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)
08-05-2009 #2So...if you're using M8 Rod....where did you get lengths from & more importantly....was it as straight as a die
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
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
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..Me
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.Nothing is foolproof......to a sufficiently talented fool!
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.
13-05-2009 #7M6 threads were exactly 1mm per turn, but they were more like 1.06
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..Me
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
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?
Last edited by HankMcSpank; 04-06-2009 at 10:14 AM.
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