The Shop => Metal Stuff => Topic started by: NormanV on July 06, 2016, 05:49:54 PM

Title: Machining problem
Post by: NormanV on July 06, 2016, 05:49:54 PM
I am machining a part from 3" diameter EN3 steel using HSS tools. I have successfully turned a tee slot around it which was quite demanding. I am now trying to reduce part of it to a much smaller diameter. All went well until I got it down to approx. 2" D. Then it started to chatter. I checked that all the parts of the lathe and the tool were tight, sharpened the tool but the chatter continues. Could it be a poor quality piece of steel that varies through the diameter? Any suggestions would be appreciated, but bear in mind that I don't have carbide tools.
Title: Re: Machining problem
Post by: hopefuldave on July 06, 2016, 08:43:47 PM
Did you increase the spindle speed (RPM) to keep the surface speed up? As you halve the diameter you need to double the RPM for the same cutting conditions, makes a big difference!
The other possibility is the tool's below/above centre height, this has a bigger effect as the diameter goes down - picture the angle from the work's centre to the tool edge, closer in the steeper it gets so if low you lose the effect of the tool's top rake. Similar if above centre, the work rubs on the tool below the cutting edge unless you have loads of relief!

Just my ha'pennorth,
Dave H. (the other one)
Title: Re: Machining problem
Post by: Fergus OMore on July 07, 2016, 03:26:00 AM
If you can cut a circular T Slot in a 3" piece of round MS, there is no criticism to make. I've done it on a Quorn bit and it isn't easy.

My suggestion is that you have hit the dreaded 'overhang' of your lathe tool. So might I suggest that you shorten the projection of your tool or change it for a more rigid one? One way to reduce the 'harmonics' is to put a bit of lead or solder near the tip of your existing lathe tool to dampen the vibration.

Best of luck from

Another Norman
Title: Re: Machining problem
Post by: NormanV on July 07, 2016, 04:52:39 AM
Full marks to Dave, when I originally set the tool height I more or less guessed it. Once I'd set it correctly and resharpened the tool it is now cutting perfectly.
To the other Norman, I am indeed building a Quorn, when I cut the tee-slot I was very careful over the tool heights as it was such a tricky operation. I need to be careful all of the time.
Thanks to both of you.