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  1. #21
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    Quote Originally Posted by turnworks View Post
    I see this often and I don't think Im understanding it or my definition is different. I know a tenth is .0001" so holding that means +/-.00005" correct?

    Ok so given that that means you can hold that tolerance all day long once the machine is up and running? Don't have to adjust after the first hour running?

    How many parts in with measuring and tweaking the program or wear comp before you can claim that you hold tenths? 1? 5? 10?

    What if its a new job and new tooling you have to probe each tool? Hold tenths on the first part? I know you can program the part big then measure and readjust the program so is that considered holding tenths?

    Very dumb and basic I know but am curious as to when you can use "my machine can hold tenths"?
    .
    .
    depends on part shape, material, size, temperature, clamping or chucking pressure, machine not oscillating, tool wear, etc
    .
    you obviously can bore a hole with a taper from temperature changes, part material hardness varies in spots, tool wear and many a part distorts or warps when unchucked or unclamped. (was round or flat but not after unclamping)
    .
    often I record measurements and if a particular part warps or curls when unchucked or unclamped between .0010" to .0020" per 20", then I can used history to predict future parts will probably do the same. many a part can be programmed to cut a straight edge and machine can move in a straight line but cause of hardness variations it might not be straight in spots to .0001", even with a .0010" finish cut cause tool deflection can vary
    .
    just saying history can show a normal variation in parts that requires variations in compensation

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    On a related note (kind of) had a teacher in one of my apprenticeship classes insist you could wedm a dowel hole +.0003" from nominal and it would be a slip fit.

    First, *normal dowels are typically +.0001"/+.0003" so there is your .0003" oversize already (mostly) used up. Then factor in roundness, straightness, alignment between different parts, etc. I always ran my slip fit dowels +.0005"/+.001" over nominal, but mostly running the program (or reamer I guess) and then checking the fit with a few different dowels to make sure I wasn't "gaging" with a wonky pin with a burr or whatnot. Also keeping in mind the function. I worked at a stamping place where some of the dies were serviced in the press meaning you wanted that just right fit of not to loose, but not needing a BFH to disassemble components.



    * I know you can buy under and over size dowels, not sure if you can get super precision or anything like that held to .0001" tolerance or less..?

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    Quote Originally Posted by Mike1974 View Post
    On a related note (kind of) had a teacher in one of my apprenticeship classes insist you could wedm a dowel hole +.0003" from nominal and it would be a slip fit.

    First, *normal dowels are typically +.0001"/+.0003" so there is your .0003" oversize already (mostly) used up. Then factor in roundness, straightness, alignment between different parts, etc. I always ran my slip fit dowels +.0005"/+.001" over nominal, but mostly running the program (or reamer I guess) and then checking the fit with a few different dowels to make sure I wasn't "gaging" with a wonky pin with a burr or whatnot. Also keeping in mind the function. I worked at a stamping place where some of the dies were serviced in the press meaning you wanted that just right fit of not to loose, but not needing a BFH to disassemble components.



    * I know you can buy under and over size dowels, not sure if you can get super precision or anything like that held to .0001" tolerance or less..?
    My pet peeve is when I ask someone what tolerance I have for their part.

    "Well, it has to be right on."

    "No, I mean how far off can it be?"

    "Well, it's a CNC machine, right? It should be perfect."


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    Quote Originally Posted by DMF_TomB View Post
    .
    .
    depends on part shape, material, size, temperature, clamping or chucking pressure, machine not oscillating, tool wear, etc
    .
    you obviously can bore a hole with a taper from temperature changes, part material hardness varies in spots, tool wear and many a part distorts or warps when unchucked or unclamped. (was round or flat but not after unclamping)
    .
    often I record measurements and if a particular part warps or curls when unchucked or unclamped between .0010" to .0020" per 20", then I can used history to predict future parts will probably do the same. many a part can be programmed to cut a straight edge and machine can move in a straight line but cause of hardness variations it might not be straight in spots to .0001", even with a .0010" finish cut cause tool deflection can vary
    .
    just saying history can show a normal variation in parts that requires variations in compensation
    Well it is NICE when yer workin' new, "virgin" metal, too, ain't it?

    "Hard spots?"

    Bearing fit in a 100 HP DC motor end bell was busting my chops one second shift. Corn-cob powdered Iron DC stick-weld cutback.

    Workin' well but this ONE "bump" as kept pushing the tool and bar. Finally had to mount a big-ass air die grinder to the four-way and wage war. We had no proper TP grinders.

    Drowsy welder had hit it a lick with what he had up ..before realizing the Manganese hard-face rod was meant to go onto an ENTIRELY unrelated location on that mining machine! Then simply buried it. BFD, no one will ever know, right?

    Stranger s**t happened when a bargain-hunting Company buyer laid-in a forging of Mangalloy figuring it was one serious-useful alloy steel. Then the previous shift foreman issued it up, having misread the unfamiliar end colours and figuring one bar was as good as any other for an ignorant axle.

    "Salvage job" fell to me. That made for another interesting shift!

    "George? You got anything in the office on the colour-code for Mangalloy or Hadfield Alloy?"

    "Why?"

    "Well.. lookin at them "wedding bands" on this, here, one, we got us a serious work-hardener!"



    Some days?

    I figured the MAIN reason we even HAD a Union was so the rank and file could call each other "Brother" .. instead of taking turnabout testing the bending moment of large wrenches on each other's brain pans over DUMB shit.

    Otherwise?

    "Hold a tenth" might mean you had half-choked nine "Brothers" already?


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    Quote Originally Posted by Mike1974 View Post
    On a related note (kind of) had a teacher in one of my apprenticeship classes insist you could wedm a dowel hole +.0003" from nominal and it would be a slip fit.

    First, *normal dowels are typically +.0001"/+.0003" so there is your .0003" oversize already (mostly) used up. Then factor in roundness, straightness, alignment between different parts, etc. I always ran my slip fit dowels +.0005"/+.001" over nominal, but mostly running the program (or reamer I guess) and then checking the fit with a few different dowels to make sure I wasn't "gaging" with a wonky pin with a burr or whatnot. Also keeping in mind the function. I worked at a stamping place where some of the dies were serviced in the press meaning you wanted that just right fit of not to loose, but not needing a BFH to disassemble components.



    * I know you can buy under and over size dowels, not sure if you can get super precision or anything like that held to .0001" tolerance or less..?
    .
    most reamers have some runout wobble and ream oversize on hole entry (0.02 to 0.25" depth) bellmouth depends on part material softness and straightness of pilot hole. soft aluminum or cast iron needs more depth or reamer in hole more to steady runout
    .
    I have seen many a hole pin gage starts but wont go in all the way especially if pin gage alot longer than the reamer. if pin gage 3.0" long and reamer has 1.5" long flutes, reamer often wont perfectly straighten a wavy or curved pilot hole. gage starts but gets stuck before its 3" in hole. basically can get a precision ball bearing to go in and out of hole easy but not a solid rigid long pin gage
    .
    I have seen 10 parts and gage goes in ok on some but not all, even gage go in ok on holes done on later parts, not like reamer was dull and small, cause later parts reamed ok
    .
    part hardness variation and varying tool deflection can be annoying.

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    '' Help me understand what holding tenths means ''

    IMHO practically speaking 99% of the time - bollox.

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    I’ve always thought that having a machine that “holds tenths” does not have anything to do with the actual tolerance of the part at all. Say you have a simple easy peasy OD lathe turning job 3.000 +\- .010, qty 50, bar puller, parts catcher. All 50 mic between 3.0045 and 3.005. That lathe “holds tenths.”

    “Hitting tenths” is a different story altogether.

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    Quote Originally Posted by upthebikes View Post
    I’ve always thought that having a machine that “holds tenths” does not have anything to do with the actual tolerance of the part at all. Say you have a simple easy peasy OD lathe turning job 3.000 +\- .010, qty 50, bar puller, parts catcher. All 50 mic between 3.0045 and 3.005. That lathe “holds tenths.”

    “Hitting tenths” is a different story altogether.
    I like that.

    Repeating within a range @ tenths for "hold"

    vs "hitting" a specfic value sought TO tenths... (easily, on the first-go..etc)

    Regardless.. I'm stayng with "what do you NEED to do" .. to get paid....enjoy a feeling of accomplishment...permit yerself a beverage ration...move on to the next without a care.... cry "bollocks" and set loose the dogs of smoke and mirrors.. wotever.


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    Quote Originally Posted by thermite View Post

    Regardless.. I'm stayng with "what do you NEED to do" .. to get paid....enjoy a feeling of accomplishment...permit yerself a beverage ration...move on to the next without a care.... cry "bollocks" and set loose the dogs of smoke and mirrors.. wotever.


    Good lord Bill. If the fucking print says to hold it to 5 Nanometers and some ass-hat in the office won the job. We're still stuck with making it to print. We can't reasonably argue fit and function to a job that we entered a bid on, and won.

    R

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    Quote Originally Posted by ChipSplitter View Post
    My pet peeve is when I ask someone what tolerance I have for their part.

    "Well, it has to be right on."

    "No, I mean how far off can it be?"

    "Well, it's a CNC machine, right? It should be perfect."

    "How close does it need to be?"

    "Make it right on."

    "I mean what kind of tolerance?"

    "Dead nuts".

    "Within .001" ok?"

    "Oh, nothing like that. 1/32" is fine."

    I forget exactly when I had that conversation...

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    Funny how much difference the machine makes. On a new Fadal I would chase close to .003" in Y and a little more in Z during the day, a 30"x16". My old Kitamura, a 20"x14", is dangerous. It holds position better than my aluminum pallets during the day, and I can't read a difference in part widths with my tenths mic. The dangerous part is I sometimes forget to even measure a part during the day since they never change, I sure don't miss the Fadal.

    As for how to get there you make a part, measure it, comp where needed, make another, measure, and maybe comp one more time. With a good machine with everything else good that should just about be it as long as your tools don't wear.

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    Quote Originally Posted by DavidScott View Post
    As for how to get there you make a part, measure it, comp where needed, make another, measure, and maybe comp one more time. With a good machine with everything else good that should just about be it as long as your tools don't wear.
    Well.. "tough job, but SOMEBODY has to do it."

    Make a second part? The same as the one you already made?

    Boooorrrrring .... !!!

    Making tooling, dies, jigs, and fixtures .. ONCE EACH ...so someone ELSE could do the same damned thing more than once in a row was a Helluva lot of fun, though!

    So was rebuild work, even when nasty. Hardly EVER more than the one item the same in a shift, if even a whole week.

    "Horses for courses" We need 'em all. Thankfully.


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    Quote Originally Posted by mhajicek View Post
    "How close does it need to be?"

    "Make it right on."

    "I mean what kind of tolerance?"

    "Dead nuts".

    "Within .001" ok?"

    "Oh, nothing like that. 1/32" is fine."

    I forget exactly when I had that conversation...
    Forgot? It's daily....

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    Quote Originally Posted by Finegrain View Post
    Half a tenth, that's a whole 'nuther ballgame. A lot of standard CNC machines aren't even programmable to partial tenths. Measuring gets very sporting sub-tenth. Temperature becomes a factor even on small parts if you are working to sub-tenths.

    Regards.

    Mike
    Hello Mike,
    Phil, AKA Machtool, described it fairly well. If, when wiping your backside, you break though and get it on your finger, after washing your hands very thoroughly, what you smell is 0.0001/2.

    Regards,

    Bill


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