Group Project: Dividing Head - The Build

[rwm]

Jeff- I am finishing up some of the gear shifts and I want to run something by you. I was considering increasing the offset of the eccentric part from .090 to .125 or so. I am concerned that if the gear teeth are not cut deep enough it may not be possible to disengage the worm. This would give us more room. Also, once these eccentrics are cut you can't really change them. I just want to make sure you think this is a good idea and will not interfere with something else.

 

[Flyinfool]

The full thread depth of the ACME thread is .15.
With the .090 offset on the eccentric, that will move the shaft .180 after 180° of rotation. Should be plenty.

 

[rwm]

I thought the tooth depth of engagement was 1/2 x pitch or .125"?
What I'm concerned about is that if I or whomever can't cut the teeth deep enough we will effectively have increased the pitch diameter and the worm may be too close to the limit of travel to disengage. Right now the gear I showed has a center to center distance of approximately 2.1" with the plans calling out 2.028. That difference already uses up .072 of travel. But maybe I am thinking about this wrong? If the teeth are less engaged it would take less travel to disengage them? Perhaps this problem corrects itself?

 

[Flyinfool]

Maybe we should do an inventory of what is done so that we know what still needs to be done. I know there was some trading of parts that went on. Yes I am lazy and do not want to reread 44 pages of this thread to figure it out. I will add a column to the BOM spreadsheet on page 1 of this thread to show what is done and who has it.
Also please confirm that if you are listed for a part not yet done you can still do it.

 

[Flyinfool]

It is the OD of the gear that needs to be cleared. So unless you are also making the OD of the gear a bigger diameter there should be no problem disengaging.

The only issue might be that as the pitch Dia changes the worm also moves off center from the gear.

 

[rwm]

Thanks for thinking this through with me. I agree it should be OK as drawn. At first I couldn't get my head around the increase in center to center distance. I see what you mean about being in the middle of the gear. I'll try to fix the gears I have to get closer. Did you calculate this distance (2.028) with some gear overlap or is that number dead on with no margin?

 

[Flyinfool]

The 2.028 the theoretical exact distance for perfect alignment and mesh. There is a bit of room that the worm can get deeper or shallower but at the expense of alignment. It is possible to shim the gear either way to accommodate some tolerance buildup.

 

[rwm]

I may shoot for something like 2.050. That will make sure the gears can be tight and not far off center. Also allows a little margin for wear.

 

[rwm]

I am signed up for the gear shift housing, clamp and shifter arm with ball. I am remaking a few parts that I was not happy with. I will have 5 sets of parts shortly.
I may be able to do some more parts within reason. I can provide acme screw stock for 1" sections of worm. I may be able to make some driven gears if no one else tackles this.

 

[rwm]

These are ready for a little mill work:



They are on spec in every important measurement. The eccentric is machined to .182 TIR. The OD of the large end I had to turn down to 1.475 because I stared with 1.5" stock. That should not be an issue. The second largest diameter section I made 1.182 to clear the eccentric. This is only .057 larger than spec which I think will be fine on clearance. (The only clearance issue would be with the notch in the Aux Base.) I did not chamfer all the small ends. I like the look without it but it could easily be done.
These parts are rather complex to make. I hope I don't screw up the mill work now!