Ball vs. Anti Backlash Acme Screws

[eman5oh]

So far I have foun the lead error to be well under the .004 that they spec. Where I have checked it it is .0003-.0005 which is damm good IMHO. I found the back lash to very a bit though. I measued as good as .0005 and up to .0025 with reloaded nuts on my mill. My router has the stock balls and is around .002-.004. I did not bother to reload them as I use the router for wood and even .004 is unnoticable in the finished parts.

[phantomcow2]

Yea ive noticed that too, the lead accuracy is actually pretty good. THe .004 is more an insurance if you ask me.
I've got some beautiful ground ballscrew, and its on a whole different level than the rolled. But, i think the rolled is pretty good myself...

I've also some got 5/8 2 start ACME with teflon coating. That means a .25 lead. With a delrin nut, it is amazing how much the nut will travel just by giving it a spin. If i had to use acme, it would certainly be multiple start! Most of our routers dont need the mechanical advantage of a 1/2-10 anyways

[eman5oh]

For this one I went with the 1/2-8 2 start acme. I was up in the air on this one so set up this poll and still ended up unsure. The deciding factor was mostly that the acme is not as hard to turn the ends to accept bearings and is a little cheaper. Turning the ends of the rolled ball screws is a real PITA.

[phantomcow2]

Rolled ballscrews are a horrible thing to machine, I agree. Just last week i thought "Well i only need to turn down 1/2", i dont need to heat it first". Well i did make it, but it was an ugly proces. You really want carbide to do it...
Or be proper and heat those ends

[PaulH]

If you use multistart ACME screws, do you plan to tap your own anti-backlash nuts, or would you buy them pre-made? If you tap them yourself, how do you plan to accomplish this?

[phantomcow2]

I have actually already made some very nice delrin nuts for them. Rather than spend 100+ dollars for a 2 start acme tap.... here is what i did:
First, i cut off a piece of the rod 5" in length.
Using a lathe, i turned a taper on the first 10 threads
Using a milling machine and a 3/16 end mill, mill 4 flutes deeper than the depth of the threads. These are not very exact, the flutes are not exactly 90deg apart.


And then i just drilled a hole in the end for a piece of 1/4" steel to slip in, as a handle.
Tap tap tap! And one thing i quickly learn, tapping ACME is hard
and it makes sense why, since your removing so much more material than 60deg threads.

The milling makes for pretty sharp edges, ideal. I had to run the tap over several times to make hte nut very smooth.

I've only done this with delrin though, would not even bother with metal. Its just so hard to tap ACME (I think that roton said its 5x harder than unified threads). Plus, metal would wear off the teflon coating

[eman5oh]

I bought the nuts from here http://www.gonebowlin.com/dumpsterCNC they were cheap enough that I won't bother to make them. If I were to make them I would cut the threads on a lathe. Doing two starts is no more difficult that duing one just takes twice as long as you are cutting two threads.

[Mcgyver]

when i first read this i was wondering why you went with 2 start for two reasons; 1) wouldn't precision ground acme screws and nuts be cheaper/more available in a regular one start and 2) the faster the helix the more leverage the cutting force has on turning the screw. If you need a faster action, couldn't you just change the gear ratios or even overdrive it - isn't that all you are doing with the 2 start anyway?

i'd say multistart threads are a little tricker than single start to cut, you've got to figure out how to index the work and the cutting tool geometry is a little tricker with the higher helix angle, still you are right they are a diy item.

I haven't made an acme tap, but if I was charge with the task I'd consider making it progressive - ie a set of 3 or maybe more with each one progessively increasing the depth of cut. imo this would give a home made tap a fighting chance in metal, although making a homemade tap is not trival, it would required grinding after hardening to be of much use on metal.

[ger21]

1) 1/2-8 2 start at MSC is very inexpensive - $30 for 6 ft.

2) 1/2-8 2 start is 50% more efficient than 1/2-10, so you waste less power. But yes, you do need a little more power to use them. Also, spinning the screw 2.5x faster could result in more whipping, depending on the required speeds.

[Mcgyver]

Gerry, thanks, didn't realize the 2 start was so cheap. still i thought it was precision ground acme that he was talking about, but ok I'll assume no price advantage. your point on it whipping around is also good as you say if it was going very fast. on the efficiency though, isn't just changing ratios, ie 12/5 geared down 2:1 is the same as 12/10 direct drive?

[ger21]

The screw itself is more efficient, so just gearing it still leaves you with a less efficient screw. Check out Nook's acme catalog for lots of great acme info. www.nookind.com

[eman5oh]

Originally Posted by Mcgyver Gerry, thanks, didn't realize the 2 start was so cheap. still i thought it was precision ground acme that he was talking about, but ok I'll assume no price advantage. your point on it whipping around is also good as you say if it was going very fast. on the efficiency though, isn't just changing ratios, ie 12/5 geared down 2:1 is the same as 12/10 direct drive?

Also with direct drive you have a much simpler drive system, less parts and less cost. For my application slowing the screw speed down is important, the longer the screw the slower the speed wherer whipping will occur.