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Question - Boring on a lathe

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  • Glenn Wegman
    replied
    Originally posted by hwingo
    The boring bar has the following nomenclature:

    NBS_12 3/4 x 10" INTRNL TRNNG TL/HLD

    and the insert has the following nomenclature:

    TPGC-321 TIN TRN/BOR INSRT USA

    Harold
    Well, I'm confused!

    The bar nomenclature above seems to be more of a manufacturers part number or designation sot it does not tell me much.

    As for the flat on the top of the bar, it usually indicates the index angle of the bar as if it were mounted in a boring head or turning tool holder, the screw securing the bar would press against the flat, so in this case the flat would (should) be horizontal.

    As for the insert being a TPGC, the C designates that the mounting hole is countersunk on both sides. I'm not sure that there is such an animal as a TPGC since a TPG should have 11° relief angle so there is no reason why you would want to be able to install the insert upside down, so there is no need for the hole to be countersunk on both sides. The insert in the picture shows what appears to be 0° relief, so the insert in the pic looks to me more like a TNGC,(just guessing) which would make sense, as you would be able to use all six points on it since it is a negative insert. I'm guessing that the correct insert for that bar is a TPGB 321 which would be 11° relief, with a hole, and countersunk on one side. The bar would be rotated so the insert and flat on the bar would be horizontal and the cutting edge on center of the bore, and the insert would have the necessary relief so it would not drag or heel.

    May be perfectly wrong since I'm not an insert wizzard, but that's my take on it anyway!

    Glenn
    Last edited by Glenn Wegman; 05-17-2009, 05:58 PM.

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  • j king
    replied
    Paul.
    I DO realize that. I was just pointing out things that most dont think of when trying to bore to a size and have problems. Same thing will happen with a boring bar on a mill. Jim

    Ps. Alot of people are using these inserted bars but arent running them anywhere close to the required speed for the carbide to perform correctly.

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  • Paul Alciatore
    replied
    He is getting a smaller hole at the end of the bore. If the problem was the tool heating up, it would be larger.

    If the work was heating up, the hole would be smaller when it cooled and contracted.

    1.25" ID, 3.6" long, the hole is several thousanths smaller at the end, AND COOLING reduced the taper. Sounds like an expansion problem in the work piece.

    1. Tool is slightly dull producing excessive heat while cutting. DO CHECK IT.

    2. Tool is rubbing, again producing excessive heat.

    3. Tool is being used at incorrect rake angle, again producing excessive heat.

    4. Cut is too agressive. You could try a slower feed or shallower depth of cut.

    5. No or too little coolant. You could try more coolant.

    I have seen videos of the fancy CNC equipment and they seem to use a lot of flood coolant. I suspect this is mostly to control the temperature of the work piece to avoid expansion and the errors associated with it.

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  • hwingo
    replied
    You guys are assuming that I know more about this than what I actually know. Negative & Positive rakes???? Negative & Positive boring bars???

    Fellows, you are dealing with an ignorant individual desiring to learn more so I can "work smarter and not harder". So tell me about positive and negative rakes and negarive and positive boring bars.

    I needed (past tense) to enlarge a hole that was 3.60" deep to an ID of 1.70". Which bar and which insert should I have been using ..... positive or negative ..... and just as importantly, why?

    Harold

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  • j king
    replied
    The insert looks to be a negative insert. I have used positive inserts in neg bars but you cant do the opposite.

    There are many things that could cause a tapered bore. One that isnt posted is a tool that heats up as you cut.If it is dull at all or rubbing it will get hot.As it heats up it will change the size of the cutter.It will grow and take a little more stock.Not much but it will happen.

    One of the main things in machining is observing EVERYTHING that is happening.You will find in time that when you look at something all the pieces will come together and you will be able to solve almost all your problems.

    Jim

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  • hwingo
    replied
    Originally posted by Glenn Wegman
    That's looks to be the incorrect insert for that bar. I believe that is a positive bar (note position of flat on top of shank) and the insert appears to be for a negative bar as it has no relief, or the insert is for a different application, sucg as a milling cutter, requiring very little relief. Is the hole in the insert countersunk on both sides?

    What is the designation on the bar? (a bunch of letters)

    Is the insert a TNxx? Or a TPxx?
    Hi Glenn,

    I know very little about carbide inserts much less positive or negative boring bars. I needed a boring bar that was at least 10 inches in length and .750" in dia (to lessen spring) so I saw this bar in Rutland Tool's big book , which came with inserts, and I purchased it.

    The boring bar has the following nomenclature:

    NBS_12 3/4 x 10" INTRNL TRNNG TL/HLD

    and the insert has the following nomenclature:

    TPGC-321 TIN TRN/BOR INSRT USA


    The insert has a hole only on one side. The hole's minor ID is .120" and uses a 4-40 screw. Years ago I used a totally different style boring bar with HSS cutters that could be easily shaped and sharpened. Now they have a two billion different cutting tools (inserts) for a billion different applications that seem to fit only one or two holders. I'm learning quickly that I had better be certain for "what I wish".

    Now, help me out here. If it looks like the wrong tool and insert for the job, what is the appropriate bar and insert that I should be using? Any information is helpful.

    Also, you mentioned the "flat on the top of the bar". Why is that important and what is the significance? Your replies will be helpful in more ways than you can imagine.

    Harold
    Last edited by hwingo; 05-17-2009, 01:14 PM.

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  • knedvecki
    replied
    Hwingo,
    Might I suggest, if letting the part cool down before making additonal passes helps, How about using a cold air gun or better yet, water soluable coolant on the tool / part?

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  • Glenn Wegman
    replied
    That's looks to be the incorrect insert for that bar. I believe that is a positive bar (note position of flat on top of shank) and the insert appears to be for a negative bar as it has no relief, or the insert is for a different application, sucg as a milling cutter, requiring very little relief. Is the hole in the insert countersunk on both sides?

    What is the designation on the bar? (a bunch of letters)

    Is the insert a TNxx? Or a TPxx?
    Last edited by Glenn Wegman; 05-17-2009, 11:00 AM.

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  • hwingo
    replied
    Hey Guys,

    You've given me much to think about. Attached is an image of my current position of the insert to the work. This position seems to have worked the best. It has provided the smoothest cut, with less chatter, and the best finish.

    I tried to raise the cutter above center but chatter became unbearable. I even tried to "grind the heel" to prevent dragging but there was insufficient support and the tip easily broke. Strangely, temperature had a great bearing on how well the boring bar cut. After allowing the work and insert to cool for at least an hour, and without changing cross feed, I reentered the bore and more material was easily removed whereas prior to cooling, no new material was removed with additional passes. So for some strange reason allowing the work to cool reduced the amount of taper I was experiencing.

    Harold

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  • .RC.
    replied
    Originally posted by j king
    I have found that a tool will wear after several feet of boring. : )




    If you used a solid carbide bar you probably would not need the stiffener bar on top

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  • Quetico Bob
    replied
    Forgot to mention, if you do use a fresh insert at .010 or there about,make sure you reset your depth of cut. If you don't you will more than likley end up over size.

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  • 4GSR
    replied
    hwingo,

    If you have a older model lathe, there is very good chance you have excessive wear in the carriage to the bed. I've seen this to add to the problem of back cutting when reversing out of the bore.

    With the right cutting tool, I get my best results by cutting on the reverse out of the bore movement. This also works very good on ID threading, start at the back of the bore and come out.

    Ken

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  • 4GSR
    replied
    Originally posted by j king
    I have found that a tool will wear after several feet of boring. : )





    Really you get tool deflection and loose your size quickly.The farther you bore, it builds up.This is a small amount but it does accumulate.Thus the need for a spring or preferably a light finish pass.
    John,

    You need a bigger boring bar, that one dosen't look like it is big enough for the job!!!

    I ran a boring operation many years ago using a 6" OD bar sticking out of the chuck about three feet. I was line boring compoents to a trepanning machine. I would have a heck of a time with that 6" bar vibrating trying to make a 8" ID cut. Boring bar was around 8 foot long.

    Ken
    Last edited by 4GSR; 05-16-2009, 03:39 PM.

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  • Quetico Bob
    replied
    Quote
    “While I agree and have experienced the same effect that a bored hole tends to get smaller toward the farthest end, the question is still why?

    The boring bar support never changes, so the amount od deflection should be the same at the start as in the end. So where/why does it get smaller?”

    You are right about deflection, unless the tool is repositioned part way through cut more like cutting tip wear and possible work hardening especially if you are not using flood coolant.

    All parameters must be maintained (or as close as possible) from start of cut to end including temperature if you want an accurate bore. Personally I do not think it is humanly possible depending on how far you want to measure it.

    I use flood coolant and a fresh insert (if critical) when I’m within .010 and have reasonable success.
    Cheers, Bob

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  • Paul Alciatore
    replied
    Originally posted by rws
    While I agree and have experienced the same effect that a bored hole tends to get smaller toward the farthest end, the question is still why?

    The boring bar support never changes, so the amount od deflection should be the same at the start as in the end. So where/why does it get smaller?

    .....
    Another possibility here is headstock alignment. If the headstock is not aligned to the ways, you will get a taper. But a second pass will not correct it.

    The most likely cause here is something is rubbing as the cut proceeds. If cutting a harder material, wear on the insert could do it, but a quick exam with a magnifier should tell you if this is the case.

    Note: A good magnifier is an indispensable shop tool. I have two high quality Hastings Triplets in my pockets at all times for at least the past 30 years: 11X in the right and 20 X in the left. And a lesser quality 2X as well.

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