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pminmo
08-17-2004, 05:36 PM
:drowning: Rough gas pipe router, absolute accuracy not hugely important. currently 5/16" -18 all thread 42" long. Stepper 127 oz in, 200 step per rev, which torque falls off quickly about 800 steps/second. Budget is a consideration, but I'd like to get to 2 or 4 pitch leadscrew rate. Is there a reasonable cost method to mechanically multiply via gears or pullys. Or do I bite the bullet and replace with new leadscrew? Is this www.mcmaster/com p/n pair: 99030A716 screw and 6350K153 nut a reasonable solution?

Phil

fyffe555
08-17-2004, 07:45 PM
Phil,

maybe a dumb question but is the motor dropping off at 240rpm or is it the driver? If it's the driver then for the cost of those drive screws maybe it's worth upgrading the drivers first? Wouldn't the 3977 drive this faster?


One of my machines (first one) is a 12x12" moving gantry mdf special. It's pretty light and uses cheap 3/8-16 allthread, gas pipe/rollerskate on X, pacsci 118oz motors and IMS 483 drivers @ 38vdc and 3.5a ea. That runs up to 70 ipm or 1120rpm quarter stepping the driver. In fact it does run faster but the gantry jumps up and down as the X screw starts to whip.....

For the McMasterCarr stuff The leadscrew and the nut you've listed are not quite the same profile...The leadscrew is 'precision acme' the nut is 'precision modified'.

I'm using similar screws in 1/2-10 acme and the MMC wear compensating nuts on a stiffer 28" x 28". Similar 118oz pacsci motors and same drivers. I started off with cheaper Enco 1/2-10 and delrin but the Lead accuracy was way off over the length. The MMC wear commpensating nuts didn't fit the enco acme, got some leadscrew from a surpls store locally which still has a slightly different profile but works well if a little tight at first. Inertia of the heavier screws limits rpm to about 900 on the 118oz motors @ 48v and 4a so that's ~90ipm or about 22 seconds end to end G00 rapid factoring accel.. With hindsight I would have got something in the 6 or 4 lead per inch range and kept the motors more in their torque range but I'd already bought the nuts.... one or more starts really depends upon where in the torque curve you want to gear your motors.

If you're going to go with Acme the quality and price of the MMC stuff is pretty good. The wear compensating nut isn't quite backlash free but over 28" I've struggled to measure the deviance after some ten months of intermittant hobby use at fairly high rpms...

hth

Andrew

pminmo
08-17-2004, 08:22 PM
Andrew,
Actually, the info comes from the motor's specification chart, but I seem to get the same results in real life.
I do intend to try different drivers and higher voltage combo's to see effect, but think ultimatley I want to get a better mechanical ratio via some method anyway.

Phil

fyffe555
08-17-2004, 08:55 PM
That makes sense. In a circular fashion I was trying to say that with an 18tpi mechanical advantage you've got 18*42" or 756 turns to your lead screw and at 240rpm it's going to take you over three minutes to move one end to the other. A 5tpi screw and 240rpm will still take almost a minute with accelerations.

On the other hand, I'd do the sums and work out the minimum force required by your machine, what gearing (tpi) your motors will stand, and see where you are for speed and torque curve. It's an interesting balancing act and more rpm, especially in bipolar parallel makes life easier.

pminmo
08-17-2004, 09:27 PM
Thanks for the help Andrew, I think I'm going to find out I want a bigger faster motor for my X. Ahh more money!


Phil

fyffe555
08-17-2004, 10:10 PM
Ah, yeah, that's exactly where I am at the moment too. What with lead screws and linear bearing costs it's almost cheaper to buy a sieg x3 mill.

I forgot to add that when figuring your 'gearing' you might want to consider that all thread is atrociously inefficient, figure 20% of the 125oz/in is actually moving the gantry. Very roughly, Acme base is 30-40%, acme precision is 35-55 and ball screw is 70-90% efficient. Putting in something other than allthread will make your motors appear more powerful, 'cos less power is wasted in the nut...

let us know what you decide..

pminmo
08-17-2004, 10:57 PM
ANy thoughts on the best price for a 2 or 4 tpi ballscrew 42" long and an associated nut?

Phil

tpworks
08-18-2004, 12:52 AM
try MSC Indrustrial Supply, thier precision acme screws are pretty cheap, I got the
5 tpi 3/4" x 72" for 28.00 and shipping they've been working for me 28 ipm.

http://www.mscdirect.com/MSCCatLookup2.process?MSCProdID=01206242

Tom

pminmo
08-18-2004, 05:10 PM
Thanks TP,
What do you think about:
http://www.mscdirect.com/MSCCatLookup2.process?MSCProdID=03777570
and
http://www.mscdirect.com/MSCCatLookup2.process?MSCProdID=03778651

Phil

arvidb
08-18-2004, 05:14 PM
I think steppers have an almost flat power output - when speed increases torque drops, as I guess most of us know. Well, if the power output is flat, then gearing will give you absolutely nothing (gearing up mean slower rotation but more torque for the same speed). My guess is higher efficiency screws or more powerful motors is the way to go.

Arvid

pminmo
08-18-2004, 05:37 PM
arv,
I'm not sure I understand "almost flat power output - when speed increases torque drops" isn't that a contradictory statement? I would think a flat power output would look like the same torque regardless of speed?

Phil

tpworks
08-19-2004, 01:19 PM
Thanks TP,
What do you think about:
http://www.mscdirect.com/MSCCatLookup2.process?MSCProdID=03777570
and
http://www.mscdirect.com/MSCCatLookup2.process?MSCProdID=03778651

Phil

I have those also on my 24 x 30 router, was at the scrap yard and came across a barrel of 5/8 - 5 oilite nuts and picked up a dozen for scrap price and bought the screws to fit. Has been working great at 23 ipm. I used a home made delrin nut for the Z axis though.

arvidb
08-19-2004, 03:30 PM
arv,
I'm not sure I understand "almost flat power output - when speed increases torque drops" isn't that a contradictory statement? I would think a flat power output would look like the same torque regardless of speed?

Phil

No, power is torque times speed, so a motor with same torque regardless of speed (i.e. typical servo motor) would have linearly increasing power output with increasing speed.

In SI units, P [W] = M [Nm] * omega [rad/s]. Written out: Power [in watts] equals Torque [in newton meters] times angular velocity [in radians per second].

Arvid