Soft Jaws For Chinese Bench Top/mini Lathe

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6061T6

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Hi, this is my first post and I am totally new to the world of machining.
I have been making parts out of 6061 aluminum with my Sieg SC4 small lathe (8.5x20) and have run into one problem: how do I grip soft aluminum without marring the threads or other features I have cut into it? I also need to maintain excellent concentricity as these parts will be used in optics where the required mechanical tolerances are quite stringent.

I realize that soft jaws are the right solution and have been reading several excellent threads on this forum archives. The jaws that come with the lathe (and after market ones too) are all single piece.

I think I will eventually have to spring for a Bison 5" (or similar) 2 piece 3 jaw chuck to use soft jaws.

How do I mount the Bison on the Sieg SC4? The backplate on the Sieg has a 100mm flange which is also the same as the Sieg mini lathes.
Does Bison or someone else make a backplate that can be easily adapted?

Alternatively are there other brands that someone here has adapted successfully to the sieg mini lathes?

All help highly appreciated!

Regards
 
If concentricity is truly important you may want to switch to collets, especially if you are working with repetitive diameters. Otherwise it's possible to simply use pieces of thin strapping as jaw protectors to protect your work from the sharp edges of hardened jaws. Any number of YouTube videos show how.
Welcome!


Steve Shannon, P.E.
 
What Steve said, especially about collets. Poor man's collets work OK, a piece of aluminium or copper pipe/tube, slit so it can open and close with the chuck jaws, if you're posh make a few and bore them to the size of the work before slitting.

Dave H. (the other one)
 
Collets probably wont work for me as I will be using diameters of upto 3" or more. Some lateral shift might be tolerable ( a few thousands) but tilt of any sort is terrible for optics.
My main question was how to mount a bison chuck on a sieg sc4 lathe. Do I have to make a backing plate myself?
 
I've used cut open soda can to protect threads and wrapped a single layer. This was on steel threads IIRC.
 
You have failed to mention what configuration the spindle mount is. Threaded, A taper, D taper, L taper or a fixed mount as supplied.

Also how many parts 1, 5, 500 or 5000 in one set up? This makes a considerable difference. If only making 1 part use a 4-jaw and dial it in for each operation using pads in the jaws as suggested above in order to protect the finished surfaces. Bored soft jaws in a 3 jaw chuck will not repeat to the level of accuracy you might likely require.
 
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Basically there are two problems as I see it:
1. I use aluminum which is extremely soft. To hold it without marring, I need something soft (like aluminum). Using cut soda cans is a great suggestion but it reduces the gripping strength of the jaws and the part comes off easily especially while turning deep. Sometimes the part spins in the chuck. I am especially worried about damaging the soft aluminum threads.
2. Precision: This I can dial in with a 4 jaw chuck. Or use a bump tool with 3 jaw chuck. Non concentricity of even 0.5 mm is acceptable when rechucking. However tilt of even 0.1mm will be easily noticed. I'd rather use a 3 jaw chuck as it does take me a while to dial in with a 4 jaw.

I tried using cut soda cans but the part came off while cutting off and got severely damaged. The contact area with the chuck is tiny relative to the length of the piece. The only way to hold it firm (without soft jaws) is to tighten it hard with the jaws but that damages the piece.

My Sieg has no modifications done to it. It is a regular spindle mount. I will likely make 1-2 parts each time. They are all approximately the same thickness (M68x1 external thread usually) with varying dimensions on the other side.

I thought bored soft jaws are much more accurate than even collets if done right? Perhaps I was mistaken
 
You may find that soft jaws in a highend chuck will work and hold .001" repeatably but do not count on it, it is entirely possible that you may have to spend as much money on a single chuck that it will cost as much as the machine itself.
 
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