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Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Sun May 08, 2011 12:45 pm
by PK
cryoscum wrote:Hi PK

I understand that tooth belts are somewhat less tension sensitive than regular drive belts,


Yes, you can run them much tighter. Loose belts give errors and backlash.
I intend to get 2m of the 10mm tooth belt, join the correct length and see what tension it'll have with the current configuration.


Timing belts come in fixed lengths because they are manufactured as a tube with a continuous winding of steel or kevlar around the outside. The manufacturers then slice the tube (like cutting sausage) to give the correct belt width.. 5mm might be just enough.

Any suggestions for the pulleys? Both the same size, maybe about 25mm dia? No need for gear ratios here I would assume?


Get T5 (5mm pitch) belts and pulleys, 10mm width for the cross slide and 15mm for the long travel.
I'd go for 25mm on the cross slide and 30mm for the long travel, just to make sure you get enough wrap.
The belt manufacturers usually have calculators and tables for required width and wrap as a function of torque.

Re gearing. You might want to consider gearing the motors up, ie having the screws turn faster than the motors. This will give you more top end speed and won't make any significant difference to cutting force. Remember that Stepper motors are (by and large) constant power devices. So halving the speed of the motor doubles the available torque. If you gear that motor up by a factor of 2 then the axis will move twice as fast.

It's for this reason, most stepper motor driven machines run with very coarse pitch screws (like 10 or 20mm pitch)
here's a video of my little router showing the difference in speed as a result of replacing 5mm pitch drive screws with 20mm pitch screws. The motors and drives remained the same:


In your case though, with ordinary screw drives, you're frictional loads will go up significantly, so you won't see quite as dramatic speed improvement.

PK

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Sun May 08, 2011 5:43 pm
by OverTheTop
The belt manufacturers usually have calculators and tables for required width and wrap as a function of torque.


Don't worry too much about wrap with the toothed belts. Almost anything will work in what you are doing. If you are going for ultimate accuracy, maybe watch the manufacturer's recommendations.

Belt tension and wrap is highly critical in V-belts (centripetal force on the belt defines the maximum power transmission) but toothed belts are quite insensitive to the tension. From my experience this is even when driving the position to ridiculous accuracies.

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Fri May 13, 2011 10:00 pm
by cryoscum
G'day gents

As PK probably thought, but thought to spare my sensitive ego, I got the T5 belt I ordered and the belt is one tooth too small, and I can't adjust it in that direction to make it work!!

Ok, after some crass language, the answer is simple: remake the stepper motor brackets and make sure the new ones have slotted holes to allow for suitable adjustment. Then order a new belt, at the princely sum of $6. Problem is the supplier only has belts with 10 tooth increments, so slotted brackets will be crucial.

Simple right? Nearly got my finger taken off by a chuck spun bracket and decided its time to end the child's play... Need a mill, now.

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Will probably have to CNC this next, but let's first get the lathe right!
More to follow...

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Fri May 13, 2011 10:04 pm
by PK
Hey, you could CNC that mill!

:-)

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Fri May 13, 2011 10:14 pm
by Space Mark
Could probably CNC the drill press too. For those too lazy to pull the handle... just press a button!

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Fri May 13, 2011 10:56 pm
by PK
Mark H wrote:Could probably CNC the drill press too. For those too lazy to pull the handle... just press a button!

Don't laugh. I'm presently considering doing exactly that!

PK

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Sat May 14, 2011 12:42 am
by ROCKet STAR
PK wrote:
Mark H wrote:Could probably CNC the drill press too. For those too lazy to pull the handle... just press a button!

Don't laugh. I'm presently considering doing exactly that!

PK


I'm interested in hearing about the reasoning for doing so. I'm guessing it's more than just "because I can". Would it be to increase tool life through being able to optimise the feed rate?

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Sat May 14, 2011 7:44 am
by PK
ROCKet STAR wrote:I'm interested in hearing about the reasoning for doing so. I'm guessing it's more than just "because I can". Would it be to increase tool life through being able to optimise the feed rate?


I'll take some pics today and explain..
PK

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Sat May 14, 2011 11:38 am
by Space Mark
Well at least you'd get exact drill depths heh.

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Sat May 14, 2011 7:51 pm
by PK
PK wrote:I'll take some pics today and explain..

Apologies. Today was a bit frantic at work. I'll get some pics real soon now!

PK

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Sun May 15, 2011 6:16 pm
by PK
Here ya go

Image
This machine will drill the same hole, in the same stuff, for it's entire life. If you get the feed right you can do it with a single retract to clear the swarf. It'd be great candidate for a single axis CNC machine.

PK

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Mon May 16, 2011 7:46 pm
by cryoscum
Much better! Easy and still have all my fingers!
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Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Tue Jun 07, 2011 8:44 pm
by cryoscum
Brackets and belts in place, adjustable and ready to go...

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Tue Jun 07, 2011 8:57 pm
by PK
You might want to consider drilling out the mounting hole on the stepper and making a tapped spacer to brace as shown in the attached pic. The reason being that the required tension on the belt will result in flex of that bracket and asymetricrical (and hence premature) wear on the belt and pulleys. If you can get another brace below then that'd be tops too, but it looks like you have a small tab welded into the corner which should help.

PK

Re: CNC'ing a small lathe - pimping the unpimpable...

Posted: Tue Jun 07, 2011 9:16 pm
by cryoscum
Already have a 50x50x6mm brace welded in there. I think you can lift the whole lathe by that bracket...