Thread: Surface Finish Problems?

Hello,

I've been (trying) putting the Hercus to good use making some spacers for the offroad buggy I'm building. Surface finish is a bit touch and go - sometimes its OK, other times it looks like the surface of an vinyl record.

Just wondering if someone might be able to help me trouble shoot?

Material Type: 1020 (I think) black steel bar.
Material Dia: 20mm
R.P.M: 1200
Feed: 0.0026" per rev.
Tool: Tungsten- Hare and Forbes banded trigon inserts in 12mm tool holders. Exact grade unknown.
Coolant: No.

To make the part, I have been drilling the 1/2" hole in the center, turning the major diameter, then the minor diameter and parting off. I have been turning with about 30mm extended from the cuck unsupported.

Any tips or suggestions would be much appreciated.

Thanks.

Tungsten inserts can be a real real real bitch to get a decent surface finish with...

As a rule of thumb as tungsten does not cut but instead shears the metal off you need to get deeper then the nose radius of the cutter and work it hard then you get a decent finish.... sometimes Your lathe can have a lot to do with it as well..... Those beautiful turned surface finishes you see that look like a mirror usually come off $50 000+ CNC machines..

My tip would be to use HSS and make a shear type tool for finishing as you are looking for consistent results..

It could be your depth of cut is too shallow for these inserts. General purpose inserts are not sharp. They have on purpose a very slightly rounded cutting edge to make them last longer. As a rule, it is not possible to take a cut shallower than the radius of a cutting edge (any cutting edge that is). If working to a dimension, you may be tempted to take a final cut only a few 1/100mm deep. A typical general purpose insert will then only "rub" instead of cutting, leaving an irregular "smeared" surface finish behind. This may explain your irregular results. Try to work in such a way, that you can take a reasonable depth of cut for your last finish cut (eg take control measurements and plan your finish cut such as to advance the cross slide no less than 1/10mm for the final cut). Or else, use HSS tools that can be sharpened to a much sharper edge and therefore can take much shallower cuts. Or use an insert designed for finish cuts (if you are now using TCMT, try a TCGT-Alu insert, these are razor sharp and have about 25 degrees top rake, designed for CNC cutting aluminium, but I find them excellent for finest finish cuts on steel too, as well as for plastics and non ferrous metals and even wood).

You can also try using a brush-on lubricant like cutting oil for the finish cut only. But do not use this for roughing (the insert will develop hair cracks from temperature shock, carbide inserts must always be used either dry or with flood coolant when roughing, never brush-on or drip coolant).

Other possibilities for irregular surface finish:

- unknown steel grade not suitable for machining (some steels leave a "cat tongue" finish no matter what you do)
- insufficient spindle bearing preload (irregular grooves)
- worn or damaged insert
- insert clamping screw loose
- bent leadscrew if using automatic feed (leaves a regularly spaced spiral finish)
- tool not at center height
- vibrations from drive motor

Hope it helps, Chris

Thanks for the help fellas .

Chris, just a few quick replies to some of the your suggestions.

-Depth of cut: On the long section of the spacer, all of those cuts were 0.5mm deep.
OD of the spacer was not critical so I was reducing the bar from 22mm to 19mm in three passes.

- unknown steel grade not suitable for machining
Steel is ordinary black bar, most likely a 1018 or 1020. I've read that it is not the best finishing material, so I will get some 4140 from work and give that a go as a check.

- insufficient spindle bearing preload (irregular grooves)
I put an indicator on the chuck, pushed/pulled it radially by hand and could get about 0.04mm of movement. I'm not sure whether this is acceptable?

- worn or damaged insert
Insert was new.

- insert clamping screw loose
My tools use a little cam. This was checked and found to be tight.

- bent leadscrew if using automatic feed (leaves a regularly spaced spiral finish)
As a rough check, I wound the carrage all of the way to the tailstock, wrapped a piece of masking tape around the center of the lead screw and put an indicator on it. I measured 0.8mm of runout,however given the dodgy way I measured it, I don't think it would be that bad.

- tool not at center height
Perfect when checked against live center in the tailstock.

- vibrations from drive motor
I put a glass of water on the top of the top cover. Perfectly still. I'm not sure what else I can check here.

RC, I dug around in my toolbox and found a piece of 3/8 HSS. I ground it up to the best of my ability as per the "Textbook of Turning" book. Speed backed of to 800 RPM
And feed increased a smiggen. I used a brush to apply some oil and below is the best result I could muster . Tool seems to be tearing the suface a bit.

I'm sure it all operator error . I better get back to the shed for some practice......

Something I observed with my students turning jobs relative to surface finish issues was using mild steel a for turning jobs which yielded a really crap finish.

A switch to1020 bright steel brought a vast improvement to surface finishes, especially when using HSS.The difference was night and day.


I perceive that many amateur or hobby turners are using carbide inserts as a one size fits all . Rake angles for inserts must to suited to the power of the lathe as well as observing the normal requirements for rigid set up, feed and speed.


To perform the standard sorts of jobs it was not cost effective to have 5 sets ( 5 lathes) of Straight, RH & LH tool holder as well as parting off tooling set up for carbide

High Speed steel on the other hand just needs a touch up on the wheel, meeting the normal requirements with set up and rigidity, care with correct speed and feed selection.


In the end the kids could see the value and the choice was usually carbide insert for heavy stock removal then to HSS for finish.

From your photos the surface finish appears the same as our standard mild steel bars gave. The tearing seems to look the same,so its entirely possible that your metal is your probably Black steel bar. I suggest a change to Bright 1020 steel.

Hope this helps
Grahame

> Tool seems to be tearing the suface a bit.

Try another steel bar. Best would be a free cutting leaded steel.

Just to verify it is indeed the material and not the machine or tool, try turning some Aluminium bar.
- If that turns with a perfect nice finish, it is your black steel bar that is not ideal for machining. - If Aluminium shows same bad finish, try honing your HSS tool after grinding. Then use a fine stone or a fine diamond lap to put a radius onto the tip of the tool (without blunting the cutting edge itself, about 0.5mm radius is fine).

What lathe is it? If it has taper roller bearings, a grossly insufficient bearing preload can indeed cause a finish like on your last picture. Chris

PS: do not worry about the leadscrew, it is only a problem if you can see it wobbling by eye, no need to measure. I did just mention, because it often gets bent in transit when moving a lathe. The effects would not be a torn surface finish like on your last pic, but a regularly spaced spiral overimpsed to an otherwise fine finish.

Are you using the clutch for feeding or using the half nuts.

Thanks again for the help.

Lathe is a Hercus 260 with 2HP single phase motor. I'm using the clutch for the feed, not the halfnuts.

I have gone away for the weekend, but when I get back I will try some othe materials as suggested. I'm pretty sure I have some 6063 aluminium bar laying around. I should also be able to find some offcuts of 4041 at work. Not sure about the leaded steel, what is it usually used to make?

Just out of curiosity, with everything set up properly with a material with good maching properties, what sort of surface finish should you realistically expect?

I see some beautiful examples on this forum, but I'm not sure if they are straight of the tool, or have been hit with fine emery tape.

> Just out of curiosity, with everything set up properly with a material with good maching
> properties, what sort of surface finish should you realistically expect?

I find the 260 is capable of a truly outstandig surface finish, especially with the automatic feed for both turning and facing. No need for emery cloth to improve!

But you mentioned earlier 0.04mm slop on the spindle, that is in my opinion very much excessive and would point to either loose preload setting, or worn out bearings. Just to be sure, where did you anchor the indicator? For this test, it should be anchored to the headstock itself (use a magnetic base). Because if you anchor the indicator say onto the cross slide or the bed and then pull on the chuck, you can easily twist the bed by as much 0.01 or 0.02mm and get wrong reading. Chris

Thanks Chris,

I think I put the magnetic base of the indicator holder on the bottom of the chip tray or maybe the cross slide?. I will remeasure and check using the correct method as you described.

Does adjusting the bearing preload require shims, or is it simply via a nut driveup arrangment like a car wheel bearing?

Originally Posted by Iggsy

RC, I dug around in my toolbox and found a piece of 3/8 HSS. I ground it up to the best of my ability as per the "Textbook of Turning" book. Speed backed of to 800 RPM
And feed increased a smiggen. I used a brush to apply some oil and below is the best result I could muster . Tool seems to be tearing the suface a bit.

I'm sure it all operator error . I better get back to the shed for some practice......

I cannot see a cutting angle on that HSS cutter... Should be angled about 10-15 degrees for mild steel..

Hi Iggsy,
What are you using to shim up your 3/8 tool in the photo? Just curious.

Originally Posted by Iggsy

Does adjusting the bearing preload require shims, or is it simply via a nut driveup arrangment like a car wheel bearing?

You loosen the lockscrew in the adjuster nut, and tighten the adjuster nut (it has a hole for a hook spanner).

How much preload is harder to tell. Lathe spindles need a lot more preload than car wheels. Simply quoting a tightening torque does not work, because a little too much and the bearings overheat, a little too less and surface finish and accuracy suffers. I know two methods, that together work reasonably well.

- First the coarse adjustment. Slacken the drive belt, put the tumbler reverse in neutral, mount the 3-jaw chuck, feed the oil cups for the spindle bearings. With just one finger on a chuck jaw, spin as hard and fast as you can. Adjust preload, such that the chuck spins for approximately one turn.

- The fine adustment is done by watching the bearing temperature rise. Let the lathe run at it's top speed for 10 minutes. feel the headsock temperature with your hand. It should feel handwarm but not hot to touch. Increase preload in very small increments until the headstock feels slightly more than handwarm after 1/2 hour at top speed. If it ever feels too hot (temperature runs away), immediately stop and back off preload and let cool before trying again. You can very quickly ruin bearings with excessive preload, and there is only a fine line between not enough and too much.

Chris

I decided to have a play with my Hercus 260 with some really cheap ###### steel to see what sort of a finish I could get...

First up the ###### black cheap steel bar.... 32mm size



I ground up a new tool out of 8mm square HSS from CTC tools.. Then did a rough cut with medium feed rate...

As you can see in the picture the chip curls towards the from.... The angle of inclination (cutting edge slopes back ~15 degrees perpendicular to the feed direction) does this, it directs the chip that way...



Next picture shows the finish I am getting on the first cut... It was not too bad, no metal tearing was evident, but was a bit rough..



I then did a fine cut with a finer feed and got a better finish, but not the best... The tool needed to be changed to get the best finish...

So I switched to the shear tool I have been experimenting with.... Everyone probably saw it in action on my shaper vid... It is the same tool..

As we can see in the picture, the finish is quite satisfactory.. it was smooth to the touch



So how do I grind my tools I hear you all asking...

I grind them up with a plain 8" bench grinder with a white aluminium oxide wheel, or if lots of metal needs to be ground I use a harder black wheel... No idea what grade the black wheel is as it came with the grinder when my father bought it in the early 80's...

Then to finish the cutting edge I have a 600 grit 4" diamond cup wheel (again from CTC tools) on another bench grinder and use that to polish all the cutting edges.... I then sometimes lightly touch the cutting edge up on a stone.. Does not need much as the diamond wheel does a good job on it's own..

Originally Posted by Jack620

What are you using to shim up your 3/8 tool in the photo? Just curious.

A mixture of things mate. I have a bit of 6mm flatbar making up most of the gap and an old set of feeler gauges making up the rest.

Chris, thanks for all that info on adjusting the bearings. I remeasured the spindle using the correct method as you described, and most I could get the spindle to move was 0.015mm. A bit better than 0.04mm. If I decide to try and adjust them, I'll be sure to borrow a temp gun so I can monitor the bearing temp accurately.

I tried a couple of of other materials today. Finish is better, but still not great. I forgot to mention before that there is play in my cross-slide - I can pull it backwards and forwards about 0.8mm. Could this be contributing to the poor finish?

RC, I would be pretty happy if I could get a finish like you have . What speed and feeds did you use?