Removing "Snipe" from Interrupted Cuts?

[evergreenblue]

Hello everyone,

It seems my participation here and with hobby machining in general is intermittent at best, but I'm back at it and have finally started working on my own "Puddleduck" 2-cylinder marine steam engine. I made a casting of some leaded red brass into a block for the cylinder head, and got it down to size, but couldn't get away from the little bit of what I'm calling "snipe" for lack of a better term in the start of the cut on each side. I squared the cylinder with my little AA109.21280 6" lathe. I quickly learned that any positive rake dug into the material like crazy, plus the little 3/16" bits that my toolholder fits just weren't up to the task of roughing down an interrupted cut like that. I found a single piece of 3/8"x1/2" HSS flat in the box of bits that came with the lathe, so I decided to stick that into the tool post and have at it with some negative rake and a large diameter nose based on what research I could gather. I also snugged up the gibs as much as practical to allow some movement to get things as rigid as possible. I roughed it down to shape pretty well this way, but it still gives a bit at the start of each cut on the rotation and produced a "snipe" on the leading edge of the block. I was able to get it down to finish size and reduce this a bit by bringing the rake closer to zero and feeding it ridiculously slow, but still ended up with some amount of "snipe" on the leading edge. I'm going to move on, as I think it's close enough to proceed with the project and I should have enough of a flat face on the top and bottom to seal the cylinders, but how do I get rid of this condition on a cut like this in this material in the future? Is it just because this machine is just not rigid enough and I've asked a lot for this particular operation? Any help or advice is greatly appreciated. I've looked through my books but don't find enough information to sort this out.

Thanks,

David

 

[markba633csi]

Pretty sure it's the lack of rigidity in the machine- the 109 has a thin spindle also so go easy
adding more side relief and keeping the rake near zero is probably best

 

[Eddyde]

I think I get what you are saying, where the interrupted cut starts, it is slightly deeper? If so, my guess is it's a rigidity issue, caused by the tool starting the cut at the set depth but then it's getting pushed back, away from the work...
Please post some pictures if you can, so we can better understand the issue.

 

[markba633csi]

Is this called "Snipe hunting"?
lol

 

[evergreenblue]

Fortunately I have a late model 109, so it’s got a 1” spindle instead of their 1/2” from earlier models. That said, I hadn’t considered the spindle as contributing to the issue. I suppose either way, this little lathe can only do so much so it is what it is. I guess I’m just trying to figure out how to minimize this issue for future reference if I were to have another situation like this again. Hopefully a milling attachment and other means will be in my future to help and I won’t need to make any similar interrupted cuts like this when the time comes, but who knows.

I’m including a couple pictures to illustrate what I’m talking about. Ha, I thought I did ok with the finish until I took these!




Sent from my iPhone using Tapatalk

 

[Winegrower]

I suggest keeping your eyes open for a better lathe. You'll thank yourself.

 

[markba633csi]

I'm going to backtrack a bit on my earlier tool recommendations and suggest trying with a sharp HSS tool with some positive rake.
If the tool cuts more easily you might have less deflection and less snipe-ing. There is more chance of grabbing/digging of course but
if you tighten up all the slides and take light cuts you should be ok
Takes some experimentation to tame the wild and savage brass beast- a tenacious metal

 

[G-ManBart]

It would help to see the setup you were using at the time. With big machines you can get away with some pretty bad setups, but with small machines you really have to get things just right.

 

[markba633csi]

No doubt- I simplify my life and just use bad setups all the time-

 

[extropic]

"Rigidity"? That looks like rubberyness. I suggest doing a survey with an indicator. Ground the indicator to the headstock or the bed. Apply the contact point to different things until you find what is deflecting (loose?). Use wooden levers as needed to apply appropriate force without damaging anything.
Do the survey with the machine unpowered.
Push and pull (in, out, up down, back and forth) on the:
1) Headstock housing relative to the bed (just to confirm the joint is secure)
2) Chuck (spindle bearing condition/adjustment)
3) Cutting tool itself and the tool post.
4) Compound slide (is the gib properly adjusted and the travel smooth from one end to the other?)
5) Cross slide (is the gib properly adjusted and the travel smooth from one end to the other?)
6) Apron relative to the bed (is it jumping up or around on the ways?)

I'm not familiar with your lathe, but I shudder to think that the headstock/bed are so rubbery as to allow that kind of finish. I'm hoping the survey finds something easier to put in order.