Chinese 4th Axis on Ebay

[JustinO]

Dan:
This one mentions "Gapless harmonic drive reduction gearbox. Gearbox made in Japan"
http://www.ebay.com/itm/CNC-Router-Rota ... 3cc129641d

[Dan]

Thanks Justin. That's interesting. I am not keen of the 0.3mm end play. The other disadvantage of this arrangement is it being very long and not suitable for some little machines and there is no through hole for bar stock. Other than that the gearbox sounds like a nice thing. Interesting what's the actual backlash.

Dan

[John S]

Be careful on the belt drive rotary drives as utting forces can easily fore the holding torque of the stepper motor to jump.

I built quite a large one with 1500 oz/ in motor but doing splines with an 8mm cutter just did a spiral toolpath. You could see it jumping as it was cutting.

Finished up scrapping it and converting a rotary table.

[SkyMoBot]

Be careful on the belt drive rotary drives as utting forces can easily fore the holding torque of the stepper motor to jump.

I built quite a large one with 1500 oz/ in motor but doing splines with an 8mm cutter just did a spiral toolpath. You could see it jumping as it was cutting.

Finished up scrapping it and converting a rotary table.

Hmm.  I don't intend to cut anything harder than aluminum so I hope the one I purchased will work.  It only took a week to arrive and it's sitting on my bench waiting for my Gecko drive to arrive. 

For the record, this is what I have.

http://www.ebay.com/itm/221272369854?ss ... 1423.l2649

[JustinO]

"...I hope the one I purchased will work..."

The other day I had an endmill drag itself across a part, twisting my Sherline 2000 at the column. Sherline 2000 mills do that if you let them. I straightened the mill and took lighter cuts, and everything was fine, even the part. Just take light cuts, and try too keep the cutting forces perpendicular to the direction the rotary table is moving the part.

I do a lot of flycutter/gearcutter cutting, and since the cutting forces are along the axis of rotation, there is very little chance of the cutter moving the stepper.

[SkyMoBot]

That's generally the approach I take, slow and easy.    Although I have learned the hard way with CNC that too slow is just as bad as too fast, so it's not always the right move.

I do have an old school machinist friend that worked in a big production shop for a zillion years,  his approach is always if the chips coming off aren't yellow then you aren't moving fast enough, but then his home shop has bigger machines that I have ever even used.    He spends three times longer than I do setting it all up and getting it all perfectly aligned and then bam, takes these huge cuts and is done in seconds.

Hopefully the rest of my parts will arrive tomorrow and I can make some progress.

[ArtF]

Usually if you cant cut fast, your cutting too deep. Speed is your friend,
the heat shouidl be in the chips..not the tool. Id say 75% of all cnc users Ive met
cut too slow, when shown he right sped they are shocked. I know I was when I was taught..

Art

[JustinO]

Saw something like this somewhere:
"...cutting with carbon steel, the chips come off blue, you've ruined the edge..."
"...cutting with high speed steel, the chips come off red, you've ruined the edge..."
"...cutting with carbide, the chips come off as sparks, you've ruined the edge..."
"...cutting with ceramic, you're cutting too slow..."

Ceramic insert video:
http://www.youtube.com/watch?v=BU14tfjUfQU

[John S]

"...cutting with carbide, the chips come off as sparks, you've ruined the edge..."

Naw,

The book says for hardened steel, slow speed, slow feed and positive insert.

However if you throw the book away and go for 1200 revs, scary fasr/mm/min and a hand ground carbide insert with about 20 degrees rake you get this.

http://www.youtube.com/watch?v=x33JG2f-mno

This is a thru hardened worm shaft at 2 1/2" diameter being reduces to fit a 2 1/4" bearing.

O a hardness scale of 0 for butter and 10 for witches tits, this was about 12

[Dan]

Nice video John  :)

Thank,
Dan

[Mooselake]

I'm not sure I'd want to get close enough to a witch to measure...

Kirk

[ArtF]

John:

you scare me... lol

Art