Help: machining quality

[Clot]

Hi Folks, I'm looking for some advice to improve the quality of my parts. Today I machined this, its aluminium, 80mm diameter, with 0.5mm deep pockets. It was modeled up in SolidWorks, run through Mastercam to generate the g-code, then run on the mill using Mach3. I have an X2 with CNC Fusion screws and brackets and a Kelling 3 axis driver and motor kit. I've had the machine for a few months and am getting more confident with it but I'm a noob when it comes to the details of the software and I'm still learning the tricks of machining.

This part was cut with a 2mm diameter end mill, feed rate of 50mm/min, cutter speed around 4000rpm. I did it with a roughing cut 0.35mm deep and a finishing pass at the final depth (overkill???). Each pocket was finished before the next was started.

Problem 1: The red boxes show the typical problem I'm seeing. The edges of the pockets are wobbly and in some cases it is almost like the cutter is not following the tool path correctly. The U shows a great example of the problem. The two inner "vertical" edges of the U should be parallel with no jogs. The two L's should also be identical but the left one also has a jog in the edge. Other areas I've highlighted are just plain rough.

Problem 2: Where the tool path is drawn as a curve or elipse in SolidWorks, it is coming out quite rough and faceted. Any thoughts on software settings to improve this?

Problem 3: The two 4.5mm screw holes came out almost as squares with rounded corners, even though the tool path and g-code shows a circle. When I ran a drill through them a significant amount of material was removed.

I checked for backlash today on both the X and Y axes. I used my vise and DTI and moved back and forth 100mm from the vise face. Backlash seems pretty good although the repeatable accuracy seemed to decrease as the feed speed increased. I had the machine rapiding at 2000mm/min but over about 1000 it looked to have a potentially noticable effect. However, there was no rapiding during the cutting of each pocket, only when moving from one to the next.

Sorry for the long story and big pic but I'm hoping someone can help to shine some light on these problems and confirm if it is software, hardware or both.

[hoss2006]

My first impression is it's a software problem.
If it was backlash or losing steps rapiding between letters,
I don't think the letters would be as inline with each other as they obviously still are.
You could try running your program through a simulator and see if the finished results are the same.
I like using the demo version of Flashcut CNC. They have version 3.0.7 available now.
Setup the Tool Table Config with your endmill size and you can watch it run the code.
Would be easy to see if the U and L are machined correctly.
If so, then you can look at mechanical trouble.

[ZipSnipe]

Looks like you need to use a smaller diameter cutter, that would explain how it bumps out on the edges like that. I would also use the engrave module of Mastercam. I would also check out your tool params in Mastercam. Maybe slow the feedrate for the tool itself(Mastercam has built in tool feeds for each tool) . Also I can't remember exactly how but in Mastercam looks like you need to adjust maybe the way the tool enter and exits the lettering. Does it make the same letters in the preview in Mastercam? If that don't work then I agree with Hoss.

[LongRat]

Agreed with some of the comments above. I'm not sure a smaller tool is going to help much though. Some of the features you have there really look like backlash I have to say. Big flats on axis reversals etc. Possibly try measuring the backlash while putting some load on the table with your hand? There might be some float in the bearings or mounts that don't show up under no-load conditions. Also check your head is not loose on the gibs and moving side to side.

[HuFlungDung]

It could also be due to poor geometry in the model, such as a lack of trim where surfaces meet, hidden overlap, etc. Do you have a method of extracting the edges of the surface geometry so that you can observe the geometric constraints that the toolpath will follow? It is usually easier to see the modelling errors in the form of poorly trimmed 2d entities.

[Clot]

Thanks for the thoughts guys. I don't think there is any issue with the surfaces as they were sketched and extruded in SolidWorks, not imported (assuming that the translations between programs are working correctly).

Will try simulating the program to see if it runs OK (that should help identify if curves are being poorly approximated).

Will also check for backlash under load. The X and Y gibs are nice and snug. Haven't checked the Z for a while...

The pockets are all at least 2.1mm wide at the narrowest point so I would expect that a 2mm cutter is fine for the geometry. I set the feed rate myself and the machine ran fine with no visible stress/overload.

I'll post back as the investigations continue...

[jfseaman]

How long is your cutting tool. Are you getting tool flex of some sort?

(forgive me if that's stupid)

[HuFlungDung]

Originally Posted by Clot Thanks for the thoughts guys. I don't think there is any issue with the surfaces as they were sketched and extruded in SolidWorks, not imported (assuming that the translations between programs are working correctly).

It might not be Solidworks' fault. If the text was translated from one of many windows fonts, there are lots of those with built in errors and poor trim.

[Clot]

Cutter is 2mm long for about 10mm, then tapers out to a 6mm shank. Should be alright with aluminium I would have thought.

Good though about the font (seen that many times when modeling at work) but its lines, arcs and splines, not font text. I couldn't be bothered looking for a font so just traced over a graphic inserted in a sketch.

[jalessi]

Do you have the Sherline Mode "Sherline 1/2 Pulse Mode" enabled in the
"Port Setup and Axis Selection" located in "Ports and Pins" in Mach3?

This is important when using the Keling 4030 drives.

Jeff...

[Khalid]

Also check the
1- The tool runout
2- Backlash
3- Increase your Feed rate a little (if it is actualy 50mm/min)and use any mist/coolent. If your feed rate is 50mm/sec then its too high.
4- Check the Steps/unit setting for all axis in Mach3.
5- Reduce the spindle speed to 2500~3000
6- Check for any loose stepping , this may be due to using low voltage /perage powersupply.

[Khalid]

Another reason may be, the holding arrangement of your workpiece. Check the holding/clamping is tight enough against the cutting forces..