Re: CNC minilathe

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "rotarysmp"
<mark.wrathall@l...> wrote:

> 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.

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Thanks for posting the lathe information.

These machines really do come in kit form, but I have the skills and
am willing spend the time to fit the parts. I already have a
minilathe that I converted into a turret lathe 7 years ago, tapered
slide gibs and everything scraped pretty close, and once they're
refitted they work well. I'm scraping the parts for the CNC lathe now.

I plan to make a new crosslide about 10 inches long, out of 7/8"
stock. The ballnut will fit in the saddle except for the tubes for
the ball circuit. My new crosslide will have clearance for the
ballnut tubes down the center, and 2 t-slots running lengthwise to
mount multiple tool setups. I want to make it long enough that it is
always fully supported by the saddle dovetail, there isn't much
bearing area there and I will be cutting hard materials.

I'm using the stock leadscrews on my minimill with 424 oz/in motors
direct drive, and the steppers peak out at about 35 inches per min.
This leads me to think I'll need a ballscrew on the long lathe axis
to get any speed for rapid positioning moves.

I have ballscrews for the minimill, just need time to put them in.