Wich leadscrew? (how much TPI)

[CnC_BoY]

Hi,

I will use leadscrew on my small 18 in by 12 in CNC with 259 oz/in motors. I planned on using 10 tpi one start screw but I saw that some use 10 tpi 2 start screw. What would be the advantages/disavantages of each (i know one is more precise and one faster but whats the real life effects of using either). Would the speed with one start 10 tpi be really slow?


Thanks a lot

[spalm]

OK, I’ll put in my 2 cents worth.
We are talking a hobby machine here right?

As you noted 2 Start screws will give you twice the speed. I believe that accuracy from the screws will not be a factor here; either type will give you very good accuracy at this level. Not having seen your design, but I’ll guess that other things will cause your inaccuracies (I could be wrong).

Speed is also probably not a factor with this small of a machine. With this size motor, let’s assume a speed of 60ipm for 1 start and 120ipm for 2 start. 60ipm is 1 inch per second. With a table of 12 inches, you get 12 seconds (vs 6 seconds) of rapid speed to transverse the table. This is maybe not a problem here, but in a larger router, speed helps.

A 2 start screw can spin at half the speed of a 1 start screw and get the same speed. This is important when trying to reduce whip, but again, with a small machine, whip should not be a problem.

So with those problems addressed, you come down to lead nuts and mounting. 1 start screws are easier in that you can make your own lead nut with a piece of the lead screw made into a tap, and then just use Delrin or a chunk of plastic cutting board. You can also buy nuts to use to mount the bearings.

2 start screws will probably require you to purchase a lead nut, and use collars or turn down the screw on a lathe for mounting.

So… 1 start are easier and slower, 2 start are a bit more complicated, are really cool, and faster. (And don’t underestimate faster).

Steve

[chuckknigh]

You forgot one important detail. Since the 2-start has less mechanical advantage (you move further with each rotation) it will require bigger motors than a 1 start screw.

-- Chuck Knight

[vacpress]

and to round it out, i beleive a 280~ipm stepper with 2start leadscrew would be fine.. i have actually seen smaller motors on small bench mill type machines.. with, i think... 2 or 4-start type leadscrew... i wasnt to into that machine.. it was like calling a chunk of alluminum a machine tool.... a minitech maybe.. something like that...

[ger21]

Originally Posted by chuckknigh You forgot one important detail. Since the 2-start has less mechanical advantage (you move further with each rotation) it will require bigger motors than a 1 start screw. -- Chuck Knight

But it's spinning half as fast, and steppers have more torque at slower speeds, so this may cancel out, and you may end up with the same force at much faster speeds.

[WhiteTiger]

Originally Posted by ger21 But it's spinning half as fast, and steppers have more torque at slower speeds, so this may cancel out, and you may end up with the same force at much faster speeds.

Hmmm... if it's spinning half as fast and moving twice as far per rev, then your ipm is identical and you've just traded resolution for torque, some of which is eaten up compensating for lessened mechanical advantage, right?


Tiger

[CnC_BoY]

thanks guys for fast answer

Yes Spalm, it's a hobby machine mainly for engraving.

I think I'll go with 1/2 10 - 2start for faster speed and accuracy still good (1 thousand vs half a thousand. By experience, a thousand is not big.

[jeffs555]

I think all Gerry was saying is that you have to look at the system as a whole, and that going from a one start with a lead of .1 inch to a two start with a lead of .2 inch would not necessarily need a larger motor. It might even generate more force at a given IPM with the same motors. Unless you are using ballscrews, friction will cause the mechanical advantage between one and two start to be much less than 2 to 1. For 1/2-10 with plastic nuts, it is only about 1.45 to 1. Depending on the motor torque curve, dropping the motor RPM's in half with the two start, might increase the motor torque more than 1.45 times, giving increased performance with the same motor.

[spalm]

If you want to try 2 start, go with 1/2" 8 (4tpi). You will get a little bit more of a kick out of it and it is cheaper than 1/2" 10 (5tpi). 6 feet of 4tpi is $31.81 while 6 feet of 5tpi is $61.33 at MSC.

Links to MSC: 4tpi 5tpi

Steve

[Kammo1]

Good thread this and it's what I am looking at at the moment. Who supplies this stuff in the UK ??

[CnC_BoY]

Kammo,

look at Mcmaster.com, I order there and they ship worldwide.

[Kammo1]

Hi CnC boy thanks for that link, man they do everything!!!!!!!!!! now I am really spoilt for choice.