6 tpi or 10 tpi?

[GPZ_Rider]

Hi Guys

Having major dificulties tracking down any 10tpi Acme here in oz... I can however get hold of trapezoidal TR 20x4 which gives me roughly 6tpi.

Now for the question. Why is 10 tpi considered the standard in all of the reading I have done on these forums?

Why couldnt TR 20x4 be used ie 6tpi??? or at least why shouldnt it?

Look forward to advice.

Cheers Mark

[fkaCarel]

You don't have to follow the crowd. Varying Tpi and transmission ratio lets you achieve the same goal in numerous ways.

Carel

[GPZ_Rider]

So is there a reason that most go for 10 tpi???

[fkaCarel]

Availability? Like you experienced in reverse? And if your metric down under, go metric to avoid exotic parts.

Carel

[Agro]

Same with the Acme - 6TPI is available in a few places in aus. After that it's the 20 tpi.

Logically, if your steppers can turn it you can run them slower to achieve the same movement.

Assuming my non-engineer and inexperienced CNC maths is correct this would yield a theoretical accuracy of 0.0008' (0.02mm), realistically a little less allowing for backlash etc. Compared this to 10tpi which would yield an accuracy of 0.0005' (0.01mm). The difference is to my mind negligable.

I imagine the steppers would be working a little harder to turn the rod, I haven't a clue what the formulae is to calculate the torque to force ratio's for comparison although I'm fairly sure some kindly soul will educate us on that shortly

I'm guessing a slightly larger stepper may be required. We can pick up the 200oz/in steppers from a few places in aus for approx $80 per.

Cheers
Jason.

[fkaCarel]

Assume 10 tpi = 2.54 mm/rev. Torque with 4mm and same configuration: = 2.54 / 4. With a gearing of 254/400, 127/200 = 0.635 the result is the same. This calculation is of course without the normally used moonshiftcompensation .

Carel

[GPZ_Rider]

Originally Posted by Agro I'm guessing a slightly larger stepper may be required. We can pick up the 200oz/in steppers from a few places in aus for approx $80 per. Cheers Jason.

WOW.... Where are you scoring these steppers from??? and on another note... if the rod was to be 18 or even 20mm dia, would the 200 oz/in be adequate to drive say a 1.5 meter lead??

Cheers Mark

[GPZ_Rider]

Carel..Assume 10 tpi = 2.54 mm/rev. Torque with 4mm and same configuration: = 2.54 / 4. With a gearing of 254/400, 127/200 = 0.635 the result is the same. This calculation is of course without the normally used moonshiftcompensation .

HUH??? not sure what you are comparing here Carel........ Country boy here remember

[fkaCarel]

Sorry, looking back it's a little obscure. I can always excuse for the language. Okay, assume you want to travel a distance of 4 mm. For the 4 mm you need 1 revolution, for the 10 tpi 1.57 revolution. For this travel you need an amount of energy. The energy of your motor stays the same. The time stays the same. So if you make a reduction from the motor so that the rod makes 1 revolution (for 4 mm) and the motor 1.57 revolution everything stays the same. Energy, time and distance.

If you are a country boy, ever thought of a GPS controlled router?

Carel

[Geof]

Originally Posted by fkaCarel .....If you are a country boy, ever thought of a GPS controlled router?Carel

I think they have them in the Canadian Prairies; except the cutter runs horizontally and they call them ploughs.

[NC Cams]

If you're going to do Imperial machining, definitely use 10 TPI or 20 TPI. Why? Do the math.

For each turn of the screw, the nut moves 0.100" for a 10 TPI. For a 20TPI, the motion is 0.050" per rev.

It is a lot easier to do the math (in head or on paper) than with the 0.166666666"/rev of the 6 TPI or some other odd ball thread unless it is 8 TPI (0.125" per rev).

All the other explanations eventually pertain but this is why I'd pay to use 10 TPI or 20 TPI instead of a free anything else (unless it was 8 TPI, but only if I couldn't get 10 or 20).....

[2muchstuff]

Is there somewhere that I can go to get the calculations to figure the mechanical advantage of a trapazoidal screw with a given motor. I have a 4 start, 8tpi coated screw and was wondering how much force that could be generated by using it.