and lower your DoC to start and work your way back up to the chatter.

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    It’s Tool pressure related. WNMG is debatably the worst geometry because of seating inconsistency but I won’t bore you with that. Go to a ground edge precision insert such as a CNGP. A single sided positive rake insert like CCGT431 or DCGT431 would be even better. The ground edge inserts designed for stainless have a super sharp edge which will alleviate some of the Tool pressure and shave the material off instead of scraping it off.

    Edge prep is everything.

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    Exkenna, thanks for the reply! That machine has limited tooling on it right now. We do have VCGX332 inserts loaded in a SVJB holder that we use specifically just for finishing tiny contours in Aluminum. I have never used that tool for heavier cuts or steels. The inserts are C520 grade, high positive, uncoated.

    Any thoughts on the bar runout and vibration playing a factor in this? I think we can do something to reduce it if that is at issue but seems that when we pull the bar back in the chuck a couple inches, most problems are solved.

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    I suppose to eliminate or check if its the stock running out as being the issue you could run a part from a short piece of rough stock?

    Why haven't you tried it with the tailstock?

    Brent

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    4 to 1 is acceptable without a tailstock center so 4.5 to 1 would be ok with the right geometry.

    I could send you a free tool holder and inserts to evaluate if you would like. Just pm me the shank size.. I'm guessing 3/4" or 1"

    I have a 3/4" SDJCR 12-3 holder and some ground DCGT 32.51 in a stainless grade that would probably solve the problem

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    Well, it looks like we will get this job done with the VCGX inserts. Running about 300sfm and .007ipr, with a .030" DOC. I pushed it on one part up to .013" ipr. Part looked fine but insert didn't like it much.

    What has me puzzled is I don't think anyone ever caught the fact that the holder we use for these inserts puts the insert about .030" lower than centerline! Like it leaves a .060" tit when you face off a bar!! I was looking for a quick fix. I don't think the lathe is off that much and no other tools do that. I am sure that is putting some extra heat on those inserts!

    Aside from that, we have ZERO chatter issues. I little whining but nothing visible in the part.


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