Thread: Turning cutting speed for Al

Good question. When I first got my lathe I made a table of RPM vs diameter for most of the common metals and plastics. There were huge variations in recommended cutting speed between the different information sources.

It is hugely dependent on the power of the lathe, DoC, feed rate and tool material. Here's a few figures for 90mm aluminium with HSS tooling:

Machinery's Handbook: HT (eg 6061): 550-640RPM
Hercus Textbook of Turning: FC: 640-1060RPM, HT:320-640RPM

I'm still a babe in the woods when it comes to turning, so I start with the slowest speed in the literature and go from there. Interestingly, for many metals the Hercus ToT recommends the highest speeds. Sometimes MUCH higher.

Chris

I use the formula* RPM=9000/d.
That's for normal (mild steel). For thermoplastics, brass and Al I multiply by 3 (copper & bronze x2, free cutting steel x1.5, higher carbon steels and CI x0.5)
That assumes HSS. For carbide, multiply by at least 3.

For 90mm Al, RPM = 9000/90 = 100rpm (steel)
3x100rpm = 300rpm (Al)

As Chris said, that's a starting point. Faster and you can overheat (destroy) your cutting edge; too slow and finish may suffer (and you will get bored)

Michael

*The full version is RPM=(300x cutting speed(m/min))/d. A typical cutting speed for steel is around 30m/min, so I use that and then adjust the result. However, the full formula is occasionally handy when faced with a cutting combination you don't know a mutliplier for - exotic tipped tools for example.

I've found the free machining grade is very forgiving and the speeds are not critical

These pages are from the Action aluminuim catalogue , the branch in Greens road Dandenong

hey mate,

at work the other day i was facing aluminium at 1600rpm with a reasonable cutting speed and turning down the outside on holding the job on a manderal between centres at 1200

which gave a good finish

You need to give more information so as the person asking the question can relate it to what they want to do.
Its no good telling what you did if there is nothing to judge it by.

Hello GSRocket,
Just between you and me, I have never learned formulae for cutting speeds. I would probably set the speed at 250 -300 rpm for that diameter. I use HSS tools and motor oil thinned out with diesel to assist cutting, and it would spray everywhere if the speed was much higher (I have a couple of pieces of cardboard which I use to stop the oil getting on the floor). Unless you are using Free Machining Al. it could weld to the tool if the speed is too high.
Russell

Methylated spirits is also a good coolant for Al. I put it in a squeeze bottle and squirt it on. It evaporates and leaves no residue. You need a nozzle with a small orifice otherwise you end up wasting a lot.

Metho? Thats a new one for me, i always use kero.....

As for formula, if you work in imp the formula is CS/(dia/4) where CS is your cutting speed in feet per min. I cut mild at 100fpm, other material i use the try it and see approach a lot.

Thanks guys. Appreciate your responses.
I was already thinking of a bottom speed of 300 RPM but I might make it a bit
slower in case I want to spin something a little larger.
I noticed the guys at a fabrication shop close by who seem to specialise
in rather beautifully made things in Al and stainless, use metho in a squirt bottle
when using a jigsaw to cut Al plate (fairly thin plate, about 3mm I would guestimate)

I didn't realise you were asking about the slowest speed available on your lathe.
I think the slowest speed available on most lathes (except for electronically controlled ones) would be about 30-50 RPM. When thread cutting, I would not like it much higher. Russell

Originally Posted by rusty steel

I didn't realise you were asking about the slowest speed available on your lathe.
I think the slowest speed available on most lathes (except for electronically controlled ones) would be about 30-50 RPM. When thread cutting, I would not like it much higher. Russell

My lathe has no thread cutting capabilities. It's actually an old pre WWII pattern makers lathes with a flat bed.
Technically it's probably only a wood working lathe. Its only got a 1" shaft so steel cutting or any heavy cutting is probably not advisable.
It originally came with a 3 speed pulley system with twin belts.
I'm reworking the whole drive system.

Originally Posted by rusty steel

I didn't realise you were asking about the slowest speed available on your lathe.
I think the slowest speed available on most lathes (except for electronically controlled ones) would be about 30-50 RPM. When thread cutting, I would not like it much higher. Russell

The slowest speed on the Hendey gearbox is 18 RPM . But with the 6 to 1 reduction headstock fitted , that brings it down to 3 RPM

All of that data is for a 60 hz 4 pole motor. The factory RPM data is thrown away when a VFD is fitted as I have done . MIKE

Surface speed for HSS would be 40 ... 80 m/minute, for carbide 200 ... 400 m/minute.
With a pulley, I'd start at the lower end, because they tend to vibrate/resonate.

With 90 mm diameter, that would be 140 ... 280 RPM or 700 ... 1400 RPM.

If you are using a form tool that cuts the full profile, you might have to go way down with RPM because of the cutting length.


Nick

Rocket,
I found these notes on turning pulleys a while ago. I've never made one myself, so I don't know how good the notes are. Also Tubal Cain (mrpete222) on You Tube has a 3 part series of videos on turing a pulley. Here is Part 1:

[ame=http://www.youtube.com/watch?v=pz_3ZESTKG4]MACHINE SHOP TIPS #65 Lathe Project Pulley Part 1 of 3 tubalcain - YouTube[/ame]

Chris