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So i've searched and searched and search and can't seem to find a quantitative answer.
My machine is designed to move .0005" per step on a motor -- I'm hoping this is achievable with ACME screws (~19" long X and Y axis, 7" Z axis)
I've heard inconsistencies can be compensated for via software...but i'm wondering can i expect to maintain this precision with regular 2G ACME screws or will 2C 'Precision' screws be neccessary?
Thanks
You nead to check the lead accuracy of the screws. The cheaper precision screws are only accurate to ±.009" per foot. These are the ones most of us use. They run $75-$90 for 6ft.
I believe Nook makes some that are ±.003"/ft. More expensive.
Thay also make ground acme that's more accurate. Much more expensive.
However, is your machine built to within .0005" accuracy?
Are you using microstepping to get to .0005"? If so, you can't really depend on microstepping for accuracy.
What kind of machine is this?
Gerry, are the errors of 0.009"/ft consistent for a given screw, and therefore can be simply calibrated in M3, or does the pitch vary from one portion of the screw to another?Quote:
Originally Posted by ger21
It's plus or minus, and can vary. So if you got a really bad one, it could be off as much as .018" in two feet.
But, they generally seam to be pretty consistent, as you never see anyone having issues with them.
Since it is a small machine you could use 5 TPI screws, so in full steps your resolution will be 0.001" per full step, and (like Ger21 said) you should not expect microstepping to yeild perfect positioning per microstep you should get pretty results to within at least quarter steps, which would be 0.00025" per quarter step.Quote:
Originally Posted by FreshMint
If you are serious about accuracy I would forget the acme screws and find some reasonable priced rolled ballscrews. You can still use a delrin leadnut if you don't want the expense of the ballnut to match the screw. I used NSK brand (a top quality Japanese brand) of rolled ballscrew, about $150 for a metre of screw length. Even though classed as "low precision" ballscrews they are made by a good company on very nice equipment and turned out very accurate. If you buy ebay (ie Chinese) rolled screws the accuracy may not be near as good but they will be cheaper.
Backlash and machine slop, flex etc will likely be MUCH more than your desired 0.0005", as much as 10 times more as 0.005" backlash and slop are not uncommon unless lots of attention is paid to removing them in the machine design. :)
RomanLini, can you say where you buy your NSK screws? I'm particularly interested if it is within Australia.
I found a local bearing shop that are a NSK agent (they stock many NSK bearings) and can order any NSK product firect from Japan. The leadscrews (I ordered 2 lengths each 1 metre) took about 10 days to arrive from Japan and were in good condition, no evidence of bending or package abuse.
I know the lcoal bearing shop are a bit expensive on most items so $150 a metre is probably not the cheapest price you could get on these NSK screws.
They were 12mm diameter 8mm lead 2-start screws. Quality and accuracy of the screws is beautiful, but then I've never seen NSK release a crap product, they are usually similar to German bearing quality ie they take pride in their reputation. I plan to buy another metre for my next CNC machine and I still have 600mm left from the first 2 metres I bought.
8mm lead screws are nice as the stepper resolution is directly in hundredths of mm (or a smaller fraction of).
I was planning on using 10TPI screws, and my .0005" are with no micro-stepping.Quote:
Originally Posted by RomanLini
Details of my machine build are here
http://www.cnczone.com/forums/diy-cn...ld_thread.html
My machine should be rigid enough to hold pretty good precision on aluminum and definitely wood.
My real question is should i invest in more expensive 2C screws, or cheaper 2G screws. Can the lack of precision in 2G screws be compensated via software, so that i can expect my .0005" step sizes?
Ballscrews are too expensive and thus out of the question.
Personally, I'd go with the precision 2C screws. But I still wouldn't count on being accurate to .0005"Quote:
My real question is should i invest in more expensive 2C screws, or cheaper 2G screws.
Why do you say that?Quote:
Originally Posted by ger21
Probably 100 different reasons.
Will you have less than .0005" of backlash? If not, the machine may not move when you want it to move 1 step.
If you push on the spindle, will it move less than .0005"? If not, the machine may not move when you want it to move 1 step.
If there's more than .0005" of flex anywhere, the machine may not move when you want it to move 1 step.
Also, everything needs to be aligned, and perpendicular to within .0005" everywhere.
Hmm I see your argument.. Makes me wonder if I should perhaps go with lower TPI screws so i can gain some speed.Quote:
Originally Posted by ger21
If you're using steppers, 10tpi screws will usually limit you to a max speed of 75-100ipm. So if you want to go faster, you'd want to use multiple start screws.
Absolutely. Somewhere around 5mm to 8mm lead is ideal for a small precise machine. 10mm to 12mm lead will get you better overall speeds but precision will start to suffer.Quote:
Originally Posted by FreshMint
Have a look at my small machine; http://www.cnczone.com/forums/cnc_wo...ll_router.html
it uses 8mm lead NSK screws with plastic nuts, and plastic linear bearings and gets excellent precision. I don't expect it to make things to within 0.0005" straight from CAD (very few machines will!) but it is capable of holding 0.0005" precision on an item after I do a test cut and then tweak a couple of parameters.