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How to get a better finish?

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WCraig

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#1
Hi:

I'm a brand-new newbie with a little Atlas 618. For the past few days, I've been trying various things to get a decent cut on some random scraps of steel. I've just ground a HSS tool bit from scratch but the finish still isn't smooth:

part_finish_1_small.jpg

The part is about 1/2" diameter and the lathe is set at 540 rpm. I'm using the power feed and I believe I'm on the slowest feed rate--need to verify. I tried to follow recommendations from 'mikey' in another thread and ground the bit with 15 degree angles for side relief, end relief and back rake. Used a coarse wheel on a 6-inch 3400 rpm grinder. Following is the bit _before_ honing:

BeforeHoning_1_small.jpg
BeforeHoning_2_small.jpg

I used a diamond plate to add the radius and a fine sharpening stone to polish all the cutting edges. Hard to get a good picture but the bit looked kinda like this after honing:

Honed_1_small.jpg

I have considerable experience with sharpening woodworking tools and I feel I got a good polished edge on each of the surfaces that come together at the cutting point.

If it helps, this is what the tool looked like in the lathe:

tool_bit_mounted_small.jpg

And these are the chips produced:
chips_small.jpg

Hope somebody can get me on the right track.

Thanks,

Craig
 

mmcmdl

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#2
Look back a week or so for a thread on this . ;)
 

samthedog

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#3
Try increase the RPM, take a deeper depth of cut and use cutting oil. The general rule is that the tool should advance the radius of the tip per revolution so you may want to play with the feed rate a bit too. It seems counter intuitive but sometimes getting a bit aggressive with the cut makes for a much better finish.

Also, some steel just machines badly. I don't much like working with mild steel for this very reason.

Paul.
 

P. Waller

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#4
Turn the spindle speed up, 540 Rpms is only 70 sfm at 1/2" diameter, I would nearly double the cut speed, 140 Sfm is 1070 Rpms at 1/2" Diameter.
You are trying to figure this out rather then producing 200 parts per day every day correct?
Wick it up and see what happens, it will be a learning experience if nothing else.

Also, use flood coolant if possible as you are fighting heat rather then lubricity, heavy cutting oils make terrible coolants.
This is 6 1/2" diameter 1045 turned at 425 Sfm using a simple carbide triangular insert, 250 Rpms, .020" Doc and .008" per revolution feed and a very small amount of coolant since I do not like a coolant bath all day.

Excellent finish and sizing, they required no further operations such as grinding or polishing. Keep the chips flying off of the tool.
 
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mikey

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#5
Hey Craig, I have a few comments:
  • First, welcome to HM!
  • Glad you decided to try HSS tools. Should work well on your lathe. In pic 2 and 3, it looks like you didn't quite sharpen the side edge all the way. See that dark part near the edge - that should be gone. I don't know if you corrected this when you honed the tool, though. Nose radius looks okay, maybe about 1/32". Otherwise, great job on grinding that tool!
  • Cutting speed for mild steel is about 100 sfm. Calculated rpm for a 1/2" work piece is 100 sfm X 3.82 / 0.05" = 764 rpm so you might speed it up a little.
  • Rule of thumb: if the work extends more than 1.5 - 2 times the diameter beyond the chuck, use a live center. To my eye, you are about 2.5 times the diameter and could use a bit more support.
  • Try angling the tool so the shank is perpendicular to the work. Be absolutely sure the tip of the tool is on center height. If I had to guess, that might be part of your issues.
  • Not sure what material that is. If it is 1018, poor finishes are a fact of life with that material. You might be better off using 12L14 or 1215 to practice with.
 

rgray

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#6
Definitely buy yourself some known material to try. like the 2 mikey mentioned or 1144 stressproof or brass or 6061 aluminum.
Then you'll have a better idea of just what's going on.
Amazon and ebay have plenty for good deals in about any quantity or length you might want.

Hobbymetalkits.com has some decent deals and are geared toward small quantity orders.
 

WCraig

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#7
Hey Craig, I have a few comments:
  • First, welcome to HM!
  • [1] Glad you decided to try HSS tools. Should work well on your lathe. In pic 2 and 3, it looks like you didn't quite sharpen the side edge all the way. See that dark part near the edge - that should be gone. I don't know if you corrected this when you honed the tool, though. Nose radius looks okay, maybe about 1/32". Otherwise, great job on grinding that tool!
  • [2] Cutting speed for mild steel is about 100 sfm. Calculated rpm for a 1/2" work piece is 100 sfm X 3.82 / 0.05" = 764 rpm so you might speed it up a little.
  • [3] Rule of thumb: if the work extends more than 1.5 - 2 times the diameter beyond the chuck, use a live center. To my eye, you are about 2.5 times the diameter and could use a bit more support.
  • [4] Try angling the tool so the shank is perpendicular to the work. Be absolutely sure the tip of the tool is on center height. If I had to guess, that might be part of your issues.
  • [5] Not sure what material that is. If it is 1018, poor finishes are a fact of life with that material. You might be better off using 12L14 or 1215 to practice with.
1 The grind for the side/back rake angles doesn't go all the way across the tool but the _tip_ was. I made sure the tip was honed.

2 OK, I'll bump up the speed.

3 If the end needed support, wouldn't the finish improve nearer the chuck? Seems to me to be same across the piece.

4 (a) This is something (tool angle) that I haven't got my head around. Only the tip of the tool is in contact with the work, right? Given that I'm working with a tiny Atlas 618, I'm never going to be taking heavy cuts. I've already stalled the lathe (belts slipping) a couple of times with what I thought were fairly shallow cuts. I've intentionally kept the belts a bit loose in case I do something 'exciting'. Anyway, the cutting tip is only ever going to be maybe 20-25 thous into the work? If the nose really does have 1/32" radius, that is 31 thous.

(b) I'm using the crappy homemade toolholder that came with the lathe. I have to use shims under the tool bit to try to get on center. I think I'll add the thinnest shim I've got and see if I get a better cut. When I first started testing, I was way below center and was getting heavy chatter. The cuts pictured above sounded much better to me than that.

5 Yeah, there is a Metal Supermarket in town. I should go buy some shorts. What I have now came from an auction box lot. There was a Myford Super 7 offered (and sold for C$ 1,400!). I got some brass, aluminum and a bit of steel that was stored beside this lathe. Cheap! (I like cheap.) But getting some known material would help.

6 Incidentally, this homemade toolholder is sized for 1/2" shank tools. The above cutter is 3/8" HSS with a 1/8" steel shim under it. Going forward, I've ordered a QCTP set from Shars. Obviously, that will make it much easier to get the cutter really at center height. However, I've been wondering what size of HSS blanks I should use for general purpose tools. 1/4"? 3/8"? I don't need 1/2" for general turning and facing, do I? It was a lot of grinding to do one 3/8" tool. It would be way less to do 1/4" tools. I imagine the smaller blanks are cheaper, too.

Thanks for the feedback (everyone). I'm going to stop typing and go try to make some chips!

Craig
PS One conclusion I've come to so far: a lathe is a very efficient tool for emptying your wallet!
 

WCraig

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#8
Re cutting oil/fluid, I've been watching Keith Rucker and mrpete222 on YouTube. They use an acid brush to dab on _something_ that then makes the cut go smoothly and results in a fantastic finish. What is the secret sauce?

Right now, I have a little bottle of Mibro Cutting Fluid that I got years ago to do some tapping. I've been using a little of it (when I remember) on my test cuts. I'd like to figure out what to use going forward.

Craig
 

WCraig

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#9
Definitely buy yourself some known material to try. like the 2 mikey mentioned or 1144 stressproof or brass or 6061 aluminum.
Then you'll have a better idea of just what's going on.
Amazon and ebay have plenty for good deals in about any quantity or length you might want.

Hobbymetalkits.com has some decent deals and are geared toward small quantity orders.
Re aluminum or brass. I got a handy pdf from steves-workshop.co.uk that walked me through the process of grinding a tool. However, it says that steel, brass and aluminum all need very different side and back rack. Eg 0 back rake for brass, 15 degrees for steel and 35 degrees for aluminum. (Suddenly, I see a LOT of grinding in my future.) I really need different bits for each of these metals?

Craig
 

pdentrem

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#10
Yes on the different metals = more tools.
 

Shootymacshootface

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#11
What mikey said and use your tailstock. A file with a handle different grits of emery cloth and some scotchbrite can work wonders when needed.
 

benmychree

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#12
Probably, 1/4" tool bits would be appropriate for that lathe, I have a 9" Monarch, and use 1/4" bits on it; plenty good for the size of machine, less grinding required. For cutting fluid, I use Tap Magic. and yes, as above, filing and polishing are part of lathe work, for sizing and appearance.
 

mikey

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#13
If the tool is ground up near the tip where all the cutting takes place then yes, that is what matters. However, I see no reason not to grind the tool properly. When you learn to fully use that tool and learn to angle it toward the tailstock to take fine finishing cuts more of the side cutting edge is engaged. At least be sure the first 1/4" of the side cutting edge is fully formed.

Insofar as tailstock support is concerned you will find that the finish will smooth out when you get to the inner 1/3 of an unsupported work piece, and the closer to the chuck you get, the smoother the cut. With a live center the cut is smooth all the way and your accuracy will improve.

I've not ever used an Atlas 618 so I'm not sure of its capabilities but I use 3/8" square tool bits on my little Sherline lathe and can easily take 0.040-0.060" deep cuts in mild steel when roughing. I would imagine your lathe would be capable of better than 0.025" cuts if everything else is okay.

I use 3/8" tools on my Sherline and Emco lathes and it works fine for me. Takes me all of 3-4 minutes to grind a tool so not too long.

The tool post is important, Craig. The tool can only cut if it is held rigidly and on small lathes with low horsepower and rigidity, it is important to get the tool on center height as well.

You can use just about any readily available cutting fluid. I use Tapmatic Gold most of the time but you can also use Sulfur bearing threading oil from the hardware store if you prefer; stinks, though. Use WD-40 on aluminum and none for brass.

Brass tools have zero rake on top so the tools are quick to grind. Hone them well, though.
 

catsparadise

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#14
Have a look on the web for details of a shear tool. Dead simple to grind (only 2 faces to grind). Only takes off a few thou at once but you can use one to get a much nicer finish on a lot a steels that wouldn't behave otherwise. I use one quite often to tidy up EN8 (medium carbon - not easy to make it shine!).

Rob
 

BaronJ

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#15
Hi Guys,

I agree about using a shear tool to get a good finish. I tend to use it when machining shafts to run in PB bearings.

This is mine. 10 mm square HSS M42.
shear.jpg
Craig: The cutting edge is the vertical one with the backwards slope. Looked at from the chuck side, it looks like a "L". Apart from the backwards slope angle, about 20 degrees, the only relief is behind that vertical edge. I forget the exact amount, but ten degrees is enough.
 

Shootymacshootface

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#16
Have a look on the web for details of a shear tool. Dead simple to grind (only 2 faces to grind). Only takes off a few thou at once but you can use one to get a much nicer finish on a lot a steels that wouldn't behave otherwise. I use one quite often to tidy up EN8 (medium carbon - not easy to make it shine!).

Rob
I learned something new today!
Thanks mate!
 

rgray

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#17
Re aluminum or brass. I got a handy pdf from steves-workshop.co.uk that walked me through the process of grinding a tool. However, it says that steel, brass and aluminum all need very different side and back rack. Eg 0 back rake for brass, 15 degrees for steel and 35 degrees for aluminum. (Suddenly, I see a LOT of grinding in my future.) I really need different bits for each of these metals?

Craig
Almost all my tools are 0 backrake for both my manual lathes. I do have a few carbide inserted tools but use mostly HSS tools and 3/8" is what I use most of on my 12x 36. Most tool holders for my small lathe (Ames) have 1/4" hss bits.
I have 1/8"-1/2" bits. The 1/8" are used in my small (and large) boring bars and I have a quick change holder with one out each end that comes in handy now and then.
I had a hss face mill for the mill that had me grinding a tool like you show as it had to cut in a circular path. Sold the face mill but had extra 3/8" tools ground for it that I kept and they work good on the lathe, but not greatly better than my zero back rake tools. So I am usually cutting with flat top tool bits.
 
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