Might have been someone else you are thinking of, my shop has been up for over 40 years now. It is not finished but finished is something that never comes, I hope.
The only things down right now are the Surface grinder and the 3D printer.
This is for sure the largest slot I have ever cut. Very stressful and lots of funny noises from the machine! Funny- read terrifying. At one point the vibration caused the vise handle to fall off onto my foot. I immediately assumed it was half of the end mill and I had to look down to see how bad I was bleeding. Fortunately not!
Roughing end mill followed by a 3/4 HSS end mill. Nice finish achieved.
Bronze bushing comes tomorrow. I need to upsize the spindle hole to fit. I have a Criterion boring head but this will be tough to hit a press fit just right. The OD is 1-11/16. The plans call out 1.688. I guess I need to hit 1.67 or so? I will likely then have to bore the bushings back out to 1.500". This may end up being a loctite scenario. Any suggestions appreciated.
I should note- because the slot is 3" deep you have to cut it from both sides and have your margins meet. I did pretty good, within a thou or two. You can feel a minimal line inside the slot.
I did some test fit up this morning and I ran into a significant problem. I have reviewed the drawings and I think it is a real issue:
The gear drawing specifies a distance from the worm center to the gear center of 2.029". All the gears are made to this specification.
The worm mount drawings specifies a center to center distance of 2.175". Unfortunately the shifter only has an eccentric offset of 0.90". All the shifters are made to this spec. (Math: 2.175-2.029-.090) The gears will not completely mesh by about 56 thou...
I am not remaking the gears I have cut! The easiest solution is probably involves changes to two parts. I could re-make the worm mount moving the hole in by .060". That makes the amount of metal surrounding the trunnion hole pretty thin, but probably workable. Right now there is 0.175" there so I could probably take that down to .100" safely. I would also have to remachine the shifter end smaller so it does not hit the aux base. That is a relatively minor operation except for the two I have already shipped out!
Please review this to see if I am creating addition problems I am not seeing. I am open to other suggestions as I ponder this.
The eccentric is offset by 0.090 that means that in 180° of rotation it will move the gear 0.180. You should be able to get an interference fit before you run out of travel and full disengagement at the other end.
I based my calculations assuming that the 1.000" Acme worm will have a pitch dia of 0.850
The eccentric is offset by 0.090 that means that in 180° of rotation it will move the gear 0.180. You should be able to get an interference fit before you run out of travel and full disengagement at the other end.
I based my calculations assuming that the 1.000" Acme worm will have a pitch dia of 0.850
Think about that. The gap is 0.146" in center to center (neutral position.) The eccentric can move the shaft .090" either direction. It is not 0.180" towards the worm. It took me a hot minute to figure this out.
To rephrase; the eccentric can move the worm towards or away from the gear by 0.090" but cannot close the gap of 0.146"
Good news is that this space is thick enough to accommodate a modification:
I think I can remake this part and move the two holes closer. Notice in the diagram the center distance hole to hole is 2.175". The eccentric can only move the worm .090" towards the trunnion. Or .090" away from the trunnion.
I'm piping up just because I'm curious. Sorry for your troubles.
The basic PD of a 1"- 4 external Acme thread is .875", per the linked on-line calculator.
That would crate .025" greater engagement than the assumed .850" PD.
The drawing #DH026 (titled Driven Gear) shows a dimension of 2.029" as the center distance between the gear and the hob.
2.175 - 2.029 = .146 - .090 = .056 clearance at closest nominal mesh adjustment.
Am I missing something here?
Is the drawing file (.pdf) in the OP the final published design?
Minimum mesh clearance should be .031" based on the difference between the two different 1"- 4 PDs shown above. Even greater clearance depending on the class of the male Acme shaft used as the worm.
Robert,
Do you think the driven gears meet the 2.029" dimension within a rational tolerance?
@extropic. Thanks for chiming in. I would say more of an interesting engineering problem than trouble! I am not sure exactly what you are concluding?
The drawings are the final designs which were distributed for the project.
I can tell you that when the worm and the gear are meshed with zero clearance on the bench, I have measured the center to center distance and it is 2.029 or slightly greater for some gears. Since the clearance is adjustable, I would like to be able to theoretically adjust it to zero and then back off to get smooth movement. The eccentric cam allows adjustment of the mesh and disengagement.
It looks like you agree with my calculation of .056 clearance. That is too much slop.
I do think this issue is easily solvable by just moving the worm closer to the gear as I discussed above.
EDIT: Also keep in mind that the driven gear was run in against a hob made from the same worm stock. I don't think thread class and minimum mesh clearance would apply the same way as conventional enclosed threads. It is desirable that the mesh here be adjustable to near zero to minimize back lash.
@rwm
My curiosity resulted in me wanting to do the simple math to see if I could understand what was going on. I've been watching this thread, and the predecessors, from the beginning, however I had previously only taken a cursory look and the .pdf and never printed out the drawings.
My question about the 2.029 dimension was because I have some recollection that there was iteration of procedure/process during the manufacture of the driven gears but didn't remember (or intend to look for) if the driven gears were essentially 'made-to-print' or other. You have confirmed that driven gears were essentially made conforming to the 2.029 dimension.
Conclusion? From my non-participating perch, I will say that I agree with your understanding and definition of both (trunnion can't pivot and worm doesn't mesh) problems. I have not checked the dimensions of your proposed fixes, but I have confidence that you will build this thing. I hope the .pdf gets updated to define your functional design changes. I shudder to think that some future pilgrim gets 50 pages into this thread to find out the design doesn't work.
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