Jaw grip length on lathe work piece - 1/3rd ALWAYS necessary?
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    Default Jaw grip length on lathe work piece - 1/3rd ALWAYS necessary?

    First of all, apologies for being a bit of a parasite on this forum. I'm new to CNC lathes, but I've worked a lot on manual machines. All of your comments are always most appreciated!

    Okay, here it goes;

    If I'm going to center drill a POM-C bar(no pecking) and thereafter bring in the boring bar - then (in my head at least) there shouldn't be too much radial loads if I run things with a careful feedrate and low RPM's. The stock in question will have Ø250mm (~10") and 340mm (~13") length.

    Do you guys consistently adhere to the 1/3rd rule of thumb in terms of jaw grip length?

    Can I reduce grip length if I use common sense and keep the RPMs low combined with a gentle feed? I presume sentrifugal load is the thing to worry about here. Assume cut steel soft jaws and hydraulic chuck.

    On the smaller manual lathe with hard jaws I have used very little grip length at times, and yes, parts have flown out, so I take this matter very seriously. It's just fun until I get hurt/killed or wreck a 100k machine..

    Thanks again,
    Cheers
    John

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    Quote Originally Posted by Schjell View Post
    ......Do you guys consistently adhere to the 1/3rd rule of thumb in terms of jaw grip length?........
    Never heard that as a rule. All depends on the job and what needs to be done.

    I've done some jobs where the stock got prepped with a "dove tail" and held onto by .1" in matching softjaws with 4" outside the jaws.

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    Quote Originally Posted by Vancbiker View Post
    Never heard that as a rule. All depends on the job and what needs to be done.

    I've done some jobs where the stock got prepped with a "dove tail" and held onto by .1" in matching softjaws with 4" outside the jaws.
    Thanks! - that's the answer I was hoping for. I was watching a Haas Youtube video on cutting soft jaws - their advice appeared to be fairly strict on the 1/3rd grip length when not using the tailstock. Being a self-taught guy I'm open to most advice!

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    The material you are using (Acetal?)with a 250mm o.d. can be more than a bit prone to 'sqeezing' out of your chuck jaws. By all means hold onto the max length you can. With a 340 length the leverage effect will also help to wiggle the part out of the chuck.
    NO BIG CUTS, you are not going to break chips at any speed/feed so don't try.

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    Agree with Red, but I'll add be very cautious with any high-positive rake tooling, that could be the source of pull-out risk if you get unlucky.

    For something that size in acetal resin, I'd want a sharp edged but low or neutral rake on my tooling, and that includes drill tips if you're going to be doing significant drilling. Remove the chisel edge from bigger drills ("dubbing") . Since you've got experience on manual equipment you may already be familiar with the process, but if not:

    YouTube

    I like to be a little deeper with the flat, so will usually do this mod on a grinding wheel rather than a flat stone.

    Once you have your tooling set up in a satisfactory manner, you can start ramping up the speeds and feeds. Depending on how you cut your soft jaws, you may be better off purposely making the jaw finish rough (final passes at a fairly high feed rate) to give some "tooth" to the surface to get a better bite into the plastic.

    Especially when boring, emphasize chip evacuation. You don't want nests of chips remaining in the hole and binding/heating up against the boring bar.

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    Quote Originally Posted by Milland View Post
    Agree with Red, but I'll add be very cautious with any high-positive rake tooling, that could be the source of pull-out risk if you get unlucky.

    For something that size in acetal resin, I'd want a sharp edged but low or neutral rake on my tooling, and that includes drill tips if you're going to be doing significant drilling. Remove the chisel edge from bigger drills ("dubbing") . Since you've got experience on manual equipment you may already be familiar with the process, but if not:

    YouTube

    I like to be a little deeper with the flat, so will usually do this mod on a grinding wheel rather than a flat stone.

    Once you have your tooling set up in a satisfactory manner, you can start ramping up the speeds and feeds. Depending on how you cut your soft jaws, you may be better off purposely making the jaw finish rough (final passes at a fairly high feed rate) to give some "tooth" to the surface to get a better bite into the plastic.

    Especially when boring, emphasize chip evacuation. You don't want nests of chips remaining in the hole and binding/heating up against the boring bar.
    Thanks Milland & Red!
    I always thought sharper was better, but that video proves quite the opposite! Seems that I may have been lucky in the past since most of my cheap chinese carbide inserts were quite blunt to start off with. The accidents I have had were actually during drilling with big twist drills in the tailstock. I always figured that it seized due to thermal expansion causing a squeeze on the drill, but I see now that it may well have been the sharp cutting edges.

    I'm trying to stay on a reasonable tooling budget so I was planning to use the same tools that we will use for stainless.
    The index drill is a Kenna Ø50 DFT and the boring bar is also a beast from Kenna. All inserts should be applicable for use on plastic. They actually do seem a bit "blunt" now that you mention it.
    Will use 200 mm tall jaws and I'll try to cut them rough as you suggested - good idea.
    As for the swarf/chips I thought I'd run the internal coolant to get it flushed out efficiently.

    Thanks again - hope you continue to have a nice Easter!

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    Thank you on the Easter wishes, and to you too.

    For acetal you do want "sharp" edges, just not at a high negative angle. The sharp edge will cut with less heat (which you want with plastics) than a duller, rounded over edge, but not grab like a negative geometry (such as for aluminum) will tend to do.

    About using stainless-grade inserts - if you can budget for it, you will get better results and less heat (so less thermal size drift) with inserts specific for plastics, and not used on metals too. If the budget's not there, then at least make sure the part is back to room temperature before making any critical measurements.

    Let us know how it works out for you.

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    Quote Originally Posted by Milland View Post
    Thank you on the Easter wishes, and to you too.

    For acetal you do want "sharp" edges, just not at a high negative angle. The sharp edge will cut with less heat (which you want with plastics) than a duller, rounded over edge, but not grab like a negative geometry (such as for aluminum) will tend to do.

    About using stainless-grade inserts - if you can budget for it, you will get better results and less heat (so less thermal size drift) with inserts specific for plastics, and not used on metals too. If the budget's not there, then at least make sure the part is back to room temperature before making any critical measurements.

    Let us know how it works out for you.
    Ok, now I see. You just change the angle by grinding - it's not a case of making it blunt. Thanks for clarifying that!

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