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  1. #1
    Join Date
    Jan 2006
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    Tolerance, how good is good?

    Hi all,

    My question is this-
    What is considered good tolerance? I'm starting a part time shop and am looking for a used CNC machining center to start out with. I have been a designer for quite some time and have been learning programming on the side. I have found a few CNC centers such as the Fada;'s and Haas, but can't decide as to what is "normal" for a used machine. How tight does a machine need to be, and what would be considered "bad"? +-.005? +-.002? In my field our design tol. are +-.05 MM in some instances. I am aware how tight this is, but is this resonable for a machine...say 1994? I doubt I'll be working anywhere near this tight, but if the job comes up.......

    Any help would be apreciated. Thanks

  2. #2
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    Quote Originally Posted by Aerobuilt
    .... In my field our design tol. are +-.05 MM in some instances. I am aware how tight this is, but is this resonable for a machine...say 1994? I doubt I'll be working anywhere near this tight, but if the job comes up.......Any help would be apreciated. Thanks
    +/- 0.05mm is not tight. This is +/-0.00197" which is easily attainable on a good manual machine. My oldest CNC is a '95 lathe and it can hold better than +/-0.0005" without any effort. No matter what age machine you get you need to aim for better than 0.0005.

  3. #3
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    Humm....I guess since I design in the "perfect world" of solid modeling, it's hard for me to actually "see" what is tight and what is not. Thanks for the info. Is there any particular test part that can be run on the machine as a deminstration to check the tolerances of a machine? Such as milling holes or anything?
    Thanks,

  4. #4
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    Quote Originally Posted by Aerobuilt
    Humm....I guess since I design in the "perfect world" of solid modeling, it's hard for me to actually "see" what is tight and what is not. Thanks for the info. Is there any particular test part that can be run on the machine as a deminstration to check the tolerances of a machine? Such as milling holes or anything?
    Thanks,
    Interpolate holes and then test them for concentricity with a dial indicator in the spindle;if there is backlash in the screws the hole will not be round.

    Install a good toolholder in the spindle and test for runout with an indicator; if you know the holder is good then any runout is probably in the spindle taper.

    You will probably get other suggestions.

  5. #5
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    Mar 2006
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    Quote Originally Posted by Aerobuilt
    Is there any particular test part that can be run on the machine as a deminstration to check the tolerances of a machine? Such as milling holes or anything?
    Thanks,
    mill a 3" circle ( The larger the circle the better idea you will have on concentricity) in a peice of alum. at the four corners of table travel and in center of table. After mill check for concentricity and true postion with and indicator before removing from machine.Check the part location from indicated machine location after mill to programmed location. The Blake co-axle indicator it is the best for this
    Attached Thumbnails Attached Thumbnails blakeco-axindica.jpg  

  6. #6
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    Mar 2006
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    Quote Originally Posted by Geof
    +/- 0.05mm is not tight. This is +/-0.00197" which is easily attainable on a good manual machine. My oldest CNC is a '95 lathe and it can hold better than +/-0.0005" without any effort. No matter what age machine you get you need to aim for better than 0.0005.
    Well Geof I kind of agree with you and somewhat disagree.
    A tight tolarance like is can be an issue on older machine when you have a true postional call out in an Orthogonal X Y Z datum plane. But as for holding a size ( even on concentricity) this should not be an issue as long as machine was not whaled on. If you can't hold the # you mostly need a service call.The machine may be out of calibration.Due to backlash adjustment and way wear.

  7. #7
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    Quote Originally Posted by lakeside
    Well Geof I kind of agree with you and somewhat disagree.
    A tight tolarance like is can be an issue on older machine when you have a true postional call out in an Orthogonal X Y Z datum plane. But as for holding a size ( even on concentricity) this should not be an issue as long as machine was not whaled on. If you can't hold the # you mostly need a service call.The machine may be out of calibration.

    A machine that needs to hold True Positioning (Least Material Condition) is going to have a hard time in any shop. Even with a new machine. On the other hand Maximum Meterial Condition is a bit more forgiving. I'll have to agree with what Mike points out. Sorry Geof. .0005 isn't good enough for a used machine unless your making AGMA 11 Gear Blanks. The spindle on a Lathe usually runs-out about .00004-.00006 from the factory.

    I was always told by a friend that has been a Tech for 32 years that you should look for a Repeatability within .00004-.00006 in all three axies. One may want to breakout the one millionths indicator for this one.

    Also do not forget to check the Spindle Taper. Some of these older machines have had inexperienced individuals loading and unloading tools. I've seen guys drop CAT50 Tool Holders on the floor and just slap them in the machine without checking for dents, chips, or cement grit from the fall.
    Toby D.
    "Imagination and Memory are but one thing, but for divers considerations have divers names"
    Schwarzwald

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    www.refractotech.com

  8. #8
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    Quote Originally Posted by tobyaxis
    I was always told by a friend that has been a Tech for 32 years that you should look for a Repeatability within .00005-.00008 in all three axies.
    Looks like we can have fun disagreeing with each other. I have a bunch of Haas and none of them have specs better than repeatability +/-0.0001" and accuracy +/-0.0002". Certainly you can get machines with the extra zero but then you also add a zero to the price.

    I think possibly I phrased it badly; " it can hold better than +/-0.0005" means it can keep diameters and lengths within this tolerance. If we watch it for temperature creep we can keep diameters within +/-0.0002" which means tha X axis is holding +/-0.0001" repeatability; in other words it is still holding to its out-of-the-factory specs.

  9. #9
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    Quote Originally Posted by Geof
    Looks like we can have fun disagreeing with each other. I have a bunch of Haas and none of them have specs better than repeatability +/-0.0001" and accuracy +/-0.0002". Certainly you can get machines with the extra zero but then you also add a zero to the price.

    I think possibly I phrased it badly; " it can hold better than +/-0.0005" means it can keep diameters and lengths within this tolerance. If we watch it for temperature creep we can keep diameters within +/-0.0002" which means tha X axis is holding +/-0.0001" repeatability; in other words it is still holding to its out-of-the-factory specs.

    Specs, what are those? Just Kidding The washer comparison is a bit on the rough side too. Lets change that to AGMA 11 Gear Blanks

    BTW: Lets also add two more zeros to the price instead of one. Now that's real Precision
    Toby D.
    "Imagination and Memory are but one thing, but for divers considerations have divers names"
    Schwarzwald

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    www.refractotech.com

  10. #10
    unless its a bankruptcy sale a used machine can and will be more trouble than it s worth , why would a company want to sell their machine ? lots of places use them till they are near dead , the older machine is always going to be the one doing to sh#t work , to save the newer ones , till they get older

  11. #11
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    Jan 2006
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    Great, thanks for all the info. The machine I'm looking at is from a cnc shop that is local and looking to use the revenue from the machine for a down payment on a high speed machine. It's still in use so I can check it in action. I'll most likely bring in a cnc tech to check it out also. Even if it costs a little money, it will save me in the long run. From what I understand they use it for the more simple stuff and utilize the faster machines for the bigger stuff. I guess that will be is good as gets when it comes to buying a used machine.

    Thanks for everthing,

  12. #12
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    Jun 2005
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    You might be able to buy a used machine. If you can only hold .001" though, do not expect to be doing a lot of the more interesting (lucrative) jobs. It might be a good machine for production jobs, which often do not require such tight tolerances.
    We have an older machine at work, which from the time it showed signs of fatigue, has seen only production jobs. It has run about 90,000 of similar parts, but only has to hold .005".
    It will be kept until it can not run anymore, still cranking out those parts. Probably not worth selling it.

  13. #13
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    Quote Originally Posted by phantomcow2
    You might be able to buy a used machine. If you can only hold .001" though, do not expect to be doing a lot of the more interesting (lucrative) jobs. It might be a good machine for production jobs, which often do not require such tight tolerances.
    We have an older machine at work, which from the time it showed signs of fatigue, has seen only production jobs. It has run about 90,000 of similar parts, but only has to hold .005".
    It will be kept until it can not run anymore, still cranking out those parts. Probably not worth selling it.

    With part quantities like those a New Machine on the shop floor would pay for itself in a year or two.
    Toby D.
    "Imagination and Memory are but one thing, but for divers considerations have divers names"
    Schwarzwald

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    www.refractotech.com

  14. #14
    Join Date
    Jun 2005
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    866
    Quote Originally Posted by tobyaxis
    With part quantities like those a New Machine on the shop floor would pay for itself in a year or two.
    You are correct. THis machine is 7 or 8 years old, and has payed for itself many times over. We keep this machine for only one customer, Thompson Center arms.

  15. #15
    Join Date
    Jul 2004
    Posts
    374
    Some facilities purchase a machine to run a specific job, then expect to sell off the machine when the contract is complete. I think this would be the ideal situation to buy a CNC machine, but I would be very cautious about machine hours and operating conditions. It's really good that you can see it in use with the original owner.


    I always cringe when someone asks me "can you hold 5 tenths?" My answer is: "on what feature?" This may be easy on some features and impossible on other features.

    Tolerance wise, there is SOOO much more [than just the machine] that will establish what kind of tolerances you can hold. Positioning the machine to a location is one thing, but cutting is another story. Tooling, fixturing, measurement, machining operations, process control are variables as well. A couple examples:

    -Milling a feature within 0.0005 that I can easily measure and adjust with cutter comp...fine
    -Reaming a hole to +/-0.0005"...okay
    -Holding tenths on a two foot dimension? I can't measure tenths over two feet, so I won't even try
    -Holding tenths between features on different operations? wow
    -Maintaining tenths on a simple feature on a production run...hmmmm

  16. #16
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    Oct 2005
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    672
    Most VMCs made over the past 15 years will hold a few tenths positioning and repeatability when set up properly. However, in the real world, jobs with those requirements are rare so few machines are maintained to hold it. For example, whenever the machine gets relocated in the shop, it's not commonly re-leveled and trammed back in to being square. That isn't negligence, simply cost effectiveness. Just as fpworks mentioned, although the machine can be set up to hold tenths over two feet, how many shops can inspect such parts?

    My experience (limited for sure) is that the machine will be very consistent and predictable. So even if a dimension is off, it will be consistently off and can therefore be compensated. It's the way the part is fixtured and the human who's loading the parts that is one of the biggest variables. Also, some machines and parts are more sensitive to temperature change, especially as the part gets bigger.

  17. #17
    Join Date
    Mar 2006
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    19
    For a couple of hundred dollars you can have a good service guy inspect a used machine. Have him/her run a ball bar on it.

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