johnsmachines

machines which I have made, am making, or intend to make, and some other stuff. If you find this site interesting, please leave a comment.

Category: CNC

CNC Mill Upgrade -8

Fitted the new VSD Friday.  Ordered Tues pm.  Arrived Thurs am.  Impressive.

$AUD315, inc shipping.   Job cost is mounting.  Still within reasonable limits.

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The old VSD, top right.  The axis controllers (top left) had not been wired when this photo was taken.

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The new VSD (variable speed drive) 4kw.  Fitted neatly with some new mounting holes, without any drama.  The rats nest looks less daunting every day.

Now, except for the main spindle motor, there are no more original major electrical components.  All have been updated and replaced, along with the cables.

Yet to be wired are the VSD, coolant pump, oil feed pump, limit switches, homing switches, and the Gecko driver and 48v power supply for the rotary table.   But the mill is useable now.   Video coming up soon.

 

CNC Mill Upgrade -5

I have been putting quite a few hours into the upgrade, but not much to show photographically.

Finally got the new servo motors installed.  Replaced the X axis belt.  The most difficult servo to access was the Y axis, and of course that was the only one where the alignment of the timing belt was out.   Finally sorted by using a fibre optic camera to see why the belt was climbing onto the flange of the pulley.  The pulley was 1.2mm too far onto its shaft.  I know that, because I solved the problem by inserting washers under the motor mounts.  1mm washers did not work, nor did 1.5mm washers.  But 1.2mm washes did work perfectly.

Today Stuart arrived and removed more of the old wiring.

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Stuart, doing another CNC upgrade wiring.

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The old 7k computer has been removed, leaving some buttons.  I might be able to use those. The computer enclosure might disappear too.  Not decided yet.

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The old CNC mill has lost some weight.  Those cartons are full of old parts.  Note that the floor has been swept.  Stuart was concerned that we might be infested with snakes, but it is winter here, so we should OK until the weather warms up.

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The rats nest is disappearing.

CNC Mill Upgrade -2

The major components arrived this week, from China and USA.  Switches, and other components which go “ping” will be bought locally as required.  I am hoping that existing pulleys, belts, brackets will be adaptable.

The motors to drive the X, Y and Z axes are 1.2kW AC servo motors which can be connected to single or 3 phase power.  Each one weighs 6.7kg (14.7lb) .  From China, they are nicely finished.   Substantially shorter than the old servos which they are replacing and slightly larger diameter.  I am hoping that the slightly larger diameter will not cause major problems.

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AC servo.  There are 3 of these.  Kitchen knife to open the box and for scale.

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Old Y axis servo on the right, and the new AC servo left.

 

And each servo motor came with a controller and cables and connectors.

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And the electronics came from USA.

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C11 breakout board.

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C10 breakout boards x2

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And the Smooth stepper control board.  It is tiny, but the most expensive electronic component.

All up cost so far is ~$AUD2100, of which shipping is about 25%.

Next step is to swap over the servos.  The old shafts are 16mm and the new ones are 19mm.  I intend to machine the bores of the pulleys.  Hope there is enough meat  Tofu to allow that.

Cutting the Gear

Today Swen and I started cutting the gear.  Here is the setup.

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The gear was centered on the rotary table with an aluminium bush, which fitted the outside of an ER40 collet chuck.  The chuck had an M3 shaft which fitted neatly into the spindle of the rotary table.  3 bolts secured the gear blank to the T slots on the rotary table.

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This photo was taken after the setup was broken down.  Showing the M3 taper ER 40 collet holder, which I used to centralise the gear blank on the rotary table.

I started the mill conservatively at 200 rpm and a 0.5mm deep cut, but gradually increased the RPM to 450, and the depth of cut to 2mm.   Later adjusted to 300rpm, 1mm cut.

The CNC table performed flawlessly, with rapid advances between the 360/77º degree cuts (about 4.6º each tooth).  The feed rate was controlled by manually winding the X axis feed on the mill.

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We initially used water soluble cutting fluid, but changed later to raw Tap Magic, which seemed to work better.

As you can hear in the following short video, the cutter teeth are slightly off centre, but working well.  Didn’t quite finish the gear teeth in this session.  Some sparks later on, indicated that a cutter sharpening was required before the finishing run.  That will happen tomorrow.

The smoke is evaporation of cutting oil.

The setup will be left undisturbed by removing the gear cutter for sharpening.

I changed my mind about sharpening the cutter in the middle of the job, and continued cutting.

Instead, I lowered the spindle rpm, and the feed-rate.  The sparks stopped.  Maybe I was just pushing too hard, or maybe there was a hard spot in the metal.  Anyway, I finished the cuts.

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The finished gear cut.  Are those teeth looking a bit skinny at the peaks?

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Showing the setup from the operator’s view.  The CNC table worked brilliantly.

 

 

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Removing the burrs with wire brush and file.

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And testing the fit with a trial run in the headstock.  Thanks Swen, for helping (actually directing) the trial run.  The new gear on the left.

I made a video of the gear being rotated through 360mm, perfectly, but for some reason it will not upload.  (did upload eventually.. see below).    It looks perfect, with a tiny amount of backlash.   Full installation in a day or two.  I was quite surprised that making the gear to the specifications worked so well.

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Me, testing the backlash.

And afterwards, sharpening the cutter on the Quorn T&C cutter grinder which I made a few years ago.  An amazingly versatile tool.

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Just in case I need to make another.

The lathe headstock will be properly reassembled in a couple of days.  But I am finally feeling a bit confident about this job.

 

 

 

New Spindle Motor for the Boxford 125TCL CNC lathe.

Reader Ben asked about the spindle motor replacement on the Boxford.   This is a small CNC lathe, and was often used for teaching in secondary schools in Australia.  Mine was made in 1985, and I replaced the electronics a few years back because they were obsolete and not functioning.  The mechanicals of the lathe were beautifully made and in excellent condition.  I did replace the ball screws, but in retrospect, that was probably unnecessary.   I also installed new and bigger axis stepper motors.

I was still getting some unreliable results, despite the the upgrades, and wondered whether the spindle motor was lacking power.  I was taking lighter cuts to try and cope but clearly a new spindle motor was required.

The space that the motor occupies is fairly tight, and initial searches for a suitable replacement were fruitless.  The new  ClearPath motors looked promising, but enquiries to the manufacturer indicated that the required power and rpm’s were not available.  Then my expert friend (or should that be “friend who is an expert”?), spotted the Ebay ad below, and bought and succesfully installed the servo motor in his 125TCL, so I did likewise.  I am afraid that the electronic aspects remain a mystery to me, so I cannot help with those.  It is a 0.75kW motor, substantially more powerful than the original, but very compact.

 

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Do an Ebay search on the code on the controller.  I paid $AUD339 but it is now plus postage and GST, so close to $AUD400

 

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The new spindle motor and servo controller

 

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A new motor pulley was required.  There is still a high and low belt ratio available, but with the extra power and torque I never use the low ratio.  RPM range is 300-3200.

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This has been a very successful modification.

Many thanks to Stuart Tankard for his generosity in time, expertise and advice in getting it going.

 

 

 

 

Project in the Wings.

While finishing the triple expansion steam engine, I have decided on my next project.  Actually, based on my past history of procrastination with the triple, I might even put aside the triple to start on this one.

Reading this article in “Model Engineers’ Workshop” gave me the inspiration to convert a rotary table to electronic control.

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Dec 2016 MEW article

So I have commenced accumulating the bits and pieces…

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An 8″ Vertex rotary table.  I have had this for years, but unused since acquiring a universal dividing head.  Should be ideal for this project.

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A Nema 24 Stepper motor, shafts at each end, so I can use the table manually as well as electronically.  The Microstep driver was supplied packaged with the motor as a kit.  $90AUD inc postage.

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From the same supplier, a 48volt power supply.  $38AUD

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The brain of the system.  A programable microcontroller “Arduino Uno”.  I bought 5 of these for $20AUD post included.

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And an easily attachable display.  To attach the Arduino.  $19AUD

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And since I knew nothing about Arduinos, a “Getting Started” book.  Excellent.  On loan from a friend (thanks Stuart)

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And to practice some circuits and get some idea about the Arduino programming, a starter kit of bits and pieces.    $75AUD, but has been very instructive and loads of fun.   The program to run the Arduino is downloadable free from the Internet, so this kit might be a bit superfluous.

And some items of kit.  Each under $20AUD.

magnifiers

A magnifier soldering station, and head light and magnifier

multimeter

A very cheap multimeter.  Previous purchase.  Works fine.  $10AUD

I have disassembled the rotary table, and ordered a 12/8mm coupler.  I am waiting for the coupler before I start designing and cutting the main part to be fabricated which is the piece which joins the stepper and the table.

Also ordered a box to contain the electronics and switches.  Havn’t yet thought about cables,  joiners etc.

 

 

 

Model Ottoman Bombard – Painting

I would have preferred that the title of this blog was “Finishing the Ottoman Bombard”, but I am still waiting for the vectors of the barrel mouth decorations and Arabic (?) writing, and the touch hole.

But I have at least painted the bombard, and the pictures follow.  You will notice that I have not attempted to reproduce the bronze or copper colours of the orginal in Fort Nelson.  Partly because I doubted my ability to make painting such variegated patterns realistic, and partly because the cannon would not have looked like that in its heyday of 1464.  It would probably have been either black, like most SBML cannons (smooth bore muzzle loading), or possibly gaudy golds and reds and blues like other medieval items.  So I painted it black.  I like it.  If I get evidence that it should be more colourful I can change it later.

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First coat – Primer.  Hmmm… interesting colour.

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Next coat – matt black brushed on, to fill the hairline wood cracks.  Incidentally, the (dirty) parquetry floor is also made from the red gum house stumps from which the cannon is made.

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final two coats –  matt black, from a spray can. 

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So there it is, finished except for the barrel mouth engraving, and the touch hole.  Now what to do with it…   SWMBO says it might be useful as an umbrella stand.

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The breech.  25mm diameter explosion chamber.  1:10 scale

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The barrel, 63mm bore.

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Assembled.  The model is 520mm long.

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It does need some decoration

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A Matter of Scale

Before I get onto a brief reflection about scale, the photo below shows 2 cannon barrels.

The big one was what impelled me to converting a manual lathe into a CNC lathe.  That time consuming, costly, and ultimately very satisfying project, started because the CNC lathe which I used to turn the big barrel could only handle the job by doing it in two stages…. doing the breech first then the muzzle.  That was due to the big barrel being too long for the lathe, at 300mm (12″).

The small barrel was a test for the CNC converted lathe just finished, being the first complicated shape which I have made.   To save on material, I made it at exactly half the scale of the big one, ie 150mm long (6″).

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Comparing the two barrels reminded me, that if an object is twice as big as another, in all 3 dimensions (height, width, depth), it is 8 times as heavy.   And any projectile, and weight of black powder, would also be 8 times the weight.  But the wall thickness of the explosion chamber is only TWICE as thick.

My point is, that if scale is maintained, the smaller the cannon, steam engine, boiler, whatever…..  the less likely it is to explode.

Not that these cannons will ever be fired.  Just hypothetically.

CNC Lathe Conversion – 17

First Test Run

After some test runs without tool or material, I performed some measurements.

500mm movements along the Z axis were reproduced multiple times with a deviation of 0.00mm!  (the Z axis has a ground ball screw)

100mm movements along the X axis deviated 0.02mm.  (the X axis has a rolled ball screw).

I was delighted to note that the lathe is extremely quiet and smooth.  The only noise is some belt slap from the very old belts, and from the stepper motors.

The video below was taken from my iphone, while I was operating the lathe controls, so please excuse the erratic movements.

The steel is 27mm diameter.  750rpm, 50mm/min feeds.

And the guards will be made next step, without fail.

The G code was generated using Mach3 for these very simple shapes.  For more complex items I use Ezilathe.

 

The lathe is 600mm between centres.  38mm spindle bore.  Swing about 300mm.

CNC lathe conversion -15

Another couple of advances in the conversion.  Today I installed the lead screw cover and the cable protector to the cross slide stepper motor.

The cable protector was easy and straightforward. It flexes in one direction only, and is fixed at the ends after the cable is threaded through it.  The length is adjusted by adding or removing links.  It was placed so that coolant liquid will drain out of it, and to minimise the accumulation of swarf.   The cables themselves have a thick covering and are well protected.  The link protector will not kink, further protecting the cable.

It was cheap.  About $AUD20 for 2 meters, posted from China.  I used about 1.1m.

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Showing the stepper motor cable protector, and the lead screw protector (one half of it.  The other half is on the other side of the carriage.)

The lead screw protector was another story.  It is a spring steel coil, about 50mm wide, and as it is compressed the coils fit inside each other.  I made a big mistake in allowing it to spring open before I had installed it (there were no instructions).  It immediately opened to a length of over a meter, in coils about 50-60mm diameter.   No big deal, I thought.  I will just compress it back to its original configuration.    Big mistake.

It was what I imagine coiling a live, oily, biting, boa constrictor would be like.  (OK, boas constrict rather than bite.  How about an anaconda, or a big eel.)

I fought it for about an hour.  And eventually succeeded.  Minus a few bits of my skin.

So I did not allow the protectors to expand again until after I had them on the lead screw.

This is what they look like.   Pretty cool IMO.  They just expanded into position when I removed the restraining clips.

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The lead screw stepper motor and protector.  The Estop box above will get some ends to exclude swarf.

It was not cheap.  The best price that I could find was from South Korea.  $AUD200 inc postage.  But it is excellent Japanese quality.

The wiring is happening, but the variable speed drive seems to be dead.  It has been sitting unused on a shelf for 2 years, so no point asking about warranty.  Took it apart to check for broken wires, fuses, burnt out components etc, but nothing visible.  Will order another one.  About $AUD200.  An unexpected expense.

 

CNC lathe conversion -14

These lathe CNC conversion posts are probably becoming a bit tiresome, but just in case there is someone out there who is interested, I will continue until the job is finished.

The latest was to make and install a spindle speed (and position – thanks David M) sensor.  It consists of a disk with a slot cut in the periphery, attached to the main spindle.  And an opto-electronic sensor which is connected to its own electronic board, thence to the breakout board and VSD.

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The disc with the slot at 8:30 and the sensor at 9:00.  I must have chosen the wrong cutter or turning speed for that disc aluminium…  looks a bit rough.  (note added 13/7    Stuart T says that I should have used coolant-lubricant).

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View from above.  Any clearer?   That gear is now superfluous except as a spacer.

So there is one electronic impulse per spindle revolution.  That is enough to measure the RPM’s.   Essential for cutting threads.

The beauty of this system is that there is no gear selection or changing, and ANY thread pitch can be selected…  metric, imperial, BA  etc…  any odd ball thread that your heart desires.

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The HTD (high torque drive, I am informed by many readers) pulleys and belts and taper lock fittings.  Unfortunately I could not find a taper lock to fit the small pulleys, so when it is all finally, definitely, absolutely, correctly,  positioned, I will Loctite them in position.  Protective covers yet to be made.  I quite like to see the mechanicals in action, so I am intending to make the covers from clear polycarbonate.(Lexan) .

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The CNC lathe has 3 belts. There is a V belt from the 3 phase motor to the main lathe spindle.  Although I changed the motor and the pulleys, the old belt fitted, which was good.  No hassle. But the stepper motors driving the lead screw and cross slide screw, and their pulleys were all […]

CNC Lathe conversion – 9

The CNC lathe conversion has been happening, despite no posts on the blog.

I have mounted the electronics enclosure, and mounted the various components inside.  No wiring yet.

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This stainless steel tool box is the electronics enclosure.  It fits the space quite nicely, and is adequately big.  The back gear cover to the right will be retained, although the back gears have been discarded.  The main switch and emergency stop will be mounted somewhere on this cover.

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The Variable speed drive (VSD) sits on top.  That will control the spindle speed.  The transformers, stepper motor drives, and Breakout board (the heart of the system) are positioned inside.  Plus cooling fan and filters.  Ready for wiring.

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Drilling the apron to attach the cross slide ball screw bearing.  One chance only at this one, so the setting up took a couple of hours.  The apron is clamped to a large angle bracket on the milling table.   M6 threading followed.

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The end result.  The bearing as attached to the apron and the ball screw is in place.  I machined the end of this ball screw to fit the bearing, cut a thread (M10x1), and machined the end to accept the pulley.  All good.  There is 0.25mm adjustment available if required, but it all seems pretty correct.  The bearing sits on a carefully machined block which is 7.85mm thick.   Still waiting the lead screw machining.(!!)

NOT MUCH GOING ON TODAY

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This is my workbench after I had almost finished tidying it.  Really.  

 

Then I thought about machining the ends of the cross slide ball screw.

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So I mounted the collet chuck and checked the runout.   0 to o.01mm.  Then I did a test cut in the ball screw.   Hard hard hard.  But it did cut.  Then I chickened out and decided to finish it another day.

So, looking around the workshop for something else to do, I decided to pretty up the new CNC lathe apron.

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Before (milled surface).

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During

 

And I forgot to take a photo of the after, but it did look nice and smooth and shiny (look at the mirror finish behind the wheel).

Being retired is great!

CNC Lathe conversion-7

I am still waiting for the replacement ball nut for the lathe cross slide to arrive.

Meanwhile, I have been busy machining the supports for the lead screw.

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Drilling the holes for the support bolts for the lead screw nut

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And gradually drilling the hole to 49mm!

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That is a 49mm drill!  First time that I have used it!  Thank goodness for the  FS Wizard app, to give me some idea about feeds and speeds.  Following this I used a boring head to enlarge the hole to 55mm.

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Gradually enlarged the hole in 20mm steel to 55mm diameter.  and here is the lead ball screw, sitting roughly in its proper position.

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So this is where I am at.  The lead ball screw is sitting approximately in its correct position.  Considerable adjustment required.  And I am yet to turn the ball screw ends to their correct dimensions.

CNC lathe conversion-6. EBay problem

My first hitch occurred today.

I was very excited to receive the nut for the cross slide ball screw.  If you have been following these posts you might recall that the ground ball screw for the cross slide came from Taiwan, and arrived in 3 days.  But I had to order the nut from a seller in USA.  The nut was advertised as new old stock, but with no packaging.  That was OK, but the postage cost for such a tiny item was ferocious.

It was the last item to arrive from overseas.   However when I looked at it, it was obviously NOT new.

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The cap screws holding the ball recirculating tube were different from each other, and the washers underneath were too big for the screws.  Obviously not the way that TNK made it.  Somebody has had the nut apart.  And the ball retaining tube was very scratched  and loose.  Again, not TNK standard.

But no biggie.  If it works — fine.

So I turned up a retaining tube to remove the previous nut from the ballscrew, and it came off without any drama.

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But when I tried to fit the “new” nut, it just would not go on.  Tried reversing the direction.  No go.  Bugger bugger.

Somebody has altered or changed the “new” nut.  Maybe installed balls which are too big, or maybe damaged the entry thread.  I do not know.

What to do.  I have been waiting 2 weeks for this to arrive.

First, Ebay email to the seller.  See what the response is.  Ask for a refund.  The postage was almost as expensive as the nut. If unsatisfactory response, they will get the worst Ebay feedback ever.

There is one other seller of these nuts on Ebay, also in USA, and 50% more expensive, and the postage is also 50% more expensive. (how DOES ebay come up with the postage charges.  It seems more related to the cost of the item rather than the weight-size etc.)  But the nuts are in original packaging.  And I want to get going with this, so fuck it.  Pay up and get it.

I will report in a later post.  (if the bad language in this post seems to reflect my state of mind, well, yes it does.)

 

PS.  Next day.  After sending photos of the issue, the seller accepted responsibility, and I am getting a full refund.  That restores my faith in Ebay/Paypal.   I hope that the next one is in better condition, and comes a bit more quickly.  Sorry for the bad language.

CNC Lathe conversion -5

This is a list of the components which I have accumulated to convert a manual lathe to a CNC lathe.  It is not quite complete, but close.

  1.  Lead ball screw and cross slide ball screw.  Both with nuts and end bearings.  (no pic yet)
  2. The electronic components.
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The electronic components, not including computer and parallel cable and manual pulse generator.

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Two stepper motors.  Nema 34, 1200 inch – oz.  With rear covers.

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A Gecko microstep drive for each stepper motor

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Cable and connectors for the stepper motors

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A transformer-power supply (48 volt)

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Another transformer-power supply (5 volt).

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3 phase 1.5kw motor (top) to replace the single phase motor (bottom)

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Timing belt gears 24 and 48 tooth, 5M.  Order belts when size is definitely established.

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FK20 lead screw bearing and Ball screw covers

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The electronic heart of the system- the breakout board.  A C11R9

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The index pulse board and sensor.  A C3.

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Manual pulse generator, wireless.

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Variable speed drive, identical to this one on the mill

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An electrical enclosure, to hold the various components.  This stainless steel box was originally an item of medical kit.  Here I am checking out one possible location.  Not yet definitely decided to use this.

Finally in the electronics section, I will need a computer, loaded with Mach3 and Ezilathe.  Surprisingly, it does not need to be a particularly powerful PC.  And there are advantages in using an older operating system such as  XPpro.  I think there are a couple of those in the attic.  If not, I should be able to pick one up for under $100.

3. Various structural items.  Most of these will be 20mm thick x 200mm wide steel, cut to size and shaped and drilled.  I will take pics of these as I make them.  I was planning to have them water jet cut, but the shapes a fairly simple so I will cut and machine them myself as I go.

So, that is most of the items for the job.  I have spent about $AUD1100 on the ball screws and nuts, about $AUD1500 on the electronics and electrics, and maybe another $AUD500 on pulleys, belts, steel, taper locks, fasteners etc. I will add it all up accurately at the completion.  The lathe was cheap, bought sight unseen a few years ago.   So all up, I should have a CNC lathe for under $AUD4000.  And many pleasant hours of design and machining.  And a great learning exercise.  Stay tuned!

CNC Lathe conversion -4

I am in the process of collecting all of the components for the conversion.  Parcels are arriving from South Korea, Taiwan, China, USA and Australia.  Next post I will take a photo of the bits, before I commence assembly.  I have spent a lot of hours on the computer drawing up the positioning of the new components, deciding which components to get, then communicating with the sellers and making the purchases.  Not to mention hanging around home when parcels are due.  If I duck out for 10 minutes, that’s when the delivery van arrives.  And of course he leaves his little card “sorry we missed you”.

This post I will show you some of the drawings of the proposed conversion.  Of course the first step is to strip the lathe of all of the old manual controls, gears, motor, Digital read out, carriage apron, lead and cross slide screw, electric control box and contents.  (taking photos of every component insitu in case of a change of mind, and restoration required later).

Then measuring the bed and carriage in minute and accurate detail, and drawing it in CAD.

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This is the lathe side on and end views, showing the new lead ball screw in red and green.  The cross slide ball screw is also there, but not well seen at this scale.  The 4 ball screws at top left are the possible combinations for eventual installation.  The second red one is the position eventually decided.  The green ones are with the nut re-installed in reverse direction.   I really did not want to remove and re-install the nut, because it is pre-loaded, and I do not want to risk disturbing the setup.

Many drawing versions are required, and as the components arrive, I find myself making changes.  This is definitely not the final version.   The two carriages show the carriage in its extreme positions on the lathe bed.

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This was an early sketch of how I thought I would arrange the cross slide motor and lead screw nut.

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This is a fairly accurate drawing of a cross section through the cross slide.  Black is existing.  Red is the new nut and ball screw.  You can see that removal of some of the cross slide bed iron will be required.  After looking at this I decided to move the nut and screw up a couple of millimeters.

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The Internet has been very useful in showing what fittings are available.  These are a few of the catalogues and tables which I have downloaded.  Some sites even provide CAD drawings so their components can be inserted into my CAD drawing to see if they will work together.

Ball Screws -2

This is a brief post to give a 10/10 rating to an Ebay seller with whom I have had dealings recently.

I have bought 2 ball screws and ball screw covers and ball screw bearings in 3 separate transactions from a supplier in South Korea.

Postage was included in the “buy it now” price.  In one case I offered a lower price, which was accepted within minutes.

After paying by Paypal, I received confirmation within an hour in each case, that the item had been shipped (actually air freight by Fed Ex).

In each case the items were delivered to my door in Australia within 3 days!  (it takes 5 days to get a parcel posted from Melbourne to Geelong, a distance of 65km).

And in each parcel there was a very nicely handwritten card thanking me for the purchase, and promising support if there were any issues with the items.

One ball screw was brand new, repackaged as advertised.  The other was used, salvaged from used machinery, but in “as new” condition.   Both were C5 grade, which is normally stratospherically expensive and has to be specially ordered.  They were priced only slightly higher than new rolled (lower grade) ball screws.

They were very carefully packaged in heavy duty cardboard rolls with metal ends, and multiple layers of heavy plastic and foam sheeting.

I made a later purchase which involved the seller removing a part from a machine.  He added it to a parcel which I had also ordered and wrote that I could pay later for the part if I was satisfied with it.  The price for the part (a THK bearing) was $UAUD32.  The same part new here would cost $AUD400.

In every case, communications have been  answered within a few hours, polite, personalised and in excellent English.

My only complaint, and it probably relates to the shipping arrangements, is that there appears to be a size restriction on shipping to Australia, of about 1200mm.  (p.s.  apparently the “do not ship to Australia” are old adverts, and there are no restrictions now.  A message to dy-global is all that is required to get any item shipped).

I hope to deal again with this company.  The name is dy-global.  To find them you need to search Ebay USA, or use the international settings on Ebay Australia.

Lathe conversion to CNC -3 Ball Screws

I have learnt a lot about ball screws in the past few days.  And I have purchased 2 ball screws and nuts on Ebay.  For those relatives and friends who follow this post, who have no idea what I am talking about, the “ballscrews” are what determines where the cutting tool on the lathe is positioned.  Very crucial.  (can something be “very” crucial?  It looks a bit like “very unique”. )

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Ball screws are the usual positioning screws  for CNC machines these days.  The alternative is Acme or square thread screws, but the few manufacturers who used to use these have all switched to ball screws (as far as I know).  Even Wabeco, the respected German lathe manufacturer no longer specifies any threads except ball screws.

Ball screws require less power to turn due to friction being a fraction of the alternatives.  Ball screws are silent.  If they are noisy there is something wrong.  They are generally more accurate than square or Acme threads.  They used to be many times more expensive than the older types, but since China/Taiwan has taken over most of the manufacturing, often using equipment sourced from US/Europe/Japan, the costs have plummeted.

And the backlash is minimal or zero.   Another name for ballscrews is “antibacklash screws”.

Backlash is annoying on a manual lathe, but it is very important on the cross slide of a CNC lathe.  Interestingly, it is less important on the CNC lathe lead screw, because most CNC lathe machining  on the leadscrew occurs in one direction only, towards the headstock.  However,  the cross slide is machining in both directions, in and out,  so the absence of backlash is necessary to maintain workpiece accuracy.

Ballscrews have grades of accuracy varying from C0 -C20.  The bigger the number, the less accurate the screw.   In general, it is recommended that industrial lathes should be C3-C5,  which means zero backlash, and accuracy of about 0.001mm.  That degree of accuracy is quite expensive, costing thousands of dollars per screw, and only attainable in ball screws which have been finished with precision grinding.  The alternative is ball screws which have been “rolled”.  These are much less expensive, costing hundreds of dollars per screw, depending on the degree of accuracy.  The best rolled screws can attain an accuracy of 0.01-0.02mm (C7), down to 0.1mm (C20).  These are approximate figures which I have gleaned from several manufacturers’ specifications.

So from scale drawings of the lathe bed and carriage and cross slide, I worked out that I needed the following…

 

IMG_3978.JPG

This is a photo of a ball nut and screw very similar to what I have bought for the cross slide.  THK brand, 14mm diameter screw, 4mm pitch, and BNT nut style.  Note the rounded channels that the balls occupy.

 

  1. Cross slide screw 14mm diameter, 400mm long with at least 200mm of thread, and a thread pitch of no more than 5mm.  The cross slide nut needed to be a THK BNT pattern in order to fit into the cross slide with a minimum of space making machining.  THK is a large manufacturer, with intermediate prices, and a very good reputation.  The screw needs to be a specific length, and one end needs to be machined to go through a bearing/ bearing housing and have a tooth belt pulley attached.  I contacted a ball screw supplier, to enquire about ground ball screws, but discovered that these were rarely specified due to their cost, and delay of 3-6 months.  A C7 rolled screw with the BNT nut was going to cost about $AUD400-450.  So I searched Ebay Australia, Ebay USA, and Ebay UK.  Eventually, I found and bought a used ground screw with end bearings and housings from Taiwan for $AUD250.  The nut was the wrong type, but I found a new correct style nut on Ebay USA for $AUD100.  I know that sounds like I have not saved much money, but that gives me a super-accurate ground ball screw!  Of the correct size and type.  I am keeping my fingers crossed that the pieces sourced from different countries will go together.  Theoretically, they should.  (same manufacturer, same size, “ground” specification, etc etc.  but finger tightly crossed).
  2. The lead screw approximately 1000-1100mm long, 25-32mm diameter, and 5-6mm lead.  Super accuracy not required in the lead screw, and I could have bought a new rolled one from China for about $AUD300-400.  But then I spotted one in South Korea, new old stock, 28mm diameter, 6mm lead, with unmachined ends.  THK brand.   Asking price just within budget.    And this was a C5, ground screw, possibly more accurate than I expected for the price, and unused, but hey, it sounded like a bargain.   So I offered about 15% less and was somewhat surprised to have the offer accepted.   So that one is arriving in a week or two.  Then to buy mounts and arrange end machining of the screw.  Although not crucial it will have zero backlash, due to the C5 designation, and the fact that it has two ballnuts bolted together in a “pre-loaded” fashion.  I expect that it will be the most accurate component on the lathe!  No decent photo to post.

(ps.  see the comments section for discussion about axial accuracy and backlash.  A super accurate C5 ballscrew has axial play (backlash) specification of 0.020mm, but the double ballnut configuration will reduce that number substantially.  “zero backlash” apparently does not really mean zero.)

 

 

 

 

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