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Topics - JohnHaine

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31
General Mach Discussion / Myford Super 7 - adding ball screws
« on: September 06, 2012, 10:40:56 AM »
Hi folks, after a lot of thought I have decided that using plain screws on my lathe conversion is never going to give good results, and as far as I can see after a little experimentation I'm not going to be able to apply backlash compensation because the play in at least the Z leadscrew is too large.  So, has anyone got any experience of replacing the myford screws with ballscrews please?  Or could point me to info on the web?

I think I can see how to do it on the crossslide without too much trouble, but getting a long Z screw aligned with the nut on the saddle, and mounting the nut, has me stumped for now.  Another problem will be protecting the screws from chips.

Thanks in advance!

John.

32
I am running the latest M3 lockdown version on my lathe and mill, and have also installed the addon for using the Xbox wireless game controller.  Backlash is (so far) not a problem on the mill which has ballscrews but is on the lathe (converted Myford) so at the weekend I decided to enable backlash compensation.  I measured the BL on the two slides and entered the right values in the DROs and set the speed to 20% as recommended.  I didn't touch the Shuttle Accel setting as the manual doesn't mention it though I see in some of the forum postings that it seems to be relevant - so probably set to the default value which seems to be 1.  The Z backlash is about 0.08 mm, Z about 0.18.

Then trying the lathe using the Xbox controller I found some very peculiar behaviour.  Continuous jogging using a joystick in one direction worked fine, but when I reversed direction, first I think it moved slightly in the same direction as before, then moved much too far in the other direction at a speed that varied wildly, first going at normal jog speed, then slowed right down, then speeded up again.  I haven't tried any actual cutting as this behaviour seems absurb, obviously something wrong.

So, questions if anyone can help please.  First, anyone with experience using BC on a lathe?  Or BC with Xbox controller?  And what is the significance of the shuttle accel parameter, is it relevant at all with the Xbox controller and if so what should I set it to?  

TIA - John.

33
General Mach Discussion / Backlash and taper turning
« on: May 09, 2012, 12:19:39 PM »
This is probably a silly question but I'll ask it anyway...

I have now got both my lathe (Super 7) and mill (Denford Novamill) working under Mach3, generally pretty successfully.  The lathe uses the standard Z leadscrew though I made a new screw and nut for the X-slide because I didn't want to start modifying Myford's components.  Because I'm not using ballscrews of course I have some backlash in both axes.  From a static point of view I could compensate this by switching on backlash comp.

The normal taper turning wizard cuts towards the headstock with increasing diameter.  This means that the X-slide is at the "wrong end" of its play, and is only constrained to the taper path by cutting forces.  In particular as you reach the end of the cut and the tool runs out of the work, there is a danger that the diameter will increase by more than it should given the taper - in fact I think I have seen this behaviour.  The alternative would be to cut away from the headstock, but then you could have the same problem with the other axis.

Ideally one would start by grooving the work near the headstock and cutting the taper inwards towards the headstock so that both axes are being driven against their play, so to speak.

Am I right in my thinking here, and are any wizards available that work in this way please?  (Or should I write my own gcode?)

John.


34
Hi all,

After adventures getting my Denford mill up and running I went back to try and see why I just couldn't get my Super 7 conversion working to the accuracy I would like, particularly on the cross slide.  I was getting inconsistent results on final diameter, with errors getting close to 0.02 mm which didn't seem right.  Eventually I decided that the problem was due to slight runout of the timing pulleys through which the feedcrew was driven - since both of these had to be bored out to fit the shafts of the stepper and the screw there was scope for error.  And doing careful measurement of feed with a digital micrometer (Mitutoyo) that can resolve a micron, and separately measuring runout with a DTI, I finally decided that both measurements were consistent. 

I'm now resigned to going to direct drive from the motor to the feedscrew, since I think there is plenty of torque available, but has anyone else had any experience of this problem; and any recommendations on how to accurately bore timing pulleys for future reference please?

John.

35
General Mach Discussion / Inserting comment in G-code
« on: February 11, 2012, 03:48:34 PM »
Well I tried writing my own G code to mill the moving jaw of a little machine vice that I'm bringing up to scratch for the Novamill.  I was being a good little programmer and commenting my code as I went but when I came to run it in Mach it stopped and gave an error message in the first line which started with a comment.

Now here is the question.  One can insert a comment at the end of a line like this: //<comment text>
which is a natural format to use.  But at the start of a line a / character is a block delete.

So, is it permissible to have a line which is just a comment that starts //...   or should one use an alternate form like ("this comment is the title of this program"), please?

Anyway, once I deleted all the comments the program worked fine but it would be a pain to have to maintain two versions one of which was commented and the other not.

John.

36
General Mach Discussion / Lathe and Mill on one computer
« on: January 29, 2012, 01:07:59 PM »
Hi, I have a licensed copy of Mach3 which I set up a while ago to run my lathe.  Now I'm bringing up a mill using the same computer, can I use the same license?  Obviously I don't run the two at the same time! 

Thanks,

John.

37
General Mach Discussion / Problem homing X axis
« on: January 02, 2012, 03:12:08 PM »
Hi, I am in the process of bringing up a Denford Novamill that I bought without electronics.  I have built a driver system using drivers / PSU / BoB from fleabay and all seems to work pretty well but I seem to have hit a brick wall with homing.

All the home switches (installed at manufacture on the mill) work in the same way - a n/c switch that opens when activated by the axis reaching the end of its travel.  I have used the autosense feature on the config/ports&pins/inputs menu to set these up so I guess they should work OK.  On the Motor Home/Softlimits menu Autozero is unchecked for each axis; the X axis is checked for Home Neg but the other two axes are unchecked (all to correspond with which way they move).

Now I find that on a "reference" request the Y and Z axes function perfectly, running to their limit until the switch opens then reversing and running slowly until the switch closes.  However the X axis just moves a very short diatance, wherever it is, then stops and the green homed LED for it lights up.

I am wondering if there could be a noise problem on the X home switch wires.  Does this sound likely to the experts please?  At the moment the input debounce interval is set to zero, and I notice that in response to a home-related question Hood recommended that this should be set to around 1000, which would correspond to 40 ms - is this likely to be a possible fix please?

John.

38
General Mach Discussion / Machturn probing and setting
« on: November 06, 2011, 01:44:44 PM »
Apologies if this has been asked before, but are there plugins or wizards for Machturn that support probing and toolsetting please?

RFA: I have developed a method to sense when a tool touches a (stationary) workpiece, for example on its periphery or end and also to measure the eccentricity using several readings.  I want to use this to sense the relative position of a piece of stock of known diameter in the chuck so that the DROs can be automatically set.  I have in mind a routine that could be run that would slowly move the tool until it touches the workpiece then stops and updates the DRO setting.

I'd like to modify an existing program rather than start from scratch, so it would be useful to know if anything is already out there.  At least some guidance - would this be a macro or a wizard please?

Once I've proved it works I'll also post details of the sensing method.

John.

39
General Mach Discussion / Problems with axis accuracy - MachTurn
« on: February 13, 2011, 06:35:01 AM »
I'm slowly getting to grips with my Myford Super 7 CNC conversion.  Initial results were promising but I have got caught up in a "problem loop" with the accuracy of the X-axis (i.e. cross slide) feed.

I have fitted a new leadscrew which is 20 tpi though I actually work in metric.  For complicated reasons this is driven through a timing belt with ratio 4/7 from a standard stepper with 8 times microstepping on the driver.  "Steps per mm" then works out to 2204.724.  The problem I have found is inconsistent diameter turning using the turn wizard.  The diameter ends up up to 0.01 mm or so different from what it should, sometimes larger and sometimes smaller than nominal.

I have set about calibrating the leadscrew pitch using a 123 block measured by micrometer to ~25.41 mm (actually measured to a micron) and using the procedure recommended in the setup instructions.  I am setting the slide to a zero with a DTI against the block, the latter mounted against an angle plate on the cross-slide, then removing the block and moving the slide under manual control to zero again and looking at the movement on the DRO.  The DTI is a very nice surplus indicator that has a travel of +/- 25 microns and indicates to 0.5 micron.

Apart from a small indicated error of about 0.08 mm over the travel, I get relatively inconsistent repeatability if I move the slide back 25 mm and then bring it back to zero, of up to about 0.01 mm.  I can reduce this by firmly pushing the slide back against the screw thread before reversing the feed to come back to zero - then I can get to a few microns.  So I was wondering if anyone could help with the following questions please?

Are my expectations of accuracy too ambitious?  If turning down to a specified diameter using the wizard, what is a reasonable accuracy to expect?

Moving the slide repeatedly backwards and forwards to the same nominal position, what repeatability can be expected?

The leadscrew is lubricated with a molybdenum grease, but it seems to me that this may be forming quite a viscous film between the screw threads and the nut, the thickness of which will depend on the force on the screw - would I be better using a light oil?

I don't think I am losing steps, but I was wondering about just how repeatable microstepping is - has anyone any experience of problems from this source?

Any answers to these or other suggestions would be most welcome!  Thanks in advance,

John.

40
General Mach Discussion / Help with ISO Metric thread form specifications
« on: December 29, 2009, 09:38:15 AM »
I wonder if anyone can help me with a reference to the complete iso metric thread definitions please?  I have the references to the ISO documents themselves but they are rather expensive!  (BTW, being in the mobile phones business I can't understand why I can download the complete set of latest specs for the 3G mobile phone system for nothing but have to pay $35 for a 20 year old document specifying screw threads - seems crazy to me.)

There are lots of thread tables on the internet but they all define the thread in terms of the major and minor diameters, and the major diameter is usually about 98 - 99% of the nominal thread diameter (e.g. for M12x1.75 it is 11.834 mm which is 98.6%).  There doesn't seem to be an exact definition that I can find of how you calculate the major dia. from the nominal diameter.  To make matters worse, the formulae for the thread dimensions (e.g. in Wikipedia) don't match the tabulated values!

The reason for this request is that I am having difficulty cutting threads that work using the thread wizard which asks you to define the start and finish diameters - I assume the start diameter is the Major diameter?  Using a pointed tool rather than a correctly rounded one, the in-feed is theoretically 7/8ths of the thread height which is itself 0.866 times the pitch; but using this value the crests get slightly turned away and the thread is too small for the nut.  (The tool is a commercial brazed thread tool but I do need to check the tip angle - but the tip has negligible radius.)


Thanks in anticipation,

John.



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