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

I'm having a mare getting some 'straight' M8 threaded rod (to use as a leadscrew - we're not all loaded!)

I'm now on to my 4th length (& this is starting to add up - so much so, that it'd probably have been as cheap to by a proper leadscrew!). So far, I've had these scenarios...

warped rod to start with (ie from the seller),
a (likely) straight rod but Royal mail sorted that out!
me thinking a rod looked straight at a local hardware store, but actually itwasn't when I put it & put it in my machine....

So...if you're using M8 Rod....where did you get lengths from & more importantly....was it as straight as a die (I need one about 44cm long...I think it's this relatively the length that is proving to be the problem)

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So...if you're using M8 Rod....where did you get lengths from & more importantly....was it as straight as a die

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Random thoughts:

Rolled threaded rod may never be straight as a die - too many stresses and no real need for it to be made dead straight.

Buy stainless steel - it's stiffer and springy so tends to stay straighter. Buy several (3)lengths in one go; they will probably be taped together which will make them less likely to be bent in transit.

Are you using servo motors or steppers? M8x1.25 will be need to be spun fast to get any linear speed - too fast for steppers without a step-up gear ?

Another option - redesign the axis to keep the thread still (taught between axis ends) then spin the nut. This will avoid any screw wobble at high speed

Hi McSpank

Have you tried stainless? Might be worth asking this guy( Ebay item 320363455329)
if he can supply straight thread, I know other people hve tried stainless and polished the thread with a fine wet n dry paper and reported good results.

Just a thought

Andy

Sorry posted after not reading Bills reply

I can send you stainless steel threaded rod thats np, i dont have the prices to hand but i think they are about £1-2 each and they would be 1 meter long. I cleaned mine up on a bench polisher as well.

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Originally Posted by HankMcSpank

I'm having a mare getting some 'straight' M8 threaded rod (to use as a leadscrew - we're not all loaded!)


So...if you're using M8 Rod....where did you get lengths from & more importantly....was it as straight as a die (I need one about 44cm long...I think it's this relatively the length that is proving to be the problem)

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I bought some M8 from eBay from these bods

Nice and straight but that wasn't really a concern for me.

Don't know what you are using for a leadnut but don't be tempted to use SS, polish the thread all you want, eventually it will bind and scrap the rod and the nut.

Jeff.

I used M6 threaded rod, and made my own 'anti backlash' delrin nuts.
(On a home-made CNC made from slabs of an old kitchen worktop)
Very cheap, and they seem to work a treat.

B&Q had some stainless steel stuff - less than a couple of quid per piece. It looked straight in the shop, and I managed to keep it straight on the journey home.

Wickes had some zinc-coated stuff. Straight, but a nightmare to use. I had to run it through a die to cut the thread back to M6 and I never managed to get it to run smoothly. I went back to B&Q to get some more stainless steel in the end.

Neither were 'straight as a die' after fitting - mostly due to my bad coordination and not cutting it to size before fitting it - but were straight enough for me.
Mine span about 600mm, held at either end with skateboard bearings from MUTR. Doing rapids at high speed (600 RPM) makes it wobble and rattle a bit.

Oh, and don't make assumptions about the number of threads per millimetre. Nothing I was routing on the machine was quite the right size... I'd assumed my M6 threads were exactly 1mm per turn, but they were more like 1.06... so calibrate your axis once it's built.

Keith.

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M6 threads were exactly 1mm per turn, but they were more like 1.06

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A common problem with rolled threads.

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.

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:

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!)....

http://img4.imageshack.us/img4/1417/skmain4.th.jpg

....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?

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.

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.

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.

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.
.

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.

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Originally Posted by korky

Well I've learnt something today!

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

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

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Originally Posted by Kip

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?

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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.

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Originally Posted by Kip

Hmmm seems a reasonable price

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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!

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..........