SX3 spindle clamp

[Tredders]

Hi All
I bought a used Sieg SX3 mill/drill a little while ago and am not getting much joy with the spindle lock/clamp.
When I lock it, it gets jammed in place and I have to use excessive force to get it free.
This is because the barrel of the clamp, where it contacts the spindle, does not have the same curvature profile as the spindle.
The diameter of the curve on the barrel where it connects with the spindle is about 40 mm, and the spindle diameter is 60 mm.
There is a very small area of contact on the outsides of the oval where the barrel contacts the spindle so the barrel tends to dig into the spindle and things get wedged together.
I've tried making a new barrel from aluminium with a contact curvature having a 60 mm diameter.
I found the spindle was still only connecting with the clamp barrel on the outside edges of the oval shape and still needed the rubber mallet to loosen it.
So I scraped and filed away at the small area of contact hoping it would become large enough so it doesn't bite on the spindle.
Before long I reached the point where the two parts of the clamp barrel on either side of the spindle were locking on each other instead of the spindle.
So I took a few millimeters off the length, but couldn't really make any progress. Just went round in circles.

Whenever I've made this type of clamp, I make the hole for the spindle with the barrel of the spindle lock in situ, and they work really well.
I'm sure your readers are all familiar with this type of clamp, though I don't know of any technical name for it.
I'm wondering if any other owners of this type of mill have encountered this problem and have any solutions to it.
The reason it bothers me so much is that the digital readout on the spindle is a nice piece of kit, but it's no pleasure to use it when the spindle gets stuck every time I clamp it.

I contacted the agent in Australia about it, among other things, and they glossed over the clamp question.

Thank you for your time and interest. I would send a sketch, but I'm not that versatile and I'm sure your members know the type if clampt I'm talking about.

[earlgo]

It is called a "cotter". Why, only history can tell.
You may have to remake the entire thing and fit it all carefully in the cylindrical hole so the two cylinders don't cock and wedge in place.
Good luck.
--earlgo

[pete]

Yep Earls correct. Another name would be a Split Cotter. That's of course the usual North American term and other parts of the world might use something different. Since "cotter" was a common name used for various designs including wedges to semi permanently lock shafts in place on some steam engines my best guess is that's where the name came from. Given the amount of use a spindle clamp would get, aluminum would very likely wear out the threads fairly quick and possibly mushroom the clamping pads creating a very tough to solve issue of getting the parts back out if that happens. I can't think of any I've seen that aren't made of brass. You should be able to machine a new blank cotter shaft to the correct diameter, carefully measure the spindle diameter and then use a boring head to machine that radius in the side of the shaft. Bore that to the correct radius depth, then split the shaft on the C/L of that radius. Face both parts to allow enough space for them to tighten on the spindle, then drill clearance on the outside one and tap the inside half. You do want a material that's durable but still softer than the spindle so it doesn't become marked. I'd assume that's why brass is usually used. Correctly fitted only a partial turn is needed to tighten or loosen. That small movement should still allow the spindles radius to help keep them self aligned in the unlocked position.

[Tredders]

Thanks Earls & Pete
After posting the question I took the original cotter and clamped the two parts together with three spacer washers between them on a piece of threaded rod.
Used a 60 mm diameter plastic jar with carborundum paper around it to reshape the curve. (my mill is in many pieces atm)
The cotter now clamps ok, & it doesn’t get stuck.
Curiously, the original one is made from steel.
I guess it has some pros & cons.
Thanks again.

[earlgo]

My 1885 Sebastian and May lathe has a steel cotter to clamp the tailstock spindle.
Ditto the 1958 Atlas Lathe
Ditto the 1966 Delta Drill press.
None have ever locked. Just my limited experience.
--earlgo

[pete]

Both my lathes and the BP clone use brass for the cotters Earl. Maybe if soft steel is used it doesn't matter?

[earlgo]

pete: You are most likely correct. Your remark about aluminum mushrooming and locking in place is relevant, too.
Admittedly I have never had to clamp any of the spindle split cotters very tightly to do the job at hand. I do not own a Vertical Mill, so I cannot comment on that situation.
--earlgo