stiffness measurements cnc mk3

[Clive S]

I just put some figures into this, FSWizard - Free Advanced CNC Speed and Feed Calculator, and it suggests 600mm/min is too slow, though I've not tried it myself.

How did you put the spindle speed in? or have I missed it. ..Clive

[EddyCurrent]

It seems like you have to change the percentage figure in the 'SFM:' drop down and this calculates an appropriate spindle speed. The maximum spindle speed is set by entering a figure in the 'Max RPM' box.
I just selected SFM percentages until it gave me a decent spindle speed.

[Clive S]

It seems like you have to change the percentage figure in the 'SFM:' drop down and this calculates an appropriate spindle speed. The maximum spindle speed is set by entering a figure in the 'Max RPM' box.
I just selected SFM percentages until it gave me a decent spindle speed.

Eddy. How can you comment on the feed rate unless you know what Routercnc spindle speed was? in the first place as I didn't see it in his post. Just wondering. ..Clive

[EddyCurrent]

Well for a feed of about 1000 mm/min the suggested RPM was about 5000 so I guessed he would be using something faster.
So for 10000 RPM it works out at about 2000 mm/min

[Clive S]

Well for a feed of about 1000 mm/min the suggested RPM was about 5000 so I guessed he would be using something faster.
So for 10000 RPM it works out at about 2000 mm/min

Ah I see you worked it backwards, then yes that computes assuming he used a calculator in the first place ..Clive

[routercnc]

The rpm I use is around 12000 (200Hz). If I put the following into that calculator:

6061 T6 aluminum
solid end mill, carbide
6mm 2 flute
DOC1 mm
WOC 6mm
slot/pocket = yes
SFM, FEED 100%
max rpm 12000

I get a feedrate of 900mm/min. I'll keep experimenting with the next parts I cut out to see if I can get a better finish.

[Jonathan]

I get a feedrate of 900mm/min. I'll keep experimenting with the next parts I cut out to see if I can get a better finish.

600mm/min @ 12000rpm generally works well with a single flute cutter, so since one would expect the chipload to be slightly lower for a 2 flute cutter, as a first approximation the feedrate should be a little less than 1200mm/min.

What are you mounting the workpiece on? It can make a big difference to the finish - for example I found it easier to get a good finish when taking a small cut with my machine when it had the MDF bed. Presumably the MDF's damping properties helped. If the parts are small you'll obviously have problems with rigidity using MDF, but if they're a reasonable size and you add a finishing pass (take off 0.1-0.2mm from edge in one pass) you can get good results, e.g:


(20mm thick parts and 25mm thick bed)

I've still not measured, but I'm pretty sure your machine is stronger than my 'old' one which cut the parts above (and below), so unless you have big problems with resonance you should be able to get similar results. Perhaps MDF is worth a shot? That's not to say you can't get a good finished clamping rigidly to aluminium, it just takes a bit more care/luck setting the feedrates, e.g:

[routercnc]

Hi Jonathan,

If you look at the last photo in post 19 you can see that I generally use an 18mm piece of chipboard if I need to cut all the way through. The part I cut out in the last photo does actually have a better finish than the parts in the first 2 photos - possibly because it is cut out of a much larger sheet.

If you look at the bed you can see gaps between the profile sections used to make up to bed. The machine is used primarily for cutting wood and in that mode it has a large sacrificial wooden sheet which sits on the bed. This works well for wood machining.
If I end up doing a lot more aluminium I can see that I may need to either add a lot more profile sections to give less gaps, or invest in a large thick aluminium sheet to bolt to the bed, skim it, drill and tap lots of holes.

I'll keep experimenting with F&S as I machine out more upgrade parts, plus try some with a finishing pass.