The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
Nothing for it then but to grit my teeth and take the angle grinder to that nice boring bar. This is of course sacrilege
Very carefully working out how long to leave it I then did the deed, and cleaned it up on the disk sander to at least look presentable. Mounting the shortened tool in the turret it at least looks less ridiculous than before it was cut, but how will it perform :scratch:
. . . Well the answer is with FAR less vibration - it might actually be satisfactory. I've only tried turning under what is called 'Overstore' on this controller, where you can set the spindle rotating at your desired speed and direction, then use the manual jog or handwheel to control Z and X
Have other duties this afternoon so next time I'll try under program control and see how good a finish I can achieve.
WeldingRod:
If you make a shrink on sleeve you could stiffen that bar. Or, a really close fit with oil could provide a nice damper. If you go for the latter, dont forget to lock it axially!
Just dreamin' ;-)
Sent from my SM-G892A using Tapatalk
awemawson:
So this morning I set up an EN8 test sample to see what surface finish I can achieve and what chatter I can see if any.
Sweet spot seems to be 1500 rpm with 25 micron DOC - not using coolant so the whispy swarf glows like steel wool falling onto battery terminals!
Finish should improve with coolant. Note that this taper is the correct 7.125 degrees wall angle but smaller diameter than the actual adaptor nose as this bit of EN8 has been turned a few times !
awemawson:
I dismounted the tool holder intending to put it on my off line tool setting rig, but I noticed a tiny flash of light from a facet on the tip.Close examination under a glass revealed a worn facet - so obviously I need to radically address feeds and speeds. Trouble is I have no idea what grade of carbide this is.
After all those dramatic glowing bits of swarf WERE a clue :ddb:
No point in accurate tool setting until I get this feature sorted - probably just far too high a surface speed added to no coolant.
I tried photographing the tip on my Baty Shadowgraph, but the image was swamped by the light - the only way I could get a result was to make a little video moving round the tip shape - hopefully this will convey what I can see with my eye :scratch:
First the worn tip:
Then a virgin tip:
awemawson:
It seems that EN19T (or 4140) likes a surface speed range of 131 to 208 m/Sec when turning with carbide. For this taper that works out as speeds between 522 and 831 RPM.
Doing a test turn at 600 RPM with a fresh tip the finish was not as good as before but acceptable. I suspect that previously it was being burnished by the worn tip.
Hopefully finish will improve when I rig up a coolant pipe for this tool.
Even after this minor test there was detectable wear on the tip - I wish I knew what grade of carbide it is. I do have some TPKN 16 03 PP tips in P40 carbide that will fit this bar, though instead of a tip radius they have two small facets approximating a radius - I may try those tomorrow!
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