Adjusting Fit / Re-Finish Internal Preps

Fit Issues?

If you are experiencing issue related to fit, there are a few things that we recommend checking:

Tool Life

Is your tool damaged / worn?

• Has the tool recently been dropped on the floor?
  • We recommend regularly checking your tools under a microscope to verify that the cutting edge is still sharp and not dull, damaged or chipped. This will also ensure that you are getting the maximum life out of your tools.

CAD Design

Did you use the appropriate drill compensation in the CAD when designing? This should always be set to accommodate (with tolerance +- 0.2mm) the smallest tool that will be used to finish inside of the prep / interface area. These are typically 1mm or in some cases 0.6mm tools, although every machine may be different. 

If you're working with a Titanium-Base / Hybrid Abutment, you can measure the radii in a 3D .STL Viewer to ensure that you're using a library designed with your tooling in mind - keep in mind that you may need smaller tooling to properly mill some geometry.

Here we can find a 2D cross section of the intaglio surfaces on a Crown near the cusps. This file has been designed with enough space for the 1mm tool to easily finish all surfaces designed:

Nesting

Did you properly nest the restoration in the CAM?

• MillBox's cutting strategies require specific curves and surfaces to be defined for every part. This can vary from one restoration type to another.
  • Failure to check these curves / surfaces can lead to undesirable milled results.
    • SEE Understanding Curves / Surfaces & Restoration Types to make sure you are following the proper nesting steps.

Material Settings / Cutting Parameters

If you are milling a new / unique material or a material that you are not accustomed to milling, we recommend checking the cutting characteristics on the machine. For milling, this typically means you're looking for potential material failing to separate / evacuate as the tool passes over the surface or material melting  and adhering to the tool. These are clear symptoms of incorrect / invalid cutting parameters.

• Each manufacturer provides unique materials, Even if the material fits into the a common material category like PMMA or Zirconia, the market product may have completely different machining characteristics. It is important to always be mindful of these changes as it will affect how your pre-programmed milling strategy will translate into physical cutting results on the machined part.
• Bad parameters / settings can leave behind residual material or promote tool / material deflection. 
  • These issues can manifest in the form of an ill-fitting restoration or a part with poor surface finish.

Re-Finishing Internal Preps

Considering the information above, a 2nd pass with the finishing tools internal to the prepared surface may facilitate a better fit on your restoration. This can only be used if the disc has not been removed from the machine or if you're using an indexable jig that can be re-inserted in the exact same spot. If you are milling Ti-Bases out of PMMA, it's recommended to always check the fit with a Base and Lab Analog while the part and disc are still mounted in the machine, before moving / removing it. Even some indexable jigs have small tolerances that could malign the part resulting in worse fit / inaccurate machining the 2nd time around.

Milling the internal prep area for a 2nd pass will typically clear out any remaining material left from tool / material deflection and may facilitate a better fit. If you notice you are having to do this routinely - you should investigate with your dealer about making a 2nd pass permanent in your milling strategy or by making adjustments to your cutting parameters (Speeds / Feeds / Step-Down) to provide more . Here is a video showing how you can initiate the re-finishing of the Internal Prep Area: