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A drilling apparatus includes a tubular body including an upper connection and a lower connection with a drill bit disposed thereon, and an axial borehole therethrough, wherein the upper connection is configured to attach to a drillstring, primary cutter blocks coupled to the tubular body and config

A drilling apparatus includes a tubular body including an upper connection and a lower connection with a drill bit disposed thereon, and an axial borehole therethrough, wherein the upper connection is configured to attach to a drillstring, primary cutter blocks coupled to the tubular body and configured to selectively expand radially therefrom. The drilling apparatus further includes backup cutter blocks coupled to the tubular body and configured to selectively expand radially therefrom, and an activation system configured to selectively expand and collapse the primary and backup cutter blocks in response to changes in weight applied to the primary and backup cutter blocks.

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1. A drilling apparatus comprising: a tubular body including an upper connection configured to attach to a drillstring, a lower connection coupled with a drill bit, and an axial borehole therethrough;primary cutter blocks coupled to the tubular body and configured to selectively expand radially ther

1. A drilling apparatus comprising: a tubular body including an upper connection configured to attach to a drillstring, a lower connection coupled with a drill bit, and an axial borehole therethrough;primary cutter blocks coupled to the tubular body and configured to selectively expand radially therefrom;backup cutter blocks coupled to the tubular body and configured to selectively expand radially therefrom; andan activation system, which in response to changes in weight on the primary and backup cutter blocks, is configured to: expand the primary cutter blocks and collapse the backup cutter blocks in a first operation sequence; andcollapse the primary cutter blocks and expand the backup cutter blocks in a second operation sequence. 2. The drilling apparatus of claim 1, wherein the backup cutter blocks are configured to cut harder formations relative to the primary cutter blocks. 3. The drilling apparatus of claim 1, wherein the backup cutter blocks are devoid of cutters for backreaming. 4. The drilling apparatus of claim 1, further comprising at least one sensor configured to detect weight applied to at least one of the primary and secondary cutter blocks. 5. The drilling apparatus of claim 4, further comprising: a first sensor configured to detect weight applied to the primary cutter blocks; anda second sensor configured to detect weight applied to the backup cutter blocks. 6. The drilling apparatus of claim 5, wherein the activation system is configured to expand the backup cutter blocks and collapse the primary cutter blocks when the first sensor on the primary cutter blocks detects a weight of at least about 35% of a weight on the drill bit. 7. The drilling apparatus of claim 6, wherein the activation system is configured to collapse the backup cutter blocks and expand the primary cutter blocks when the second sensor on the backup cutter blocks detects a weight of less than about 35% of a weight on the drill bit. 8. The drilling apparatus of claim 1, wherein the primary and backup cutter blocks are configured to be in a retracted position as the drilling apparatus is inserted into the borehole and run downhole. 9. The drilling apparatus of claim 8, further comprising a main sleeve;a ball-drop sleeve; anda shear pin configured to maintain a position of the main sleeve relative to the ball-drop sleeve, the shear pin configured to shear in response to an increase in hydraulic pressure thereby allowing the ball-drop sleeve to move and align a ball-drop activation port with a main sleeve activation port so as to provide a fluid flow to a primary cutter block activation port. 10. The drilling apparatus of claim 1, further comprising a switch or valve to toggle a direction of fluid flow to either a sleeve activation-down port or a sleeve activation-up port. 11. The drilling apparatus of claim 10, wherein the switch or valve is configured to toggle the direction of fluid flow each time a motor or pump electronically coupled to the switch or valve is shut off. 12. A method of selectively activating cutter blocks in a drilling apparatus, the drilling apparatus having a tubular body with an axial borehole therethrough, the method comprising: expanding primary cutter blocks;detecting an increased weight applied to the primary cutter blocks;collapsing the primary cutter blocks; andexpanding backup cutter blocks;detecting a reduced weight applied to the backup cutter blocks;collapsing the backup cutter blocks; andexpanding primary cutter blocks;wherein the expanding and contracting of the primary and backup cutter blocks is performed with a closed loop control system based on the detected increased applied weight and the detected reduced applied weight. 13. The method of claim 12, further comprising inserting the drilling apparatus into the borehole and running the apparatus downhole with both the primary and backup cutters in a retracted position. 14. The method of claim 13, further comprising shearing a shear pin thereby allowing a ball-drop sleeve to move and align a ball-drop activation port with a main sleeve activation port, and providing a fluid flow to a primary cutter block activation port. 15. The method of claim 12, further comprising toggling a direction of a fluid flow to selectively expand and contract the primary and backup cutter blocks in response to the detected increased applied weight and the detected decreased applied weight. 16. A method of enlarging a borehole drilled by a bit, the method comprising: inserting a drilling apparatus in the borehole wherein the drilling apparatus comprises a tubular body, primary cutter blocks, backup cutter blocks, and a drill bit;rotating the drill string and applying a weight on the drill bit; andalternatingly expanding the primary cutter blocks and the backup cutter blocks in response to a detected weight applied to at least one of the primary cutter blocks and the backup cutter blocks, such that the primary cutter are expanded when the backup cutter blocks are collapsed, and the primary cutter blocks are collapsed when the backup cutter blocks are expanded. 17. The method of claim 16, wherein the detected weight is in a range from about 20% to about 35% of the weight on the drill bit. 18. The method of claim 16, wherein the alternatingly expanding comprises: expanding the backup cutter blocks and collapsing the primary cutter blocks when the detected weight is above a preset weight limit in the range from about 20% to about 35% of the weight on the drill bit; andcollapsing the backup cutter blocks and expanding the primary cutter blocks when the detected weight is below a preset weight limit in the range from about 20% to about 35% of the weight on the drill bit.