How to get a better Finish

The usual question from a beginner. The only way I can get a reasonable finish on steel is to run the lathe on high speed at about 3000rpm using a HSS cutter on 1.2" steel round. Is this normal for a small lathe I spent a full day swopping cutters carbide and HSS with fast feed slow feed. I started of at about 600rpm and slowly wound it up once I switched to high speed and wound it up to about 2500rpm things started to look some thing like.

Any advice appreciated.

I just checked my lathes max speed is 2500 so it must of been about 2000rpm my digital read out keeps going blank.

There are a few questions.

1. Are you using auto feed?

2. What grade of steel?

3. What depth of cut?

4. Radius tool?

5. Edge clearance?

6. Tool overhang?

starter for 10

Bob

As far as this relative begginer understands the ideal cutting rpm for 1.2in mild steel is around 350RPM .. so running HSS at 2000 or 2500 suggest to me that your burnishing rather than cutting.

Also as far as I understand the finish will depend on the grade of steel..leaded giving a better result and the radius of the tip of your HSS tool as well as depth of cut and feed rate. It;s also going to depend on the rigidity of the lathe.. to have the option of 2500rpm - what lathe? My crusader only goes to 1800+ rpm and I've noticed that on some gash mild steel I get a better finish with a heavier cut on a wide radius HSS tool than i do with a finer cut tool...but that may also be due to a better stoned finish on the larger radius tool. For practical reasons using an indexable tool I can polish it nicely on the lathe with some scotchbright rather than keeping on fiddling.; the depth of cut and speed of cut making up the time.

As a relative beginner myself I find the auto feed that Bob mentions is way better than by hand on my small lathe when using HSS cutting tools. If I want a really good finish I turn the diameter to within 0.05 to 0.1mm an use P120 sandpaper (ally oxide) to remove machining marks, then P400 emery paper to finish.

Using carbide insert tooling on the smaller lathe gives an ok finish when taking ~0.5 to 1.00mm cuts.

Another popular tool for finishing is the vertical shear lathe tool. Plenty of references on this forum and on youtube.

Ed.

Normally, it is all about 'Feeds & Speeds' Dave - but a sharp HSS tool (with a slightly rounded nose) should be able to get you a good finish at a lot less speed than you are describing. I manage to do so and none of my lathes will rotate the work that quickly!

Check your tool height and eliminate all unnecessary 'overhang'. Remember that a finishing cut is generally a light one. If the tool is sharp enough and your feed rate less than the nose radius (so you are not cutting a very fine thread), then you should get a pretty good finish. Of course, different people have different definitions of "good" - but "good" to me is that there is no need to use any secondary polishing, filing or sanding.

Some materials are more difficult than others to turn well. So try to get some leaded (e.g. free turning) material and experiment on that. The key thing to is to 'know' (e.g. believe) that it's possible to get a good finish. Once you believe that fact - then it's much easier to persevere and work out why you are not getting one. If you don't believe it's possible, then it's very tempting to just give up before you've discovered the real reason things are not working well.

In my experience, it will be something very simple causing the problem, which (with hindsight) will have been entirely avoidable. Of course hindsight is a wonderful thing after you've spent a goodly while going around in circles trying to figure it all out.

So far, you've tried "Speed" - so look at feed rate, tool height, sharpness, rigidity (work/tool extension), gib adjustments and that (maybe) it's simply very crappy material and all the other (relatively minor) things that can cause a poor finish. It may well be more than just one problem combining to cause the overall poor result. Try to methodically eliminate any possible causes.

Good luck!

Regards,

IanT

.

I (as a relative new comer myself) have also been down this road.

I am going to make a guess that the HSS tooling has not got a 'proper' edge on it. I had a similar problem and was convinced that because I had purchased some ready ground tools they must be OK. I was wrong, very wrong.!

I got a pro machinist friend of mine to come round and assist me in my problem. He looked at my prized new tool bits, gently shook his head and sighed sympathetically at me. . A few seconds on the grinder with the tool and the lathe was producing ribbons of uniform swarf leaving a super finish on the bar.

I should add that the guy has well over 40 years under his belt as a machinist so to produce such a tool is second nature to him. Unlike me who still has to be the worlds worst tool grinder.!

I would say regrind your tool as it is highly likely that at such a high speed you will have wrecked the edge (if it was any good in the first place.) Slow things down considerably and try again.

Above all don't get disheartened. As the saying goes "Rome was not built in a day." Also if you are at the practicing stage where did the steel you are trying to turn come from.? Some dubious origin 'stuff' I have found will simply just not produce good results period. And you are more likely to get a better result turning Brie cheese.

Nick

Hi Dave, I wouldn't mind betting if you have used a HSS tool at those speeds on steel, the edge has over heated and gone a bit soft.

When I started I couldn't get a good finish on anything, it was because when I sharpend the HSS I over heated it on the grinder. Most off the books for begginers seem to gloss over this fact and move straight onto tool shapes and feeds/speeds.

Keep a pot of water next to the grinder, dip the HSS steel in frequently. You should be able to hold the steel without any risk of burnt fingers, if its getting uncomfortable to hold then dunk it. Never allow the steel to change colour.

Heat treatment of steel should be lesson 1 in my opinion.

Sorry if this is teaching you to suck eggs but it is often over looked by beginners.

John

Edited By John Bromley on 30/08/2015 09:38:32

Edited By John Bromley on 30/08/2015 09:40:42

I had a similar experience with one of those lathe tools with the brazed on carbide tips. "Harry - I can't make any sense of this lathe tool" Response: "You're learning then!".

Steve

.......and a drop of oil. I don't mean flood. Really, just a teardrop.

Dave I discovered an old armstrong lathe tool holder, remembered from my apprentice days, these have made it easier to set the tool centre height, the tools go into an angled slot, this and as has been mentioned, cooling the HSS when grinding has improved the finish of the work and the life of HSS. Just a thought.

Gordon

To day I moved the tool closer to the work that helped. I turned the speed down as I slowed the lathe the finish got worse. To answer a few questions.

Im not using a auto feed just a very slow feed by hand.

The steel was purchased from a model engineers supplier its 1/2" mild steel.

Im taking very light cuts.

The tool is HSS rounded tool.

The tool is on centre.

I have moved the tool so there is the minimum of overhang this helped.

Im not using flood coolent.

I am using a cutting fluid applied with a brush.

The finish is reasonable but only if I have the lathe running at the top end of its speed range.

.

How much is sticking out from the chuck.?

Are you using a live centre in the tailstock.?

Nick

More often than not trying to get a good surface finish by hand usually takes a fair bit of practice.

The last time I did a finishing cut by hand was when using the compound slide for which I use both hands to minimise any stops in the tool travel for all other cuts I use auto but then my AUD does have auto feed.

Bob

It is strange to hear that cutting steel in a lathe will heat up the HSS tool enough to dull the edge! Strange that HSS will still cut steel when glowing red ! Which was one of the selling points used many years back. This was demonstrated by the HSS makers.

The items that affect the finish, 1. speed. 2. depth of cut. 3 height of tool. 4. rate of cut. and finally the condition of the tool.

Any takers?

Oh I forgot yes I'm using a live centre to support the work in the tailstock. I also forgot to add that my HSS tools are ground for me by the guys at work they are toolmakers/machinists by trade and they have all been at it for at least 30 yeas so I'm guessing its not the tooling that's at fault.

Dave, above you say the tool is rounded this could be the problem.

You need a knife tool ground up, with a small 10-20 thou radius on the tip of the tool.

The tools that have a large radius are fine on big heavy lathes where big cuts can be made, on small lathes I've never had much joy with this type of tool. I tend to use sharp knife tools on all materials with no problems.

John

Hi John, what is a knife tool? What does it look like?

Hi Dave,

Search this forum for Vertical Shear Lathe Tooling. I made a tool out of a bit of round bar and a triangular insert. With the insert set near vertical and on auto feed I can get a mirror finish.

I'll take

As a for instance M42 is often stated as having a better performance when the cutting edge is glowing red hot than other grades of HSS. Some even suggest it's a cheaper option to replace carbide with. People also used to think that the earth was flat. The trouble is that it will stay sharper for a lot longer when it isn't but will still wear out at a rate mostly dependent on cutting speed, the material also the "friction" and cutting stresses generated by the tool shape. It's pretty obvious that from a cutting stress point of view a razor blade at an angle set by the feed rate would be ideal. Not that this would give the best finish so compromises and a balance is needed.

My view on allowing a tool to colour when grinding it comes from using a lathe 8 hours a day 5 days a week and not sharpening them periodically with a stone in the lathe. If they weren't allowed to colour they lasted a lot longer. Actually I can't achieve the same life with the HSS I have at home. i have tried M42 as well. Work had to be nice and shiny to gain maximum brownie points and emery and files were not allowed. This area came up in another thread and I posted a link to some info on how tool bits are heat treated. The 2ndry treatment is still at a rather high temperature but the cutting edge of a tool is vanishingly small and does get hot when it's cutting. It will while grinding as well. Don't expect to see a few thou of a cutting edge glowing but in principle that's all it takes.

My personal view is that quoted cutting speeds for various tool materials should be regarded as maximums and sometimes much lower ones can be beneficial. Boring bar chatter for instance also vibrations within the machine.

Tool rads are an odd area. Conventional wisdom states that large ones shouldn't be used on smaller lathes because they consume more power. They also have a tendency to promote tearing. On the other hand shallow cuts with rather large rads may give a better finish. The state of the lathe comes into this area. It's not unusual to find that the finest feed doesn't result in the best finish or that deeper cuts provide a better finish than shallow ones. Some combination will give the best results. Machines need a certain amount of cutting effort to remove play in various parts of them. This is why hand feeding can be tricky. If the feed rate drops off so does the cutting pressure and rings usually appear on the work. The work may deflect as well - same result.

I tend to stone tool rads on unless I want large one for some reason. In principle it just needs to be n times the feed used. Finish is important as it will be transferred to the work. More friction too so I stone all parts of the cutting edge. This also helps prevent aluminium melting and sticking to the tool - that will wreck finishes.

To be honest I have never seen any decent information on lathe tool bit shapes. What is missing is the fact that angles are changed because of the nature of the material being used and being cut. This and cutting speed just relate to tool life really and in the case of carbide for instance just how fine the cutting edge can be before chipping gets to be a problem. It's as simple as that. A lot of the info that is about is aimed at production work. A balance of tool life and metal removal rates. In that area it's not unusual for the thing that is being made needing serious departures from these figure to get acceptable results.

John

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