By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
Last week, we finished fabricating the bronze pivot, discussed material selection and made the TeflonÂ® thrust washer. This week, weâ€™ll discuss recovering from mistakes assemble the mechanical parts for a â€œForm, Fit and Function Verification Testâ€.
Form, Fit and Function Verification
After finishing the bronze pivot, phenolic base and TeflonÂ® washer, I decided to assemble the parts to verify form, fit and function. Donâ€™t you just love when everything fits together perfectly? What a thrill, right? Well, it didnâ€™t quite happen in this case. The trouble began when I press-fit the bronze pivot into the base with the 3-ton arbor press. Yes, with the LoctiteÂ® #262 too. The pivot was not vertical. It was perhaps off by 0.005â€ or so. The steel base still rotated well, but that pivot was not quite right. When I drilled the hole, wasnâ€™t the drill press table perpendicular to the drill axis? Did I check beforehand? Did I forget to clamp the drill press table again? Well, who knows? If this were an exam, we might have the dreaded last choice â€“ â€œDâ€ â€“ all the above. This is one of those instances where you just have to take a deep breath and think things over. For example, the drill press is used often and its table-to-drill axis perpendicularity can get out of adjustment. I verified this fact with a large Forstner bit â€“ its rim cut deeper on one side. I know. Circles donâ€™t have sides. One of its sectors was cut deeper. Better?
Recovering from Mistakes
So, now it was time to recover from yet another mistake. Being able to fix a mistake is a combination of science and art. Sometimes we can just start over. Other times, this approach is not practical given the partâ€™s complexity or cost. Perhaps we just donâ€™t want to scrap something weâ€™ve worked on. There are other times when we just get stubborn and donâ€™t want to give up. This is an excellent trait, but one must also recognize when starting over is the best approach. In this case, I decided to counterbore the base with a larger Forstner bit and make another bronze pivot. The pivot was not at fault, but it was a little too loose â€“ undersized by ~0.003â€ and its major diameter was not strictly concentric with its bearing section â€“ a consequence of trying to use a piece of scrap bronze thatâ€™s been sitting in a drawer for eight years.Â No, Iâ€™m not stubborn. Having made a pivot before, the new one was done quicker and fit the steel discâ€™s 0.3755â€ diameter center hole nicely. As a matter of fact, the pivotâ€™s bearing section was 0.375â€ for a 0.3755â€ hole. Not bad. Well, what next?
Phenolic Base â€“ Almost Done
I counterbored the phenolic base with 0.75â€ diameter tungsten carbide type Forstner bit. The hole depth was 0.325â€ to accommodate the new bronze pivotâ€™s major diameter. Spindle speed was 300rpm. Carbide Forstner bits are more expensive than the common tool steel variety, but they are good for phenolics and other abrasive materials. Though tool steel versions are perfectly acceptable in this application with spindle speeds low enough to minimize frictional heating. They just will not last as long before they become dull. Before removing the phenolic base from the milling machine table, I checked the pivotâ€™s fit within the counterbored hole. Afterwards, I applied 2-part epoxy to the counterbored cavity and pressed the pivot into place. The blue tape protected the base from any errant epoxy in the photo above.
Phenolic Base and Pivot Assembly Tensioner
The phenolic base is now done. The last task was adding four (4) clear polymer feet â€“ one in each corner. These feet were 3/8â€ thick and about 3/4â€ in diameter â€“ they will help dampen ambient vibrations. This cross section view of the pivot assembly shows its tensioner comprised of six (6) Belleville disc spring washers in series. The thumb screw tensions the Belleville washers and they exert an on-axis downward force on the rotatable steel disc base. Tightened enough and this force locks the base in a given position. A slight loosening allows one to rotate the base smoothly on its TeflonÂ® washer. I selected the disc washers since they have a smaller profile compared to a coil type compression spring with similar characteristics. This particular arrangement appears like it will work well â€“ the rotatable steel base movement is very smooth. The thumb screw is not knurled since thereâ€™s not much room to manipulate it with the electromagnet coil discs in place. With â€œtestingâ€ the smoothness of rotation done, the base is now complete. The next step is to begin work on the electromagnet coils themselves. Though at this point, I must deviate from the project itself and build a new coil winder. The existing coil winder is too small for the ~4.5â€ diameter magnet coils. The other alternative was winding the coils on the lathe. However, given its lowest spindle speed is 150rpm, coil winding in the recent past for another project yielded less than optimum results. This was especially true when applying 2-part epoxy to the rotating coil. Yes, a little messy. Just a little.
Next week, weâ€™ll continue this stupendous series and see some progress on the coil winder and begin discussing the electromagnetâ€™s electrical aspects. Hope to see you then!
For those of you who like lots of photos, here is a gallery of additional images from this project: