lathe vs cast iron

[Henrik]

I've just been out in the garage trying to true up an old lathe back plate I got with an old chuck.

Anyway, machining it is a right pain in the rear. All I seem to get is chatter and/or a high pitched scream. Tried about 300 and 500 rpm.

I've tried various tools (all carbide tipped), though I'm not sure how sharp they are, it has to be said.

Does HSS generally work better on cast iron, or what else should I try? All I want to do is true the surface up and cut a register for a new chuck.

Am I going too fast?

 

[stormin]

Fresh cast iron has a hard outer skin you have to get beneath with a heavy first cut but should then machine easily. The exception is "chilled" iron castings which can be hard throughout, or have hard spots. As its supposedly been machined before wouldn't have thought you should be having problems.

 

[DanielW]

Cast Iron needs to have a low speed,chatter and sparks is the norm if its too high in speed.

 

[Digger6135]

Vital that the tool is at centre height, the feed should be relative slow 300 should work ok for high speed steel tools, but to use tipped tools the speed can be higher.

Digger6135

 

[DanielW]

you want a speed around 120rpm with a good carbide tipped tool,hss would need to be slower with light cuts.

 

[Flathead_Dave]

I'm guessing 120 RPM is a typo.
The speed depends on the diameter.

 

[DanielW]

120 rpm for about 6" to 8" ? back plate.

 

[Pete.]

It does indeed but when facing a backplate the diameter is variable. You only need to true up the register, not the whole backplate so for a 6" backplate turned only near the edge with carbide in soft cast iron 120rpm is about right. For HSS it would be less than half that.

Henrik - keep the tool tight in to the toolpost. Make sure the tip is good and have a small radius.

 

[Wedg1e]

Whenever I've turned cast iron it was always with backgear in to get low revs; learned the hard way with my very first lathe (a Mellor), kept wrecking HSS tools (at least it was good grinding practice!) within seconds until I eventually tried it on the lowest speed and hey presto, knife through butter(ish ). I'm guessing that'd have been around 100-150rpm.
On my Boxford VSL I have a Bison 3-jaw mounted to a cast iron catchplate; the chuck was given to me new and the catchplate cost me £4 as I recall (a new L00 back plate would have been £90 and that's a Chinese one)... turned a register on the catchplate, drilled 3 holes and the Bison is more accurate than the old 3-jaw Burnerd that had seen decades of schoolboy abuse. You know, on second thoughts maybe I should rephrase that last bit...

 

[piman]

Hello Henrik,

by my calculations, a 6" diameter back plate should be at about 50 rpm. (80 ft/min divided by (pi x 0.5 ft), obviously when facing the diameter drops steadily so the speed can be increased part way through if you wish. Whatever your 300 to 500 rpm is far too fast.

Alec

 

[Pete.]

That would be correct for HSS, but he is using carbide tools.

 

[Henrik]

Thanks for the comments! I will reduce the speed anyway and see how I get on.

I'll grind up some HSS and give that a go, as well as give the TCT tools a tickle, and make sure I'm on centre height.

The lathe is an old atlas 10f, but the motor is a 2880rpm one instead of the 1725 one it originally came with, but using back gear I should be able to get it down to about 50'ish rpm.

 

[gordon stephenson]

I would use a slow speed and a tip tool to get under the "skin" some cast iron has hard spots, and these can be a glass hard in some cases,

 

[DAPPH]

Cor all this takes me back to when I were a lad .....
I seem to recall that when truing/cleaning up brake drums an even the brake disks our fitter turner got down to 20 RPM using a very fine auto feed , you'd hear them gently singing throughout the workshop , the finish on them was truly amazing . Happy day's indeed .

 

[Gritineye]

As you have power cross feed, use it and watch carefully as the cutter moves across the face, the speed reduces towards the centre, usually there will be a point when the tool starts to cut well. This will be the correct feed/speed, use an average of this to match it best you can.

Start cutting from the centre out as the speed is lower there and if the speed is wrong the tool won't get ruined as quickly as when you start at the rim of the plate.

 

[Henrik]

As you have power cross feed, use it and watch carefully as the cutter moves across the face, the speed reduces towards the centre, usually there will be a point when the tool starts to cut well. This will be the correct feed/speed, use an average of this to match it best you can.

Start cutting from the centre out as the speed is lower there and if the speed is wrong the tool won't get ruined as quickly as when you start at the rim of the plate.

This is a good idea, but sadly the cross feed is broken


Anyway, I tried machining this again tonight, this time in back gear at about 75 RPM, and it worked so much better. I didn't go mad, but I could take a "10 thou" cut without any problems.

The tool I used was one of these:

This is the finish (although not finished item). It was completely hand fed

Thanks for the help, everyone! Mig-welding.co.uk to the rescue again

 

[Gritineye]

That's not bad hand feeding, the finish should be Ok for the use, I've taken chucks off and found a lot worse..

 

[DanielW]

I guess it would have been better than that if the handfed was constant all the time....good you got the speed down,but do try a HSS round on a light cut.