Messages

771

General Mach Discussion / Re: Machturn probing and setting

« on: November 09, 2011, 09:46:06 AM »

Kerry, not just yet!  It's a technique I'm developing which isn't proven yet.  When it is I'll post details.

John.

772

General Mach Discussion / Re: Machturn probing and setting

« on: November 08, 2011, 11:58:51 AM »

Well, that's a good point, but my reasons are several.  First, this effectively provides an accurate X axis home switch which is needed for setting up the tools in the first place.  Second because I want to measure eccentricity.  Third because I'm fitting the same system to my mill(s).

John.

773

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.

774

General Mach Discussion / Re: Problems with axis accuracy - MachTurn

« on: February 15, 2011, 04:22:43 AM »

Thanks for the comments Rich...all goes I think to confirm that given that I'm using a standard screw rather than a ballscrew I'm probably getting as good as I can expect. 

This was all a bit of a diversion back to the lathe from my main project at the moment which is converting an X1 mill to CNC, so I will stop worrying and get on with the job!  Maybe one day I will return to the lathe and fit ballscrews to that.

Thanks again,

John.

775

General Mach Discussion / Re: Problems with axis accuracy - MachTurn

« on: February 14, 2011, 03:45:06 PM »

Thanks, that's good information though as it tells me I probably can't get mine much better than it is now so I should stop worrying and start making chips! 

As always, thanks for everyone's help and advice.

John.

776

General Mach Discussion / Re: Problems with axis accuracy - MachTurn

« on: February 14, 2011, 03:23:03 PM »

Hood - thanks, will go with the oil option.  I should have chosen my words more carefully - Mach is capable of very high accuracy, I was really asking what people had achieved in practice with typical lathe conversions.

Stirling, yes, it should be but doesn't seem to be!

777

General Mach Discussion / Re: Problems with axis accuracy - MachTurn

« on: February 14, 2011, 04:50:04 AM »

All, thanks for the good advice, I have now at least re-calibrated the "steps per" for the X-slide and things are much more understandable now.  I can see that microstepping is not working exactly as one might naively expect and I'm seeing an effective "average" single step size of around a micron which is double what it "ought" to be.  I have also reduced the axis acceleration - I wonder if the slide was slightly over-running when the motor slowed.

Dresda, as I mentioned, the indicator has a full-scale range of +/- 25 micron and resolves to better than 0.5 micron.  And it cost a few pounds from a surplus stall - a good buy (though this is the first time I've had a use for it).

Could I (re)ask a couple more questions please?

Is my surmise about leadscrew lubrication correct - is it better to use a light oil rather than thick grease to ensure that one doesn't get a variable thickness layer of grease which could affect the position?  And what is people's experience of the accuracy achievable with Mach?

Thanks again,

John.

778

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.

779

General Mach Discussion / Re: Help with ISO Metric thread form specifications

« on: December 29, 2009, 05:25:41 PM »

Thanks Rich...I just discovered that my local library (Cambridge UK) has all the BSI specs accessible on-line from home and I can save them (but not print) as a library member!  Glad I renewed my membership now (and it's free!).  Now to get puzzling...

BClemens...didn't see your post...?

Thanks all,

John.

780

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.