[Ashley Cobb]

Hey guys,

I recently CNC'd a little chinese lathe.

I am using 55 degree diamond inserts mounted at 93 degrees to the lathe. I have left and right holders. The inserts have a .4mm radius on the tip.

Whenever there is a shoulder, the cutter gouges and destroys the job.

I just discovered in the mach3 manual a section on tool tip radius compensation, and programmed this tool in. However when I went to try it, I discovered that the intermediate shaft pulley had broken.

Is the tip radius compensation what I am after or am I barking up the wrong tree?

I have attached a crude image in the hope it will provide some clarity. The toolpaths were generated with mach3's built in wizards.

Cheers

Ash

[fizzissist]

rigidity.

[Ashley Cobb]

Thanks fizzissist,

Unfortunately that is something my little lathe is sorely lacking.

I know a few people remove the compound slide and replace it with a solid block, so I might try that.

Thanks

[txcncman]

rigidity.

+1

Seems your tool is nose diving when it encounters a shoulder. You might try a different cutting pattern as a work around.

[Ashley Cobb]

I will try putting a decent radius on the internal corner and see if that helps, because at this stage I don't need a perfect sharp shoulder (it won't be sharp anyway, because of the nose radius)

Cheers!

Ash

[txcncman]

It is not just a matter of the tool nose radius. It is tool nose radius + depth of cut + lack of rigidity.

[Ashley Cobb]

I was taking 0.2mm cuts which I didn't think was excessive, running at about 1000rpm, it is also 2011 aluminium which should be free machining.

I can see that the RPM is too high, which is my mistake also.

Thanks for your help,

Ash

[neilw20]

Tool gets to the corner and the extra side force rotates/moves the tool to a smaller diameter, then the load increases, then it digs in more then it all turns to cr4p in the blink of an eye! WTF?
It happened to me the other day. Tool holder was not tightened adequately.
When it happens, sometimes takes a bit of detective work, if any evidence is left.
1000 RPM is OK. The more the merrier, with a spray of WD40.

[john-100]

I have a Sieg C2 lathe
the first attempted to part off a part the tool deflected downwards
and I hit the stop button

after close inspection I decided compound & top slide gibs were under size , bent and looked like the they had been chopped out of a lump of slag
the adjustment for slide plates for the saddle hit and miss
no rigidity !
I removed the 2 smaller set screws from each plate and used shims
to adjust the clamping pressure

a few days later the lathe was transformed by these new brass gibs ,
part numbers 94 and 107 near the bottom of this page :-

C3 & Super C3 Mini Lathe Upgrades - Arc Euro Trade

a value for money upgrade

replacing the spindle ball races with taper roller bearings was not as great an improvement and is a lot more work

John

[Ashley Cobb]

Tool gets to the corner and the extra side force rotates/moves the tool to a smaller diameter, then the load increases, then it digs in more then it all turns to cr4p in the blink of an eye! WTF?
It happened to me the other day. Tool holder was not tightened adequately.
When it happens, sometimes takes a bit of detective work, if any evidence is left.
1000 RPM is OK. The more the merrier, with a spray of WD40.

Thanks for the tips, the tool holders were definitely tight.

I have a Sieg C2 lathe
the first attempted to part off a part the tool deflected downwards
and I hit the stop button

after close inspection I decided compound & top slide gibs were under size , bent and looked like the they had been chopped out of a lump of slag
the adjustment for slide plates for the saddle hit and miss
no rigidity !
I removed the 2 smaller set screws from each plate and used shims
to adjust the clamping pressure

a few days later the lathe was transformed by these new brass gibs ,
part numbers 94 and 107 near the bottom of this page :-

C3 & Super C3 Mini Lathe Upgrades - Arc Euro Trade

a value for money upgrade

replacing the spindle ball races with taper roller bearings was not as great an improvement and is a lot more work

John

Thanks for the tips on the brass gibs, I will grab some once this infernal christmas is finished with.

The plan going from here is to completely eliminate the compound slide, since it is not needed, and to replace the gibs in the cross slide to brass.

In the mean time I will try lapping the gibs per the mini-lathe site.

I also think that the backlash is causing some of the problems, so I am going to adjust the nut on the cross slide, that should help.

Cheers for all the advice guys, I really appreciate it.

Ash

[neilw20]

Ash,

Thanks for the tips, the tool holders were definitely tight. 

Might have been tight, but once something flexes further than the thickness of the chip it gets worse within 1 revolution, and BANG!!

Quicker then the eye.
Actually you can run the test at a super slow speed, (if that is possible) and see it happen.

[Ashley Cobb]

Ash,

Thanks for the tips, the tool holders were definitely tight. 

Might have been tight, but once something flexes further than the thickness of the chip it gets worse within 1 revolution, and BANG!!

Quicker then the eye.
Actually you can run the test at a super slow speed, (if that is possible) and see it happen.

Hey Neil,

After your reply I went and inspected the lathe, and sure enough, the compound slide had moved from the 0 degree indication!

It is tight, but it confirms that a) the rigidity of the machine is less than adequate and b) that the sideways forces are significant.

Cheers

[neilw20]

And my toolpost rotated exactly the same way.
And I've been doing this for 40+ years.
Only difference is I've done it more often than you, and the others won't admit it.

[Ashley Cobb]

Looks like removing the compound slide will be a big help, then.

If we didn't admit to our mistakes that embarrass us, we would have no incentive to avoid further embarrassing mistakes in the future!

Cheers