Advice for surface finishing

So I turned a part for a reverse tumbler gear I am making. Its ok and will probably do but I'm wondering what the technique is to achieve a shiny smooth finish?

I used a brand new cutting tip and had the lathe speed at around 700rpm. I took the final cut at 0.25mm. Do I need to speed things up for the final cut and take a smaller cut?

Thanks Stuart

BTW the steel was a piece of 25mm bright steel rod I had laying around

Stuart the rod lying around might be the problem lots of guys wont use stuff unless they know what it is. Also I seem to get my best finish with HSS. I can hone it with a diamond lap and then take a nice light cut with lots of cutting oil to bring it to size and finish.  I normally use a tangential tool for this.

Edited By Buffer on 19/01/2021 19:59:57

+1 on the HSS cutter.

Cannot comment on using old bits of steel. I do it a lot.

Steve.

Ah, ok. Lesson learned!

Thanks Stuart

The finish looks to be poor on all faces and the spigot at the left doesn't look parallel? So I suspect the problem is generic. Unknown material is one potential problem. Where did the insert come from and what scale is the part? On some materials you might get away with a shallow depth of cut, on others you will need to increase speed and depth of cut to get a decent finish.

Andrew

The shininess can be improved with emery cloth, of course - but that finish looks a bit rough, even for that.

Looks like EN3 - use an aluminium insert

So there are specific inserts for cutting aluminium?

The spigot is actually parallel, I think the photo makes it look not. However the surfaces are poor hence the post.

The insert came from Amazon, so China I would presume. The piece is 10mm diameter and 75mm long to give it some scale.

Thanks Stuart

I think Dave is referring to a CCGT type uncoated small radius insert for aluminium and stainless. They work well for lighter cuts in harder materials.

Andrew.

Before worrying about where the insert came from what size and shape is it? Inserts come in bewildering variety because they are race tuned to suit industry whose goal is to remove metal as fast as possible whilst getting a good finish at the same time. Usually means a fast powerful rigid machine with the insert carefully chosen to match the material. Typically inserts are driven much faster and deeper than a hobby machine can manage, and the effect is spectacular - rather than producing swarf in spiral ribbons, smoking hot chips come off in a spray.

To get performance inserts are rather blunt and the finish improves with more speed, faster feedrates and.deeper cuts, which isn't always possible.

On a slow hobby lathe, it pays to use inserts of the sharper type, which is why Dave suggested using an insert desiged for Aluminium on mild-steel -being sharper they work well on steel at hobby speeds.

Ordinary mild-steel tears and smears rather than cuts cleanly - rather like the photo. Some steels are worse than others - always suspect the material when poor finish occurs on unknown scrap. I usually cure poor finish by cutting more aggressively, but this undermines another trick, which is to sneak up on a dimension by taking fine cuts. For that a sharp insert or HSS tool is better.

I prefer inserts but they take a little getting used to. HSS is more forgiving but you have to learn how to sharpen it.

Dave

Thanks for the advice everyone.

As always, much appreciated!

Your inserts may be at fault, some have a negative top rake and are more suitable for industrial machines which can take heavier cuts and are sturdier than hobby machines. These are my DCMT 070204 for aluminium, they have a positive top rake. I use them to cut everything, stainless, mild steel, brass, plastic etc. very sharp and leave a great surface finish, ideal for small machines.

Your photo looks like there is vibration to the left, but improving towards the threaded end where I presume it was held in the chuck, I couldn't tell from the photo but your part may need centre drilling for tail stock support, which will improve surface finish.

IanJ

Thanks Ian, your right I didn't use tailstock support. I'll try that next time and I'll also get myself a set of DCMT070204 tips to try.

Steady now most inserts for aluminium and non ferrous are **GT, So the one pictured would be DCGT070204

As said the very cheap inserts can be of variable quality so that won't help with getting a half decent finish, at 10mm dia you could be running faster try double your 700 to 1400 for starters.

Mine in action taking 1thou cut on EN8, 10mm dia

Edited By JasonB on 20/01/2021 07:34:00

Edited By JasonB on 20/01/2021 07:36:04

Thanks Jason. Do you have a supplier/brand you could recommend for good quality inserts?

Stuart

I have bought them from JB Cutting Tools. I came across them because other people on this forum said they used JB and were happy with their offerings.

Martin C

PS I would just add that when you get inserts in boxes of ten as opposed to singles often they have a chart on the back of the box detailing their recommended depths of cut, feeds and speeds for various materials. It is probably worth looking at these for whatever you get and try to work with them. Also RPM is often a maximum not a recommended value.

Edited By Martin Connelly on 20/01/2021 08:58:06

If it is the DCGT type you need then the one I'm using in that video is from APT, and is the DCMT 070202 which has a smaller tip radius than Ian's and is the one on the right of the second row. The price is for a pack of two for hobby use.

If you go to the full pack section then that gives speeds for the inserts but as industry won't be using them on steel you will only find ali and non ferrous speeds