# Thread: I've built this machine 100 different times in my head!!!

1. Originally Posted by JoeHarris
I saw the 10mm pitch threads too but now I have the 1605 I'm sticking to my guns. The results dean gets from 1605 are plenty good enough for me and if I can get to anywhere close I'll be a happy man!
The thing is, it's not just a question of does the machine work well enough with 1605. If 1610 is used where appropriate, then you will get better acceleration and probably higher feedrates with the same motors. The maths to prove that is simple - moment of inertia (J) of 1610 and 1605 is pretty much identical, but to get the same acceleration 1605 needs to spin twice as fast. Torque is moment of inertia multiplied by angular acceleration (Newton's 2nd law), so since we require double the acceleration the torque is twice as great.
Now you may say that you don't need these benefits, since the 1605s are 'good enough', but the point is if you leave it at the same speed and acceleration as you get with 1605 and use 1610 the required torque from the motors is lower. That means you're running further from the limit, so are less likely to have problems since the system isn't being 'stressed' so much. That's not to say you should immediately sell the 1605 screws, since it's no loss to try it and see.

2. Originally Posted by Jonathan
That's not to say you should immediately sell the 1605 screws, since it's no loss to try it and see.
That's exactly what I plan on doing, stick them on and try. If 1605 doesn't work well for me, a pair of 1610 at 1300mm will cost an extra £100 + p&p so its not too bad. What I'd like to clarify, is resolution regards pitch, and micro stepping, and pulley reduction. here's the situations:

1. stick to my 1605 and gear 1:2 to get effective 10mm pitch but, will my nema23 have the torque needed for this gearing? I got the option to gear 1:1 when I need resolution,

2. buy 1610 and gear down 2:1 for high resolution, and stick to my nema23

3. buy 1610 and use micro stepping when high resolution is needed.

Thanks

3. Microstepping helps with resolution, but not as much as it initially appears to. The datasheets for many stepper motors specify that the tolerance on the angular position for each step is 5%, for a 200step/rev motor. That means you can say with some confidence that if stopping on a full step, the motor will be accurate to 1/(5/100%)=1/20th of a step. That implies that down to around 1/20th microstepping you'll gain resolution, however when stopping on microsteps the tolerance is worse since the relationship between the reluctance of the rotor, and angular position, is not ideal. It's hard to say exactly how much resolution you can gain. My micro lathe has 4mm pitch ballscrews, geared 2.5:1, so the effective pitch is 1.6mm and with 1600step/rev (1600 is good number to use in general) that's 1um per microstep. If I put an indicator on it and try and measure this, the axes do appear to move about 1um per step, which is promising but bear in mind that's in a static situation. Whilst the motor is spinning continuously it will be less accurate. Arguably the main bonus from microstepping is it means the motors are exciting the system at a higher frequency, so problems with resonance are attenuated.

I can think of very few parts that would benefit from the 2* resolution gain got from using 5mm effective pitch instead of 10mm. There's only one part I've made where I decided to swap my pulleys round to get higher resolution, and that wasn't the Mayan calender. However there are numerous other reasons to use pulleys, so it's still a good idea to plan for the ability to swap them to get the higher resolution. If you stick with the 1605 screws then option 1 (1:2 to get 10mm effective pitch) will work substantially better than 1:1.

Unless you'

4. Originally Posted by Jonathan
If you stick with the 1605 screws then option 1 (1:2 to get 10mm effective pitch) will work substantially better than 1:1.'
You can't say that without knowing what's being done with machine.? 1610 doesn't work better it just allows higher speeds and if those speeds aren't needed then there's no point.! . . . Infact it's wasted resolution/torque.

That said I didn't realise Adil bought 5mm pitch screws for all axis and believe he mainly wants to cut wood in which case 10mm pitch would have been better suited. But as we know it's not the end of the world and with belt gearing easy sorted.

Regards Micros stepping then Really you shouldn't consider micro stepping for resolution it's biggest help is with smoother operation of the motors at lower speeds and like Jon says it helps with resonance. Just remember the higher you set the resolution the harder the Parallel port has to work putting out more pulses's for the same movement so you'll need a good PP port other wise you could easily lose steps. More than 2000(10x) micros steps is pointless really has most of the Cheaper motors can't resolve to higher than that.

5. Jazz, I bought my 1605 screws 2-3 years ago, don't think 1610 would've been available then, maybe, can't remember.

You are correct, I will be mainly cutting wood and MDF, next perspex, and dabble in ali. So I do hope my design will cope with the stress of cutting ali now and again.

Am I right in thinking for wood/MDF and plastic, 10mm screws are ideal as motors will be working in their ideal region giving 7mtrs/min cutting speeds, but 5mm geared up will just about work without getting to close to the motors critical speed and screws whip speed. The calculations suggest 1200mm 1605 screw will whip at around 1000 rpm. So even with gearing up, the motors can now spin at 500rpm, giving the screw 1000rpm which is still 5mtr/min. Will I be able to go up to 7mtr/min cutting or just for rapids.

Then for machining ali, 5mm pitch is best, but can we cut with 10mm screws, or do we have to gear down? What is the cutting speed for ali? I had a number of around 2-4mtr/min. When you mainly machine ali why are you using 1:2 on your 5mm pitch screws?

6. Cutting aluminium will be fine regardless of which screw you use since it's only possible to use around 0.6-1.2m/min. With regards to resolution 5mm is better, but it's not a big difference and you're unlikely to notice it. If the critical speed is 500rpm, then yes that means you can get 5m/min, but not 7m/min since that would be exceeding the critical speed. The critical speed formula should be treated as a guidline, since it doesn't take into account the support from the ballnut and relies on a constant you select for the end-fixity, which is a bit of a guess. However I still wouldn't be happy with operating much above what the formula states, even if it looks OK. Have you seen irving's motor calculation spreadsheet on this forum - it may help clarify things.

7. Has Jon says the critical speed is a guide and the reality is that you'll possible double that and still get away with it.?. . My 1500mm x 20mm screws do.!

Regards me using 1:2 mostly well that's not exactly correct. Since the last time I changed back to 1:1 it's been left that way and when I do change back it's going to be 1:1.5 because I don't the speed. I don't really need the torque or the resolution either if I'm being honest but that little overhead helps when I do need higher feeds.

8. Seems to me from reading (and that is all I can base this on) that both 5 and 10mm pitches will work pretty well on most materials if set up correctly. I was getting all worried at one stage that my 5mm pitch screws would mean I was going to be burning up the wood I was cutting due to the low feed rates then I remembered back to when I started getting into all this lark seeing machines that cut wood pretty reasonably being driven by threaded rod with pitches so small you can hardly see them! Reckon our 5mm screws will be all right. But I may live to eat my words...

9. Originally Posted by JoeHarris
I was getting all worried at one stage that my 5mm pitch screws would mean I was going to be burning up the wood I was cutting due to the low feed rates then I remembered back to when I started getting into all this lark seeing machines that cut wood pretty reasonably being driven by threaded rod with pitches so small you can hardly see them! Reckon our 5mm screws will be all right.
Ermm Ye understand what your saying Joe.!!. . . BUT don't be fooled.? . . . If truth was told there bull shitting you.!! . . . You'll see many examples of flimsy machines cutting Aluminium with dremels etc but what they don't tell you is the finish is shit and the machine/spindle curled up and died 2hrs after the video.!

Same goes with threaded Rod machines the finish is rubbish the cutters wore out in 30mins and the machine fell to pieces 10mins after that.!!

Your correct thou that 5mm will be ok if you already have them but if your mainly cutting wood then I'd look at gearing at least 1:1.5 or 2:1 has that will allow the speeds needed to cut MDF/plastics at correct feeds.

10. Oh yes I moved away from threaded rod a LONG time ago!!! Just comforting to know my &#163;500 worth of ball screws were not a waste of money!!

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