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.

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

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

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

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

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

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

–

No Problem Dave I should have done the extra bit and added some links.

Ed.

Hello Dave,

A couple of points; firstly get yourself a copy of Lathework A Complete Course by Harold Hall This book assumes no previous experience and secondly If you've a model engineering society nearby, make contact and there will be a wealth of knowledge there to tap into. What you really need is to watch someone setting up and using the lathe, you'll learn more in ten minutes watching someone. We all get there in the end, but in the beginning its all a bit of a mystery.

Regards,

Bob Youldon