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Advice Needed- retrofitting lathe to CNC
« on: January 13, 2017, 07:01:38 AM »
Hi everyone,

I am considering converting this lathe to CNC. I want to convert this lathe into a gang tool production lathe for making small parts in plastics and wood. Part size will be 16mm dia. max and 70mm long. The initial questions I have for you all are as follows:

Collet Closer: Having the ability to advance the stock through the spindle is essential, which is why I chose this lathe. Is there a mechanical way of operating the lever, without relying on compressed air? Like a large stepper motor or servo? If air is the best option, what recommendations do you have for setting this up and integrating it into the control.

Spindle Motor: Would it be advisable to use a servo motor as the spindle drive to simplify controlling speed, on/off etc?

Linear Guides: What size would be best for this setup? The bed is only about 60cm long and I will not need to have guides nearly that long, would HIWIN MGR15 be appropriate? Also, could I mount the guides directly to the bed or would they be too close together? Would it be better to mount them to a plate that then gets attached to the bed? I am concerned about mounting them precisely, as the only machinists I have access to are not known for precision work. Is it really true that the guides themselves are not straight enough to mount directly?

Ball Screws: recommendations for the exact specs and where to locate them? For the Z axis should it be in the center of the bed or in the front?

Stepper Motors: size?

Planning to use Mach 3.

Thanks in advance for your ideas and suggestions,

Justin
« Last Edit: January 13, 2017, 07:15:21 AM by Tweakie.CNC »
Re: Advice Needed- retrofitting lathe to CNC
« Reply #1 on: January 15, 2017, 01:14:24 AM »
Hi,
cant answer all your questions but maybe some.

Is it necessary to use linear guides at all? If the existing ways are OK and don't bind up at either end of the travel indicating
wear in the ways, you could use them, saves time and money and will probably be more accurate than mounting ball ways
if you don't have skills/resources. Ground ways are much draggier than ball ways so you would need steppers with serious
grunt to overcome the drag.

If you do use guides it is recommended that one rail, the 'reference' rail be butted up to a lip that needs to be dead straight.
Not easy to achieve unless you have access to a very good bed grinder. I used 15mm Rexroth rails without a lip, ie just
relying on the 'straightness' of the rail and it turns in 0.006mm over 200mm from straight. Happy enuf. Would like to
redo that part of my design but it would COST! Probably have to hand scrape to get it right, no mean feat without really
good (expensive) granite straight edges.

What are the existing leadscrews like? If there is appreciable backlash then you will probably need to change them.
Rolled ballscrews are widely available and are pretty good, you can pretty much rely on 0.05mm per 300mm error.
If that is sufficient accuracy for the parts you want to make all well and good. If you require better you will need
ground ballscrews, C5 is the entry level accuracy, 0.018 mm per 300mm. Even C5 ballscrews are EXPENSIVE, easily
five times that of a rolled screw. The price of more accurate C3,C1 and C0 will make you wince!

Without resorting to screw mapping the absolute accuracy over 70mm (the length of your part) is 0.012mm with the
C7 rolled screw or 0.004mm with C5's.

Looking at you pic guessing the collet is C5? You could use a motor and screw with limit switches to do it but a
pneumatic ram would be much easier assuming you have a compressor. Would require a double acting ram and a
change-over solenoid, not cheap if you have to buy new but if you buy good you'll never have to fiddle with it again.

I would like to see some more pics and measurements before I comment on where to put the Z screw. In the centre
of the bed is good for accuracy but MAN do they get covered in S...T.

Cant really tell the scale of your machine or how hard and fast you need to push it to get it to make money for you.
I got reasonably lucky and got four C5 Kouruda ground screws/ preloaded ballnut/ end bearings, 400mm travel, 5mm
pitch,20mm dia in good order for $600 plus $150 shipping off Ebay. They're great. Even with small steppers thru 10:1
gear reduction get 750kg thrust..plenty enuf to break things! I think you should be thinking about 16 or 20 mm dia
screws particularly if they have to work for a living!

Craig
My wife left with my best friend...
     and I miss him!
Re: Advice Needed- retrofitting lathe to CNC
« Reply #2 on: January 15, 2017, 03:11:20 AM »
Hi Craig,

Many thanks for your reply. The lathe does not have any leadscrews. It has a lever operated slide that has 2 lever operated slides mounted on it, one is compound and both travel in the z axis by means of gear racks. So I am pretty sure I will be having the linear guides installed. Would it be an option to have some shallow tracks milled in the ways to register the rails? I am considering the HIWIN MGR15 series. I have read that these are for 3d printers etc, but am wondering if the largest one, the 15mm, will be sufficient for machining soft, small diameter materials? The reason is that I think I might need to conserve on height as i start stacking rails and blocks for the 2 axes or else I might reach center and have no room left for tool holders.

Any thoughts on the spacing of the z axis rails? It seems like they would benefit from a wider spacing, but then...there is never any load on them except for directly at the spindle. I am thinking of desktop or even bench top cnc lathes that have a narrow bed but seem to do fine.

I am pretty sure rolled ballscrews will suffice, as the accuracy I require is not that high.  What pitch would you recommend for speed?

Yes, I will post more photos and measurements soon,

Thanks again for your thoughts, much appreciated,

Justin
Re: Advice Needed- retrofitting lathe to CNC
« Reply #3 on: January 15, 2017, 06:35:29 AM »
Hi Justin,
having a look thru the Hiwin catalogue, I don't think the MG series is going to be up to it. The MG series has just two recirculating
ball tracks, one on each side. Dynamic load rating about 5kN. The EG series has four ball tracks and is much stiffer, the same 15mm
rail dynamic load rating about 10kN and the HG series about 12kN.

The guideways and cars I selected for my mill are equivalent to the 15mm EG series and they're fine but I'm glad I didn't choose anything
lighter. 3D printers have vey little load; a lathe has cutting forces and might as well come from a different planet! Don't be duped, rigidity
is paramount, even if you get everything else right if the ways/guides are not rigid you've wasted your money. I can understand the
height limitation. The WE 27mm rails/cars have a height of 27mm and a dynamic load of 12kN. Given their width you may get away without
having a lip to clamp it to. Additionally the extra width will help if the rails are closer together than ideal. It may also allow you to have
the ballscrew along the front of the machine without it skewing.

The 15mm EG rails/cars have a height of 24mm, but should probably have a lip to which to clamp and be as widely spaced as possible.
Would almost require that the ballscrew be centrally located.

Quote
but then...there is never any load on them except for directly at the spindle

That's wrong, once you start cutting there's PLENTY of load.

As for a spindle motor, certainly servos are good and just about mandatory if you want to do any serious threading but they're not
cheap. Given that you aren't intending to cut hard materials (yeah right, you will try..I sure have!) modest power (1-1.5kW) should
be enuf. If you want to save a few bucks then a standard 3 phase motor and VFD are pretty good. Don't buy a cheap Chinese or
Indian made one tho...hook them up to a VFD and they go bang. You could probably get a 2-3 hp American/British/German/Japanese
one second hand and mostly they tolerate being driven by a VFD. I have a little 1/2 hp 3 phase 4 pole motor, with 50Hz input rated
1475 rpm for grinding tools. I regularly push it up to 3000 rpm. The rotor is as far as I know the same as is fitted to the 2950 rpm
motor, in fact probably indentical to the 3600 rpm rotor in countries with 60 Hz. Haven't blown it up yet!

Stepper motors run out of torque real quick at speed. Most two phase steppers are likely only have 5% of rated torque at 1000rpm.
Best to consider max speed of a stepper of about 500 rpm and still have enuf torque to accelerate the cross slide. With a 5mm pitch
gives you a rapid of 2500 mm/min. Clearly you could go faster with 10 or even 20mm pitch but then run out of thrust at cutting speeds.
My steppers have 10:1 planetary gear reductions and rapids are fairly slow, 1200mm/min but the thrust at cutting speeds is huge. I probably
went a bit overboard but I sure do like it when I start hooking into a bit of 316 SS.

Post a few more pics and some measurements, I'm wondering if one or both rails could be mounted on the sides of the bed and therefor reduce
the height, I wouldn't consider dropping below 10kN dynamic, I'd have to find another way.

Craig
My wife left with my best friend...
     and I miss him!
Re: Advice Needed- retrofitting lathe to CNC
« Reply #4 on: January 15, 2017, 11:06:30 AM »
Hi Craig,

Great, thanks for clearing up the issue of the guides, interesting idea mounting them on the sides. Is it significant that the ways were designed to, and presumably were capable of supporting cutting with this slide setup? Or is it not relevant because the z axis would always have been fixed st the time of cutting, while if we used linear guides mounted to the ways, the slide would not be as rigid with a moving z axis? Regarding a servo spindle motor, what sort of specs would I be looking for if I decided to go for one?

Some photos below,

Thanks!
Re: Advice Needed- retrofitting lathe to CNC
« Reply #5 on: January 15, 2017, 12:55:34 PM »
Hi Justin,
looks like a fairly tight installation.

How about just one rail for Z, say WE 35mm rail with it spanning the gap down the middle? Height 35mm. Probably still want two
cars, nose to tail as it were.

Craig
My wife left with my best friend...
     and I miss him!
Re: Advice Needed- retrofitting lathe to CNC
« Reply #6 on: January 16, 2017, 12:09:11 PM »
Hi Craig,

Interesting, still trying to figure out the advantage over a pair of 20mm rails though. Perhaps it is a solution to minimize the chance of problems with the installation?

Offline RICH

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Re: Advice Needed- retrofitting lathe to CNC
« Reply #7 on: January 17, 2017, 06:56:50 AM »
Suggest the following:

Get an idea of retro fit total cost for what you idealy want in the end.
Seems you can't do the work yourself so be practical on having machining work done by others. You may come to the conclusion that it's financialy smarter to
get something else.

Only your operation and number of parts to be done will define the level of automation required. I assume you will be selling the parts so do some number crunching.

Gang tooling can become restrictive ( my opinion ). Flexibility is gained with a small turret or even quick change. Just someting to consider.

Intent is to machine plastics and wood. Determine the rpm operating range required for you want to do. What is rated spindle operating range? BTW,
may as well consider spindle bearing replacement.

Just some thoughts,

RICH
Re: Advice Needed- retrofitting lathe to CNC
« Reply #8 on: January 17, 2017, 10:39:38 AM »
Thanks Rich,

Great suggestions. In terms of cost, I think I have quite a cushion, as I could not find much in the benchtop size for cnc, gangtool, with automated collet closer. The retrofit Hardinge machines seemed to be in the 15 K range and up. Perhaps i am missing something? The machine I have cost me $500.

Target spindle speeds: I seems to be usually working at around 5000 rpms and above manually, any thoughts or suggestions for ideal speeds for these materials?

Thanks for the spindle bearing suggestion!

I will try to post some of the options and parts I am considering soon and would be most appreciative for any further thoughts or suggestions,
thanks again,
Justin

Offline RICH

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Re: Advice Needed- retrofitting lathe to CNC
« Reply #9 on: January 19, 2017, 04:56:08 AM »
any thoughts or suggestions for ideal speeds for these materials?

Nope, look them up and see what is recommended.

I would suggest you do some design for the retrofit and to arrive at what you will need first.
A 3/8" thk precision ground steel plate would provide for easy attachment to the current bed and allow size options
for creating what ever you come up with for the X and Z axis.
As a starter, may want to consider using assembling purchased parts from current produced small lathes.

Lay it all out!

RICH