Holding Tenths On Cnc?

[aceofspades25]

I was wondering if it is possible to hold tolerances within tenths on CNC machines without grinding? Say to keep it between .5455 and .5463

[HuFlungDung]

Its a matter of economics, so yes and no.

The higher the precision of the machine, the more capable it is of holding the tolerance.

If the part is very simple, high dimensional accuracy is not too expensive because the unit cost of the reject units is low. It usually takes a few units to get the tooling set right, and the
program adjusted, etc.

If the material is easy on the tools, then you can expect quite a sustained period of accurate production. That is, if you can predict the performance of your machine spindle as it warms up during the run.

If the material is tougher on the tools, then you've usually got to endure a cycle life on the tools, and it sometimes takes quite a number of units to determine what the typical tool life is. This will include frequent gaging to observe the evolution of the tool wear. Whenever a new tool is substituted for a worn one, the cutting action can be markedly different while the new tool "breaks in", then settles into its "typical performance" for the rest of its wear life.

A good quality machine is very repeatable so far as its own positioning is concerned, but how tools behave is like running a new experiment for every program, and every tool.

[psychomill]

Also, it comes down to if you're trying to hold tenths in position or feature size (ie: a bore/hole). Some "beat up" machines can't hold "a thou" to save its life in position, but it can bore a hole to tenths. This is also vice-versa.

[WayneHill]

I was wondering if it is possible to hold tolerances within tenths on CNC machines without grinding? Say to keep it between .5455 and .5463

0.5455 to 0.5463 ?
Wow. I wish we had that much tolerance on our medical parts We are holding +-.0002 on some jobs. +-.0001 on others. Using a Hardinge Conquest SP with a coolant chiller in an air conditioned shop.

No grinding required.

[Chris D]

That's an incomplete question that really can't be answered. Are you talking about a lathe (much better chance) or a mill (less of a chance).

How big of a part? What material? Is the shop climate controlled?

Etc.

I was wondering if it is possible to hold tolerances within tenths on CNC machines without grinding? Say to keep it between .5455 and .5463

[aceofspades25]

Lets go with a lathe,small part(5 inch),stainless,shop not climate control. Only room climate controlled is the QC room and the lathe is not in there of course.

[DieGuy]

That will be a strugle, Steels generally grow about 5 millonth per in per degree, so a 5 inch part X 20 degrees is going to 5X5X20 millionths or .000500

machine @ 90F and inspect @ 70 F you have a problem

[HuFlungDung]

Add to that, stainless steels have a thermal expansion near 12 millionths of an inch per degree F, and you've really got a serious issue with temperature.
(I wonder if he meant 1/2 inch? )

[BillPSu]

The real answer is it depends on the quality and condition of the machine tool. A manual Hardinge toolroom lathe can hold those tolerances and a Hardinge CNC can also do it. You may have to use some temperature compensation for the size measurement. If the machine is in poor condition or it is some run of the mill CNC lathe you will probably have problems. You will also need the correct tooling and setup to hold these tolerances.

[JFettig]

I was turning castings at one time at work when it had to be held within .0009" and it wasnt so fun I was having to change the offset every part, I had to predict how much to change it to be right on.

It is possible, just not always fun This was on a romi-bridgeport ez-path lathe.

Jon

[mxtras]

Why, yes - it is possible.

Probable? Not under your mentioned conditions.

In addition to the material itself being effected, the machine changes, too. Depending on how the position of each axis is encoded, your dimensional errors could be as much as doubled. With the addition of temperature, the screws get longer, the table gets bigger, the frame gets bigger, the spindle gets longer, etc, etc.

If the machinist is measuring a part that is 85 degrees with a mic that is at 92, and it is QC'd at 72 - there goes your tolerance.

Climate control the room the machine is in, set it on isolation mounts or a isolated foundation, calibrate it periodically and keep it well maintained and your chances to hold tight tolerances improve dramatically. Establish temperature records and quidelines. Eliminate the variables.

Plus you might have happier operators/machinists and that alone can yield substantial gains. Never under estimate moral.

Scott

[meckardt55]

It can be done if you are willing to spend some money. You need to look at the OEM specks of the machine you are using and the condition of the machine. You would need to air condition the shop and get the machine laser calibrated. You need to keep the coolant at +- 1 degree and need a machine with scales. We laser calibrate machines with scales and when all of the above and more are looked at you can hold 0.0002"

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CNC Machine Services, Inc.