Myford super 7 Positioning servo's on Spindle and main infeed

Hello All.

I am a drives and controls engineer, I have applied this knowledge to customer with machine tools all my working life. I AM NOT A MACHANICAL ENGINEER, but have a reasonable understanding and a real interest.

I have recently purchased a Myford Super 7 to restore and convert the spindle and main feed screw to CNC with positioning servo’s on both axis. I am fitting a 16mm dia, 5mm pitch ball screw to replace the main screw, this is the axis that will have one of the servo’s.

As a learning process, I restored a basic lathe and fitted a vector spindle, just to understand the mechanics and understand what I needed from the next lathe to deliver the quality I needed.

YouTube - **LINK**

When the Myford is finished, I will sell the first lathe.

The main use of the lathe will be to produce clock gears and pinions, but also parts for my vintage motorbike.

The lathe is stripped and the bed is away being ground.

I am going through each assembly and restoring as I go along.

I selling the clutch and counter shaft, motor, all pulley’s including spindle pulley, rack, screw and apron and all other bits that I do not need. Some bits have all ready been sold.

My first question is the top and bottom slides. There is backlash, I think it will be the nuts as these are made of an alloy. Where would I get 2 bronze nuts to replace the original nuts?

I understand that backlash is not always a problem as it can be overcome with good practice, but I want to reduce it to a minimum

I am tempted to replace these primitive screw’s with rolled ball screw’s and include a locking mechanism on each axis as the ball screw will not hold position as it does not have enough friction. This would allow servo’s to be added on these slides. But this may be a job for latter.

I have many pictures I have taken along this journey and I will have many more before I reach the finish line. I am not sure about the norm of this forum but I am happy to share them.

I would welcome the answer to the above question.

(Due to my lack of knowledge about this forum, I posted this elsewhere. I have decided to create a new thread).

Edited By Andrew Davies 4 on 25/09/2018 09:42:08

Another question.

The S7 is designed to use a tool holder about 8mm square. I want to use a 10mm tool holder.

As I am doing so much work I am happy to mill 2mm from the bottom of the main tool holder that sits on the compound slide.

I am going to get the 3 saddles ground during the rebuild, hence the 2mm may vary to bring the tip height to the exact position.

Can anyone see a problem with doing this?

Thanks

Andrew

Common practice to mill a bit off the bottom of the tool holder. My ML7 was made to take 3/8" tool bits or smaller so I had to knock 20 thou or so off it when i converted to modern 10mm insert tooling. Did it in situ with an end mill cutter held in the lathe chuck and suitable packing under the toolpost.

If you are getting the carriage and cross slides reground, you may drop the tool holder sufficiently in the process anyway and not have to bother.

Comments withdrawn - I was against grinding the toolpost but then read Hopper's post and realised I was being over cautious...

Edited By SillyOldDuffer on 25/09/2018 12:05:11

Why not just make an extra toolpost or even clamp the tool with packing straight onto the topslide like Myford owners did before they got a 4-way or QC toolpost.

the bigger tooling is stiffer when you get a situation where you need a lot of tool overhang such as working with tailstock support or around the edge of  a large diameter such as a flywheel not just when you want to take a heavy cut.

Edited By JasonB on 25/09/2018 12:09:02

If you are regrinding the carriage you must remember to also grind the surface where the apron fits, otherwise the leadscrew alignment will be too low. Similarly if the bed is reground.

Or just mount the new ballscrew nut to take into account any loss of height.

Buy a kit from cncyourmyford.com

Martin.

Many thanks for all your comments.

The apron will no longer be there. I am making a bracket to mount the ball nut to the underside of the bottom saddle. Due to this, I am going to mount the ball screw and both end bearings and then measure up bracket.

20 thou is only 0.5mm, I have a difference in tool holders of about 2mm. Have I missed something?

Andrew

I compliment CNCmymyford for the work they have done, but I think I can have more fun doing it myself, complete for a much lower price, and have a better solution.

This is not arrogance; it is just that this is what I have done for 30 years.

Please note the operative word is think, as I have not seen the full specification of cncmymyford.

Issues I have:-

It still uses an asynchronous motor, albeit with a cheap encoder. Not sure you can position the spindle with full torque

No dedicated HMI

No real time CNC as it is running on a laptop

Not a big fan of stepper motors.

…………..

Wow. £3300 plus painting and a lot of fitting. Pigs and lipstick come to mind. For that cost plus the cost of a Myford you could probably pick up a proper CNC machine and spend your time bringing it back to life, with some likelihood of ending up with a decent machine.

Murray

Hi

Is that Muaray from Kenilworth??

Andrew

No, that'll be Murray from Lancashire (currently) - Lytham St Annes (posh name for Blackpool).

Murray

If you replaced cross slide and top slide standard screws with ball-screws you could use a harmonic drive gearbox to link a handle to the screw, that would eliminate the need for a lock or brake.

Hi

This is true, and may be the way to go.

Many thanks

Andrew

Just starting to paint main bed with enamel

By top and bottom slides I suppose you mean the cross and top (or compound) slide? For CNC you have no need of a topslide any more. You can then make a properly rigid tool post that fits the cross slide. This makes it possible to used indexed tool holders and properly apply tool offsets.

The cross slide screw is best replaced with a ballscrew, it is very hard to get rid of backlash with a normal nut, and backlash on the X slide is a real pain - for example when cutting tapers. Depending which S7 you have you may need to find a small diameter ballscrew for the X slide to fit the available hole - my conversion was of a power X feed 9pxf) machine that uses a larger feedscrew so I was able to use a 12mm screw supported only at the outboard end. It isn't essential to have a ballscrew on the main Z feed ("leadscrew" and I haven't fitted one (it's on the to-do-one-day list). If you do use standard nut, the Myford non-pxf nuts are made of mazak alloy as standard and you'd need to make or have made a bronze one - even then you'd be lucky to get a low enough level of backlash. My ballscrew gives about 0.02 mm IIRC. The Myford pxf nuts are hardened steel, but even with those the as-new backlash is ~0.2 mm.

Why are you fitting a servo to the spindle? Normal practice is to have a spindle sensor, either 1 per rev for Mach 3 or a multi-slot sensor for better systems such as LinuxCNC, but to use a standard drive system. As long as the controller knows where the spindle is it can control the tool position to suit at least for screw cutting. If you are thinking of using this for dividing, I think you're better off having a separate digital division head. Most readily available CNC controllers expect a standard drive spindle, not a servo positioned one.

These days it is recommended to have a separate motion controller between the controlling PC and the servos/steppers (by the way please note that you DON'T use an apostrophe when an "s" is indicating a plural). This interfaces to the PC using USB or Ethernet, and actually generates the real-time control signals - I can't recall exactly but I assume that's what cncyourmyford does. With this approach you can use a laptop because all the real-time stuff happens in the controller.

Hope this helps - I first converted my S7 nearly 10 years ago and have made a number of improvements since and it works very well.

Hi

By fitting a servo on the spindle I have infinite position capability, For cutting different clock gears or threads this is perfect, all gears will be removed and the servo is on the end of the ball screw.. Spindle and additional axis will interpolate.

I will be using a position control integrated into one of the dives with PLC (IEC1131) capability, this will talk to the second drive over SERCOS, the HMI will both read and enter variables. I want the CNC to be real time, hence not from a lap top.

The solution I will be using is MLD by Bosch Rexroth.

I want to fit a high speed spindle for the gear cutting at a later date; this allows a great deal of flexibility.

P.S. I am and engineer, hence grammar is not perfect.

Many thanks

Andrew

Edited By Andrew Davies 4 on 02/10/2018 12:38:19

Edited By Andrew Davies 4 on 02/10/2018 12:43:07

Just finished the final bit of painting. Start propper engineering in the morning

Sorry to say that looks like a crappy regrind you got there.

Did they skim the feet first and also do the vertical shears & undersides?

Martin.

Edited By blowlamp on 03/10/2018 13:21:19

Edited By blowlamp on 03/10/2018 13:22:03