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First CNC machine - help!

darcus

Plastic
Joined
May 24, 2014
Location
Birmingham, UK
Hi Super Noob here

I am still waiting for our first cnc machine to arrive so haven't had a chance to try this yet.

I am wondering if it's possible to turn the outside of a bush with just one tool?

I am using bar fed 35mm diameter C45 material, the od of the bush is going to be 31.185mm maximum. It has a 30 degree chamfer going down to 27mm on both the front and the rear face.

Initially, I was thinking of using one tool to do the front chamfer and reduce the diameter down and then use the parting tool to produce the rear chamfer and then part off.

Is this how you seasoned machinists would approach it?

Or could I use a WTENN2020K16 type holder with a TNMG1604 insert to do both front and rear chamfers as well as reduce the diameter:
1711025556270.png
I was thinking of using G71 to produce the profile I need but I am unsure if this is the right way to go about it.

Any help would be greatly appreciated.
 
Initially, I was thinking of using one tool to do the front chamfer and reduce the diameter down and then use the parting tool to produce the rear chamfer and then part off.

Is this how you seasoned machinists would approach it?
For 1 piece, sure.
For multiple parts, no.
You want 1 tool to rough, and 1 tool to finish.
Ideally I would use a CNMG to face, chamfer and turn, then use the part off tool to do the back chamfer and part off.
 
For 1 piece, sure.
For multiple parts, no.

I would add the caveat for very easy to machine, short chipping materials, like free machining brass, one tool will work great and reduce the cycle time. For tougher stuff and tight tolerance you need to rough and finish. Roughing the skin off will relieve internal stress that may be present. One thin bushings or tight tolerance bushings, it is also advisable to part a groove almost through to break the internal stress transfer that can occur on the stock end of the bushing.

For my work, I prefer to use the turning tool to face, turn, and make front and rear chamfers. Since all the external features except the length are made with one tool, the rear chamfer size is easier to hold. In addition, you don't have to worry about the finish on the rear chamfer as the parting tool wears different than the turning tool.

Emphasis on "for my work." I just like this way better. Either way will work.

I would not bother with a roughing cycle for such a simple part.
 
I would add the caveat for very easy to machine, short chipping materials, like free machining brass, one tool will work great and reduce the cycle time. For tougher stuff and tight tolerance you need to rough and finish. Roughing the skin off will relieve internal stress that may be present. One thin bushings or tight tolerance bushings, it is also advisable to part a groove almost through to break the internal stress transfer that can occur on the stock end of the bushing.

For my work, I prefer to use the turning tool to face, turn, and make front and rear chamfers. Since all the external features except the length are made with one tool, the rear chamfer size is easier to hold. In addition, you don't have to worry about the finish on the rear chamfer as the parting tool wears different than the turning tool.

Emphasis on "for my work." I just like this way better. Either way will work.

I would not bother with a roughing cycle for such a simple part.
While that is true and I agree, in a cnc lathe you're typically going to have tools dedicated to od roughing and od finishing that usually stay in the turret most of the time.(Speaking from a job shop perspective).
So it only makes sense to use 2 different tools unless he really needs the turret space for a special tool.
 
While that is true and I agree, in a cnc lathe you're typically going to have tools dedicated to od roughing and od finishing that usually stay in the turret most of the time.(Speaking from a job shop perspective).
So it only makes sense to use 2 different tools unless he really needs the turret space for a special tool.

We agree!

For sure you've got a roughing tool handy. But in high production jobs where a seconds add up, I eschew the rougher when I can to save time, and when easy machinability allows it. It's not unusual to save a day or more in those cases.

In short, do what works best for the particular job! Don't struggle to eliminate a roughing pass if it doesn't make sense.

To the op, use a D or V style tool in a holder that allows facing and turning. Only use a neutral holder as you showed when it's all that will work.
 
To the op, use a D or V style tool in a holder that allows facing and turning. Only use a neutral holder as you showed when it's all that will work.
This is awesome help.

It's our first cnc machine, only a cheap one from China. The turned parts we want to make on it will be very simple and tolerances don't need to be too tight - just want to turn parts quickly and out the door!

I am planning on making hex nuts from round bar with the machine as it has a polygon turning head too - which is pretty easy to program on the thing although speeds and feeds for this process makes absolutely zero sense!
 
Wow, what a flex on that diagram, it looks beautiful and the handwriting is spotless. I appreciate that you took the time to draw it, I understand what you mean now.

My follow up question would be, since the material is now only held by a thin bit of material, would this not increase the chances of chatter being induced during the finishing operation?
 
First I'd say don't do this unless the bushing goes out of round. This will often help.

If you use a small tool nose radius without a big hone, chatter is usually not much trouble. Of course, if the part is particulary long, the chatter danger increases.
 
Hi Super Noob here

I am still waiting for our first cnc machine to arrive so haven't had a chance to try this yet.

I am wondering if it's possible to turn the outside of a bush with just one tool?

I am using bar fed 35mm diameter C45 material, the od of the bush is going to be 31.185mm maximum. It has a 30 degree chamfer going down to 27mm on both the front and the rear face.

Initially, I was thinking of using one tool to do the front chamfer and reduce the diameter down and then use the parting tool to produce the rear chamfer and then part off.

Is this how you seasoned machinists would approach it?

Or could I use a WTENN2020K16 type holder with a TNMG1604 insert to do both front and rear chamfers as well as reduce the diameter:
View attachment 433647
I was thinking of using G71 to produce the profile I need but I am unsure if this is the right way to go about it.

Any help would be greatly appreciated.
Are we to assume there is no turret or tool changer?
I believe that must be the basis of your inquiry.
 








 
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