Milling Strategy Question

[PDC]

Can someone explain why the underside of these crowns gets milled differently? I outsource to Argen using their settings. Sometimes it duplicates the surface of the die and sometimes it looks like circular patterns as shown on the other die. I don't change anything. Everything fits well, but why the differences?

 

[zero_zero]

Those crowns are milled the fastest way possible hence you see the scalloping from the increased stepover distance (shorter tool-path)...they prolly overmill the inside a bit to assure fit. That pattern is visible on both crowns...

 

[JohnWilson]

I got a gold crown they milled yesterday and it was amazing marginally, seemed loose on the die but it was a short prep. I did mill the same file in PMMA and it was obvious they adjust the CAM to compensate as the PMMA fit different.

Still was very impressed the external step was tight with minimal tiger stripes and the secondary anatomy was very sharp.

 

[PDC]

Thanks guys. I don't really understand the milling lingo but it sounds like everything is being done properly. It just seems like sometimes there is a little bit of overkill with the milling of those circles as it often gets into the sides of the crowns pretty good.

 

[RCKSTR]

The crown is roughed with a 2 mm bur, inside and out, then reduced further with a 1 mm bur, finally a .6 mm and or .3 mm. The internal should get a second pass with a 1 mm and .6 mm to smooth it out and have a better fit, maybe in an effort to increase productivity, they skip that step. What you see is the bur radius compensation.

They either do this purposefully to reduce mill time, or are milling on the wrong setting (not finishing the underside).

It certainly isn't normal, and as a paying customer Id be calling and complaining about it. Cement gap is one thing, but that top crown is a whole new ball game.

 

[CoolHandLuke]

sloppy

 

[rkm rdt]

I've never had that happen. Could it be your drill compensation settings ?

 

[PDC]

I sent photos to Argen...will see what they have to say.

 

[Tayebdental]

Argen is good, but on many occasions I design a titanium custom abutment / split file "to do the crown at my lab " and make sure the margin on abutment is sub gingival, what I get back is the margin at tissue level. Now I have to go back and prep margin again. So the split file is not advantageous in these cases.

 

[Fogember]

Been using Argen for the last three years and have never had that.

 

[RCKSTR]

It could be your drill bit compensation settings, but you said you're using argens DME, so they would have set that

 

[PDC]

Been using Argen for the last three years and have never had that.

Well it seems like it just kind of worked its way into my workflow. It's interesting to note that I can look under the initial crown proposals and see the circles as soon as the case is brought in to CAD. Maybe Argen changed something in the parameters. They did tweek things a while back on the margin settings saying they were getting chipping.

 

[CoolHandLuke]

Well it seems like it just kind of worked its way into my workflow. It's interesting to note that I can look under the initial crown proposals and see the circles as soon as the case is brought in to CAD. Maybe Argen changed something in the parameters. They did tweek things a while back on the margin settings saying they were getting chipping.

still sloppy. they have added (as everyone does) milling bur size compensation so that the machine that cute the unit can correctly calculate the toolpath for the tools it uses. in this case it is compensating for sharp prep corners by super-inflating that corner to match the size of the cutter.

you should see it being used on every unit, albeit less pronounced as the first photo.

that much compensation is just sloppy manufacturing. leaves a gigantic amount of room for cement and with so few areas of tight contact with the die, means the unit will very likely come loose.

yes the unit can be manufactured, no making it in zirconia wont serve the patient's best needs. doesnt matter how strong or nice looking it is, this unit will have so little retention it wont stay glued.

it would be best to refine the milling strategy to utilize smaller finishing burs on the intaglio surface. but that introduces too much time for this machine to spend on your unit. time is money. argen can't compete on cheap if they invest too much time. so quick and dirty, thats the name of the game.

 

[PDC]

So here are the parameters. If you could change something to avoid the "over-milled look", what or even is there something that could be done?

 

[CoolHandLuke]

this isnt something you should be trying to adjust, given you arent matching these parameters to your own CAM and Mill.

when you do get a mill, we'll talk. but its on Argen to match parameters to their mill.

 

[PDC]

this isnt something you should be trying to adjust, given you arent matching these parameters to your own CAM and Mill.

when you do get a mill, we'll talk. but its on Argen to match parameters to their mill.

Understood. I just thought you might see something that might raise a red flag. I am waiting on a reply from Argen as to how this situation can be remedied...or not. So it sounds like they are not allowing for smaller burs to mill out the corners in order to save time with tool changing.
It just seems that this is something they have recently incorporated into their workflow. I don't remember it being this way before.

I agree with you on the cementation problems that could crop up with this. It's already a very technique sensitive process to get these things to stay in.

 

[CoolHandLuke]

thinking logically here, these units are designed in real dimensions. designed 1:1 scale. when milled, due to disc shrinkage during sintering, they are expanded by the same shrinkage factor as the puck, a la 1.25:1

so the bur size value can be decreased 25% (0.6mm becomes 0.45mm),but that assumes they will have a 1mm bur doing the internal finishing.

also, you'll want to leave some overcompensation for the bur diameter to accomodate for two things: the amount of bur deflection (as you put force on it, it bends) and space for zirconia dust to extract. otherwise you will break burs. usually 10% of the cutter diameter is appropriate.

so its fairly fanagle-y and probably safer to leave it as is.

 

[PDC]

Finally got the issue fixed. The "apply tool compensation" box had been checked for some reason. After "unchecking" that and recomputing the case, everything was fine. This stuff just blows my mind sometimes.

 

[CoolHandLuke]

aha. so they are overmilling on purpose. very interesting.