Tolerance, how good is good?

[Aerobuilt]

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

[Geof]

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.

[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,

[lakeside]

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.

[tobyaxis]

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.

[Geof]

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.

[lakeside]

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

[Geof]

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.

[tobyaxis]

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

[Geof]

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.

[tobyaxis]

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 . 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?

[Geof]

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.