Saturday's fun bits...

Saturday's fun bits...

Joined: January 1st, 1970, 12:00 am

April 8th, 2018, 9:22 am #1

Got a couple of fun things done today, although not everything is photoworthy just yet. 😁

One thing I did kind of for myself  was another bit for the "new" mill. The knee-lift crank has been seriously pissing me off, since the teeth are badly worn. The two parts want to "cam" apart from each other, and it takes a lot of push to keep them held together. Which isn't helped by the fact that the knee and table on this monster is considerably heavier than your typical Bridgeport units, which means there's a lot more effort required to raise and lower it.



Anyway, I need to be able to use the damn thing, so it was time to fix it. I'll have a fresh page added to the build blog over on the Projects Pages as soon as I have a chance to write one, but for the time being, it's worth a quick note on how to cut the engagement teeth.

Here's the old piece, with a new blank ready for teeth:



Seems simple, right? Just run an endmill in there a few times? Nope, the sides of the teeth are tapered. They form a "pie shape" that points to the center of the part.

Well, turns out the trick is actually pretty easy: Because the teeth are the "fat end" of a wedge shape, that means the sides of two opposing teeth are in line with each other. All you do is offset an endmill so the edge of the cutter is on the centerline of the part, and make a cut across the entire face of the part.



The other tricky part is that only works with an odd number of teeth. Bridgeports, and a lot of similar clones, including my Grizzly, have nine teeth. Nine divided into a 360 degree circle is a nice round 40 degrees.

But noooooOOOOooo. This Spanish-made monster has seven teeth. 360 divided by 7 is... 51.428571 degrees.

Not exactly a nice round number. More like a number the engineers put in there just to piss off the guys in the machine shop. 😁

I'm not sure even my indexer can do a fractional degree to that kind of accuracy, but fortunately, that sort of accuracy isn't needed. I cut the actual part at 51.5 degree intervals- just eyeballing the half-degree as halfway between the tick marks on the scale on my rotary table- which means I was only off by half a degree after the full circle.



The other part was I used a chunk of "mystery metal" out of my bins. The bar was almost exactly the right size, but had been nickel plated- and sections of the plating peeled off under the wheels of my steady rest.



That caused it to rumble and thump as it was turning- I couldn't peel it all off, so I just had to put up with it 'til I was done. 😁

Keep that in mind in case you ever have to use a steady to turn some plated bar. 😋

Doc.
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Joined: November 30th, 2014, 1:36 am

April 8th, 2018, 4:48 pm #2

Sounds like the knee crank on my Lagun.  Beefy but a PITA with the 7 lugs, I had to modify the hand crank for the Y axis power feed, handle had to be bent over to not impact the drive.  There's a fellow somewhere who makes an adapter for a drill/driver so you can speed the knee up & down.
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Joined: January 1st, 1970, 12:00 am

April 9th, 2018, 1:18 am #3

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Joined: March 8th, 2004, 11:48 pm

April 9th, 2018, 2:39 am #4

DocsMachine wrote: But noooooOOOOooo. This Spanish-made monster has seven teeth. 360 divided by 7 is... 51.428571 degrees.
The Spanish use metric degrees. 100 divided by 7 is 12.285714285714. Much easier to machine. 😂
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Joined: January 1st, 1970, 12:00 am

April 9th, 2018, 3:14 am #5

Oh yeah, trying to eyeball a quarter of a mark on my rotary table would be so much easier. 😁

Technically, though, a "metric degree" would be a Radian... which is something like 57.2 degrees. So I'd have to crank my table... what, 0.893 of a radian? 😋

Even though the job's done, I keep thinking about it when my mind wanders- why seven? Nine is easy, and given the ubiquity of Bridgeports, common. They must have gone with seven because it was easy to machine for them- keeping in mind this particular mill I suspect having been made in the late 60s to early 70s, so it wasn't simply a matter of programming a computer to cut it.

I'd wager they had a fixture to do the actual milling, so the actual spacing was irrelevant. But still, I keep trying to think of some reason that was chosen over a more even number- like 5 (72 degrees per notch) 9 (40 degrees) or 15 (24 degrees.)

Doc.
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Joined: March 8th, 2004, 11:48 pm

April 9th, 2018, 3:42 am #6

Maybe they couldn't do 9 because of patent constraints? Maybe 7 is the boss's lucky number. I would think 5 would be preferred since it leaves a lot of strength and you can count them on one hand. Maybe the boss had a mutant hand?
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Joined: October 8th, 2014, 2:05 pm

April 9th, 2018, 12:55 pm #7

Maybe they wanted thicker teeth. Could it be a strength issue?
If it ain't broke, I'll fix it!
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Joined: January 11th, 2016, 8:57 pm

April 9th, 2018, 9:45 pm #8

Not exactly a nice round number. More like a number the engineers put in there just to piss off the guys in the machine shop.

Or they had a set of dividing plates on their rotary tables, and working with micro-degrees wasn't a hassle for them....

(We used to train apprentices, and often gave then a task where they had to use a piece of equipment to its full capabilities, even though it wasn't the simplest solution to a problem. "9 teeth? Nah, use the divider, and make 7 - or 11, or 13, or 23 teeth..."  Maybe the mill manufacturer had apprentices, and wanted to make sure they knew which way was up for getting funny angles on things.)
Breakfast.com halted. Cereal port not ready.
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Joined: September 13th, 2014, 9:47 pm

April 10th, 2018, 1:36 am #9

It's too late, and I don't want to do the math...  or the geometry...  But my age got me thinking - and maybe Doc's vast vintage machine knowledge can dig up something.
Is there any way that a SHAPER setup would have an advantage with that number?
The only time I ever did a cut at all like that was on a shaper.
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