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Thread: Newbie needs help with mini-lathe

  1. #21
    Join Date
    May 2019
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    I don't understand. What do you mean by "pull the stickout of your tool in about 1/2"?

    So, a 90 degree cut will be better for face cutting?

    Just curious, how do carbide bits compare since the geometry can't be changed based on material?

  2. #22
    Join Date
    Dec 2015
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    Chilliwack, BC, Canada
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    D4E, looking at the pictures the first one looking pretty well straight down shows what I warned about in my previous post. Notice how in that picture the cutting tool is hanging out over the bed and the carriage is to the right by some amount? On a big heavy lathe this would be fine and the force of all but the heaviest cuts would not lever up the carriage. But on these 7 inch lathes the carriage is simply not heavy enough to resist that sort of levering up. And when it starts to dig in any rise in the carriage due to flexing or simply lifting up will force an even worse dig in. And brass LOVES to grab and dig.

    What I'd suggest is to swing the compound rest (top slide) around to the usual 29 degree angle normally used for threading. That will swing the tool post around and more in line with the cross slide and bring the tool post and tool around closer to center of the carriage and cross slide. You could also screw the top slide back a little so the front edge of the slide and base for the top slide are even. That again will pull the tool in closer and more over the carriage below. The goal is that when you look down from above you want to see the tip of the cutting tool being at least within the "footprint" of the carriage. And more ideally it should be sitting within the footprint of the cross slide. That way any downward loads are supported directly instead of being cantilevered out to the side.

    The geometry of that cutting tool is not so extreme that it won't work well. It's not ideal to have the top rake but the angle on the tool in your pictures isn't extreme either.

    The tool bit could be pulled back in the holder a little more so it doesn't stick out. It looks like it's a 5/16 (8mm) or maybe 3/8 (10mm) size? So maybe pull it back another 5/16 or 3/8 to match the size of the tool. That's about all you can do anyway.

    Your overhang issues are not helped by the fact that the tool post appears to be too big for the lathe. Not so much in terms of vertical size since it is clearly doing the job there. But in the horizontal size which positions the tool so far out from the mounting point on the cross slide. So it's important to try to swing things around in your case to get the cutting tip located at least over the metal of the carriage.

    It's still not ideal though. Consider when you're hammering on something on your bench. If you're trying to drive a stubborn pin of some sort on the bench on the portion between the legs on each end the parts bounce around a lot and it's noisy as blazes and you work twice as hard to get the work done. Move the same parts so they are over a leg of the bench and suddenly the noise changes and the parts move a lot more for each blow of the hammer. That's what I'm suggesting here for your lathe. I know it's a lovely toolpost and all. But the tool post to my eyes is part of the problem and while you might fix it for this job it is likely to haunt you forever with chatter that occurs when trying to take even moderately aggressive cuts in other materials besides brass. In this case you can fix things with some minor adjustments of what you have. But the longer term solution to what I see and what I've found would be a tool post that positions the tool holders more directly over the top slide so you've got a shorter path to direct metal from the tool all the way to the bed.

  3. #23
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    Dec 2015
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    Oh, one more thing. Looking around at some of the videos for the small 7x series of lathes one of the factory shortcomings is noted as the bearings not being ideal. And in fact there are upgrade bearings for the head stock that change them over from ball to tapered roller bearings. It's possible that at least part of the chatter you're getting is due to some play in the ball bearings.

    But start with adjusting the top slide angle to swing the tool post inwards and more centered over the cross slide. From there try a totally flat top tool. From there try actually tightening the gibs of the top slide to lock the slide as a temporary test. Don't go nuts, just tighten all of them with a "firm pinch" to cause a firm bind of the gib as a test. And before you do that shift the top slide so the tool post is directly over the foot of the top slide.

    If it still chatters then I'd suggest you might have misadjusted head stock bearings. There's a test you can do with a mag base and dial indicator to test for "lift" of the chuck. And if you have any loose lift amount there's a couple of lock rings you can tweak to adjust the preload on the bearings to eliminate that play. And snug but not overly tight head stock bearings is certainly one step towards avoiding chatter.

  4. #24
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    Jan 2007
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    Minnesoa
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    Hi,

    I use "stickout" to refer to the amount of tool over hanging your tool holder. You have at least 2x's more than needed. A good rule of thumb is only have as much tool sticking out of your tool holder as you really need to do the job. Less stickout = less chatter and better finish. More stickout means more chatter and worse finish.

    Yes turning your tool to the face of your part will give you a much better chance at an improved finish. And stoning a radius on your cutting tool will help to minimize that "threaded" look of a rough finish.

    Most carbide inserts have a radius of some size molded into them. This is done to increase edge strength to make it harder to chip or break the cutting edge. It will also increase surface finish. The drawback to choosing a radius on inserts is the power often required to use them - a larger nose radius tends to need more power to use. Not an issue with large machines, but can be an issue with small lower powered benchtop machines. Brazed carbide tooling can be ordered with a variety nose radii, from sharp like HSS to radii as big as can be ground on them. Brazed carbide has lost a lot of popularity because of the ease of making inserts and convenience of use.
    If you think you understand what is going on, you haven't been paying attention.

  5. #25
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    Oct 2013
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    USA MD 21030
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    When I tried knurling on my lathe, I found that the QCTP would rotate when pressure was applied. It appeared that the top of the compound, and/or the bottom of the toolpost, were not quite flat, so I added a piece of drywall sanding screen to provide better grip.




    And, yes, my 200 series (BXA) QCTP set is larger than recommended for my 9x20 lathe, but it works OK. I got a good deal on the set at the LMS booth at Cabin Fever a few years ago - I think I paid about $150 and now the list price is $300.

  6. #26
    Join Date
    May 2019
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    I had great success! I ground a 3/8" tool blank without touching the top surface. It's not pretty, but it works. I reduced the overhang to a minimum, rotated the cross-slide and centered it to give maximum support. I increased speed a little and raised the bit to slightly above the center-line.

    The cut was smooth and has a nice finish. Thanks to all! You guys are great!






  7. #27
    Join Date
    Aug 2004
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    York, PA
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    I do not think the tool stickout is too big to cause the chatter at .005" depth of cut. But spindle bearings are under suspicion - I agree with BCRider here. The design of this little lathe is already deficient. If you divide the distance from the chuck jaws to the front of the bearing by the spindle diameter inside the bearing, you will most likely get a ratio grater than 3. If you ball spindle bearings (2 single bearings in front and back), it will not help either. Such design is very flexible, it lacks rigidity.

    But your lathe should be able to take .005" cuts. Therefore I think the bearings are loose or not preloaded properly. In a lathe spindle the rigidity is very important. You preload the bearings to increase the rigidity even so you sacrifice the bearings life by doing so.

    You can easily test my theory. Take a light cut on a piece of scrap material held just in a chuck. Use the same tool and same stickout as on your picture. If you feel chatter, try to support the part with a tailstock. If you see better results now, you have a spindle problem. Try it and let us know.

  8. #28
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    Sep 2004
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    Oregon Coast
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    WOW, you guys that contributed to this post need a pat on the back. You took your time to help the OP with his problem. I think BCRider"s post was great, actually they was all great, but I think I learned the most from BCRiders the most.
    It's refreshing to see a post like this where the OP gets the help he was asking for without ridicule of his small lathe and his admittance of his lack of the proper technique. Not one photo of corndogs or pizza in the replies. Welcome to the forum Devils4ever.
    _____________________________________________

    I would rather have tools that I never use, than not have a tool I need.

  9. #29
    Join Date
    Dec 2018
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    My guess would be to put a dial indicator on the face of the chuck, and push/pull/jerk on the chuck looking for endplay.
    I had chatter problem with my Logan, and when I indicated the face of chuck I saw movement.
    I setup the head bearings properly and all is fine now.

  10. #30
    Join Date
    Dec 2015
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    Chilliwack, BC, Canada
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    Quote Originally Posted by Ringo View Post
    My guess would be to put a dial indicator on the face of the chuck, and push/pull/jerk on the chuck looking for endplay.
    I had chatter problem with my Logan, and when I indicated the face of chuck I saw movement.
    I setup the head bearings properly and all is fine now.
    Yep, that's the trick. But using a lever and fulcrum to do the lifting and pushing so the movement is positive.

    What to look for in the dial gauge is a sudden initial step or jump of the needle. THAT is the free play in the bearings being telegraphed through the structure to the dial gauge. After that further pressure will result in a smooth motion of the needle which is the head shaft flexing like a spring from the pressure. After all when we're working down to thousandths or even fractions of a thou (or hundredths of a mm) EVERYTHING is a spring of some sort. But any play in the bearings will show as a sudden jump of the needle even if you can't feel an associated "click" in the lever or hear a click when things shift.

    Lugnut, happy to help. And thanks for the vote of confidence. I've picked up enough hints from the others here that have helped a lot in my own shop or even just made me think more about what I've done. It's nice when I can pay it back here and there.

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