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Rif
05-02-2008, 10:38 AM
Hello,

This maybe a newbie question but I have never seen it discussed...

I have noticed that if I use a tool to made 0.050 cuts (this is on a lathe) that I usually cannot make a 0.001 cut without resharpening the tool as the tip will get rounded off. If I try to use the tool, without resharpening, there will either be a poor finish or it will just rub on the work piece. However, the tool can make 0.050 cuts all day without any problems. I always have to re-sharpen it to make the final, finishing passes. Does anyone else have this problem?

I was thinking that maybe I should have a tool for the larger cuts and a tool for the smaller (finishing?) cuts.

Thanks,

Brian

A.K. Boomer
05-02-2008, 11:21 AM
I was thinking that maybe I should have a tool for the larger cuts and a tool for the smaller (finishing?) cuts.

Thanks,

Brian




I think thats a good assumption -- what i believe also happens is a slightly duller tool does not care as much with a deeper cut because its "peeling" and separating the two metal surfaces, the more you grab the more this effect is evident -- its actually using the rigidity of the metal to help separate the two surfaces as its being cut, very small cuts dont get to do this so it takes an extremely sharp tool with the right reliefs...

Rif
05-06-2008, 12:39 PM
I think thats a good assumption -- what i believe also happens is a slightly duller tool does not care as much with a deeper cut because its "peeling" and separating the two metal surfaces, the more you grab the more this effect is evident -- its actually using the rigidity of the metal to help separate the two surfaces as its being cut, very small cuts dont get to do this so it takes an extremely sharp tool with the right reliefs...


That makes sense. I'll have to grind some tools for course cutting. It appears that the same goes for end mills, as well.

Thanks,

Brian

Carld
05-06-2008, 01:00 PM
An important issue is to have the cutting edge at or a few thousandths below the axial center line of the lathe. When turning a small diameter shaft it is more important than with a larger diameter.

If the cutting edge is above the center line the front edge of the tool is rubbing on the work, especially on light cuts. It is better to set the cutting edge below center and I have never had issues with that.

It takes some time when using a lantern tool post and the tailstock dead center, but if your using a QC tool post you can set the cutter for each holder using a dead center in the tailstock if you know your tailstock is not high. I always put the top edge of the tool just under the sharp point of the dead center and that makes a good cut with as good a finish as your machine will get.

If your lathe has deflection issues and the cutter tool tends to move down under load you could set the cutter higher but I have not had a problem with lathes like that even setting the cutter a little under center.

The thing is you can't assume the cutter is at center line, you have to have a way to check if it is.

38_Cal
05-06-2008, 01:40 PM
The way I was taught by my step dad is to use a 6" steel rule/scale placed lightly between the lathe bit cutting edge and the workpiece. If the edge of the bit is higher than center, the centered rule will tilt away from you at the top, if it's low, the top will tilt towards you. Doesn't matter if you're using a lantern tool post or a quick change.

David
Montezuma, IA

Fasttrack
05-06-2008, 01:59 PM
Yep - I use an old school ID card that I carry around in my wallet. Works for "centering" bits and works as a straight-edge if you need to sketch a line. Very handy!

Usually roughing bits have a radius on them while finishing bits have a much sharper "tip" as AK pointed out. Do you have a copy of "Machinery's Handbook"? There is a whole section on tool wear and tool life. Very interesting and it will help you find an optimal DOC, speed and feed to keep your bits alive for a long time.

Also, I was reading an interesting book, Manufacturing Technology Vol. II ... can't remember the author though ... that had some interesting information about chip formation. Based on the chip formation, you can tell if your bit is ground correctly and your feed/speed/DOC is correct or not. Most machinist probably have a feel for this just from expierence, but I think the book would be useful for new guys.

Michael Moore
05-06-2008, 02:02 PM
Brian, I think you've hit on a plan, Have roughers and finishers and save them for the appropriate job.

cheers,
Michael

Carld
05-06-2008, 11:29 PM
There's no point in having a tool for rough cuts and a tool for finish cuts. Grind a sharp edge on the cutter and use it. on a heavy cut many times it will get a false cutting edge and the real edge will stay fairly sharp. A heavy cut will most always dull the edge even with the false edge. Before you use a cutter for a finish cut be sure to remove the false edge.

When you are using a sharp cutter for finish cuts it will always get dull and you don't want a false edge to form as it will give a rough finish. For finish cuts you may have to sharpen the cutter several times. Sometimes it helps to grind a slight radius to get a smooth finish. I use a QC tool post and I don't take the cutter out of the holder to sharpen it. I set the holder on the tool grinder table and sharpen the edge as needed.

If you are using insert type holders be very careful to not set them above center as most of them have no front clearance and will rub the work.

With brazed carbide cutters you can grind a front clearance on the tool.

toastydeath
05-06-2008, 11:53 PM
Another tip is to simply not take .001" cuts. It's not really a great practice.

I leave .020" or so on light finish cuts. Works great, even with an abused cutter. Your lathe, and this mileage, may vary.

I sort of agree on carld's point. Roughing and finishing tools are really only appropriate in either heavy machining or anything more than "short" production runs (with length based on in^3 removed per part rather than qty).

b2u44
05-07-2008, 12:13 AM
Another tip is to simply not take .001" cuts. It's not really a great practice.

I leave .020" or so on light finish cuts. Works great, even with an abused cutter. Your lathe, and this mileage, may vary.

I totally agree. No less than 0.010" for a finishing pass. If you absolutely have to take a 0.001" pass, then you'll probably have to re-sharpen, but that's a practice that I avoid.

Fasttrack
05-07-2008, 12:44 AM
Yeah good point - i was assuming that the .001 was unavoidable for some reason. I hate taking that light of DOC.

JRouche
05-07-2008, 01:29 AM
Great question and post.

I also have trouble getting a silky smooth finish trying to shave a thou off. My problem isnt machine rigidness, plenty there. Usually when Im turning a small diameter part and I think it is moving in and away from the cutter, even with a small amount of pressure. I usually switch to a super abrasive such as 3M lapping sheet to get it really nice. And a thou will come off quick, even with the lapping film..

Oh!!! On a side note. I have recently come into several hundred CBN and PCD inserts. Mostly in TCMT configuration.. I got these for a steal so I have been playing with them. CBN for steel and PCD for aluminum and brass..

Talk about SHARP!!!! These guys are sharp. Not a point shape, actually kinda round nose, largish radius. But the break from the top surface to the front surface is very sharp. Like a brand new cemented carbide bit, ten fold. So this sharp edge really cuts. And even at a thou (.001) cut. Any amount of metal, no matter how thin or small that happens to get in the way of the edge comes off. So if the part is turning round, and the 10EE turns parts round, it shaves the metal off.

The inserts are something new for me and I like them.. And they are hard!! Just messin around to get a seat of the pants feel for how hard they are I tried some stuff.

Just took an insert and did a scratch test on some steel, hmmm, scratches it easily, Did the same on a HSS milling cutter, same, nice deep scratch, lil pressure. Then a nice hard, new SGS, mirror finished carbide cutter (just on the shank). Same thing, both PCD and CBN made a nice sharp scratch. I expected it to kinda grip in but mostly slide off as any other material would. Nope, I could have etched my name in the shank if I wanted to..

So, having so many of these I thought the next step is upon each other. Just on the top of the tips, nuthing that would hurt or chip the cutting edge. These appear to be carbide with a chip of either CBN or PCD attached. Well, they both cut into each other, the diamond seems to be harder though. It left a deeper mark than the CBN..

And I know, its like rubbin rocks on metal at this point. But they also cut OH so nice.. Got enough to last my lifetime as much hard turning as I do. LOL Fun stuff!!!! JR

Mark Hockett
05-07-2008, 03:17 AM
On a lathe the best way to get a good finish is maintain a sufficient chip load for the cutter being used. The reason you don't get a good finish when taking a light cut is the cutter is scraping not cutting if the chip load is not high enough. It is very hard to get the proper chip load taking a light cut on a tool with a large radius. The way to increase chip load is raise the RPM's and use a smaller tip radius on the cutter. This is all assuming that your cutter is sharp. So there is a reason to use separate roughing and finishing tools. A roughing tool will have a large radius (.030" +) and less side angle to withstand heavy cuts. A finishing tool should have a small tip radius (.015" or less) and a sharper tip angle. One reason for the sharp tip angle is to keep the chips from getting caught between the work and the cutter. Most of the time my finish cuts will be at the max speed my lathe turns which is 2000 RPM for steel 2" and smaller. Also for light duty machines use a positive rake tool.

The material being cut also effects the surface finish. If it is not free machining material chances are the finish will not be great.

A while back Lane asked why a DRO on a lathe, well this is a good reason. A programmable DRO and a quick change tool post makes this very simple. I can use the same roughing tool for many hours and I don't care how much wear it has because I will use my finishing tool that has been programed in my DRO to make the exact size finish cut every time. The finish tool will last for a very long time because it only gets used for one pass per part profile.

JRouche
I can't believe you would have problems getting a good finish on a 10EE. If used properly that lathe should get mirror finishes on steel even taking a light cut due to the high RPM's available. That is the reason the 10EE is such a great tool room lathe is the speed range. Also check with the manufacturer of the CBN inserts as they require some very high SFPM to work properly and not self destruct. I payed $90 for one insert for a big plastic job I had. The insert lasted for 12 parts and came apart. My industrial supplier sent it to the manufacturer and they told me my lathe would not turn fast enough at 2000 RPM to use that insert, they said 4000 RPM or higher was recommended. They replaced it with a box of carbide inserts. Here is a good article on the CBN inserts,
http://www.mmsonline.com/articles/049605.html

gregl
05-07-2008, 11:06 PM
I once asked this question of a friend of mine who had worked in a tool room, and his answer was, "That's what emery cloth is for." If all you're taking off is .001, emery cloth should do the trick and leave you with a nice finish to boot.

quasi
05-07-2008, 11:30 PM
IF I need .001 off of something, I get out my lathe files.

Rif
05-08-2008, 12:32 AM
Yep - I use an old school ID card that I carry around in my wallet. Works for "centering" bits and works as a straight-edge if you need to sketch a line. Very handy!

Usually roughing bits have a radius on them while finishing bits have a much sharper "tip" as AK pointed out. Do you have a copy of "Machinery's Handbook"? There is a whole section on tool wear and tool life. Very interesting and it will help you find an optimal DOC, speed and feed to keep your bits alive for a long time.

Also, I was reading an interesting book, Manufacturing Technology Vol. II ... can't remember the author though ... that had some interesting information about chip formation. Based on the chip formation, you can tell if your bit is ground correctly and your feed/speed/DOC is correct or not. Most machinist probably have a feel for this just from expierence, but I think the book would be useful for new guys.

That is a good tip to center the tool! Right now, if I am unsure, I usually just eyeball it or check it with a tailstock center. Though, centering the tool is quite easy with a rise-and-fall cross slide.

Yeah, I have a copy of the "Machinery's Handbook." I'll have to check it out. I usually just refer to it for the tables.

I really should take a few minutes, some day soon, and determine what the actual speed is for all 10 speeds of my primary lathe. Since I haven't rigged up some sort of exhaust system, I have been cutting without any coolant. My lathe is in my basement, so if I use any oil I end up smelling up the whole house.

Thanks,

Brian

Rif
05-08-2008, 12:46 AM
Reading through the replies, I get the idea that maybe I have been doing things wrong?

If I need to cut a diameter of 0.500, for example, first I'll turn it down to something like 0.525. Then, I'll let it cool so I don't cut under size due to expansion. (I've had that happen to me a couple of times....I hope I learned my lesson.) After it cools, I'll mic it out and it may be something like 0.520. Then, I'll take 0.005, or so, off until I get it down to 0.505. Next, I take it down to 0.500-0.501 by turning 0.001 off at a time. Once I get it down to 0.500 to 0.501 (usually the part has a slight taper, that is why I am describing the diameter as 0.500 to 0.501) then I'll mic out small sections that are usually the width of the anvil on the mic. Any sections that need more attention get marked with a sharpy. Then, I'll use emery paper to take off the final 0.001 (or less) where it is required.

What I am reading is that I should probably turn the work piece down to 0.525 and let it cool. Then, after I mic it again and get maybe 0.520, I should take off 0.019 in a single pass and then use emery paper to take off the final 0.001 or less.

Oh, yeah, for my example I am considering my tolerance to be something like +0.001 -0.000.

Thanks,

Brian

Paul Alciatore
05-08-2008, 01:21 AM
.....
Thanks,

Brian
__________________
There are only 10 types of people in the world: Those who understand binary and those who don't.


But that's 11 types.

Or is it 12?



Sorry, I just couldn't resist.

toastydeath
05-08-2008, 02:44 AM
Reading through the replies, I get the idea that maybe I have been doing things wrong?

If I need to cut a diameter of 0.500, for example, first I'll turn it down to something like 0.525. Then, I'll let it cool so I don't cut under size due to expansion. (I've had that happen to me a couple of times....I hope I learned my lesson.) After it cools, I'll mic it out and it may be something like 0.520. Then, I'll take 0.005, or so, off until I get it down to 0.505. Next, I take it down to 0.500-0.501 by turning 0.001 off at a time. Once I get it down to 0.500 to 0.501 (usually the part has a slight taper, that is why I am describing the diameter as 0.500 to 0.501) then I'll mic out small sections that are usually the width of the anvil on the mic. Any sections that need more attention get marked with a sharpy. Then, I'll use emery paper to take off the final 0.001 (or less) where it is required.

What I am reading is that I should probably turn the work piece down to 0.525 and let it cool. Then, after I mic it again and get maybe 0.520, I should take off 0.019 in a single pass and then use emery paper to take off the final 0.001 or less.

Oh, yeah, for my example I am considering my tolerance to be something like +0.001 -0.000.

Thanks,

Brian

On a manual machine and for something as small as .500, I personally try to blow the whole roughing depth down in one pass if at all possible, leaving twice my finish allowance and some change above the final dim for one off parts in the +.005/-.000 range. I leave three times or more the finish pass depth for anything closer. For close stuff, I usually take .020", and for anything past +.005 I leave .050". If it needs to cool after roughing, I let it cool.

If I let it cool, I try to get it to 2x or 3x the DOC above the finish dimension. Then I take the first practice cut - .050 or .020. Mic it, note the difference from what I set it to. Dial in the next cut and adjust for whatever error there was in my trial cut. If it's a +.005 dim, that's the finish pass and I'll bet you fifty dollars I put it in tolerance. If it's less, I take the next cut and it should come out right on the money. Make any remaining tiny adjustments, and then the last cut.

I have held +.0005/-.0000 on a small production job on a manual lathe using .050 cuts, with a really heavy roughing operation. As long as you are consistent in feed rates and depths between parts, you don't need to let anything but the first part cool down. Mic it hot so you have a hot reference, and just do all your measurement at temperature. .5052 hot translates to .5000 at room temp, for instance. If you didn't screw up, they'll all be right.

Rif
05-08-2008, 11:44 PM
But that's 11 types.

Or is it 12?



Sorry, I just couldn't resist.


10, in binary, is 2, in decimal

0000 = 0
0001 = 1
0010 = 2
0011 = 3
0100 = 4
0101 = 5
0110 = 6
0111 = 7
1000 = 8
etc.

The first time I saw that statement, I think, was on a T-shirt. It took me a while to figure it out and I work with computers. :o

Brian

Rif
05-08-2008, 11:46 PM
On a manual machine and for something as small
.
.
.
your measurement at temperature. .5052 hot translates to .5000 at room temp, for instance. If you didn't screw up, they'll all be right.


Thanks for the advice. I'll try cutting a part this way, the next time.

Regards,

Brian