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cropwell
03-10-2013, 02:17 PM
I need to cut a thread on (say) 30mm brass rod that engages with m20 threaded rod (2mm pitch?). The 30mm is only a nominal diameter at this ideas stage. Anybody got any formula to calculate the diameters according to the pitch and number of starts on a thread ?

OR am I being stupid, will a 2mm pitch external thread on just fit regardless of the diameter of each rod ?

I only did O level maths over 50 years ago, so don't go into much beyond basic trig svp.

I have a mini lathe (in bits at the moment) with a metric thread cutting leadscrew btw.

Cheers,

Rob

birchy
03-10-2013, 06:40 PM
All threads have standard drilling and shaft sizes which MUST be followed. So for a standard M20 x 2.5 coarse thread, you need to drill out 17.5mm before tapping. The O.D. of the shaft is irrelevant as long as it's bigger than the thread diameter. For male threads, the O.D. is 20mm, so if you had a 30mm shaft, you'd have to turn part of it down to 20mm before using a threading die. Tap Chart Metric Threads (http://www.carbidedepot.com/formulas-tap-metric.htm)

mekanik
03-10-2013, 06:58 PM
Hi Rob Can't get my head round what you are trying to do ?
You can put a 2mm pitch thread on practically any O.D. (within reason)just make the depth correct as per your Zues book, lots of manufacturers make odd size threads on some of there equipment but they would manufacture special taps to match,are you trying to make a nut ? or a male thread on 30mm bar ? I do have some info on specials but i think they are Imperial, not sure if there is info in Machinerys Handbook, i have seventeenth edition.
Mike

birchy
03-10-2013, 07:08 PM
Yeah, it's not totally clear what you are trying to achieve? If you decide to search for the (essential!) book mentioned above, it is ZEUS and not Zues. Zeus Precision Engineers Metric Data Charts Reference Tables Book Drill/Threads | eBay (http://www.ebay.co.uk/itm/281165324602)

Threads may not be the correct solution for your problem, so maybe you could tell us what you are ACTUALLY trying to do so we can bounce ideas of each other?

cropwell
03-10-2013, 07:52 PM
OK - here's the problem : Velleman make a 3D printer kit for 700, it is cheap, but nasty. The Z axis is a piece of threaded rod with a standard nut. That strikes me as sloppy by design and I am looking at ways of improving the design by creating a nut with three bits of threaded rod with ends turned to fit into bearings (3x10x4), putting the whole thing together with some end caps so the thing becomes a sort of ballnut that fits on the existing Z axis. I am playing about with the whole idea at the moment. That's what I do (play with ideas, since I lost interest in playing with myself ages ago). I didn't want to go public as the idea may be plain stoopid and I don't want the position of Forum Clown, even if it is vacant.

Having said that the Velleman K8200 kit is crap, it still looks like a cheap way in to 3D printing. You get a lot for the money.

Maybe someone has already done what I am trying out and got far enough to reject the results, or maybe you can buy one from China :crushed:

Cheers

Rob

mekanik
03-10-2013, 08:12 PM
Hi Rob
Gotcha now, i was looking into this the other weekend on one of the US CNC forums for a cheap ballscrew alternative. so all you need is 3 pieces of the bar threaded with 2mm pitch, diameter will have to be bigger than the smallest bearings you can get unless you were just going to put oilite bushes in the end plates, still relying on accuracy of the studrod but as it's rolled it should be near enough for making bits of plastic.
Mike

John S
03-10-2013, 08:32 PM
I don't want the position of Forum Clown, even if it is vacant.



Rob


Nope, someone has just filled it :joker:

cropwell
03-10-2013, 08:50 PM
Sorry John, I meant even if it WERE vacant.:very_drunk:

mekanik
03-10-2013, 08:57 PM
Had a picture of a roller screw that i had saved when i was looking @ them the other week but dumped it the other day when having a clear out of my desktop.think it was CNC zone but it was a huge thread.

mekanik
03-10-2013, 09:01 PM
Olsen Engineering - Roller Screw Linear Actuators (http://www.consultolsen.com/roller_screw_linear_actuators.html)

FatFreddie
03-10-2013, 09:06 PM
You can get a 3d printer much cheaper than that if you don't mind assembling it yourself - Home - RepRapPro (http://www.reprappro.com/) from 400ish. Or look for Mendel90 (a better design).

The z axis doesn't need a ballnut because gravity (and a spring) takes out the backlash. I have a RepRapPro Mendel and it seems pretty crude but produces quite reasonable results - the Mendel90 is much more rigid and I would expect very good results from that.

cropwell
03-10-2013, 09:58 PM
Bound to have been done before. It is good to know what they are called anyway.
Cheers,
Rob

cropwell
03-10-2013, 11:32 PM
Hi, Thanks Fat Freddie.

The Velleman Kit requires assembly anyway. On first glance I like the Mendel90 especially with the Dibond frame.


Cheers,
Rob