What Is The Biggest Diameter Of Steel You May Cut?

A. Fig Lee

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Hello guys,
I'm deciding on whether to improve my 7x12 metalworker or sell it and buy South Bend.
So far was impressed by SB - some people claim up to 7" diameter of steel successfully cut on it.

I only capable to do 1/2" of steel. 1" of steel very hard to cut off, though just a regular turning doable with
0.5 mm or 0.02" (diameter) for each pass.

Anyone had a luck to turning/cut off 3" steel?

P.S. I have 30206 bearings, have not installed it yet. To install I need a tool. Which is silly to buy since I have lathe and may make it. But lathe is not capable of making that diameter. Secondly, you need working lathe to cut spacer width..
Anyone in Toronto/GTA area wants to upgrade bearings? We may use each other lathe and do both.
Thank you
 
The maximum diameter which you can practically cut is going to be limited by the available torque, the lowest spindle speed, and the size of your chuck. As the diameter increases, more torque is required to make a cut. Small lathes don't have a lot of torque to spare. Also, as diameter increases, the surface speed increases, requiring running at a lower spindle speed. Finally, small lathes have small chucks which have shallow jaws. The ability to safely hold a large diameter workpiece is diminished. Using a center in the tailstock would, IMO, be an absolute on a workpiece approaching the swing of the lathe.

You mention cut on in the first paragraph and cut off in the second and third. If you are referring to parting when you say cut off, that is another issue. Parting is not the easiest operation on a lathe as can be attested to by multiple posts on this site. Small lathes have small parting tools and extending the parting blade enough to part even a 3" diameter in steel is a challenge. I would saw the piece with a band saw or (ugh) a hack saw and face the piece on the lathe to finish.
 
Parting off on a lathe can be a totally different beast than regular turning......the relatively wide tool face, the extended tool overhang, the changing sfpm all make parting more difficult. It really seems to exaggerate any slop/flex in the entire lathe; ways, cross-slide and tool post. Chatter and noise are common, but so is the tool getting jammed in the cut and the work piece trying to climb up the parting tool.

There are some very good threads here dealing with most of the issues:

http://www.hobby-machinist.com/threads/parting-for-idiots.32938/
http://www.hobby-machinist.com/threads/help-parting-on-a-curve.36268/#post-310396
http://www.hobby-machinist.com/threads/rear-tool-post-parting-tool-holder.36455/#post-310961

With my current setup (Southbend 9x54" lathe and AXA-style quick change tool post) I would not attempt to part-off anything over about 2 inches. However, I am considering a back tool post just for parting.....that should help.

-brino
 
As Bob mentions it's not clear if you mean simple surface turning or parting (cut-off).

To be very clear:

Do not part-off (cut-off) on a lathe while using a tailstock centre!
The cut off part needs a place to go otherwise it will pinch the cut-off tool and bad things happen.

-brino
 
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I worked in one tool room or another for almost 30 years, never parted off anything over 1 1/2 inches. We had horizontal and vertical band saws for that.
 
Thank you all.
Indeed.. I used to work 3 years as a lathe operator, but it was back 30 years ago and lathe we had was 10 kVA motor, 5000-10000 pounds ..
So, never had a problem with anything. But do not remember parting big diameter. I think anything bigger then 1" was pre cut.
That solves one problem. I may leave with it. Cut it with saw.

But what a practical diameter for turning (not parting) of steel?
Would it make on 3" diameter?
I saw examples SB could turn even 7".

P.S. Yes, I do understand cutting speed etc..
 
I've turned 3" steel and aluminum on my 7x10 HF mini. I do have angular contact bearings and a reduction pulley set installed. The angular contact bearings don't require any changes, they are a direct replacement. If you look up the approximated spacer difference you could cut one now and later use a belt sander to get it to the dimension you need for the tapered roller bearings. I believe the spacer modification is to get correct alignment of the gears?
 
Bingo, royesses, that what I wanted to know. So, I may stay with mini lathe. Just improve it a little bit. I have 30206 bearings, and if I remember correctly, they altogether wider something like 1.5 mm then original. Problem is, I do not know dimension of the spacer until I remove original bearings and get access to the spacer. Besides, I was told to replace the spacer with a metal one. But not sure which one was mentioned, that one, or other, which sits close to double nut on left side.

Edit: spacer in question is white one on picture 6 in http://www.arceurotrade.co.uk/machineguides/Mini-Lathe-Angular-Contact-Bearing-Change-Guide.pdf
But make a metallic one I think was on pic 3, black one. Same URL
 
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I only capable to do 1/2" of steel. 1" of steel very hard to cut off, though just a regular turning doable with0.5 mm or 0.02" (diameter) for each pass.

Anyone had a luck to turning/cut off 3" steel?

Something is off - a 7X12 should be capable of much more than this. I own a Sherline lathe and have turned a 3" steel work piece up close to the faceplate and regularly turn steel up to 1.5"OD over the cross slide. I regularly take a 0.05" roughing depth of cut in mild steel and part easily with a rear mounted parting tool in every material I've mounted on the lathe. A 7X12 is a much larger lathe in comparison so I'm curious to know why you can't do it.

If the problem is with the lathe then you can decide to upgrade it or move to another lathe but it could also be adjustments (gibs, headstock bearing preload, etc) or perhaps something about your cutting tools or whatever. It might allow us to be more helpful if you posted a pic of your set up so we can see what's going on. Something is just not right here.
 
I have parted off up to 1 inch steel stock on my HF 7x12 with no problem. I have also parted off up to 3/4" Stainless in it.
 
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