Speeds when facing

Ed

Surface feet per minute decreases as you move toward center.

I don't think too many people stop and change speeds as they face though. Start out at the right speed and accept that you will be slow at center.

My Clausing has a varidrive so I actually could give the speed control a turn as I head in if I was in a hurry ;)

CNC lathes have a constant surface speed mode that ramps the spindle up as you approach center. Also have a G code to say do not go about a certain rpm.

Wes

Yes, increasing the speed would be the preferred method.

On a machine with a fixed spindle speed, if the speed was selected for the smaller portion of the diameter, I expect that there are few lathes that won't demonstrate some chatter at some point on a 5" diameter facing cut if the spindle speed remains constant.

As the cutting tool approaches the center, the surface speed (SFM) decreases, reaching zero at dead center.

This same effect is taking place at the center of a twist drill. The SFM is essentially zero at the center of the drill point. The cutting edge SFM increases toward the outer edges of the cutting edges.

WB ......... metalworking projects

Yes.

Most CNC lathes have a feature that does this for you when you program the appropriate command

You also have to do the same when using a cut off tool

However...in most HSM applications, you really dont need to do so.

Gunner

I've never seen a problem when cutting at low speed with HSS.

Well, surface finish can go to pot when speeds are too low, and the variation in speeds across a large face is one of the classic problems in machining. When CNC came in, a very big deal was made of the fact that you could program for constant surface speed to overcome the problem.

In some old textbooks you'll see different comments upon it, one of which was a suggestion to preferentially cut from the center out, adjusting speed for the minimum that would give you a clean cut near the center and counting on the heat tolerance of HSS to carry you through to the outside diameter. This is iffy, of course, but it's the way it frequently was done in production, back in the days of HSS tools and manual machines in high-volume manufacturing.

I don't think there's any really good solution with a machine that can't change speeds during a cut. It's been a while since I read the old articles and books that talked about it but I don't remember any clear answers. I've run into the problem myself, which was one of the first things that motivated me to try using brazed, carbide-tipped inserts on my old South Bend. They handled it pretty well, especially in cast iron, which was giving me fits.

-- Ed Huntress

some times its the metal...

I got some metal that welds real good, but turning/milling no good, as the engineer at the steel place, where I bought it, "that stuff don't machine very well, but welding is good"

So, its kept separate from the other metal.

xman

The other side of the coin is 12L14 ;)

Wes

The specific problem tends to be that, if you are using carbide or HSS at high speed, you can get a beautiful finish on the outside. Swarf coming off yellow or blue (for carbide). At some point towards the centre the speed will fall into the region where the Built Up Edge phenomenon occurs. The surface finish will go to hell during this range. As the tool gets closer to the centre the finish will still be bad, because there is a rough gob of steel welded to the tip of the tool.

Running HSS below the BUE region all the way will avoid the problem, but can still benefit form changing the speed as the cut progresses if possible e.g. from maybe 50rpm at 5" to 250 at 1". This is easy if you have a variable speed drive. It's a waste of time if you need to do belt changes.

Running carbide above the BUE region can get a bit scary towards the centre of the part :-)

PS. The best way to get a nice finish is to slap the part on the surface grinder after facing

Mark Rand RTFM

Just so I understand: There is a definite BUE range of speeds, you are OK above or below it (as far as the BUE phenomenon)? Would an alternative solution be to run a fly cutter over it on a rotary table? Thanks,

Michael Koblic, Campbell River, BC

You might want to explain the type of surface fininsh you need, which may or may not be the same results as the type you might be anticipating (depending upon numerous variables).

Variables.. type of metal, overall size of the workpiece, the type and condition of the machine(s) being used/available, cutting tool (typically referred to as bit) geometry and material, operator's skills, and also speeds and feeds. Also, using a cutting lubricant, which is nearly always a preference.

With the correct application of variables, the built-up-edge (and cracking/chipping of the cutting edge) conditions should be completely avoidable.

The usual rotary table designs are very slow, and unless it's motorized (and built solidly enough to be used for feeding material while it's being machined), turning it by hand will result in variations in speed of the workpiece which will result in changes in the machined finish.

If the application demands that the finish of the surface has to be precisely flat and exceptionally smooth, then cutting it with a cutting edge might not be the correct application. Precision grinding will produce much better finishes, so second or third operations might be required to attain the required results.

Whether your available equipment can produce the fit and finish that you actually require can't be anticipated by others without more information from you.

WB ......... metalworking projects

That's what I've seen, too, but I'm not sure enough of the details to post them for posterity. I try to keep the speed below yellow chips with HSS and up in the blue chips with carbide. If you stop to change belt speed the bit will leave a ring on the work. Good HSS will take the heat. A lot of my bits are Medium Speed Steel from Enco, which at least are easy to grind.

With practice I've learned how to recognize and mostly avoid built-up edges and their rough finish. In general I think it's from following the bit grinding and honing instructions in old lathe manuals and water-annealing bad steel.

For some reason facing out seems to leave a better finish. Maybe it has to do with wear and play on my 1965 lathe, or because it's a shear cut.

Before I bought one I filed and sanded out the tool marks with a single-cut pillar file and fine SiC paper on a steel block. The amount of work involved is -why- I bought the $100 worn-out surface grinder, which has been very useful for finishing and for sharpening tools.

I have read here and other places that leaded steels don't weld well . How do they braze ? Though it's not likely I will be using any , since my main source doesn't carry it . Inquiring minds and all that ...

Mild steel disc - 5-8" diameter, 1/8" -1/4" thick. The desired surface finish is: a) Flat b) Even c) Equivalent to 300 - 600 grit d) The reverse side finish is less stringent - 200 grit would do.

At this point there is *no* machine available for this. I am trying to design one to construct from used matchsticks and double-sided sticky tape (you have to be British to get that reference :-). I am sorting out in my mind what is the best way of going about it.

So far all I have come up with is an old gramophone player - 33-1/2 to 78 rpm would be just right :-)

There is, it's called a Blanchard grinder. A wood lathe with a large faceplate might work, but don't sand or grind on a metal lathe unless it's too worn for anything else.

The machines used to polish samples in metallurgical labs are approximately what you need. They look like record turntables but are much more solidly built since you press the sample against them to polish it.

No go on the potters' or motorcycle wheel and a pad sander, like we suggested?

Brazes fine. I tend to use high silver content braze but I'm pretty sure copper based braze works fine.

Wes

There is, it's called a Blanchard grinder. A wood lathe with a large faceplate might work, but don't sand or grind on a metal lathe unless it's too worn for anything else.

The machines used to polish samples in metallurgical labs are approximately what you need. They look like record turntables but are much more solidly built since you press the sample against them to polish it.

No go on the potters' or motorcycle wheel and a pad sander, like we suggested?

On the 24th there is a big tool auction coming up here. I had a look what they had so far and there *is* a lathe. Looks ancient with bits hanging off it. It is quite big and space would be an issue. Also could not find anyone to tell me the details.

A chap near here had a lathe on Craig's list for $600. The info was sparse to say the least: It turned to be something like a 7X10 Pratt&Whitney bu that is about it. No pictures ("camera batteries low") and unable to demonstrate that it actually turns. I gave it up as a bad job.

Local Canadian Tire has a wood-lathe with the lowest speed 500 rpm. Two reviews gave it 1/5, the thing fell apart on them.

I have also got a 1/3 HP induction motor (I think) sitting on the top shelf. I have been looking into various modes of transmission (a subject I am even more ignorant about than others) with a view of pressing it into service.

Yours and others comments about using wood for at least some of the construction sent me thinking along different pathways.

It's nothing that has to be sorted by tomorrow...