CNC conversion project

I am in the throws of planning the best way to convert my Myford VMC Mill to CNC, specifically the fitting of ballscrews is at the forefront
of my deliberations.
Once dismantled, I will have no other milling capability and so I am considering making the ballnut housing and the ballscrew bearing blocks with oversize mounting holes and to tighten and shim everything using a guide rod through bearing and nut housing. Once everything is aligned, I will drill and ream for dowels. My uncertainty is whether I have any chance of obtaining a 900mm length of silver steel or PGMS that will be straight enough for the purpose.
Can anyone comment on what typical stock straightness is like??
TIA
Bob
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Bob Minchin wrote:

You could try round rail, eg
http://cgi.ebay.co.uk/W12mm-h6-Price-100mm-Round-Rail-Carbon-Steel-/310061828359?pt=UK_BOI_Metalworking_Milling_Welding_Metalworking_Supplies_ET&hash=item4831200907
But I'm not too sanguine about your plan - maybe because I don't really understand it.
-- Peter Fairbrother
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Peter Fairbrother wrote:

http://cgi.ebay.co.uk/W12mm-h6-Price-100mm-Round-Rail-Carbon-Steel-/310061828359?pt=UK_BOI_Metalworking_Milling_Welding_Metalworking_Supplies_ET&hash=item4831200907
Maybe I can try and explain using the x axis as an example. The feedscrew has bearings at either end and a nut in the middle. The new ballnut fits into a bored block and the ballscrew runs in new borings in the end castings that bolt to the table. I plan to make the mountings oversize. A rod to suite the bearing bore (20mm) also passes through the nut mounting block which is fitted with a concentric sleeve to reduce the bore to 20mm. My theory is that I can adjust the tightening of the mounting bolts until the table slides freely and then fit dowels to hold the alignment.
The guide rod and bushes are removed and the ballscrew & nut fitted and then everything should line up.
Thanks for the links to Marchant Dice - I will see if they have 20mm diameter available athough I could use more bushes to accept 12mm if needed.
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Bob Minchin wrote:

Ah. Still don't quite get why you need the rod at all, but good luck.
Yes. MD do 20mm rod (though it's called rail for some reason). There are also cheaper sources on eBay, and elsewhere. Eg:
http://cgi.ebay.co.uk/SFC20-Precision-Round-Rail-20mm-OD-DIY-CNC-/220733819295?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item3364c2c99f
-- Peter Fairbrother
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Bob Minchin wrote:

I fitted the ballnut and blocks loosely, then moved the axis to one end, so the ballnut was close to the block. The ball nut was tightened and checked for free running, and the first end block was then shimmed into place.
Then I moved the axis to the other-end, and wiggled the screw up and down, using a dial gauge to measure the wiggle (a couple of hundredths iirc). The other-end block was then shimmed to be half-way in the wiggle.
I intended to repeat this half-wiggling with the first end, but never got around to it.
One thing though, I had to recut the seats for the blocks a few times to get the angle right - I figured I'd need it flat and the angle accurate to within one part in 3,000 for my small mill.
-- Peter Fairbrother
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Peter Fairbrother wrote:

Thanks Peter
That is helpful Bob
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On 12/07/2011 18:52, Bob Minchin wrote:

Have you considered that the 900mm rod, supported at it's ends, will sag almost .4mm at the centre due to it's own weight.
--
Regards, Gary Wooding
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lemel_man wrote:

Hm! I suppose the same will be true for the ballscrew when fitted
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On 14/07/2011 08:57, Bob Minchin wrote:

That's true, but if you line up to the middle of the rod, then each end will be almost 0.5mm too high. If you line up to an end, then the nut will support the sag in the middle.
--
Regards, Gary Wooding
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