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Neil Jones
03-03-2009, 09:29 PM
Could someone describe how a "split cotter" works and how you make them. I've seen mention of them along with pictures of projects that use them but it's still unclear to me how they work and how they are made.

JCHannum
03-03-2009, 10:11 PM
I'll take a shot at a hand waving description. This is a split cotter I made for holding a 3/8" boring bar in a boring head. The hole for the bar is 3/8" of course and the cotter is 7/16". No particular reason, but it helps if the cotter is a bit larger than the retained part.

http://i320.photobucket.com/albums/nn351/jchannum/P1060541.jpg

This is the three pieces of the cotter;

http://i320.photobucket.com/albums/nn351/jchannum/P1060542.jpg



The holes are drilled at 90 degrees to each other, with about 1/3 of the cotter in the insert hole. The cotter hole is drilled first and a blank for the cotter is inserted and retained with a parallel clamp or screw and washer. It needs to be held securely as you next drill the intersecting hole, making the scallop in the cotter.

The cotter is then removed, drilled through with a tap drill, drilled halfway with a clearance drill and, in this case, counterbored for the socket head cap screw. The SHCS in this case is 10-32. The cotter is then sawed or parted through the center of the scalloped area. The kerf should be wide enough to provide clamping clearance.

John R
03-04-2009, 07:36 AM
The Bed Side Reader has a good description on how to design and build cotters
Good luck
John R

MLAToolbox
03-04-2009, 07:54 AM
Jim Hannum's description is pretty complete and I can't add much to it except to say that in my experience from having made a few cotters the kerf should also be wide enough to prevent wedging. Andy L.

J Tiers
03-04-2009, 08:47 AM
In most cases it may not be required to drill the cotter, you can do fine by simply cutting a suitable angle on it. That also allows for making the cotter from two smaller pieces, if necessary, which can be a pain, but is perfectly possible.

Atlas, for instance probably never made a curved cotter in their zamac-laden little lives...... All the Atlas ones I have seen that were curved were merely indented from years of being crunched onto the held shaft, otherwise identical ones showed a plain angle. Atlas just loved the split cotter.

I have replaced a large number of big Atlas split cotters, up to about 30mm diameter, using the cut angle, and they all work fine.

SGW
03-04-2009, 10:22 AM
As John R notes, one of Guy Lautard's "Machinist Bedside Reader" volumes goes into great detail about split cotters.

I've made several, and I like them a lot. They are a bit of trouble to make, but IMO worth the effort. I draw it first to get the proportions and figure out the offset so the scallop won't cut into the clamping screw.

JCHannum
03-04-2009, 11:39 AM
I used the Lautard writeup for my first couple, but just wing it after that. Mainly, dimensions will depend on the diameter of the part being retained. The hole for the cotter needs only to be offset enough for the clamp bolt to clear the retained part. The larger the collet, the higher the gripping power, but they have surprisingly good holding power and do not damage the part being retained.

The dimensions of the cotter will be determined by the size of the clamp bolt. It needs to be large enough to accomodate that, and a counter bored SHCS, while not necessary, makes a nice clean installation.

The collet can be angled and machined separately, but machining it while drilling the cross hole makes things easier to figure out.

BadDog
03-04-2009, 11:55 AM
Along the lines of what J said, you can also just take two small pieces of suitable diameter and turn a good sized OD chamfer on the end. Easy, fast, and works well if there is otherwise enough length to keep them well aligned in their bore.

The drilled version mentioned by JC is likely the best for max hold for minimum closing torque/pressure (from cam toggle, screw, etc.). J's version would be next because it still has a bearing surface directly behind the ramp, but has a smaller contact area (but higher psi, not good if held rod is aluminum or something). The one I suggested also allows forces to try to cock the cotter halves in the bore, but for many things, it works well enough and is by far the fastest and easiest. For instance, this is exactly the style I have on my lathe chip guard.

BobWarfield
03-04-2009, 11:58 AM
More split cotter work here for a Kurt vise stop I made:

http://www.cnccookbook.com/MTMillKurtViseStop.htm

Best,

BW

Frank Ford
03-04-2009, 12:19 PM
I still haven't learned to draw anything, so I continue reinventing ways to work and figure things out as I go. With that in mind, here's an article on my Web site that includes two of the ways I've made split cotters:

http://www.frets.com/HomeShopTech/Projects/TailstockToolHolder/tailstocktoolholder.html

http://www.frets.com/HomeShopTech/Projects/TailstockToolHolder/tailstocktoolholder25s.jpg

Neil Jones
03-04-2009, 06:25 PM
More split cotter work here for a Kurt vise stop I made:

http://www.cnccookbook.com/MTMillKurtViseStop.htm

Best,

BW

How well did the split cotters work out for your workstop? Is your workstop very rigid with very little thumb screw pressure?

Neil Jones
03-04-2009, 06:26 PM
I still haven't learned to draw anything, so I continue reinventing ways to work and figure things out as I go. With that in mind, here's an article on my Web site that includes two of the ways I've made split cotters:

http://www.frets.com/HomeShopTech/Projects/TailstockToolHolder/tailstocktoolholder.html

http://www.frets.com/HomeShopTech/Projects/TailstockToolHolder/tailstocktoolholder25s.jpg

Frank, thank you for posting this. :)

Neil Jones
03-04-2009, 06:34 PM
This is the three pieces of the cotter;


The holes are drilled at 90 degrees to each other, with about 1/3 of the cotter in the insert hole. The cotter hole is drilled first and a blank for the cotter is inserted and retained with a parallel clamp or screw and washer. It needs to be held securely as you next drill the intersecting hole, making the scallop in the cotter.

The cotter is then removed, drilled through with a tap drill, drilled halfway with a clearance drill and, in this case, counterbored for the socket head cap screw. The SHCS in this case is 10-32. The cotter is then sawed or parted through the center of the scalloped area. The kerf should be wide enough to provide clamping clearance.

Thanks, Jim :)

Neil Jones
03-04-2009, 06:35 PM
The Bed Side Reader has a good description on how to design and build cotters
Good luck
John R

Thanks, John.

Neil Jones
03-04-2009, 06:37 PM
As John R notes, one of Guy Lautard's "Machinist Bedside Reader" volumes goes into great detail about split cotters.

I've made several, and I like them a lot. They are a bit of trouble to make, but IMO worth the effort. I draw it first to get the proportions and figure out the offset so the scallop won't cut into the clamping screw.

I'll make sure I draw them first in CAD. Thanks. :)