Group Project: Dividing Head - The Build

Shotgun said:
I used a bolt as a mandrel and mounted it in my lathe chuck. The bull gear on the Craftsman Commercial lathe has 60 dimples and a pin that locks into them. I used that to index the part every 1.5 holes. That is, pin in the hole, make a slit, skip one, pin between holes, make a slit, skip one, pin in the hole, make slit, etc, so that I ended up with 40 slits.

To make the slit, clamped a block of wood to my cross slide, such that the top of the wood was flat and at center height. The height isn't critical, but you want every slit to be the SAME height. Then I used a cordless oscilating tool to make the slit.

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You don't need to go deep. You don't even need to cut more than the center. If you see the teeth of the blade disappear in the middle, you've gone plenty deep. The teeth of the hob just need to find the slits on the first few passes, and five thou will be a big cut at that point. The hob is both cutting and pushing the blank around, and the slits are really just to help the hob find its place and give it something to push against. Even with the slits, you may need to help the blank spin on those first few rotations. Gotta make sure the hob teeth find the slits. 60 RPM on the hob will be fast initially.

Once the hob cut is established, the hob will have no problem finding it's own groove and you can advance to deeper cuts and speed up the spindle.
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Do you have to cut ALL of the teeth, or just a couple to get the initial cut working?

Also, the slits have to go at an angle, right? What did you calculate as the angle? I have to use the dividing head/rotary-table anyway, and don't have one of those multi-tools, so I likely will be using a rotary table on the mill with a blade that way, so I think i have to set the angle with a sine bar or something...
 
ErichKeane said:
Do you have to cut ALL of the teeth, or just a couple to get the initial cut working?
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Yes, else the hob will lose its place and not spin the blank.
ErichKeane said:
Also, the slits have to go at an angle, right? What did you calculate as the angle? I have to use the dividing head/rotary-table anyway, and don't have one of those multi-tools, so I likely will be using a rotary table on the mill with a blade that way, so I think i have to set the angle with a sine bar or something...
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No. I cut them straight. The hob will find it and keep things moving. Keep in mind that the first few times around the blank, the hob cut will be just a dimple. But that dimple is all it needs to push against to keep the blank going.

Another point. most of the work will be done by just a few of the hob's teeth. Move the carriage back and forth a little to switch to a sharper tooth.
 
Just spent a few hours in the workshop. I'm amazed what a 4x6 bandsaw can do. I just cut about 8ft of 3/4" A36. Wore out the blade that I got from the Tractor Supply, and broke one of my two new Lenox blades. They were rather short, so I had to modify the saw a bit to get them on there. But, they cut so much better than what I can find locally.

Good news is that I have one more cut on each, and then I'll have the Main Base roughed out. Holes are next, and I got most of the way through the construction of my hole saw mandrel. A couple holes to drill and tap, then a taper to turn, and it'll be ready.
 
Observer's $0.02:

Any eccentricity in the gear(s) will result in backash (slop) where the mesh isn't ideal.

The best way, I can think of, to cut the gear is to mount the blank on it's mating (what have you named it?) spindle and rotate the spindle on it's bearing journals. The mating gear/spindle diameters should be matched to the best of your ability and the setscrew engaged during gear tooth machining. The Gear/spindle should then be a matched set.
 
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For the record, I will be working on the spindles at least 8 hours every Sunday until they are done. M-F I can’t do anything. I work 7+ hours on Saturdays, but I will try to do a little bit on Saturdays, too.

I have ALL the tools & materials on hand.
 
extropic said:
Observers $0.02:

Any eccentricity in the gear(s) will result in backash (slop) where the mesh isn't ideal.

The best way, I can think of, to cut the gear is to mount the blank on it's mating (what have you named it?) spindle and rotate the spindle on it's bearing journals. The mating gear/spindle diameters should be matched to the best of your ability and the setscrew engaged during gear tooth machining. The Gear/spindle should then be a matched set.
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So I'm not sure I need to be concerned about it actually, and don't think it causes backlash or anything. Since the gear blank isn't fixed, any 'play' during the hobbing will be consistently taken up by the pressure of the hob, wouldn't it? There will be a consistent single-point of contact between the gear blank and the rod that it is attached to, and it should rotate on that, right?
 
ErichKeane said:
So I'm not sure I need to be concerned about it actually, and don't think it causes backlash or anything. Since the gear blank isn't fixed, any 'play' during the hobbing will be consistently taken up by the pressure of the hob, wouldn't it? There will be a consistent single-point of contact between the gear blank and the rod that it is attached to, and it should rotate on that, right?
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And when that gear is mounted to the spindle, the setscrew will push all clearance to one side of the gear/spindle fit. Thereby creating eccentricty.
 
extropic said:
And when that gear is mounted to the spindle, the setscrew will push all clearance to one side of the gear/spindle fit. Thereby creating eccentricty.
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The gear/spindle fit should be one of the tighter ones that we're working on. Any eccentricity from the set screw will be half the clearance between the gear and the spindle.

As @ErichKeane is hobbing, he is pushing the gear blank against its axle (which happens to be a fixed, vertical shaft). The blank is freewheeling on the axle. There is no set screw, yet. So, as the hob turns, it pulls the blank around, with the inside diameter of the blank rubbing against the shaft. The hob's DOC is set by the distance between the axle (maybe post is a better word?) and the hob.
 
rwm said:
Sigh. Long story short; I scrapped my first part.
I drilled the hole for the lever handle in the wrong place and it intersects one of the mounts for the index plate. It is not fixable because a hole in the correct place would overlap the incorrect hole. I will have to make a new one.
Having said that, my plan is to send this to Jeff (or whoever is receiving?) for the QA part since that is the only error, with the understanding that the lever will be correct on the next 8 parts. This will not end up in a finished device.
I did finish the clamp without an issue. Here are some pics.

View attachment 367959


Hard to tell in the pic but the lever hole is well above 35 deg from the 3 o'clock position (referenced as 0 deg)

Robert
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All is not lost yet.
How bad are they intersecting?

On the clamp the slot into the other side is still missing. There is nothing magic about the exact depth or width of the slot, It could be done with a saw if your slitting wheel does not reach that deep.

You can try it out on the Housing you have made to see if it will still clamp up tight without it?

As for the eccentricity of the driven gear once the set screw is tightened, Yes tightening the set screw will push it to one side, ASSUMING all parts are in tolerance and you use the worst case tolerance stack up the added backlash from the worst case would add ~0.002° of backlash, or ~0.00015 of linear travel at the pitch Dia. I personally don;t think I can even measure that repeatably.

Ideally the slits should be angled, The angle needed will be about 7.12°. The angle is not that critical that you need sine blocks, a plastic protractor will be plenty close. Even an eyeball guess and by gosh will be good.
 
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