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Poor finish, aluminium

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emtor

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#1
Friday evening, turned the large wheel on my lathe using carbide insert tool.
Saturday I turned a smaller wheel (placed on top of the large wheel).
I started the lathe and I got something that sounded like the belt is slipping (a short squeak every revolution).
Squeaking gets worse when the tension is released and also has a tendency to increase slightly with increased loads.
The finish is, as can be seen in the image, very poor. Metal looks torn instead of cut, but please note the shiny section
on the small wheel. Every now and then the lathe produces shiny sections, some of them just narrow lines and some of them wider sections.
I'm thinking this may have something to do with belt slipping.
Tried cleaning both the pulleys and the belt,- not much success. Belt has only run a few hours.
-Any thoughts on why the finish looks like it does?
The large wheel does not have a good finish, but at least it is consistent. The small wheel has a poorer and very inconsistent finish.

aluminium.JPG
 
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derf

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#2
Are you sure that is aluminum? It looks like steel to me. The surface finish will vary with tool height using carbide inserts. Larger diameters will be more tolerant than smaller diameters.
 

Karl_T

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#3
yep, that sure looks like steel. I suspect you have too small depth of cut and too small a feed and too slow a speed. You have to make carbide work to get a good finish. Get a speed feed chart for your insert. Light cuts and feeds are HSS territory.
 

Nogoingback

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#4
You also might check to make sure your tool is on center as well.
 

emtor

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#5
I've tried the whole speed range from 125 rpm to 2000 rpm,-no change.
Light cuts, heavy cuts,- no change.
Manual feed, auto feed,-no change.
Ground a HSS tool today,-no change.
Ground a HSS vertical shear tool, took very light cuts,-still poor finish.
I've used a valve gauge blade between the workpiece and the tool tip and the blade is vertical. No tit in the center when facing . . . tool is at center as it seems. The strange thing is the fact that while turning the finish suddenly gets mirror quality for sometimes a few revolutions, sometimes half an inch of width, then goes back to poor finish.
Looking at the large wheel, the finish is not great but uniform and acceptable. Lathe did not squeal when turning that wheel.
Next day, belt squeal, probably due to slipping, and the finish got as seen on the small wheel. Varying between mirror finish to torn up.
Compound rest is locked down hard, and I could try the same with the cross slide and do a pass to eliminate movement/backlash in the cross slide.
However, I'm sure it has something to do with the squealing belt,-if it is the belt that makes the noise that is. Rotating the chuck by hand does not produce any sound or rubbing feeling, so I'm sure it's not the chuck.
 

Tozguy

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#6
Mine does that when I forget to tighten the tool holder on my QCTP
 

mikey

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#7
I would either tighten or replace the belt. I've seen this when the belt stretches and sets up a harmonic. The squeal/squeak also suggests a drive issue.

That aside, if this is aluminum, I would not consider that large disc a great finish but if it is acceptable and on size then that's what counts.
 

petertha

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#8
The strange thing is the fact that while turning the finish suddenly gets mirror quality for sometimes a few revolutions, sometimes half an inch of width, then goes back to poor finish.
Looking at the large wheel, the finish is not great but uniform and acceptable. Lathe did not squeal when turning that wheel.
I assume you are power feeding. If so, I think this is a clue that there could be a problem with your lathe & not the cutting tool since you have tried many options. If there is resistance or binding as the carriage is traversing down the ways, you may be picking up vibration. The bely squeaking may be telling you its seeing higher (traversing) load. Power feed bar is bent or out of alignment. What happens when you just traverse carriage by hand? Is it perfectly smooth? Does the same bad finish occur at the same position in the lathe? Need more information to help you.
 

emtor

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#9
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).
The shims I use are blades from valve clearance gauges. I thought that was a good idea since each blade has their thicknesses stamped on them.
(Good for future reference).
Now, these blades are tapered and not the same width as the insert holders, also there are quite a lot of them needed to get the tool up to center of the workpiece. I need to find something thicker that fills the gap without using a lot of separate shims as my current arrangement may be causing the tool to move while turning.
Also I'll disconnect the gear banjo to rule out loose/overly tight gears.

An update before going off to work: -Disconnected the feed screw gear train. One of the gears were loose,-I could tighten the nuts with my fingers.
Belt is still squealing though. After work I'll remove the belt to rule out motor/spindle problems.
 
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tq60

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#10
Noisy belt is a slipping belt.

Belt slips due to excessive load for the transmission system.

Others suspect binding someplace.

Chuck up a chunk of wood and try it.

It will be messy but should work.

If belt still noisy repeat with nothing to troubleshoot.

Your material may be work hardened and if so will do as it is.

You state aluminum...are you sure?

Test with a hacksaw to see if the weight of the saw will be enough pressure to cut.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 

emtor

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#11
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.

insert.JPG
 
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BaronJ

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#12
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.
 
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kvt

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#13
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

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#14
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.


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

emtor

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#15
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:

toolpost.JPG

shims.JPG
 

BaronJ

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#16
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

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#17
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

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#18
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

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#19
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.
 
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BaronJ

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#20
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 !
 

kvt

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#21
Yes there are gassing compounds you can use to help, and Mix of diff metal that helps, but most of that is more than just the normal backyard foundry person like me is doing.
I am now trying to stay with things that have been machined, and cast for my AL sources. And like SAVARIN said use plenty of WD40 or whatever to help keep it from sticking to the tools. From what I have done, once it starts sticking it takes a lot to get it cleaned up and not sticking again.
 

macardoso

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#22
As a few have mentioned above, there are aluminum specific inserts. I have gone to these over the last year and haven't looked back. I use primarily the CCGX insert (cousin of the CCMT, fits the same holders), but I've also started using the TCGX triangular inserts, and aluminum specific GTN-2 grooving inserts (although admittedly I like the HSS blade cutoff tools better). The inserts are electropolished on the top surface and ground to a razor edge on all sides. They are usually pressed with an aggressive positive rake, and have a chipbreaker which starts to break chips once you cut deeper than the nose radius.

Couple of thoughts... On a given size of insert, you will usually have your choice of nose radii. The larger radius will be stronger and allow you to feed more aggressively, the finish will also be better for a given feedrate. The larger radius requires a larger depth of cut to engage the chipbreaker, usually 1-1.5x the radius in depth of cut. This can be a problem for small lathes. The larger radius increases tool pressure causing more deflection. This can make it hard to dial in very tight tolerances (talking tenths here guys). The *minimum* depth of cut is also increased as the tool will rub rather than cut.

A small nose radius tool will make a cut even when your DOC is .002 or less, however it is more delicate and will require a much slower feedrate to achieve a desirable surface finish. I typically use a large nose radius tool when roughing or when finishing where tolerances below .001 aren't required. The smaller radius tools come out for precision finishing and boring when trying to hit .0005 or less.

I often cut dry with these tools and experience little to no built up edge in 6000 and 7000 series aluminum. If you are machining a 1000 or 2000 series (think about where you got your scrap) then all bets are off as these are MUCH more ductile for the purposes of bending sheet metal. I'd try to get your hands on some practice non-cast stock (look at buymetal.com, they're pretty inexpensive) and a few aluminum specific inserts (try shars.com, not as cheap as they used to be but still only $5-6 an insert).
 

macardoso

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#23
Not exactly "mirror" finish, but the inner bore of this part was cut on the lathe. Bored out using a 3/4" CCMT boring bar (with CCGX insert). On size within .0008" in a single finish pass at .010 depth of cut. No polishing was done afterwards.

1.jpg 6.jpg 8.jpg
 

emtor

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#24
Nice finish on those parts.
I read on the web that the 2000 series aluminium has the best properties for machining. This series has some copper in it.
Some guy on youtube states that high temps are not needed to melt the coppert since liquid aluminium helps to melt the copper at a lower temp than the melting point for copper.
 

BaronJ

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#25
True !
 

Techee

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#26
I suspect the chemical composition of the Al scrap pieces you melted is affecting the machining capabilities.

As Emtor indicated, some grades of Al are easier to machine...& give a better surface finish than others.
 

Dan_S

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#27
The issue with casting aluminum at home is, that even if you use use known material of all the same grade, and de-gas the melt, you are still going to end up with sub par results.

The reason for this, is because as cast, you have no temper/heat treatment. take raw 6061, and then compare it to some of its common tempers.

Hardness Brinell
  1. 6061 : 30
  2. 6061-T4 : 65
  3. 6061-T6 : 95

Yield Strength in ksi
  1. 6061 : 8
  2. 6061-T4 : 16
  3. 6061-T6 : 39

6061-T6, is probably the most common aluminum on the market, but 2024-T3, and 7075-T6 are better to work with.
 

macardoso

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#28
Nice finish on those parts.
I read on the web that the 2000 series aluminium has the best properties for machining. This series has some copper in it.
Some guy on youtube states that high temps are not needed to melt the coppert since liquid aluminium helps to melt the copper at a lower temp than the melting point for copper.
1000 & 2000 series are softer grades of aluminum typically used for drawn or formed sheet products. The 2000 series has a decently high strength for use in aerospace and is considered machinable. 6000 series is a general purpose machinable grade and is quite a bit harder than the aforementioned grades. 7000 series is a high strength aluminum alloy and is harder than the 6000 series. Generally the harder the alloy, the easier it is to achieve a quality finish (when using the right tooling). An exception is the 4000 series silicon alloy aluminum which is very hard and abrasive. 3000 Series are used less often, and are good for higher temperatures. 5000 series aluminum is considered a structural high strength grade is is not often machined.

The 4 digit aluminum numbers are extruded, rolled, or wrought. This typically increases their strength and forms a fine grain structure. 3 digit numbers are cast, and often have lower strength and a large grain structure. They machine very easily, but don't necessarily give a good finish as the aluminum grains can pull out at the cut interface, leaving micro voids which appear a a dull appearance.
 

pontiac428

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#29
Someone above mentioned it... it looks like a "steel" insert. Looks like TiAlN, to be exact, which does stick to aluminum. Uncoated or TiN might help with getting that finish right (for the record, I prefer HSS).
 

emtor

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#30
The issue with casting aluminum at home is, that even if you use use known material of all the same grade, and de-gas the melt, you are still going to end up with sub par results.

The reason for this, is because as cast, you have no temper/heat treatment. take raw 6061, and then compare it to some of its common tempers.

Hardness Brinell
  1. 6061 : 30
  2. 6061-T4 : 65
  3. 6061-T6 : 95

Yield Strength in ksi
  1. 6061 : 8
  2. 6061-T4 : 16
  3. 6061-T6 : 39

6061-T6, is probably the most common aluminum on the market, but 2024-T3, and 7075-T6 are better to work with.
Well, being a knife maker I'm building a programmable heat treating oven. Knowing the temps and time durations to heat treat aluminium would make it possible to do just that. I found the methode for 6061-T6: After smelting, let the aluminium cool to room temp. Then re-heat to 980 deg. F.
Hold for one hour and quench in water. Heat to 400 deg. one hour or 350 deg. five hours or 325 deg. eight hours.
The only problem is that the aluminium will deform quite a lot, but this is no problem in my case since I will be casting round stock, not parts.
 
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