Planning stages of a CNC conversion

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WJH
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Planning stages of a CNC conversion

Post by WJH » Sat Jun 18, 2016 3:00 pm

I already setup two computers for Linux CNC, debating which one I will use. One has better jitter results for the Mesa Ethernet card, the other works better for the Parallel port or Mesa PCI card.
Will be converting my LatheMaster 8x14 lathe, as I am only in the planning stages, I've been reading the comments from Bill discussing a proper home switch for repeatability. So I take it that I should use decent quality inductance proximity switches, and a Z pulse... I will be going with steppers, I won't have a Z pulse unless I modify a stepper with an optical encoder wheel.
So the premise is, the carriage or cross slide move towards the proximity switch, stops at the proximity switch, then moves forward to the Z pulse and stops?
What if just using steppers, the carriage or cross slide move until it hits the proximity switch, then single steps until the proximity switch turns off, then single step backwards until it just comes on? Am I splitting hairs on a flies butt?
I have no practical experience yet with a CNC lathe, so I am trying to picture how to accurately do the cross slide, since that is more critical than most things it seems. I don't know yet what I can get away with.
As for the spindle, right now I am leaning towards a VFD, 3phase motor combo, controlled by the Mesa card, using a quadrature encoder on the spindle, so I can do rigid tapping if I so desire.
I know theres other things you can do for a spindle, like a full blown servo spindle, and live tooling on the cross slide, I have a product in mind that could use that...
Last edited by Harold_V on Sun Jun 19, 2016 1:10 am, edited 1 time in total.
Reason: to remove undesirable language

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SteveHGraham
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Re: Planning stages of a CNC conversion

Post by SteveHGraham » Sat Jun 18, 2016 3:21 pm

I converted a mini lathe to CNC, and when it was over, everyone told me I was stupid for not buying a ready-made CNC lathe. The screws have to work really well.

I was also advised that a mill was a better idea, and I have to agree. Much more versatile and productive.

Just thought I'd toss these things out so you will hear about them before you build and not after.
Every hard-fried egg began life sunny-side up.

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Dave_C
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Re: Planning stages of a CNC conversion

Post by Dave_C » Sat Jun 18, 2016 3:55 pm

I'll chime in with Steve since I converted my G4003G lathe to CNC as well. I wish I had done a mill instead but my mill wasn't worth the effort or expense.

On a lathe the Z registration is not as critical as the X (cross slide) Z can be set for the the part end after you home the machine but the X determines the dimension and is used in the tool offset tables.

On a cheap or very small machine you can make them quite accurate by taking a test cut and setting the DRO to that dimension. The prox switch then can serve as a travel limit and not have to be very accurate as far as repeatability.

There are lots of ways to do CNC. Cheap or accurate but not cheap and accurate! IF you just want to play (and CNC is fun to play with) you can do a cheap CNC conversion. However, if you need to make parts with close tolerances you will need good ball screws and steppers that don't miss steps. Hybrid servos solve that issue as they have feedback encoders on the end to tell the controller if they made the step or stalled.

If you have the option, do the mill as it is easier to do. On a mill you can locate your point of origin and the top of the part and set your zeros. No accuracy needed on your limit switches! (Or use a hole center or whatever you like as the point of origin, as long as it agrees with how the CNC code is written)

Dave C.
I learn something new every day! Problem is I forget two.

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SteveHGraham
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Re: Planning stages of a CNC conversion

Post by SteveHGraham » Sat Jun 18, 2016 5:24 pm

Some day I'm going to bite the bullet and buy a Tormach mill or something similar. The lathe I made can probably be made useful if I spring for better parts. It's already useful for stuff with loose tolerances. I should use it to make wooden tool handles!
Every hard-fried egg began life sunny-side up.

WJH
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Location: Florida

Re: Planning stages of a CNC conversion

Post by WJH » Sat Jun 18, 2016 6:35 pm

My Bridgeport is not worth converting, needs to be reground, and scraped back to tolerances, but as a manual machine, very easy to work around its issues for my needs. This lathe is for the learning experience and the challenge. No reason why it can't be made to be useful. If I can make useful parts, great! If I can sell them, even better. What I am not going to do is buy a 16,000 lathe when I have no products or customer base yet.

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