This video details the extensive process of upgrading a previously built tool post milling spindle to perform precision grinding operations on a lathe. The creator begins by enhancing the spindle's own taper through external grinding, achieving a significantly improved surface finish (RA 0.4) and runout (2 microns). A custom, larger grinding arbor is then designed using CAD, fabricated with precise turning and milling techniques, and carefully balanced. This project involves the complex steps of hardening steel components and then using the new grinding setup to finish a suite of hardened accessories, including internal grinding arbors, a chuck adapter, and a slitting saw arbor. Through meticulous external and internal grinding, these new tools achieve exceptional surface finishes (RA 0.2, RZ 1.5) and near-zero runout, surpassing commercially available alternatives. The video showcases the challenges and solutions in high-speed machining, including effective dust management, precise angle setting, and addressing thermal issues at extreme RPMs (up to 30,000 RPM for internal grinding), ultimately demonstrating the spindle's versatility and high-precision capabilities.
Upgrading a Tool Post Spindle for Precision Grinding
0:00 - Introduction to Grinding: The creator introduces the goal of adapting a previously built tool post milling spindle for precision external and internal grinding, expanding its capabilities beyond milling steel.
0:48 - Initial Spindle Taper Grinding: The first task involves grinding the spindle's own taper, which previously had a poor surface finish and 0.02mm runout, to improve its accuracy.
1:14 - Dressing Tool Holder Fabrication: A custom dressing tool holder is built using a magnet base, allowing for adjustable angle and enabling dressing of the grinding wheel in-between cuts without disassembling the setup.
2:13 - Critical Dust Management: Before grinding or dressing, the lathe's sensitive surfaces are meticulously covered, and a dust collection system is set up to protect the machine from harmful abrasive dust.
2:29 - Grinding Wheel Dressing: The grinding stone is dressed by taking 30-micron passes, ensuring a sharp and accurate cutting surface.
2:49 - Test Piece Grinding Success: A test piece with a taper is successfully ground, achieving a very smooth surface with no discernible steps, demonstrating the effectiveness of the grinding process.
3:35 - Spindle Rotation for Grinding: A custom plastic pulley is made to allow the main spindle (the part being ground) to rotate while the chuck remains stationary, powered by the same motor used for the tool post spindle.
4:24 - Precise Angle Setting: The compound slide's angle is precisely set to 8 degrees using an indicator, crucial for accurately grinding the taper. A step is dressed into the wheel to reduce grinding pressure.
4:52 - Spindle Taper Grinding Outcome: The actual spindle taper is ground with 20-micron depth cuts and a slow feed, correcting initial runout and taper angle. The final runout is reduced to an impressive 2 microns.
5:34 - Surface Finish Analysis: A surface roughness tester measures an RA 0.4 finish, considered average for cylindrical grinding, which the creator is highly satisfied with given the setup.
6:09 - Designing a Larger Grinding Arbor: The ambition to use a larger 150mm grinding stone (requiring 3-4000 RPM) necessitates designing a proper flange in CAD, which integrates balancing weights.
6:35 - CAD Software Advantages: The creator highlights On Shape's cloud-based nature, emphasizing its accessibility, unlimited version history, and crash-free operation, based on a previous negative experience with another CAD software.
8:36 - Flange Shaft Fabrication: The shaft for the larger grinding arbor is machined from high-strength stainless steel, requiring multiple setups and turning between centers for high accuracy, despite challenges with stringy chips.
10:19 - Taper Angle Refinement: The taper angle of the shaft is meticulously checked using a dry-erase marker and the spindle, requiring several fine adjustments to the compound slide for an even contact pattern.
11:40 - Flange Disc and Axial Groove: The large flange disc is machined from C45 steel, including boring a precise central hole. An axial groove is cut using the milling spindle, though an initial attempt failed due to excessive chip load and belt slip, requiring adjustments to RPM and endmill type.
14:50 - Shrink Fit Assembly: The flange disc is shrink-fitted onto the shaft by heating the disc to 250°C, ensuring a secure and precise connection.
16:16 - Draw Bar and Repair: A draw bar is created to secure the grinding stone. A machining error leads to an undersized section, which is ingeniously fixed by pressing on a bronze bushing and re-turning it, making the "mistake" appear intentional.
18:20 - Guard Fabrication Challenges: Construction of a safety guard for the large grinding wheel leads to significant problems when an aluminum hub collapses due to internal stress during slot cutting, clamping onto the blade. A multi-step repair is needed to make the part usable, highlighting lessons learned in material stress and flexible design.
23:16 - First High-Speed Spin and Dressing: The assembled large grinding stone with the new guard is spun up to 3000 RPM. Dressing is performed, but high-speed camera footage fails to capture the action effectively due to the wheel's extreme speed and insufficient lighting.
24:29 - Balancing Weights and Procedure: Balancing weights, designed into the flange, are meticulously fabricated. A custom shaft and bushing assembly allows the entire arbor to be balanced on the lathe bed using gauge blocks, achieving perfect balance.
29:12 - Hardening New Tooling: Five new accessories (internal grinding arbors, chuck adapter, slitting saw arbor) are machined from 4140 steel, left slightly oversized, then hardened by heating to 860°C and quenching in water, followed by tempering at 150°C.
31:16 - Grinding Hardened Parts: The hardened components are then precision-ground, correcting any distortion from the hardening process. The result is a beautiful surface finish with exceptional accuracy.
34:19 - Final Surface Finish and Runout Results: The ground parts achieve an RA 0.2 and RZ 1.5 surface finish, which are excellent results, even outperforming a store-bought Morse taper adapter. Runout on these new tools is consistently under 0.02mm.
35:05 - Internal Grinding Demonstration: A Morse taper 6 to 4 adapter sleeve with poor original runout is internally ground using a medium-sized arbor at 20,000 RPM, successfully achieving a perfect surface finish and zero runout.
36:48 - Extreme High-Speed Grinding: The smallest arbor is tested at 30,000 RPM, demonstrating the spindle's capability, but highlights thermal issues with the belts and 3D printed components at such high speeds.
38:28 - Slitting Saw Test and Conclusion: A slitting saw is tested, proving its functionality. The creator expresses satisfaction with the external and internal grinding arbors, confirming the success and versatility of the tool post spindle.