Free Surface Grinder Downfeed & Spark-Out Tool
Calculate downfeed per pass ($a_p$), crossfeed stepover, table longitudinal traverse speed, spark-out passes, and Ra surface finish for toolroom grinding.
⚙️ Workpiece & Grinding Parameters
📊 Recommended Feeds & Spark-Out
Recommended Tools & Equipment
Tested hardware and components for high reliability
Precision Surface Grinding & The Physics of Spark-Out
During surface grinding, normal cutting forces pushing downward between the abrasive wheel and the workpiece cause elastic deflection of the grinding spindle, machine column, and wheel grains. As a result, the wheel removes slightly less metal than the dialed downfeed setting on the handwheel.
Why Spark-Out Passes Are Mandatory
When the final dimension is reached, disengaging the downfeed and allowing the table to reciprocate across the part without applying additional depth is called spark-out.
During these "spring passes", the machine's stored elastic deflection relaxes as residual microscopic high spots are shaved off. Grinding must continue until sparks completely cease exiting the wheel, ensuring:
- True flatness across the entire magnetic chuck without end-lift convex crowning.
- Achievement of sub-16 micro-inch ($Ra < 0.4\,\mu\text{m}$) ground surface finishes.
- Relief of surface grinding residual tensile stresses that cause thin plates to curl.
Frequently Asked Questions
How does crossfeed stepover affect surface finish?
For roughing, crossfeed can be 1/3 to 1/2 of the wheel width. For precision finishing, reduce crossfeed to 1/10 to 1/5 of wheel width (e.g. 0.050" to 0.100" on a 3/4" wheel). Overlapping cuts smooth out individual grain cusps, producing a uniform satin sheen.
What causes workpiece thermal burns and cracking?
Dull, glazed, or loaded wheel pores generate extreme friction heat without shearing chips. The local surface temperature spikes above 800°C, causing re-hardening and microscopic thermal stress cracks (crazing). Dress the wheel with a sharp single-point diamond and ensure flood coolant reaches the contact arc.
Why should thin parts be flipped repeatedly during grinding?
Grinding induces compressive and tensile surface skin stresses. If you grind 0.010" off one side of a thin plate, it will bow like a banana when released from the magnetic chuck. Remove equal stock from alternate sides (0.002" per flip) to keep stresses balanced.