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South Bend 9B having issues with dials.

Rhodian45

Plastic
Joined
Apr 25, 2019
I have been trying to figure out the dials on my south bend 9b that I rebuilt last year. I tried measuring cuts once before and it seemed that everything was double, 5 on the dial took 10 thou off; 10 took 20 thou off. Not sure if that is how it was designed, but now I am not sure that is even correct.

Just now, I chucked up a piece of scrap rifle barrel I had to mess around with threading the muzzle.

Initial measurement was .595" so figured I'd take it down to an even .5

Now assuming that the dial measures in thousandths, I did the following

First cut was 10 thou, then a second cut at 5 thou, and a third again at 5 thou. Total of 20 thou, with the dial on 20. Measurement after those cuts was .572, which not exact, but not terrible for the old lathe.

After that I set set the dial at 30 for another 10 thou cut. This time it took roughly 30 thou total off(did not rezero) measuring .538". Not good at all. Rezeroed the dial, turned it to 10 thou, measured it an this time it took 38-40 thou off. measuring right about .5". So while I did reach my desired diameter, it was by pure luck.

Anyone have any thoughts on what could be going on? I thought about setting up a DRO on it, but not sure if it would be worth the money vs just buying a newer lathe
 
The tool angle of attack ie the tip of the cutting leading into the cut can cause the tool to dig in and over cut the depth. IE the tip ofthe tool at 11 0 clock rest tip to be at 1 O clock. I learned this years ago on a similar light duty lathe with a rocker tool post. a dulling tool will take more force to cut and get progressively worse assuming your tool is leading into the cut. A picture of your setup would help.
 
Thanks for the quick reply John. I am still trying to wrap my head around all this geometry and angles of things. This does make sense though, since I just switched my tooling out from HSS I was using to the indexable tooling.
 

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Couple of things:
First, from the photos your lathe has the "small" dials on the cross-feed and compound feed. That means that the dial indicated how far the tool post moves, and NOT the diameter being taken off. So for a feed of .010" on the dial, your diameter is reduced by twice that or .020"

Second, you need to be really careful with carbide index tooling. A wide variety tip geometries are available and many of them don't have enough clearance angle to allow for light cuts on a light-duty machine like a South Bend 9". If you get the wrong geometry you have to really load the tool point to get it to cut and then once it does start to cut it will be taking off more than you'd planned for because of flexing of the machine.

Be really, really careful about the height of the tool point against the work. If you're above center and you have an unfortunate point geometry you'll get a result just like what you saw. High Speed Steel tooling is often a much better choice for the smaller South Bends
 
If you are curious at all, check the pitch of the cross slide screw sometime (a super easy way to do this involves a dial type indicator with enough travel)

What ever its pitch is is what is taken OFF THE SIDE of the work piece in a full turn - like zero to zero

Let's say it was 16 TPI (it probably isn't) . The reciprocal of that is .0625"

A full turn will reduce (or enlarge a bore) work piece diameter by .125"

Being curious is what makes the mental light bulbs come on
 
Yep ! Your tool is at 11 Oclock and will tend to dig in and cut under size. Rotate TP or compound until the point of the cutter is just to the right of 12 ,don't need to be much. That way if you get flex from tool load it will push away from the material and cut over size rather than take off more than is wanted. Actually the same thing can/will happen even with HSS tooling if the tool leads into the cut as your picture shows. If you must use carbide a insert with a chip breaker and positive rake will give best results in a worn light duty lathe like your old SB. BTDT !! And DO NOT set the cutter above center !!! That will cause finish issues as well as what you have now.
You did not do yourself any favors going to carbide insert tooling ,can get HSS inserts from Warner. Not to say that carbide can not be used on your SB but it will take some experiments to find what WILL work. And that can be expensive !! Positive rake inserts for aluminum will work with steel just fine and is really what you need for your lathe.
Member Exkenna sells tooling optimized for the light duty old home shop machines. Talk to him and see what he recommends.
 
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Thanks for all the info. As I said, I am still new to this, and trying to learn little bits at a time. I will adjust the compound when I head out to the garage later on and see how things go. As for the carbide inserts, they are cheapos that came with the holders that I planned on changing out in the future anyways. I only switched to the index tools because I am lazy and didn't feel like grinding hss bits.
 
Put a dial indicator on your carriage and measure how far the tool is moving in when you adjust your dial. That will tell you exactly how far it's moving in per cut.

Ted
 
As for the carbide inserts, they are cheapos that came with the holders that I planned on changing out in the future anyways. I only switched to the index tools because I am lazy and didn't feel like grinding hss bits.[/QUOTE]

Well the cheepos are most likely negative rake which will not work well with light duty lathes but, the inserts do look like they have a chip breaker which makes them act more like a positive rake insert. My 2 lathes are in the 1000 lb range and I do use some negative rake inserts but with chip breakers with good results. However I had to experiment with different styles before I found ones that worked good. I first tried a set of the cheepy holders/inserts and had poor results.But, a lot of your current issues revolve around the leading angle of atact of your tool. Just remember LAZY has a price ! LOL
 
So I went out and adjusted the tool post as suggested. This made a world of difference. I also put a Dial indicator on the carriage, and it corresponds with the dial on the lathe as well(10 is 10, 20 is 20). After making the adjustment, I repeated the testing I did last night and everything lines up as Dobermann stated. Attached is a photo of the end results. I think it worked out pretty well. Thanks again for all the advice.test barrel thread.jpg
 
I can only reinforce what everyone else has said. Been there done that. I make sure my tools are absolutely dead on center height, using a dead center to line them up. When I switched to carbide tooling it disn't like the negative rake tooling very much. I'm getting amazing good results with CCGT 32.52 inserts though, because they were meant for aluminum, with a high positive rake and a sharp ground edge.

I also have smaller (3/8 shank) tooling which is doing good with CCGT and DCGT inserts. For threading I still prefer my 3/8 HSS, it leaves a buttery smooth finish even in 1018.

When I switch to the positive rake inserts you could feel and hear the difference -- the machine wasn't working nearly as hard. Cuts came out right where I wanted them, on target. I had to improve my math because the machine would actually do what I told.

Cuts come out with a nice finish and the part doesn't get so hot any more. Sizes are right on.
 
Old trick I read about and put to use:
Get a dead sharp HSS tool perzactly on center.
Now move over and scratch the paint on the tailstock with it.
 
do you know how much backlash you have in your screws ? are you turning your dials far enough to get rid of any backlash before ya start making your cuts ?
animal
 
do you know how much backlash you have in your screws ? are you turning your dials far enough to get rid of any backlash before ya start making your cuts ?
animal
about 5 to 10 thou backlash. I turn past 15 to 20 thou past 0. This was one of the first things I figured out. It was 20 to 25 before i replaced the cross feed nut.
 








 
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