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Newbie needs help with mini-lathe

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devils4ever

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Hi all,

I'm a newbie when it comes to machining and I need a little help.

I have a MicroMark MicroLux 7x16 mini-lathe. I've been trying to machining some brass (C360) on it and I'm getting what I would call "catches" or "grabs" when I'm turning. The work seems to catch and either leave a deeper than wanted gouge or the work jams and the motor stalls. I'm looking for why this is happening. It happens on face turning and length-wise with a slow feed rate.

I'm taking very light cuts (0.003" - 0.005") and the RPMs are low (not sure the exact amount). Everything seems tight on the lathe. All gibs are tight without play as far as I can tell. I'm using HSS bits that I'm sharpening myself on a high-speed grinder. I'm fairly certain that the bit is very close to the center line of the work.

Any suggestions?

Thanks!
 

markba633csi

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Brass is a "tenacious" metal. Tighten up the gib screws on your carriage and compound as snug as possible and still be able to move, any slop will increase the tendency to grab. Grind your tool bit to have a minimum of rake. Keep the tool within the carriage footprint and not hanging out too far. A tool with a narrow tip as opposed to a broad tip may work better, with a small sacrifice of surface finish. There's probably a couple more things but I haven't finished my first coffee yet :)
Mark
 
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francist

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A zero-rake tool shape is often preferred for brass as well, ie: no back rake on the cutter. Less tendancy to grab and try to pull the bit in.

-frank
 

higgite

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....... the RPMs are low (not sure the exact amount).
Have you tried running higher rpm? I use approx. 200 SFM for brass on my bench lathe with an HSS tool.

RPM = SFM times 4 divided by Diameter of the work piece.

For a 1/2” OD workpiece, RPM = 200 x 4/0.5 = 1600 rpm. That's just an approximation. Play around with it a little.

Tom
 

devils4ever

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Thanks for everyone's suggestions.

I'm using brass because it's soft and I thought it would be easy to machine. I guess not!

I think all the gibs are tight, but I'll check again.

I'll try re-grinding the bit and see if it helps.

The brass work piece is 1.250" in diameter X 0.750" long. So, RPM is 200 X 4 / 1.25 = 640? I'm probably running it under 200 RPMs.
 

homebrewed

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In addition to previous comments, enlarging holes in brass can be problematic. Standard drill bits will grab and try to self feed into the hole. That's bad if you want a precise depth, ditto if the bit breaks off in the hole or if the bit pulls the drill chuck out of the tailstock....or pulls the work out of the headstock chuck.

To address this, you need to dedicate some drill bits to drilling brass, and flatten their cutting lips (a procedure called "dubbing"). I use a 600 grit diamond sharpening "stone" to do the job.
 

ericc

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Do you have some pictures? Both of the setup and the tool. Especially close up pictures of the tool and its grind. You may want to spend the money, just once, for a pre-ground tool to get this out of the equation. A few thou on brass is not much. It should cut fine. As long as you KNOW that this is how much you are cutting. A typical mistake is to have insufficient clearance ground on the nose of the tool, and driving it in with the feed. By the time the bit catches and starts cutting, it is much more than a few thousandths. Look very carefully at the dig and check if the chip is thick.
 

higgite

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The brass work piece is 1.250" in diameter X 0.750" long. So, RPM is 200 X 4 / 1.25 = 640? I'm probably running it under 200 RPMs.
Since that lathe doesn't have a tach, I’d try running it at about 1/4 of full speed on the dial, which should put it near 600 rpm, and adjust from there if need be.

Tom
 

brino

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also consider finding a piece of aluminum for practice. It cuts a little easier than brass.

Don't give up, there are lots of people here that want this to work for you.
any extra info you can provide would help us to help you (close up photos of tool bit, surface finish, etc.)

-brino
 

devils4ever

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I do need to drill some holes in this, but not yet. Hopefully, that will easier.

I am using a pre-ground bit. I've been trying to keep the angles and just touching up the surfaces. All the surfaces have positive relief and rake angles. It was probably ground for steel initially?

I'll try to get some pics tonight.

Thanks! You guys have been most helpful.
 

mikey

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Brass is one of the easiest metals to turn. When things are right, it cuts like butter. When things are wrong, you dig in.

The things that must be right include having the tool dead on center height. If the tool is too high, you rub. If too low, you will dig in, no matter what your tool geometry is. I suspect this is you key problem - the tool is low.

Ideally, you want a tool with zero to maybe 5 degrees of side rake only - no back rake. You also want fairly large relief angles, on the order of 12-18 degrees. Brass likes really sharp tools so grind the relief angles and then hone the sides and the flat top and the tool will cut well. Also hone a nose radius; for brass, this is not that critical but I prefer a smaller nose radius, maybe 1/32". Find a 1/32" drill bit and make the nose radius like the bit. With all this said, a standard tool with side and back rake will work but your set up must be rigid and it must be on center height.

I don't think cutting speeds are all that critical for brass. I tend to run fast, maybe around 1000-1200 rpm on a piece like yours and I run much higher for smaller work. The material is so soft that just about anything will work fine.

As for drilling, I think dubbing your drills is a good idea if you insist on using a pilot drill. If you go straight to your main drill without a pilot then it will drill just fine as is.

If any of this is unclear, let us know. For now, look up using a 6" rule to find center height.
 

mikey

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It looks like you're using a knife tool, which works fine for some stuff (facing, very light cuts) but for general use, a more conventionally shaped tool with a bit more side cutting edge angle would work better for you. May I suggest you look at the tool we ground in this thread? Grind it the same but don't grind the top rake angles. Put a nose radius on it, hone it and I bet it will work well for you.

Also, its hard to tell but it looks like you have a little nub on the face of the work. This suggests that your tool height is a little low. Fix that and the tool will work better for you.
 

devils4ever

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Knife tool? I thought this was a basic turning tool?

I did have a very small nub in the center of the face. I'll try raising it up a bit.
 

Latinrascalrg1

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I didn't read the entire thread so i apologize if this is a repeat.....

I had the following similar issue on my lathe which turned out was caused because the "torque" setting pot was apparently never properly adjusted from factory which caused me to experience exactly what you are experiencing on your lathe!

If your lathe is anything similar to the harbor freight benchtop lathes and you continue having motor stall problems then you may need to get into the control circuit board to see if there are 2 or 3 adjustable "pots" on that circuit board which will allow you to fine tune (increase or decrease) both speed and Torque settings which will keep the motor from stalling out so easily!
 

markba633csi

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Devils: Looks as though your tool is hanging out there too far- these small machines are limiting in that the carriage "arms" aren't very wide. Try rotating the compound to be at around a 45 degree angle and crank it almost fully back. Bring the tool bit closer in to the tool post also, that should help.
Do whatever you can to bring the cutting edge as close to the center of the carriage (when looking down on it) as possible
 

devils4ever

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I don't "think" the motor has a power issue. It was jammed when it stalled, but I can take a look.

I'll try making the tool bit closer to the tool post.

Just curious, how do carbide bits compare since the geometry can't be changed based on material.
 

mikey

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Knife tool? I thought this was a basic turning tool?

I did have a very small nub in the center of the face. I'll try raising it up a bit.
Yeah, that tool is called a Knife Tool. Have a look here; we discussed this tool and others in some depth.

Regarding carbide inserts on a little 7X lathe, you can certainly use them. Lots of hobby guys do. However, you lack the speed, power and rigidity to use them well. HSS will typically outperform inserted carbide on small lathes. Another option is brazed carbide tooling that can be sharpened to near-HSS sharpness. You can alter the geometry of these tools if you have the right equipment and know what you're doing and the tools will easily outlast HSS; they probably won't outperform HSS, though.
 

ericc

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To catch a rubbing tool, color all cutting faces with sharpie marker. If you get any more than a thin shiny line on the face, you are rubbing. That will make the cutting terrible.
 

devils4ever

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I had great success tonight! I ground a 3/8" tool blank without touching the top surface. It's not pretty, but it works. I reduced the overhang to a minimum, rotated the cross-slide and centered it to give maximum support. I increased speed a little and raised the bit to slightly above the center-line.

The cut was smooth and has a nice finish. Thanks to all! You guys are great!

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