Lathe conversion from stock to CNC

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18 Jul 2012 15:22 #22144 by andypugh
Soundreflections wrote:

I have a 4 jaw chuck, for which I have started turning an adaptor plate, but stopped when I realised I need highest precision possible for the recesses, as the plate needs to fit my lathe's faceplate snugly, and the chuck needs to snug in as well

You just need something that can screw onto the spindle nose. It need not be terribly precise at first. (though will be an interesting excercise in internal threading).
You then mount it on the spindle, and machine OD and register in-situ to take the chuck. Then it _has_ to be true.

If you want to then clean up the back face, re-mount the three-jaw, mount a bar in that then mount the4-jaw on the bar.

I am keen to know how you think I can mount the compound vertically


As JT says, a bit of sturdy angle would work (possibly with a welded gusset).
Bolt it to the cross-slide (I am not sure how, this is the hardest part) and face it off with a lathe tool held in the 4-jaw chuck (crosswise, like a big fly-cutter).
Then bore a hole to suit the compound-slide pivot pin. Probably with a drill mounted in the chuck, followed by a lathe boring-bar mounted eccentrically in a 4-jaw chuck (an adjustable boring bar would be better, of course). Maybe this one:-)
global.ebay.com/Wohlhaupter-UPA2-5702/261069311985/item (actually I was going to post a joke link to a $2000 tool, but that one is a significant bargain at the current price)

Drill more holes at the correct centres for the hold-down bolts, then mount it properly.

Then face it off with the scary fly-cutter again, and drill a second hole. The size of this one doesn't matter, but needs to be smaller than the compound slide register.

Then put the lathe back together, and turn a spigot to fit in the second hole snugly, and to suit the recess in the compound slide base.

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18 Jul 2012 15:23 #22145 by Soundreflections
OK, Only one pic survived - I must still figure quite how pictures work in this forum.
Attachments:

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18 Jul 2012 15:33 - 18 Jul 2012 15:34 #22146 by andypugh
Soundreflections wrote:

OK, Only one pic survived - I must still figure quite how pictures work in this forum.


You can only insert one pic using the button, but you can link to several using
[img] your image URL here [/img]
tags. There is a button above the edit window that you can use to not really help with the tags.
Last edit: 18 Jul 2012 15:34 by andypugh.

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18 Jul 2012 15:41 #22147 by Soundreflections
andypugh wrote:

You just need something that can screw onto the spindle nose. It need not be terribly precise at first. (though will be an interesting exercise in internal threading).
You then mount it on the spindle, and machine OD and register in-situ to take the chuck. Then it _has_ to be true.


My machine has a face as the end of the spindle - it cannot be removed. So I have a 125mm 4 jaw chuck (100mm 3 jaw), so that I can have an adaptor plate that fits the 3 jaw's profile, then has the 4 jaw bolted on. I hope this makes sense?

As JT says, a bit of sturdy angle would work (possibly with a welded gusset).
Bolt it to the cross-slide (I am not sure how, this is the hardest part) and face it off with a lathe tool held in the 4-jaw chuck (crosswise, like a big fly-cutter).
Then bore a hole to suit the compound-slide pivot pin. Probably with a drill mounted in the chuck, followed by a lathe boring-bar mounted eccentrically in a 4-jaw chuck (an adjustable boring bar would be better, of course). Maybe this one:-)


My biggest problem here is I don't think I will have the desired vertical travel for the hole. I see what you mean by the bit in a 4 jaw, with an offset to whatever size one wants.

I do not follow hundred percent about where I need to face, though I realise the importance thereof. I will only have vertical adjustment once the compound is mounted. I could possibly face the 10mm steel plate, depending on travel, and then mount the vice - or how feasible would it be to have T slots? I suppose 10mm is getting a bit thin here!
Regards
Peet

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18 Jul 2012 15:44 #22148 by Soundreflections
andypugh wrote:

You can only insert one pic using the button, but you can link to several using

[img] your image URL here [/img]
tags. There is a button above the edit window that you can use to not really help with the tags.


Can one tell the system where in the text the pic should be?
I do not have an online storage for my pics, so I will stick to 1 pic/post.
Thanks
Peet

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18 Jul 2012 17:18 #22150 by BigJohnT
I always just edit the post and move the tag where I want it...

John

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18 Jul 2012 17:26 #22151 by BigJohnT
The vise appears a bit on the large size compared to the rest of the components. You could just have a plate mounted to the compound with some tapped holes in it and clamp your work down. For fly cutting like Andy said you just need to adjust the vertical axis (the compound) to center your material then using the crossfeed you can fly cut the material to size. More ways to skin a cat as we say around here than the cat cares to know about. You can also get machinist angles... I assume that you use mm so your not in the US?

John

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18 Jul 2012 18:14 #22152 by Soundreflections
I am in South Africa, I am thinking to try and get a mill table top - with the grooves for the clamps. I have a +-120mm 10mm plate that is currently surplus to requirements. So holes drilled and tapped seems a good idea, except for the pattern / spacing. I can see that a mill table, with T grooves, makes a lot of sense. I can just about have a mill attachment before I have a pulley to drive it all!
Regards
Peet

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19 Jul 2012 12:18 #22183 by Soundreflections
I have this morning bought a pulley, so if the belt still fits I should soon be running again!
What PC card would you recommend, to be able to handle 2/3 axis, I want to put dual encoders on the spindle, but the encoders I have available may have too many holes, it will be high frequency if it works.
Any problem using stepper drivers I put together myself? I assume one just needs step / dir inputs? There are a number of chips on the market for that. Putting shaft encoders on to the steppers should be fine at a later stage?
I am a bit surprised by the pricing of steppers, but I see why not to go servo!
How do I determine what size / rating servos I would need? Or does one just get the biggest one can afford and hope?
Thanks
Peet

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19 Jul 2012 16:02 #22211 by andypugh
Soundreflections wrote:
[quoteWhat PC card would you recommend, to be able to handle 2/3 axis, I want to put dual encoders on the spindle, but the encoders I have available may have too many holes, it will be high frequency if it works[/quote]

Why dual encoders?

If you want to go much above 20kHz count rate on the encoder then you can't use the p-port any more, so that would be ) only 300 rpm for a 1024 (4096 count, allowing for quadrature) encoder.

My lathe uses the Mesa 7i43 card, which connects to the parallel port and can measure 10MHz encoder counts, and produce similarly fast stepper pulses. I am not sure that I would use the same thing again, as other boards have been developed in the meantime, but at $80 and easy cabling it is still quite an attractive option. You would probably also want to budget for at least one $45 7i42TA with that.

Other options would be the Pico PPMC (Also connects to the p-port, but moves the counting and step generation to hardware): www.pico-systems.com/univstep.html
It is a bit more plug-and-play than the Mesa 7i43 system, and the price ($250) is similar by the time you have added connector cards to the Mesa.

Mesa also offers the 5i25/7i76 combination for stepper machines. That's a low-profile PCI card and a remote breakout . for $200 including the cable. (there is also a 6i25 PCIe card which is equivalent)
Quite a popular combination but only handles 1 encoder in the normal configuration. Swapping the 7i76 for a 7i77 makes for a very easy conversion to a servo system. You can connect all sorts of other IO to both the 7i76 and the 5i25 in addition, up to several hundred IO pins

I wouldn't bother with anything other than Pico or Mesa, just because both offer support here on the forum and mailing list.

How do I determine what size / rating servos I would need? Or does one just get the biggest one can afford and hope?

Good question. I have a big NEMA 23 stepper on my lathe Z rated at 3Nm, and that works fine most of the time, but can't really handle toolpost drilling. The X axis is a much smaller 2Nm NEMA23 stepper, and that is very much OK. The problem with bigger steppers is that they become more sluggish, losing torque more rapidly with speed than smaller ones. At some speed point there is a crossover where the little stepper makes more torque than the big one. My steppers are all connected via belts, which adds to the complexity because then you can play around on the speed/torque curve.

My next lathe will probably be servo, but that is mainly because I have servos found cheap on eBay.

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