Lathe - Poor surface finish

[Hawke]

Gents,

Recently, I have not been able to get a decent finish on anything I turn. I have tried slow and high speed, deep and fine cuts, manual and machine feed, ceramic and HHS bits, dry and lubricated and the results are always similar.

I have also tightened up all the gibs, and used a center in the headstock.

I have put a dial gauge on the chuck, and cannot find any slop. The chuck turns OK when out of gear.

The material is a grade 5 mild steel bolt.

Any ideas chaps.

 

[A_Pmech]

I see three things:

1) Some chatter and resonance.

2) The faceting might be from a loose chuck or loose spindle bearings.

3) Very light feed and potentially wrong tool angles.

What machine are you using and let's see some photos of the setup and tool. Also, what speeds and feeds have you been using?

P.S: A Grade 5 bolt is NOT mild steel but heat treated medium carbon steel. They are heat treated to 120,000 PSI minimum, which works about to about 25-29 RC. The steel can also be rather abrasive.

 

[doug.j]

Have you tried an easy steel like 12L14? Some metals just want to fight.

 

[holt2ton]

Also, what is you depth of cut on your final pass? Some materials just plain don't like a light finish cut. Tool nose radius and positive/negative/zero degree rake can also play into this. What type of lubrication Oil or water soulable coolant? What are you making this for? Have you explored other material selections like a free machining type of steel?

 

[zkling]

Lets see a pic of your finishing tool, setup and some running numbers like DOC, feed and speed.

 

[Steinmetz]

Gents,

Recently, I have not been able to get a decent finish on anything I turn. I have tried slow and high speed, deep and fine cuts, manual and machine feed, ceramic and HHS bits, dry and lubricated and the results are always similar.

I have also tightened up all the gibs, and used a center in the headstock.

I have put a dial gauge on the chuck, and cannot find any slop. The chuck turns OK when out of gear.

The material is a grade 5 mild steel bolt.

Any ideas chaps.

Try a different alloy. As someone pointed out, you can hardly fail to machine Ledloy (12L14) successfully.

 

[Steinmetz]

Gents,

Recently, I have not been able to get a decent finish on anything I turn. I have tried slow and high speed, deep and fine cuts, manual and machine feed, ceramic and HHS bits, dry and lubricated and the results are always similar.

I have also tightened up all the gibs, and used a center in the headstock.

I have put a dial gauge on the chuck, and cannot find any slop. The chuck turns OK when out of gear.

The material is a grade 5 mild steel bolt.

Any ideas chaps.

I think you mean "center in the tailstock", unless the headstock and the tailstock are reversed in the southern hemisphere.

 

[Pudge87]

Just to mimic everybody else, some stuff just doesn't like to make a good finish. Lots of times in a situation like that, a .050 cut will do better than a .005 cut. Just pull out the old belt sander and smooth her up, quick and easy.

 

[justanengineer]

Are you remembering to tighten the cross-slide lock? On some lathes/setups you can get away without, others need it.

 

[Zeke]

I think you mean "center in the tailstock", unless the headstock and the tailstock are reversed in the southern hemisphere.

I think he meant what he said. He was looking for runout at the point. Not very good technique but you could always run a piece of something clamped to the compound over to the center just to see.

 

[DocsMachine]

Nope, your setup is fine. As others noted, it's the material- mild steel simply doesn't produce a good finish. It tends to pull and tear, or even smear, more than cut, and really, there's no easy way to fix it. I've tried all manner of HSS tips (negative and positive rake) several styles of insert, turned it fast, turned it slow, tried deep DOC and shallow, you name it. It's simply the nature of the material- it's "gummy", and tends to smear rather than cut.

If you need a part in steel and want a good finish, try a leaded steel, as noted above, like 12L14, or even 1144 if you need more strength. These steels produce amazing finishes with hardly any special effort. Just keep in mind that some of them don't weld well, if the part is going to be attached to an assembly.

Doc.

 

[zkling]

Geez, sounds like we have alot of "legal folks" (I'm trying to be nice) on here. If the solution is slightly challenging, why not just change the problem (in this case material) so it is easier to solve avoiding the initial problem all together.

OP, don't give up yet at the very least you will learn something from this experience. I've turned bolts many a times (cheapest way to find short hex stock) and have obtained much better surface finishes than that.

Nope, your setup is fine.

How do you know that for sure? Have you seen his setup? Sharpness and geometry of cutting tool? Checked the lathe over for mechanical issues. Notice how he says...

Recently, I have not been able to get a decent finish on anything I turn.

Hawke, can you elaborate on this?

 

[Hawke]

Gents,

Thanks for the quick replies. Its a HAVCO AL-320G lathe. I got it about 6 years ago new, and it has only been used for domestic stuff.

The speeds have been between 300 and 1600 rpm. There has been little change in finish. The feed is 0.005" per rev. The lubricant is soluble oil.

What I am making are some spacers and sleeves that will adapt a different type of flexible drive donut, used on the output of Formula Ford race car.

The tip is a TCMT-SM "Triangular 7° positive flank inserts for semi-finish and finish turning. Chip breaking in moderate feed range. Positive rake, low cutting forces. Used for a wide range of materials, including soft and exotic alloys. Designation : TCMT 110204-SM"

I have also used a sharp HSS tool.

The last cut was very fine - 0.001", but others were 0.15 mm with no apparent difference in finish.

 

[Hawke]

Forgot photos.

Have to admit, that I meant headstock - not tailstock.

The only things I have been making, are spacers and sleeves, and I normally use bolt material. When you run a race car, there are usually plenty of used bolts - aircraft quality and otherwise - just lying around.

 

[gtermini]

Hone your tool razor sharp with a slight radius (put on with the oil stone) on the end.

Bolts are ****** steel to machine a nice finish on, plain and simple.

Greyson

 

[A_Pmech]

I'll begin by saying I dislike TCMT inserts. They're an old design and the SM chipbreaker is finicky. That said, your feed rate and depth of cut is far to low for that insert. Increase the feed to about .012 per rev. Speed about 300 FPM. DOC about .03 inches or so.

For light finish cuts I prefer CNMG inserts with the FF chipbreaker profile.

I still see faceting on the surface of the part. I'm guessing it's the live center bearings. For what you want to do, there's no need to use the tailstock to make that cut.

 

[DocsMachine]

How do you know that for sure?

-Besides long years of getting the exact same kind of finish in mild steels?

The photos speak for themselves; The finish is not "shiny" smooth, but it's also relatively uniform along the length- that indicates the cutter didn't go dull midway through the cut. The smoothness also indicates the cutter wasn't periodically "digging in", which could happen with a worn leadscrew or loose gibs.

The diameter of the work is consistent (to the naked eye anyway) from end to end, which also indicates a reasonably tight carriage, no slippage of the work, and a good chuck.

The grooving of the cut at least shows he wasn't taking too heavy a DOC or too fast a feed. There's no signs of chatter, which might indicate too high a spindle speed, or bad spindle bearings. Ditto there's no real sign of the cutter being excessively high or low, and the finish indicates the traverse speed was consistent.

Really, there's nothing to indicate an excessively bad setup or any noteworthy wear. That is, as others noted, a common result when turning mild steels.

Doc.

 

[Dave455]

I reckon it's most likely a blunt tool as well!

Give it a touch on the grinder, or failing that an oilstone, but ensure you keep the rake angle about the same!

Alternatively, get onto MSC and get a replaceable insert tool! Give their 'tech team' a ring, tell them the size of your tool holder and wht you are cutting and they will sort you out!

I regularly turn mild steels and get good results with either insert tooling or a good (not Chinese) HSS tools! To make life easy get some free turning mild steel. It's got a tiny bit of lead so doesn't wld well but is gorgeous to turn!

 

[NASTYZEN]

Washers and sleeves for the doughnuts and your using old bolts?
You may want to look at DOM tubing for that use. Its 1045 Alloy if memory is good. Same stuff as for roll bars. Machines nicely as well.
On more powerful cars like the old Can Ams, I use 4340 tubing in the N condition for the long sleeves to keep them from crushing when the bolts are torqued.
You can get many size combinations of this type of tubing.