Newbie needs help with mini-lathe

[devils4ever]

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]

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

 

[francist]

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]

....... 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]

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]

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]

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]

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]

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]

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.