Manufactured object is 1 mm too big - machine, Fusion or bit error?

[donjuanciu]

Hi all

I've been fighting with a precision problem for a few months now that's driving me mad.

I'm struggling a lot with precision when manufacturing small puzzle like parts. I need precision of up to 0,1mm. I keep changing between thinking that it's the machine that's the problem or the program/CAM (Fusion). But now, I start to suspect that it's actually the router bits.

I'm using this high end custom build linear guide CNC from Makersupplies: https://makersupplies.dk/da/cnc-maskiner-kits/lineaere-guide-cnc-routere/piranha-cnc/piranha/se-hele...

Precision shouldn't in theory be a problem with the machine, and in tests it's been off with up to 0,2mm. Not perfect but good enough I guess?

The bits I use are these chinese made 3,175mm straight flutes: https://www.amazon.com/gp/product/B07F3LGSS7/ref=ox_sc_rp_title_rp_1?smid=&psc=1&pf_rd_p=ba5b5f9b-5b...

My problem is that all circles and ovals seem to be just fine and more or less precise, but rectangles and squares are off in total by 1 mm. Fx. an 8x8mm rectangle in Fusion CAD/CAM is 9x9mm IRL. As I don't have this problem with other bits I've tried, I suspect that it's the bit that somehow get's "smaller" and bends in higher spindle speeds. Can this be the case, is this a normal reaction from bits in general?

Would be great to know if this is yet another factor to calculate with when aiming for high precision, as by now it feels like I've tried everything and can't find any other explanation.

Hope someone can help! Thanks a lot.

Best,
Erik

[seth.madore]

Straight flute bits produce a lot of tool pressure and it's likely a combination of the bit flexing (carbide will only flex so much before it breaks) and machine deflection. Have you tried any spiral flute bits?

Can you share some posted code so we can verify that the dimensions being passed thru are correct?

[donjuanciu]

Thanks for your reply!
Wouldn't flexing make the object smaller though? In my case they're always bigger, by almost 1 mm.
I've uploaded the gcode as txt. The object in the file should be 8mm wide and approx 88mm long with curves and some 2d shapes along the way.

[seth.madore]

 

[seth.madore]

The g-code checks out correctly:

 

[donjuanciu]

Good to know!
Given that the problem doesn't occur when working on ovals that are often made with helix ramp and therefore has less force on the tool, I can probably conclude that the issue is indeed from the bit flexing? The part itself should in most cases be quite well fixed, so I don't think it's machine deflection (unless I've understood this concept wrong).

[seth.madore]

I mean absolutely no disrespect to your machine or your decision to buy it, but; in general, desktop gantry routers are not the most rigid beasts in the world. If a machine costs below 5-7k (USD) it's likely going to have some flex in it, whether that's in a ZX motion or ZY.
If your ovals are being cut with a Ramp motion (small depth of cut, many times around) and your OD's are being cut at a single depth, then this is showing that your machine is flexing. You're not going to get a 3mm carbide bit to flex a half MM without breaking (in my experience). I suggest using multiple depths of cut or Ramp at all times.
Alternatively, you could also just use a "spring pass", where the tool makes a second trip around the part to remove any deflection that has occurred. Be warned though; deflection may not always leave "stock on", and you may find internal corners to have been cut oversize as the machine switches directions

[donjuanciu]

I see. Thanks for sharing your knowledge on this.

So the conclusion must be that the problem is partly the bits and probably mostly the machine deflecting. I will try to respect lower speeds and always do multiple small depths and ramps.
This is quite a big issue for me though, as I bought the machine with the prospect of being able to masse produce many small parts rapidly and was promised a rigid linear machine that could do the job. But, isn't 1 mm a lot of deflection, even for a lower end machine of this type?
I've just ordered other higher quality bits, including some compression end mills, to see if they react differently.
Is there anything else I can do with my machine to limit flexing?.. I regularly check all screws etc and all should be more or less fine 😕
Thanks again for your help, so much appreciated.

[seth.madore]

Yes, bit deflection is real, but machine deflection is even more real. Consider the point of flex in the axis:

It's all located around the center of the gantry. While it's possible that there is something loose (lead screws perhaps), it's possible that it's just the type of tool in use is producing too much radial load and forcing the machine to flex. For example, your gantry plates are only 6mm thick, which isn't the stoutest construction. Again, I don't want to be perceived as criticizing your purchase (I've been in your shoes more than once), I just want to properly set expectations and give you some constructive feedback on how you can make this work for you

[donjuanciu]

Thank you, so helpful with a visual explanation/example. I will go through those areas on the machine more in depth and see if anything can be optimized. I don't take it as critique at all, real expectations and proper guidance is much to be preffered.
Last question: This Piranha machine is stronger as more rigid than a Openbuilds LEAD CNC though, right? At least that's what I was told, and I couldn't find better solutions for my budget (5-7k $ max) here in northern europe. Fingers crossed I can make it work.

[seth.madore]

Your machine is most certainly more robust than the Openbuilds, based on my quick review of that machine