Problems with spring back when machining square edges

Hi All,

First post here just after some advice, I’m relatively new to CNC machining.
We have a new job at work, where we are required to machine two edges at 200mm and 300mm as square as possible. When machining the parts we have managed to active a squareness of less than 0.005mm on the machine but as soon as the part is unclamped the part is springing to around 0.1mm at the most. Does anyone know anyway to get around this. The current strategy is a 0.5mm step over climb milling.

Any help much appreciated, can provide more info if needed.

Thanks in advanced for the help.

Details would help us help you.

What is the material? What does the 200x300mm section look like? One large boss/island? Thin walled feature? How deep is this feature? What is the current cutting data (Tool, SFM/RPM, Feed Per Revolution?). What machine are you doing this on?

Change how you hold the part.

or

Clamp tight to rough, then unclamp and reclamp lightly to finish

or

Send it to someone else to do.

Thanks for the reply’s,

Some more information below;
Material is S275 Hot Rolled mild steel 5mm thick but this can be easily changed if needed there is currently no spec’d material (we are working with the customer to try and prove the process at the mo and price isn’t much of an issue either, could be be a more expensive material if needed)
The profile is basically a 50mm wide L-shape,
Unsure on the F/S for the roughing pass but it’s an 8mm YG XSPEED ripper running on recommended speeds and feed, something like 9000rpm and 1600mm/min, with around a 1.75mm step-over. Cutter can take up to 50% according to cutting data. Then to finish it’s a YG K2 8mm running at around 3000rpm and 600mm/min again all recommended f/s taking the final 0.25mm off, DOC for both is 5mm.

The machine is a 2018 XYZ 500LR 3 axis mill w/ Siemens 828d on

The clamping is something I’m playing with at the moment, didn’t get much time to play with it today but hoping that’s the solution should be the easiest.

Thanks for all your help,
Much appreciated

If either of the cutters is dull, you may be building up a compressive stress along the cut, which could then "spring" as the restraining clamps on the machine are removed.

For what you're doing, you may find a rough - semi-finish - finish arrangement works better, with semi-finish being a good, sharp cutter, and finish being the best you have (and watched for dulling) with a re-clamp done before its cut. That way spring in the material should be mitigated. At worst, the finish cut is done twice, first pass leaving 0.15mm, re-clamp, final finish pass.

Yes, it's a pain, but if it is cutting stress causing the issue that should take care of it.

Another option is to rough all the parts, send them out for a controlled thermal stress relief, then refixture and finish cut.

You pay with time, or you pay with extra processing - the joys of high precision...

I second the clamping. Like dandrummerman21 said. Clamp tight and rough. Leave .25mm and clamp real light. Toss an indicator on the part when you're clamping for the finish pass and watch the needle. Stop tightening when you're at an acceptable amount of distortion.

Thanks again for the help all.

Looks like the overall opinion is a clamping issue. Will have a look see what I can do, may end up taking the suggestion of 3 passes but full unclamp before the final finishing pass.

Thanks again.