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

[BillTodd]

This sounds similar to my little engraver/mill. I had the same trouble with bearing alignment - I faced the nuts in the lathe while on the lead-screw then manually adjusted them to the centre of the bearing (i.e. tapped them with a hammer).

I'd recommend some kind of compliant coupling twixt motor and screw (I used a rubber tube reinforced with plastic tube cut over the top)


[edit] re: Dremel mount

Wrap it in cling film use it as a mould to cast a epoxy (e.g. car body filler) mount? Or, have look at Polymorph, a low temperature thermo-plastic that can be easily moulded by hand.

[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]

Well I've learnt something today!

But it's not how to post up links ;-) ...

http://www.mekanizmalar.com/oldham.shtml

[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.

[HankMcSpank]

Hmmm seems a reasonable price

On the face of it yes, but then dig deeper. It works out at £1.98 per 100mm delivered

I'd need at least 1m (this would be for all 3 planes in total)

So £20 vs £4 for a 1m length of M8 rod from my local fixings merchant

But wait that's not the end of it.

I can make delrin nuts myself (about a quid each, therefore £3 in total)...I don't want to imagine how much the nuts are going to be for that rod.

then you'd likely have to get the rod machined to fit with your couplers. Then you'll likely need custom rod mounts (ouch)

I reckon you're looking at the guts of £200 ...vs £20 for the cheap ' dirty M8 threaded rod route!

Horses for courses, but I'd imagine most new to CNCs wouldn't want to spend that type of money on a hobby that might not even have longevity of interest!

[John S]

Hank,
Ask you local nut and bolt / fixing man if they stock trapezoidal, ours do, never bought small only 16mm and I can't remember how much it was, they also have like a barrel but to suit.

I accept you will have to get some machining done but this post is also a heads up for other people,

If I have a length left I'll get a pic later, still after that bloody cat..........