I'm intending to purchase a large 2nd hand CNC milling machine in the not too distant future (bridgeport interact or similar) so I was wondering if I could get some pointers here so I dont get ripped off.

Whether it be at an auction or private seller, these things are normally sold as seen and is up to the buyer to discover any faults when viewing before parting with any money.

As far as the electronics go, I dont care about the condition of them as I intend to replace the servos and drivers with new stuff anyway. Its just the mechanical gubbins that are more of a mystery to me.


I know backlash on the axes is a big issue and perhaps my single biggest concern. So how does one go about testing for backlash in a scientific manor other than trying to move the table? Is there a way of measuring it without acutally running the machine ? Also if the machine is running, is there a better/alternative test I could do and how would be the best way to go about it ?

Is there anything else I should be overly concerned about other than backlash ? Ovbiously I can check the general condiditon of the machine just by looking, but are there any other specifics ?

Make sure it has ball screw and not acme, if you can move the axis to each extreme, check the ways for scoring, do they have a film of oil? (lube system working)? check backlash, preferably with a dial guage, power up, put the mode switch to handwheel .0001 increments if using a dial gauge or .001 if not, and move the axis a fair amount in one direction, stop and take note of the dimension on the display, then move the axis in single increments in the opposite direction and stop immediatly you see or detect the axis start to move, deduct the final move from the initial move as per the display and this will give you an idea of backlash present.
If you cannot power up and it is a smaller mill, you may be able to apply pressure on one end of the axis and then the other and see if it is noticable, make sure the motor/ballscrew is not backfeeding when you do it.
If it has vari-speed head or gearbox, check for excessive noise when running.
Al.

Thanks, thats very helpful :)

Just one thing though .... What is a dial gauge and how do they work/how should they be used?

Go here for picture of a dial gauge and how it's used.
http://www.5bears.com/cnc08.htm

Perhaps I'm just being thick, but it doesnt seem too clear from the 3bears site what one is supposed to do with it.

Also, he was using it to measure the parallelism between two rails rather than backlash.

On a manual mill you would lock down the table and put the dial indicator against the table in the direction of the test. then you would turn the hand wheel for that axis and the indicator would read the backlash....you would do the same for both axis'. With a CNC....I would do some pragmatic test...like mill a square of x dimensions and measure it....do the same thing with a circle....and a circle inscribed by a square....a 30 degree ramp and a 60 degree ramp.....

Perhaps I'm just being thick, but it doesnt seem too clear from the 3bears site what one is supposed to do with it.


Usually they have a magnetic base and a dial that is operated by a moving plunger to indicate the degree of movement so it is relatively easy to attach the base to either the fixed part of the machine or to the moving part itself and arrange it so the plunger is operated by the opposing part, the plunger registers on the dial whenever movement takes place in whatever it is calibrated in .001" or .0001" increments.A dial micrometer.
So in the case of checking backlash, the backlash is taken up in one direction then the axis is moved in the opposite direction and as soon as the needle moves one increment this is deducted from how the display amount and this is the amount of backlash.
Al.

Be sure that it comes with all supporting documentation ie: wiring diagram, mechanical parts list, etc. Not having these thing can create future headaches. Also, if you are buying it from an auction house, try to find out where the machine came from and contact the person or company to ask your questions about the machine. Thirdly, have a look around to see if spare parts are readily available for the make of the machine. If you are going to perform an electronics retrofit, steer clear of automatic tool changers unless you are a hydraulics and PLC wizz.