Best way to machine features facing opposite directions on 5 axis

[ckguru]

Hello,

I am designing a block type part which must have features accurately placed on opposite faces, say +Z and -Z. Use 180° rotation feature on 5-axis CNC machine, no problem you say.

The CNC programmer and operator of the machine says he will not be able to achieve the accuracy I require this way though. (I need true positions to better than 0.025mm) He wants me to redesign my part to allow machining from only one direction, not two.

Discretely, I found the spec's for one of their machines on the internet and "positioning accuracy" is stated as 0.008mm. I've measured some test pieces for them before and found they had problems in maintaining alignment between turret positions; they've often had the manufacturer on site for service.

Does the claim he is making add up? Can we conclude a significant collision in the past with this machine?

 

[BillPSU]

Positioning accuracy is not the only accuracy you have to deal with when machining on 2 ends of a part. Centerline accuracy of the X-axis to the centerline of the index/rotary table. Squareness and parallelism of the index/rotary axis to the X and Y axis. Backlash in the slide assemblies. Squareness of all axis to one another.

Here is an example, say the index table centerline is out of parallel with the Y-axis .0001" in a 12". You are machining this part 12" above the table top. At the table top the X-axis is in perfect alignment. You machine one feature one the first side and index 180. You machine the second feature and the two features even if perfectly positioned and indexed and zero backlash will be off .0002 to each other. Your postioning error will also be doubled when you index 180 degrees or .0006.

Unless you have some special things done to the machine tool and you take very special precautions a true position of .025 is very difficult to machine in a double ended horizontal machining operation.

 

[ckguru]

Bill,

Thanks for the information. I have a better grasp of the obstacles now. I have resolved myself to just simplify the design to allow machining all from one direction to get what I'm after. I'll have to drill through the part to locate a hole for a pin to be made up separately and installed in the hole from underside. Not too big a deal.

I could push for a one-piece design as my original design called for a boss with two flat sides; that would offer more positive location to the mating part. I thought of asking them to leave these two locating faces high to allow us to measure where the boss was at and then correct the faces individually. But even that wasn't greeted very well.

 

[knobhead]

Hmm, you could use a spindle probe to check your position before machining the second side. If you're feeling ambitious, you could even create a feedback loop from the probe to automatically correct itself.

A decent 5 axis machine shouldn't really struggle to do this, it is what they're for after all! I would contact the manufacturer of the machine, tell them what you're trying to do, and ask their advice. (Which is what you're programmer should be doing!)


Excessive accuaracy is a sign of poor breeding. -Socrates.

 

[Cogs]

When I was running a 5 axis system we devised a regular test and measurement cycle to determine the positional accuracy of all the axis. This allowed me to see if (and when) re-allignment was necessary. There were occasional knocks which did have an effect on accuracy. If there is one axis in particular that is vulnerable to error this will help identify it.

Machine tool specs are often given to 0.00xx mm but as wear and tear plus different tooling configurations all play their part, not to mention tool wear and component material variation, it can be practicaly impossible to achieve the degree of accuracy stated for some applications.

What is realy usefull is to have a process capability index for the type of feature/tooling/material combination that you need to design for and use that to guide you in what method will give you a satisfactory and consistent result, ie; machine through from 1 face vs two face machining 180' opposed.

I have done much work with processes that were statistically "out of control" and it takes an operator that is on the ball a lot of effort to keep things from ending up scrap. Processes that are "in control" are much less prone to problems and will still produce good results with operators that are less able.

Colin

 

[aamoroso]

This doesn't seem like it should be too difficult to accomplish. I would run tests on the machine on a scrap part and see what it is physically capapble of holding by testing and insptections. If the machine cannot hold the tolerance then you have a dilema, if it can your programmer has a dilema.

My experience with multi axis machining has shown me that the new automated rotary axis are limited in quality, compared to a manual indexing setup where the accuracy could be adjusted in minutes and seconds with the high end equipment.