Re: CNC minilathe

I'm in the middle of doing a 7x12 minilathe conversion. So far I have
the Z running under power. (Your reference to the X in your posting I
take as meaning along the bed, which is actually Z). I am using a 2A
L297/298 based stepper controller like from Camtronics, 185 oz-in
Vexta PK-268-2.0's in half winding mode geared 2:1 with 9mm HTD belts
and replaced the standard chinese leadscrew to a 16mm x 4mm pitch
trapazoid screw with nylatron low friction nut. This new lead screw
is currently still running in quick bodged up bronze bushings in the
chinese cast iron bearing blocks.

The Saddle gib strips have been shimmed quite tight (and the mickey
mouse set screw adjustment thrown out), and the bushing blocks are
not ideal. There is a lot of friction and poor alignment in my system
as it stands. My rapids are currently only 18 ipm, which can be
increased to about 25 ipm when I loosen up all the mounts. Once you
have driven a stepper with a half step driver, you realise why you
should have bought a microstepping driver. Check out Xylotex drivers,
or Geckos.

Once I have finished the cross slide (X) conversion I will make up
some proper bearing blocks for rolling element bearings for the lead
screws, have a closer look at the alignment of the apron and nut
carrier and see if that improves things. If threading proves
differcult due to acceleration issues I may bite the bullet and put a
ball screw on Z.

If you are in the states you can get a cheap standard ballscrew from
Mcmaster Carr, bore the center, and braze in shaft ends, then turn up
to fit your bearings, avoiding the need to grind the hardened faces.

As it is, 18ipm would be too slow for production work, but is not a
limitation when you are just learning and using the MDI.

Using 200-300 ozin motors, 2:1 belt reduction, and the standard fine
pitch lead screw you will need high amp motors which maintain good
torque at higher rpm to get fast rapids. Given that the standard lead
screw drives the sloppy half nut, you will be better off long term to
replace that lead screw, although I know Dave K just lives with it.

The cross slide nut runs in a slot milled into the saddle which is
only 16mm wide and deep. Will your ball nut fit in here?

TurboCNC is a great choice for a lathe as it supports single point
threading with a simple encoder.

Regards,
Mark.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "knives01us" <knives01@a...>
wrote:

>
> I'm planning to convert a 7 x 12 minilathe to CNC. I have a

basement

> full of machine tools so making parts for the conversion is no
> problem.
>
> Briefly, I'm scraping the lathe for bearing and alignment, and I'll
> make a longer crosslide with T slots to allow mounting multiple
> tools. I envision using the lathe to turn small repititious parts,
> and also with the tailstock in place to do taper or contour turning
> on longer parts.
>
> I have a 3/8", 8 turns per inch ballscrew with 6" travel I would

like

> to use for the crosslide. I understand this will rule out manual
> operation, other than that any downside? I was planning to mount

the

> 16 turns per inch Acme that came with the machine in ball bearings
> and make a bronze nut for the X travel.
>
> Need guidance as to appropriate step motor size from someone who

has

> CNC'd a similiar machine. I'm thinking that the rapid travels
> wouldn't need to be as fast on a lathe as a mill, so I'm tending
> towards 200-300 oz/inch motors belted 2-1. Any suggestions as to

what

> has or hasn't worked well are welcomed.