CNC (DIY) - ideas on trapezoid acme threads.

[DTJ]

I have a small DIY router running on M8 x 1 all-thread (threaded builders rod). The steppers are PV269-02BA-C1 (data attached).

It's mainly used for routing enclosures and plastic parts, its quite slow and I'm looking at better lead screws.

All axes need about 300mm of lead screw. Ball screws are too expensive so I'm looking at acme trapezoid lead screws.

These ones are priced OK at AU$25 / 300mm.
Being a 4 start thread the lead is pretty high @ 8mm per revolution - it will be 8x as fast but the cutting force will be way down.
It looks to be nicely machined and has brass nuts so I guess the friction will not be too high.
http://www.ebay.com.au/itm/Lead-Screw-ACME-Leadscrew-Tr8x8-300mm-Brass-Nut-RepRap-Nema17-RAMPS-3D-Printer-/201277801789?var=&hash=item2edd17853d:m:mnnfysxkeo7nl5wNMb72oyQ


I've also found this single start acme trapezoid lead screw.
Its a single start 10mm x 2mm lead so should be twice as fast as my current lead screws.
I'm waiting to hear back from the seller on what nuts, if any, he sells to go with these.
Being single start I'm guessing the friction may be much lower than a four start acme thread.
http://www.ebay.com.au/itm/TRPZ10x2-100mm-TRAPEZOIDAL-SCREW-ACME-LEAD-CNC-ROUTER-MOTION-3D-PRINTER-MACHINE-/291728797505?hash=item43ec642f41


Has anyone used the acme threads? How did it go, I'm cautious about spending my money only to find out the acme thread friction is too high.

[Dago]

You can get ballscrews from China nowadays for next to nothing http://www.ebay.com/itm/222115460307 They will surely be superior to any threaded rod solution.

[H.O]

Yeah, I really wouldb't bother with threaded rod or ACME or anything like that. Even the cheapest ballscrews will give you higher speed, more linear force (due to the much higher efficiency), better accuracy and less backlash than anything else.

If you don't have access to a well equipped machine shop make sure to order them with pre-machined ends and watch out for bad bearings in the end supports, they've contributed way more to backlash than the screw/nut in the sets I've bought.

Then of couse the 16mm ballsecrew and nut (which, by far is the most common size on EBAY etc) might be too big to actually fit in the machine. But if you shop around you might be able to smaller ones.

[DTJ]

You can get ballscrews from China nowadays for next to nothing http://www.ebay.com/itm/222115460307 They will surely be superior to any threaded rod solution.

Yeeeeah, you're twisting my arm. Maybe I need to look harder at ball screws.

Yeah, I really wouldb't bother with threaded rod or ACME or anything like that. Even the cheapest ballscrews will give you higher speed, more linear force (due to the much higher efficiency), better accuracy and less backlash than anything else.

If you don't have access to a well equipped machine shop make sure to order them with pre-machined ends and watch out for bad bearings in the end supports, they've contributed way more to backlash than the screw/nut in the sets I've bought.

Then of course the 16mm ballscrew and nut (which, by far is the most common size on EBAY etc) might be too big to actually fit in the machine. But if you shop around you might be able to smaller ones.

It's starting to look like ballscrews are the way to go. Perhaps part of my speed problem is down to friction in the drive assembly anyway.

I have a lathe to machine the ends of the screws but if I could buy them already done for not much more money then I'd go that way.

16mm would be too large, I'd need to go for something smaller.



Looks like it's back to eBay. Thanks for the ideas guys.

[langwadt]

You can get ballscrews from China nowadays for next to nothing http://www.ebay.com/itm/222115460307 They will surely be superior to any threaded rod solution.

Yeeeeah, you're twisting my arm. Maybe I need to look harder at ball screws.

Yeah, I really wouldb't bother with threaded rod or ACME or anything like that. Even the cheapest ballscrews will give you higher speed, more linear force (due to the much higher efficiency), better accuracy and less backlash than anything else.

If you don't have access to a well equipped machine shop make sure to order them with pre-machined ends and watch out for bad bearings in the end supports, they've contributed way more to backlash than the screw/nut in the sets I've bought.

Then of course the 16mm ballscrew and nut (which, by far is the most common size on EBAY etc) might be too big to actually fit in the machine. But if you shop around you might be able to smaller ones.

It's starting to look like ballscrews are the way to go. Perhaps part of my speed problem is down to friction in the drive assembly anyway.

I have a lathe to machine the ends of the screws but if I could buy them already done for not much more money then I'd go that way.

16mm would be too large, I'd need to go for something smaller.



Looks like it's back to eBay. Thanks for the ideas guys.

I've not bought anything from them so I have no idea of the quality but http://www.robotdigg.com/
seems to have a lot of ballscrews/linear rails etc. for crazy prices

[DTJ]

I'll have to see what reviews I can find but they certainly look ok. Thanks.

http://www.robotdigg.com/product/614/1204+Ball+Screw+w/+Machined+Ends


They have a Aliexpress store. Might be some protection ordering via that route.

http://robotdigg.aliexpress.com/store/all-wholesale-products/228043.html

[H.O]

I have a lathe to machine the ends of the screws but if I could buy them already done for not much more money then I'd go that way.

The screws are pretty freakin' hard on the surface so you need a sturdy lathe and proper tooling to dig in below the hard surface or use grinder to remove the bulk of the material. If you can get them pre-machined I highly recommend going that route, most EBAY sellers even offers custom end machining for no or a small (extra) fee.

Your screws are pretty short so it's probably not an issue but I once ordered a 900mm screw and it came wrapped in bubblewrap (only). Needless to say it wasn't straight which totally sucked. It's worth asking the seller how they're shipping. Bubblewrapped and placed in a thick cardboard tube is good.

[langwadt]

I have a lathe to machine the ends of the screws but if I could buy them already done for not much more money then I'd go that way.

The screws are pretty freakin' hard on the surface so you need a sturdy lathe and proper tooling to dig in below the hard surface or use grinder to remove the bulk of the material. If you can get them pre-machined I highly recommend going that route, most EBAY sellers even offers custom end machining for no or a small (extra) fee.

I've seen some videos of using an induction heater to anneal the part of the screw that needs machining

[DTJ]

I have a lathe to machine the ends of the screws but if I could buy them already done for not much more money then I'd go that way.

The screws are pretty freakin' hard on the surface so you need a sturdy lathe and proper tooling to dig in below the hard surface or use grinder to remove the bulk of the material. If you can get them pre-machined I highly recommend going that route, most EBAY sellers even offers custom end machining for no or a small (extra) fee.

Good point, I'd thought of making a clamping sleeve that would go over the end. I think for the sake of a few dollars buying pre-machined is the best way.

I have a lathe to machine the ends of the screws but if I could buy them already done for not much more money then I'd go that way.

The screws are pretty freakin' hard on the surface so you need a sturdy lathe and proper tooling to dig in below the hard surface or use grinder to remove the bulk of the material. If you can get them pre-machined I highly recommend going that route, most EBAY sellers even offers custom end machining for no or a small (extra) fee.

I've seen some videos of using an induction heater to anneal the part of the screw that needs machining

Good idea, if I do need to machine them I'll anneal with a gas torch. Thanks.

[DTJ]

I've not bought anything from them so I have no idea of the quality but http://www.robotdigg.com/
seems to have a lot of ballscrews/linear rails etc. for crazy prices

I'm part way through an order with Robotdigg now. Its a weird system - status of my order is "Pending". I'm not sure if i do something or wait for a response from them!
Prices certainly look good.

[cncjerry]

Couple of points to consider in no  particular order:

1) You might need a brake on your Z axis if you  switch to ball screws.  The low resistance and low twist rate combine to allow the head to drop when the steppers aren't energized.   This is a bigger problem with servos and it depends on the weight of your headstock.

2) You can anneal the end of the screw by using a hot torch, acetylene is  preferred because it will heat it quickly, while wrapping a wet wrap around the screw threads to keep them hard.  You can also put the screw in a bucket of water or oil.  Let the screw cool slowly.

3) Everyone jumps to screws but your table slides are probably also causing drag.

4) Rotrax (I think was the name) made a great solution in that they put oversized balls into the nut to take the backlash out.  Ball screws typically have higher backlash without some type of ball adjustment.  Putting larger balls in though increases drag a little, still much, much better than an acme depending on pressure angle.

5) It might be cheaper to just change out the motors and drivers but there is a limit to stepper torque for practical NEMA sizes. Also, open loop steppers will always have a problem with lost/extra steps.  Servos do to, since they are always a little behind the drive.   Most drivers have a number of pulses that you can set before reporting an error so there are trade-offs there as well. 

6) Overall,  and I use 1100 oz/in servos on one of my mills and steppers on the other, I've found Acme screws to give acceptable performance.  The anti-backlash nuts are easy to make and adjustable; they come in a higher thread count per inch/mm;  they are easy to replace as they wear;  and can generally be tuned to lower backlash (and people will debate this point to no end).

7) If you stick with acme screws, the best thing to do is make your own nuts.  Buy an extra piece of Acme and grind channels to make a tap.  This will give you a tight fit.  Alternatively, you can mold zero backlash nuts using one of the many self-lubing epoxies.  Don't try using an epoxy graphite mix, it won't work.  For low-torque applications like drilling (without a side load) you can make really, really tight nuts from delrin and nylon.  I use these on my engraving machine.

[Koen]

How did you make the jump from him having a tiny router currently moving with M8 screws to axis brakes, a complete drive change, backlash reduction and so on ? He's beginning, no need to cloud his mind. Go simple, go cheap, learn. 25USD pre-machined chinese ballscrews are easy, cheap, efficient and reusable.

[H.O]

And the statement that open loop steppers always "have a problem with lost steps" can be a bit misleading for someone not familiar with stepper motors IMHO.
I often feel there's a belief that a stepper motor can just randomly lose a step here and there which of course isn't the case. As long as the load placed on the motor is less than the torque available at that speed the motor will never lose a step.

With that said it can be hard to determine what the load, and therefor the torque requirement, is so you do your calculations or make an estimated guess and the add a safety margin or you simply need to find the limits and stay below it. If you do that a stepper motor is a very reliable device.

But yeah, we're drifting OT.....

[DTJ]

Thanks for the replies guys, lots of useful information.

Hopefully I've got enough drag in my Z-axis linear rails to hold the axis in place. It's mainly aluminium, and is pretty light.

I've ordered a couple of 400mm & one 500mm 12mm ball screws with ball nuts, as well as the support bearings etc from Robotdigg. What started as a $75 patch up has turned into a biggish upgrade.
The ball-screws were US$24 each with machined ends. The end mounts and ball nut mount added about another $25 per axis.

I run my steppers pretty lightly to try and avoid missed steps. So far so good, I did a run of 60 poly-carbonate enclosure lids and over the whole job it didn't lose one step. I set the tool back to a reference point after each piece which allows me to do a quick visual check before clamping the next part down.

The replacement interface board (see below) arrived today. If it works OK it will be $10 well spent.

[H.O]

I run my steppers pretty lightly to try and avoid missed steps. So far so good, I did a run of 60 poly-carbonate enclosure lids and over the whole job it didn't lose one step. I set the tool back to a reference point after each piece which allows me to do a quick visual check before clamping the next part down.

And if you do that now, with threaded rods, you'll be more than fine once you fit the ballscrews since their efficiency is much better than the threaded rods. The motor speed will also be lower for the same federate due to the higher pitch which means that you get even further back/up the motors speed/torque curve and $49 per axis for complete, machined ballscrew with end support it's a total win.

[DTJ]

I run my steppers pretty lightly to try and avoid missed steps. So far so good, I did a run of 60 poly-carbonate enclosure lids and over the whole job it didn't lose one step. I set the tool back to a reference point after each piece which allows me to do a quick visual check before clamping the next part down.

And if you do that now, with threaded rods, you'll be more than fine once you fit the ballscrews since their efficiency is much better than the threaded rods. The motor speed will also be lower for the same federate due to the higher pitch which means that you get even further back/up the motors speed/torque curve and $49 per axis for complete, machined ballscrew with end support it's a total win.

I used to use an very old flex shaft drive Dremel as the spindle, it was was too fast and melted a few different types of plastic (using 2 flute 2mm carbide routing bit).

I turned up a new spindle and now run it from a reasonably heavy 12V DC fan motor (probably of automotive origin).
I still get some melting on some plastics and am hoping to move the axes quicker to reduce the melting.

[DTJ]

The ball screws are in & running. Much faster than the old M8 threaded builders rod......