Where did you buy your threaded rod?(cheapo leadscrew!) - was it straight?

[Lee Roberts]

Yea zinc is no good, if you need any of the 608zz i'v got 100 of the ZZ type or 100 of the RS type. £3.99 & £4.99.

[HS93]

If brass will do try of all places B+Q they have meter lenths and nuts, not shore of price but they where not that expensive and the one I got was stright till I got it home, they also gave some metals that come in handy for bodging

Peter

:dance:

[HankMcSpank]

Wow...I'd forgot about this thread...but a lot of reply activity....thanks chaps!

I realise now that I'd not given enough info in my original post.

I am using stainless steel (A4) rod, along with with homemade delrin nuts (delrin rocks!)

I have a fair degreee of misalignment showing at the motors, that I initially thought was just my M8 rod leadscrew being slightly bent I now think is a combination of that with the following 'errors' too....


1. I support my M8 threaded rod leadscrew at each end, by feeding it through a roller skate bearing & then it's all held held by an SK12 mount (these things rock - get some!)....



....The mount in conjunction with the outer skate bearing surface is a perfect fit , but there's about 0.3mm play between the M8 rod & the skate bearing inner circle which it feeds through (meaning a potential 'slop' factor of 0.6mm). I place a nut either side to tighten the rod onto the bearing inner race...but obviously, it's pretty random as to how 'central' I get the rod when the nuts are tightened. I'm at a bit of a loss how to address this...best I can come up with is perhaps use plumbers PTFE tape to help 'take out' this 0.3mm error allowing me to tighten the nuts & get more central)

2. I reckon my jaw coupler holes are also slightly out of whack. I went & bought jaw couplers with a 6.34mm hole on each jaw...one side was fine as that's the exact diameter of the stepper shaft, but the other side needed the hole enlarging & then tapping to thread it onto the M8 rod leadscrew ...I think this might have introduced a margin of 'untrue-ness' (I though having the 6.34mm pilot hole would make enlarging/tapping this hole easy - uh uh!)

The end result, is that the motors aren't turning smoothly...they bind. The workaround (for now), is to loosen the motor mounting bolts to allow the motor some play to physically move & 'go with the flow' so to speak.

What I learnt from this episode...

1. Don't buy your leadscrew rod online! Make sure you can physically inspect it. Tale a long time while inspecting it.....don't just eyeball it....roll it on a completely flat surface.

3. Have someone with the proper tools/experience to drill/tap your motor jaw coupler!

4. Think of a better way of 'holding' your leadscrew (whilst skate bearings are a great idea, the lack of snug fit in the inner race...however small, will simply cause you great headache!)

For all the negativity, my machine works ok, it's just not something I'd want to show off to everybody!

But, hey, this was my first build...we all make major mistakes with that one eh?

I'm now on to my final bit - of my CNC build trying to design/cut a Dremel tool holder...woah - what a palaver. As a temporary measure, I've clamped mydremel to the Z plane (what a kludge - plastica cicular shapes held against flat smooth surfaces is not a good mix)... the Dremel is an awful shape (it's eliptical further up the body!), so even designing it in CAD was grief. But my goodness, that I've come to cut it, I'm findingthe 'clamped' Dremel moves slightly...*very* annoying - especially if you're nearing the end of the cutting run (& esp since I have such slow feed rates)....why did I ever get involved with CNCs?

[korky]

Can you tell us more about the couplers you use?

I've no experience with couplers, but I thought that the whole point was that they allowed for some mis-alignment between the screwthread and the motor?

Would some PTFE tape around the threaded rod before you clamp it in the coupler help to centre it?

I've got an interest in this because at some point I'd like to replace my hosepipe and jubilee-clip 'couplers' with some real ones!
If they need accuracy to within tenths of a millimetre, then I'll need a rethink.
(Perhaps I could join the screwthread and motor together with one of those flexible drill extensions?! :heehee: )

Keith.

[HankMcSpank]

Bill,

I'm not blessed with a lathe - I used a pillar drill at work, figuring I can't go wrong enlarging the hole & then tapping it - like I say, simple in principle, but when you're new to engineering type stuff (I'm more electronics h/w in background),...well, let's just say I wish I'd paid that little extra & got Lee (who sells the same ones I ultimately bought!) to do the M8 tapping aspect! Thanks for the idea about casting - I had considered it, but then I saw how much casting kits cost & I'm unfeasibly tight...I had some 100mm acrylic & wanted to learn CAD anyway, so that's the path I've cjosen (retrospectively, I wish someone sold these!)

I have no idea what you mean by "compliant coupling twixt motor and screw"

korky, I bought 3 x L050 lovejoy type jaw couplers (made by challenge) - these come with a 6.35mm hole drilled in the middle of each jaw. One side will slide onto your stepper shaft perfectly, but the other side of the jaw needs 'modding' to fit what ever diameter/fit your leadscrew is. Mine is M8 threaded rod...which has a 1.25mm thread, therefore 8mm minus 1.25mm, means the tap hole should be 6.75mm. I therefore drilled it to that specification using a pillar drill & tapped the hole with an M8 tap. You'd think this would be a simple job - it probably is to someone who more of an engineering type...but as it turns out, I reckon my inexperience has resulted in me adding in a degree of 'error' making it untrue, which certainly gets magnified when coupling to some M8 rod that slightly bent too!

I'd also read that such couplers allowed a degree of leeway...the two couplers push together incredibly snug with their insert...I estimate my leadscrew -> motor setup is out of whack by 1mm, but the 'forgiving' jaw couplers didn't forgive! I should say though that I have one of those SK12's holding my M8 leadsrew about 2cm from where the M8 rod couples to the motor, so this is probably why I'm not seeing any 'give' supposedly allowed by the coupler.

[John S]

Lovejoys are resilient type couplings and don't handle misalignment as you have found.

For misalignment you want Oldham couplings, these also have the advantage in they are the narrowest couplings available that work, handy if you are pushed for space or want to build a machine that doesn't look like Princes Charles ears.
.

[korky]

Well I've learnt something today!
Here's a link to a flash animation of how an oldham coupling works:
http://http://www.mekanizmalar.com/oldham.shtml

Most impressive.

Keith.

[HankMcSpank]

Have you learnt that threaded rod isn't leadscrew yet?

Seems cheap enough to buy the real thing instead of using the TR in it's stead?

No, I've not learnt that! (I use the term generically, else I'll be saying things like "the threaded rod that drives my planes back & forward"...not exactly punchy is it?)

If you mean buying leadscrew instead of threaded rod - again no I've not learnt that - Why? Because the leadscrew rod isn't the end of it...for example, diy I can't make drive nuts for proper leadscrew (I can for M8...cost about a quid per nut)...& proper drive nuts cost a fortune x 3. Then there's the getting the leadscrew machined to 'dovetail' with your own mounts/ couplings, then the mounts themselves etc....& on it goes.

I'd love to have proper leadscrews, but the attraction of cheap threaded rod is *very* powerful (even after all the grief - but that's why I posted up ...to save others who in these credit crunching times perhaps haven't got that much money to throw at a hobby).

Seems there are two types when it comes to CNC building...

1. Hobbyists who aren't even sure if they'll use their CNC much & are perhaps just up for the challenge (that'll be me)...they don't want to throw too much money at the project (but have tiome a plenty to get creative & re-use bits & bobs towards their build)

2. Those that are serious about CNC'ing and are prepared to throw some dosh at it!

At the end of the day, I'm a bloke who's made a CNC machine out of old dot matrix printers to cut little bits of plastic up into suitable shapes...my pockets aren't very deep for this project....so proper leadscrews & drive nuts are out.