Poor finish question???

[JackG]

Can someone explain to me the cause of this (see the photo) and what I should be doing to avoid it?

http://home.comcast.net/~jtgartner/Misc/DSC01443a.jpg

As I cut it, the feed seems to jerk as if I’m hitting soft and hard places in the steel. I’m using a carbide inserted tool on a Sherline lathe, and the metal is cold rolled steel.

Thanks,

Jack

 

[doc1955]

It kinda looks like you have you tool slightly higher than the spindle center line.
To check this quick bring your tool up to your stock (spindle off) and put a 6 inch scale between you stock and tool and bring your tool up until it just touches your scale now notice if the top of your scale leans toward you or away from you it should be just about straight up or just a bit of lean toward you if it leans away from you your tool is to high.

 

[arnoldb]

Jack, there could be many causes...

It looks like you have some work-hardening occurring - that will account for the "hard and soft" spots.

Can you add a photo of the tool you are using ? Are you using any kind of cutting fluid/coolant ?

A carbide tool likes to take a fair depth of cut and generous feed rate to work well. I've never operated a Sherline, but they are small machines and might not be suited to the forces needed for running carbide tools on steel - I could be wrong here though.

Have you tried a sharp HSS toolbit on the same CRS ? - it might give you better performance.

Regards, Arnold

 

[JackG]

>>> It kinda looks like you have you tool slightly higher than the spindle center line.

Thanks Doc. No, I checked the tool height using your method, and the tool is very very slightly low.

>>> It looks like you have some work-hardening occurring - that will account for the "hard and soft" spots.

Thanks Arnold. I was wondering if it might be something lke that (work-hardening) because I always notice a hard spot at the beginning of the cut where I had previously cut the bar with a band saw. I am using cutting fluid, but taking what are probably fairly light cuts of about 10 thousands (The spindle speed is about 200 to 300 rpm, and I don’t really know the feed rate because I’m cranking the feed by hand. I can get a rough estimate tomorrow).

I’ll take a photo of the tool and post it tomorrow.

Assuming that work-hardening is the problem, how do I remedy it? Is it the metal (i.e., the incorrect type of cold rolled steel for working on a lathe), too high a spindle speed or feed rate causing the metal to heat up too much? Or something else?

 

[shred]

Hardware-store steel or from a metal supplier?

How sharp is the carbide tool? You might try tuning it up with a diamond hone.

 

[Mike N]

Sometimes CRS is a little gummy & the chips builds up on the tip of your carbide tool bit & creates cuts like your photo. A larger radius on the tool bit tip will help. Try a piece of 12L14 & you will find it easier to get a smooth finish.

 

[JackG]

>>> Hardware-store steel or from a metal supplier?

Hardware-store steel. What's the difference and what should I have used? Assuming that poor quality (or the wrong type) steel is the problem, can that type be cut on a lathe?

 

[gmac]

Jack;
I'm the rookie here, not the pro, but for what it's worth - I too had a lot of trouble with mystery steel (and store bought 1018). Like you I have a small lathe (Taig). Two things improved finish for me - lubrication and this cutter;

http://xa.yimg.com/kq/groups/2573836/1737539945/name/FinishTool.pdf

Most of my cutting has been done with HSS.

Garry

 

[steamer]

Hi,

If your cutting with a Sherline, you don't have copious rigidity to begin with, and carbide really needs that regidity. I would stick with HSS here.

The finish tool shown in the previous post is interesting, it's called a shearing tool, but they are a lot of work.

A bit of radius will probably do the same thing and take a lot less time to grind.

For roughing, I would not use a radius or at least keep it as small as possible.

This will reduce the cutting forces.

Make sure your tool over hang is as small as possible.

Hone the edge until it is razor sharp and has a near mirror finish...this helps a great deal with any good finish, but especially with small machines.


Power feed will give better results than manual feed

Touch back and let us know.

Dave

 

[shred]

>>> Hardware-store steel or from a metal supplier?
Hardware-store steel. What's the difference and what should I have used? Assuming that poor quality (or the wrong type) steel is the problem, can that type be cut on a lathe?

That stuff is pretty much bottom-of-the-barrel steel and you never really know what you're getting-- some cuts ok, some not. If you can get your hands on some 12L14, it'll make the cutting part a lot easier and more consistent, leaving you less variables to sort out finish-wise. 6061 Aluminum is good to practice on too (hardware store Al is almost certainly not 6061)

 

[JackG]

Ok, here's a photo of the tool I used for this.

From all of your responses I’m getting the idea that the basic cause is the metal heating during the cut, which in turn is caused by a combination of the type of metal and the way it’s being cut.

Ignoring the type of metal, if I had a tool with a smaller radius, the point of contact would be smaller therefore the amount of heat generated (not necessarily the temperature) would be smaller, and therefore there would be less work hardening. Is this correct, or is my logic in the weeds???

Maybe I should switch to HSS. The main reason for using the carbide tool is I don’t have a decent grinder or the skills to make one from HSS. I guess I need to get back to basics and learn how to make my own tools.

 

[nesikachad]

Can someone explain to me the cause of this (see the photo) and what I should be doing to avoid it?

http://home.comcast.net/~jtgartner/Misc/DSC01443a.jpg

As I cut it, the feed seems to jerk as if I’m hitting soft and hard places in the steel. I’m using a carbide inserted tool on a Sherline lathe, and the metal is cold rolled steel.

Thanks,

Jack

If this is redundant, sorry. I didn't take the time to scroll through the other contributors.

Poor surface finishes:

1. Cold Roll can be a challenge sometimes. If you can use cold rolled for your project, chances are you could get away with a leaded steel that'll turn very nicely. Just something to consider.

2. Tooling. It should be on center. Easy way to check is to face a part. If it leaves a tit in the center adjust the tool height accordingly until it wipes the face clean.

3. Tooling. Carbide is great stuff so long as it's used properly. It wants to run! If your machine is capable of it, buzz the rpm up a bit and let it eat. Too shallow on the cut depth will also cause poor finish. On my CNC turning centers I typically finish pass at .05" depth of cut on 4140 HT to get a good finish. The insert wants this. Granted, my machine is probably a little more "robust" than yours but it's worth experimenting with.

4. Coolant. Either flood it, or run it dry. Maybe a squirt of oil to mitigate chip weld. Thermal shock is real and it wears out inserts. A mist coolant delivery on insert carbide is generally not recommended by the tooling manufacturers.

5. Speeds/Feeds. Check your SFM and see where your at.

6. Tool radius and insert orientation. Fiddling with this can also be a big help.

Good luck.

C

 

[gmac]

Jack;
You do have the skills to make HSS bits. The grinder doesn't have to be fancy and you don't need high tech holders/gauges etc. I hadn't done it since high school and borrowed a friend's generic garage grinder (scary looking grinding wheels and all). I found that the material doesn't come off so fast that you can't slowly approach your shape. You want to frequently cool the bit in water. Don't worry much about angles - I used a small protractor to get "close enough". Just hand held the bit, letting the heat remind me to quench.

There's lots of info on the net about angles and shapes for the various materials we use - enough to confuse, but just a few bits can do a wide range of things to get you started. Here's a starting point;

http://sherline.com/grinding.pdf

Get at it and git 'er done...... ;D

Garry

 

[arnoldb]

Jack

I agree with Steamer and others suggesting HSS for cutting - it will be well worth your effort to get into grinding up your own bits, and it really is not difficult.

A sharper tool with less of a radius on the tip will place less strain on your lathe and will remove metal easier with less friction, and as a result less heat build up.

Work hardening material wants to be cut aggressively; usually at low rotational speed but at a good feed rate.

The cutter you posted will put a fair bit of strain on your lathe - that may be the cause of the problem you get at the start of the cut; the toolbit must first take up a fair bit of loading before it will start to cut resulting in it appearing that the material is hard when you start the cut.

If you can get your hands on free machining steel like suggested, go for it. If, like for myself, your choices of material is limited, then it's a bit harder and requires some practice to get the finishes.

Whatever you do, don't give up - experiment a bit with different cutters, cutting fluids, machine and feed speeds and materials, and things will soon come together for your own specific environment.

Regards, Arnold

 

[Maryak]

Jack,

As a fan of mystery metals, (why buy when you can scrounge), I mostly use HSS tool bits, ($ again). 9 times out of 10 slower is better but 1 time out of 10 quicker is better. As our Patron Rick often says, "Listen to the metal and the machine - they WILL tell you when you're doing it right."

For a good finish the width of the cutting face in contact with the work must be bigger than the pitch of the feed otherwise you are cutting a small thread. The other points about centre height etc are spot on although sometimes a good finish can be helped with the tool slightly and I stress slightly, above centre height to allow it to "rub" without grabbing.

Hope this helps

Best Regards
Bob

 

[steamer]

Hey JackG,

How ya making out?

Gaining on it?

Dave

 

[JackG]

Thanks, yes. The next time I head off the the tool store I'm going to pick up some HSS blanks and try to make my own tools (also stop using hardware store mystery metal). It's something that I've been wanting to do for a bit, and this gives me an excuse to just do it.

 

[jpeter]

I know the point of the tool is supposed to rounded but on a light machine or small part the radius needs to be really small. After I grind the tool to a point I ever so lightly touch the point straight on the wheel removing only the slightest amount of point, keeping in mind the needed relief angles. This flattening of the tip produces a tool that gives a nice finish without loading the machine too much. Junk steel is hard to manage. Properly setting up for carbide tools is also hard to manage. They're much less forgiving when making poor choices of feed, speed, changes in material hardness, interrupted cut, etc. than HSS. And too, when you foul up a HSS tool a quick trip the the grinder has it working again in a heart beat. Small machines generally don't have enough power to drive carbide tooling. For example, 1/2 inch bar should be turned with carbide at maybe 3200 rpm. I doubt a sherline can do that. Smaller diameters have to go faster yet.

 

[steamer]

Hi Jack,

Just another shout out....having any luck?

Dave