Question about cuts.

I have a little 10" I bought late last year and have a question about cut finishes.

If I take a .020 cut out of a piece of stock it produces a relatively smooth cut. Now when I go to take a smaller cut say .005 the finish on the piece looks more like a record with the grooves. Same speed & feedrate on both cuts.

Now I'm admittedly new to all of this with my only real experience from one college manufacturing class but this one has me stumped. Any ideas?

what sort of toolbit?

allan

What is happening is that with the heaver cut you are taking all the "spring" out of the lathe so it doesn't allow the tool to move in and out. This in and out movement is what is making the groves in the work when you are taking a smaller cut.
Gary P. Hansen

garyphansen said:

What is happening is that with the heaver cut you are taking all the "spring" out of the lathe so it doesn't allow the tool to move in and out. This in and out movement is what is making the groves in the work when you are taking a smaller cut.
Gary P. Hansen

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What he said. You might try honing the cutter razor sharp with a very tiny radius, see if that helps.

Crew cut looks nice.

Most lathe cuts are a thread.
A radius nose makes the the lines less pronounced. Smooth cuts are much better than chatter.
Think about a .010 nose radius and a .015 per revolution feed rate.makes nice lines going round.
Chatter ... lines horizontal can be many reasons like length of part, diameter. A slug of children's clay can help if not having a steady.
*Sharpness of tool bit,cutting edge angle. position height to center, rake angle, nose radius.

kitno455 said:

what sort of toolbit?

allan

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Currently using a precut carbide triangle blade. I will try to order a smaller radius next time I place a tool order. I figured maybe it was because I wasn't taking all of the backlash out of the cross slide but trying to remedy that proved ineffective.

Anything else I should try? Anything possibly worn out on the lathe that might need replacing?

Material?
Size?
RPM?
Feed rate?

The above can effect the looks of a cut even with the proper cutting tool and good machine.

For instance, a heavier cut can look better than a light cut because the heavier cut produces enough heat at the tip
to make the material machine differently with a light cut producing little heat.

If your turning to slow on soft material with a light cut it will leave a crappy looking finish.

If you're kind of new to this lathe operation stuff, I'd strongly recommend that you put the carbide tooling away for a while and switch to high speed steel tools which you can grind yourself and get sharp enough to give you the finish you want. Carbide tooling can be difficult to use at low turning speeds.

Look in the How to Run a Lathe handbook for some guidance on what HSS tool bits should look for the various turning operations.

Also, it you're turning a bar with a lot of "stick out" from the chuck jaws you should give serious consideration to using a center in the tailstock to support the end of the workpiece to eliminate as much of the spring back of the work piece as possible. Either a dead center or live center will work equally well for this.

Anything else I should try?

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Forget the carbide and try hand ground and honed razor sharp high speed steel - the difference will be night and day, plus you will have learned something about cutting tools that won't be gleaned from "ordering" them

Dobermann said:

If you're kind of new to this lathe operation stuff, I'd strongly recommend that you put the carbide tooling away for a while and switch to high speed steel tools which you can grind yourself and get sharp enough to give you the finish you want. Carbide tooling can be difficult to use at low turning speeds.

Look in the How to Run a Lathe handbook for some guidance on what HSS tool bits should look for the various turning operations.

Also, it you're turning a bar with a lot of "stick out" from the chuck jaws you should give serious consideration to using a center in the tailstock to support the end of the workpiece to eliminate as much of the spring back of the work piece as possible. Either a dead center or live center will work equally well for this.

Click to expand...

The lathe came with some HSS blanks that I tried to grind myself (although made in China). I read up on grinding and watched multiple videos on youtube about the subject but every time I tried to cut with them it the workpiece would take more metal off the tool. Gave up after going through 3 bars of the stuff. I will order some and try it again.

Currently I have setup 10in of 1.5 diameter cold rolled steel chucked into a 3 jaw chuck with a live center in the tailstock. I am currently trying to turn some pins for farm equipment that have out of spec holes. The pins I have currently made work fine but just not "pretty" so I figured there were people smarter than me who could point me in the right direction. The consensus is that HSS would work better for my needs so I will give that a try.

IronReb said:

Material?
Size?
RPM?
Feed rate?

The above can effect the looks of a cut even with the proper cutting tool and good machine.

For instance, a heavier cut can look better than a light cut because the heavier cut produces enough heat at the tip
to make the material machine differently with a light cut producing little heat.

If your turning to slow on soft material with a light cut it will leave a crappy looking finish.

Click to expand...

IronReb,

It pains me to have to admit this but I am honestly do not know how the RPM and feed rate factors into lathe operation. On a mill it is no problem for me just two equations and I know exactly what rpm to spin my tool bit and what feed rate to set my machine to make sure I am not snapping bits left and right. I have looked but maybe I missed something somewhere. Again guys relatively new on the lathe so take it easy on me.

Forget the HSS.

Just add surface speed.
For 1" D bars, about 700-1000 rpm.

If the chips are not coloured, you are using too little rpm.
You want some color, blue or black is too much.

Also, probably:
The material is crap.
The toolbit is crap.
If either is the case, you may struggle with any tool.

IE use known steel from an industrial metals suppler.

I havent used HSS in 10 years, and would have no desire to do so.
I can take single micron level cuts with positive rake carbide tools (small CCMT type, size 21.51).
(Tool tip wears fast in tiny cuts).

Finish is good to excellent on standard tool steels, and universally excellent on hard steel (ballscrews, sliver steel aka drill rod, hardened bolts, all stainless 303, 304, 316l).

Title could have been "groves in lathe turning"

Inserts can be the very best for hard stock and production work. For one ups, mild work and special shapes you cant beat HSS.

http://www.steves-workshop.co.uk/tips/toolgrinding/tool-grinding-poster.pdf
Tool bit sharpening is very simple and CRS should cut like butter. A simple bench grinder with a course wheel on one side and a fine wheel on the other.(just the wheels that come with are OK) The table on the fine side set at about 10*. Side cutting angle perhaps 30*, front angle perhaps 30* both at 10* (about) ok (that way you can turn the bit to make a 90* corner if needed). dip the bit in water often to keep cool. Grind the two angles rough then finish wheel..Turn the bit to grind top and holding so you grind an angle off the side cutting edge slopping away about 8* (7 to 10 OK)and there you have a simple tool bit. Turn the wheel off and as it goes a little slower make a smooth turn to make perhaps a .020 radius at the very point. Now you can take a hone and holding at slight less angle make a bright shinny land at the very edge on the side cutting edge.( with a little experience you hone the radius also)
Note that when two angle come together the make a clearance angle greater than the two angles.. That is why often a grind bump cat the nose makes the bit last longer. (Yes if the part allows an radius.)You can use that one tool bit bit to turn an OD, make a sharp 90* corner, face , nick a center or drill hole start or make a after thread under cut all with one bit.

You will see the way the chip flows away from the cut. Next bit you can add some back rake and the chip will flow away from the part. Soon you will be so expert at sharpening you can just hold the bit not even set on the table and make good tool bits. Best to buy US, Brit or German bits to know they are good steel,

Buy one good HSS bit, maybe a left hand turning tool. Get a good one; if you can't find one local, you can always look in McMaster-Carr. Then, use that to learn to set the grinder to make the several angles. Check to see how sharp the edges are, and try to duplicate that.

Check whether your tool edge is on center with the work by using a thin machinist's rule between the two. Advance the feed until the rule is held lightly between the two. If the rule is vertical, you're on center. If it tilts back, you're above. If it tilts toward you, you're under.

ISEN AG said:

IronReb,

It pains me to have to admit this but I am honestly do not know how the RPM and feed rate factors into lathe operation. On a mill it is no problem for me just two equations and I know exactly what rpm to spin my tool bit and what feed rate to set my machine to make sure I am not snapping bits left and right. I have looked but maybe I missed something somewhere. Again guys relatively new on the lathe so take it easy on me.

Click to expand...

No shame in not knowing bro.
The fact your were able to turn a pin with no knowledge at all of an engine lathe operation says a lot.

I am not the best teacher online, hard for me to convert what I know and feel into text sometimes so here goes...

Tooling
It makes not much difference at all what tool you decide to use. HSS blanks are great for forming a feature, radius or grooves.

They also can do turning just fine but at much slower RPM than carbide.
The trick to it though is proper grinding and tool holding.

Brazed on carbide is a great next step up https://www.mscdirect.com/browse/tn...Holders/Single-Point-Tool-Bits?navid=12105896

These tools will machine right out of the box.
They can be re-sharpened but it takes a special grinding rock and is a bit more of a pain to grind, not much but
for sure more messy because the rock breaks down so fast.

Inserted tooling for one who uses a lathe often is the way to go, depending on the style of insert, you can get 6 cutting tips per insert..$3-$18+ each.
Dull the tip, rotate insert and a minute later your machining again.

All tooling have parameters they operate best at in a given material.

And also very important, tool nose center.
Its important that no matter the tool that it is on the center line of the part, to low or high will cause issues like broken tool edges
and crappy finish.

Boring on the other hand seems to work better if it is just slightly higher than centerline.

A quick easy way to check is to eyeball the tool tip to center of the tip of your live center which is on centerline of the machine, or should be.

RPM
Tooling is rated at surface footage. The numbers I use next are just for examples only and are not correct math or from the book....

HSS machining mild steel is something like 60-90 surface feet a minute (SFM), meaning if you were to drag that tool along a straight line it can handle
the heat generated if it traveled 60-90 feet in one minute.
If you were to drag it 180 feet in one minute you would generate to much heat and melt your tool tip.
If you dragged the same tool 20 feet in a minute the material will tear and stick to your tool tip and leave a crap finish.

Its the same game with even a high dollar inserts except instead of running at 60-90 SFM like the HSS, the carbide insert will need to run at say 800 SFM.
Meaning your lathe would need to be max RPM for smaller diameters.

There are tons of apps to convert SFM to RPM, or you could use the pie X diameter stuff...or is it pie times radius? Hell, I forget now.

Feed
1/32 tool nose radius
.006-.007 and a little sand paper = seal fit.
.008-.010 = nice finish with slight tool marks.

1/64 radius...about half

Hope this helps till one that better at explaining chimes in.