Poor finish, aluminium

[emtor]

tq60 --> Yes, aluminium. Melted it myself into round stock.

This is the result of my investigation:
First the chirping sound . . . disengaged the lead screw drive train,-still chirping.
Removed the belt,- same thing.
There are two belt pulleys at the motor end. One for the low speed range and another for the high speed range.
I pulled the low speed pulley and the axle was bone dry and covered with brown dust.
Ran the lathe without the pulley and the chirping went away.
The lathe was purchased not long ago and hasn't been used more than a few hours so the workers that put it together didn't bother to lube the thing.
I put on some grease, mounted the pulley and no noise when running.
Problem solved.

As for the poor finish I cleaned the insert and had a close look at the tip (see image).
Looks like build up after turning aluminium dry.
Turned the insert around, and I still don't get a mirror finish, but it is at least consistent and the lathe runs effortlessly.
This is a step forward, and the next steps are correct rpm, correct DOC, correct feed rate, casting round stock out of known good aluminium known to machine well.

 

[BaronJ]

Power feed or manual makes no difference, also the bad finish appears randomly.

The comment regarding failing to tighten the QCTP is interesting, and something to look into.
My toolpost is tight, but in order to get the tools on center I have to shim quite a lot.
(That's right . . . no adjustable QCTP here).

Please can you post a picture of your QCTP.

That insert looks to be badly chipped.

 

[kvt]

I see build up on the insert. Also what did type or items did you melt to make the bar. Cans, Extruded, Cast etc. They make a big diff in what you get when you try to turn. Cans and extruded stuff has a tendency to be very gummy and will stick to the insert quickly, Where some cast stuff makes a better machining AL bar.

 

[Dan_S]

Yes, aluminium. Melted it myself into round stock.

what did you melt down to make it?

depending on what you used, you could get anything from bubble gum, to something fairly decent to work with. usually though home cast is always going to be pretty inferior to stock produced in an industrial mill that's gone through heat treating etc.

View attachment 277686

That looks like a steel insert to me, you want something more like this for aluminum.

 

[emtor]

Yes, it is a steel insert. Haven't gotten around to buy alu-inserts yet.

The aluminium is melted using pieces from a wheel rim, motor housing and cylinder from an air compressor and a few sticks of something that looked like extruded.I stay away from cans. Those are manufactured from alloys that needs to be gummy. In the future I'll be taking notes of what I melt down to get an idea of what produces the best aluminium for turning.

I increased the speed to 1000 rpm and removed 0.2 mm of the shims. Workpiece is no longer torn up but I'm not yet at a mirror finish.

Here's the pic that BaronJ wanted:

 

[BaronJ]

Hi Emtor

Thank you for posting the picture of your lathe tool post. Unfortunately that is not a QCTP, it is a four way tool post ! Yes you can fit four tools and quickly rotate the tool block to get a different tool. In practice you can realistically only get two tools in there using insert tool bars.

Have a look at these pictures,
https://www.hobby-machinist.com/threads/tool-post-holder.69487/page-3#post-615263 post No:67 the bottom picture is my "Norman Tool Post".

Not QCTP in the normal sense, but there is a hight adjustment screw hiding behind the big nut, allowing the tool hight to be very quickly set.

It is a long post to read through but the Norman Tool Post was designed to overcome the problems with setting tool hight and its ability to improve rigidity. Its easy to build as well.

 

[savarin]

I have never got a "mirror" finish on any of my home cast aluminium, close, but no cigar.
I get the best results with a very sharp honed HSS tool bit with a very small nose radius, a fair bit of rake and clearance. Lots of WD40 when cutting.
It is gummy but the wd or kero really helps here.
The same tool on purchased aluminium produces a superb mirror finish.

 

[emtor]

Hi Emtor

Thank you for posting the picture of your lathe tool post. Unfortunately that is not a QCTP, it is a four way tool post ! Yes you can fit four tools and quickly rotate the tool block to get a different tool. In practice you can realistically only get two tools in there using insert tool bars.

Have a look at these pictures,
https://www.hobby-machinist.com/threads/tool-post-holder.69487/page-3#post-615263 post No:67 the bottom picture is my "Norman Tool Post".

Not QCTP in the normal sense, but there is a hight adjustment screw hiding behind the big nut, allowing the tool hight to be very quickly set.

It is a long post to read through but the Norman Tool Post was designed to overcome the problems with setting tool hight and its ability to improve rigidity. Its easy to build as well.

The Norman toolpost looks interesting. My insert holders are too long to make it possible to fit four tools, but as everything else on my lathe changing things takes time.

 

[emtor]

I have never got a "mirror" finish on any of my home cast aluminium, close, but no cigar.
I get the best results with a very sharp honed HSS tool bit with a very small nose radius, a fair bit of rake and clearance. Lots of WD40 when cutting.
It is gummy but the wd or kero really helps here.
The same tool on purchased aluminium produces a superb mirror finish.

I've read about hardening and ageing aluminium but I've never come across exact methods for doings so.

The large wheel had next to no porosity and got a way better finish. The small wheel is gummy and porous like a swiss cheese.
The large wheel will be the drive wheel for a belt sander, and the small one will be one of two idler wheels.
But, seeing how bad some of my aluminium casting are, I want to make the idler wheels from steel instead and spend some time melting alu from different sources and taking notes for future reference.

 

[BaronJ]

Hi Emtor, Guys,

I read somewhere about aluminum smelting, where they added some stuff to the melt that caused impurities and other stuff to float to the surface where they could be skimmed off. Unfortunately I never took a lot of notice at the time, so I don't remember or know what that stuff was.

I also have vague recollections of a chap on one of the other forums discussing how he was improving the quality of his castings by using additives to the melt prior to pouring.

Sorry not a lot of help I know !