split cotter dimensions

[russ57]

Hi all
i have a need for a clamp, and i think a split cotter type would work well.
http://www.thewarfields.com/cnccookbook/MTMillKurtViseStop.htm

This uses a cotter dia 0.5" with a 0.25 thread. I want something a bit smaller.

I want to clamp onto a bar around 8-10mm, and i was thinking a cotter the same size (say 8mm), with a 4mm thread inside. But that means at most i can take a 2mm 'crescent' out of the side. Is that enough? (the half-inch version would have 1/8" deep).

has anyone done this? 
- thanks!
russell

ps, its for a tilting LCD monitor stand. Its a touch screen, so it needs to tilt much more than the supplied stand.

[mattinker]

I'm sorry, but that's not a split cotter in the CNCcook book!

The "split cotter" described here is much better.


http://bbs.homeshopmachinist.net/archive/index.php/t-33623.html

Regards Matthew

[russ57]

thanks for the link, matthew. I'm sorry, however, but i cant see the difference. In fact, the 3rd link on that page is to the same project i pointed to, and at the bottom of the page i linked to is the description taken from Lautards bedside reader that your link refers to.

http://www.cnccookbook.com/img/MillStuff/KurtViseStop/SplitCotterDrawing1-1.jpg

What have i missed? (there are variations described which i hadnt seen, ie, the cotter 'notch' doesnt necessarily need to be circular, it could be just cut at an angle; the circular notch seems to have more gripping area to me)

regards,

russell

[75Plus]

Russ, You may be able to increase the crescent by off setting the cylinder in a four jaw chuck for drilling. Maybe gain .5 to .75mm.

Joe

[mattinker]

My mistake, the link provided takes you to a Kurt vise stop, from what I could see, it didn't seem to be the split cotter that I've made, I didn't scroll down the page.


Regards, Matthew

[russ57]

;-)

sorry matthew, i should have noted the cotter bit was way down the page. its the best description i've found.

Joe, good idea, i'll draw it out and see what i can gain.

I have enough trouble drilling centrally, so i could end up with offset anyway! regards

russell

[mattinker]

Russell,

here's a .pdf on split cotters, I don't know if it'll help, I found it interesting when making my split cotters.

Regards, Matthew

[andyf]

Russell, going back to your original post, I'm sure a 2mm crescent would work fine. But if you want to increase the area of contact between the two sides of the cotter and the bar they are gripping, the 4mm hole through the 8mm dia. cotter could be offset by (say) 1mm from centre, allowing you to make the crescent up to 3mm deep.

Andy

[russ57]

I should measure properly,  but owing to a slight 'miscalculation ' the crescent ended up only 1 mm or so,  clamps fine.  I think the most important factor is the fit between the clamp body and the shaft to be locked.  However,  wear over time will also be a consideration.

Hopefully next one will be slightly more accurate!

[Lew_Merrick_PE]

I have been following this discussion for several days now, but have had my back to the wall with a project release schedule closing in while my customer makes those wonderful last minute changes.  The project was accepted for release yesterday.

I learned this technique under the name split collet, so please bear with me if I slip up in my nomenclature.  The first thing to remember is that what you are making is an anti-rotation clamp.  The clamp works because of the resisting moment developed by the area of contact and coefficient of friction between the parts through the force applied by the locking feature.  Only those areas wherein the collet/cotter actually intimately contact the shaft can apply a resisting moment to it.  The better the fit smoother the finish between the collet/cotter and the shaft, the greater the intimate contact over which the coefficient of friction will work.  The force of the locking element should really be nearly irrelevant as the loads applied to such a mechanism should be fairly low in proper design practice.

Thus, assuming the fit is good (boring or reaming the holes makes good sense rather than depending on the smoothness and roundness of a drilled hole), the issue almost always boils down to surface finish and coefficient of friction between the collet/cotter material and the material of the shaft being clamped.  Whereas leaded steels machine really nicely, their relative coefficient of friction can become a real issue in such an application.  A trick to remember is that high-zinc aluminum (i.e. 6061 alloy) has quite a high coefficient of friction with respect to steel.

Does this help?

[russ57]

 

I think you are suggesting that perhaps a cotter of 6061 may clamp better than steel,  against a steel shaft, and that the fit of the shaft against the cotter is important.
Discussions elsewhere suggest even a notch cut in the cotter,  rather than a milled crescent works fine.

More Experiment required!
Thanks,  I'll be back!

[Lew_Merrick_PE]

I think you are suggesting that perhaps a cotter of 6061 may clamp better than steel,  against a steel shaft, and that the fit of the shaft against the cotter is important.
Discussions elsewhere suggest even a notch cut in the cotter,  rather than a milled crescent works fine.

Russ,

It is less that I am suggesting using 6061 aluminum than it is that I am suggesting that you think about the variables affecting your work.  It is true that using a higher friction material (such as 606xx aluminum) can work in your favor (though there are drawbacks in co-machining aluminum and steel), but that is not the only variable.

If you make "ramps" (avoiding the general term "notch") in your collet/cotter, you will end up with a wedge-lock device.  This may help you in terms of applying a restraining force against rotation, but it also may for you to "rap" the unit to let it go.  It will self-unlock so long as the effective angle of contact between the face of the wedge and the cylinder being clamped has a tangent value greater than the coefficient of friction between the two materials.  Going from memory (always dangerous at my age), this is generally a wedge angle (with respect to the centerline of the collet/cotter) greater than 18°.

One thing to remember about aluminum on steel contact is that aluminum oxidizes into (gee whiz, what a surprise) aluminum oxide -- a very fine abrasive.  Aluminum oxide will rather quickly abrade non-hardened steel at rates that often surprises people.  One of the, I was a hero (for a day) situations occurred with a company I did work for when I "solved" the problem of their aluminum router "skates" wearing out the CRS sheet "tracks" upon which they slid them.  (A UHMW polyethylene "bearing" was the answer.)