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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 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.

  5. #5
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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

  6. #6
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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.

  7. #7
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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  

  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
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    Quote Originally Posted by tobyaxis
    .....BTW: Lets also add two more zeros to the price instead of one. Now that's real Precision
    It is also not really a starter machine uinless you have a very rich uncle. Not to mention the climate controlled siesmically isolated facility to house it.

  11. #11
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    Quote Originally Posted by Geof
    It is also not really a starter machine uinless you have a very rich uncle. Not to mention the climate controlled siesmically isolated facility to house it.

    Yup!! That is a pricey situation (chair) . Do you have the programable coolant lines on any of your VMC's? Shopping for a Good Used CNC Machine has to be one of the most difficult tasks for anyone. There are so many things to consider and check it's not funny. Can we agree on that? :cheers:
    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

  12. #12
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    Quote Originally Posted by tobyaxis
    .... Do you have the programable coolant lines on any of your VMC's?.....
    Yes on three but many times we just use the Locline nozzles snaking around to come from all directions.

  13. #13
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    Jan 2006
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    Well, most of this sounds interesting(some over my head). My only worry is that I were to buy a machine, and then produce an item that will be laughed at when it comes to tolerances. Idealy I would like to purchase a "spare" machine from a reputable CNC shop. Possibly the chances of keeping a good repore with them in the hopes of getting some "pick up" work when things get too busy for them(if that ever happens). I don't know if this would be a valid expectation or not. But it sure sounds wonderful on paper! I've heard of how tough the industry is. I'm really just looking to help pay for my hobbies, if I can help out in the form of overflow work, or just some simple stuff for "cheap", it would be for the better. I've got great designer job and enjoy it much. Learning CNC with UG along with FEA and such is my true goal.

    Thanks for all the great ideas and thoughts!

  14. #14
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    Mar 2005
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    This post has changed my mind about looking for a used mill. The prices seem right on eBay but not knowing the condition would be a small shop killer. 98% or bust nono I mean 100% or bust. I think just about everyone on these forums thinks about getting that mill to pay for their hobby I know I have as well.

    There was a 4000 brass pieces job posted to RFQ that has me interested in a new machine. I just for fun made a quote to see how close I would be to the ones that got it. I was $2000 above the guy that got the job and thought I would barely be making ends meet. Another problem is a months work doesn't pay for a new mill, tooling, inspection, cleaning and finishing.

    My location being kinda central in the country (Nebraska) kinda sucks as well the shipping and handling charges for materials would just stack up. A bigger shop might be able to offset the costs by placing large orders but a single machine is not it.

  15. #15
    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

  16. #16
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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.

  17. #17
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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

  18. #18
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    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.

  19. #19
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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,

  20. #20
    Join Date
    Jul 2004
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    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

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