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gary wrote:That's looking great Harry, when I saw your first attempt I was going to advise you of the fact that solid wood (especially Bunning's structural construction timber) can swell, warp, and bow enormously with changes in temperature and humidity - that's why most people use MDF or plywood (the latter is better but, as you know, in Australia plywood has strands of gold running through it which can dull your cutting tools quickly - I should point out the I only know there are strands of gold from the fact it is so expensive here, I actually have never found any - it must be well hidden).
However I really like your new solution (steel) and is the way I intend to go on my next machine. Keep the pictures and videos coming. Well done.
harry dalek wrote:This does have me wondering do you have the wood or plastic you are cutting off the table the base a bit as it would be sort of be cut each time you use it other wise ?
M3DVQ wrote:If you are using threaded rod and a nut you will probably want to devise some sort of anti-backlash system otherwise your accuracy will suffer. It depends how accurately you need to be able to cut of course!
gary wrote:Indeed, and there are many different solutions to that from the very simple to the outrageously complex.
I found that the use of an all-thread union nut, being very long in comparison with an ordinary nut, averaged out the backlash to a level less than the resolution of the system as measured with a dial indicator.
The biggest problem, surprisingly, is that I found them very difficult to obtain as zinc plated all-thread in Australia is imperial whereas the union nuts are all metric - they are very clever at Australian hardware stores (NOT!). I eventually found a source at a specialist fixings store.
Harry, you may have better luck in Melbourne.
In addition to this most CNC software allows you to enter a backlash value which is taken into account when moving the position.
It is also important to note that all-thread can vary a bit over it's length - this variation must be measured and entered into the software as a scale value - there is a very good tutorial on how to do that, Harry, on the BuildYourCNC site.
gary wrote:No harry this has nothing to do with how well you build the machine or what you are cutting - "backlash" refers to the little bit a nut on a threaded rod can move back and forth (without turning it) - what it means is when the axis moves forward and then back to the same place there will be a little bit of difference as the nut has moved from one end of it's "backlash" to the other end - the larger this movement the worse the results because they error can accumulate.
Try it yourself - place the nut on the rod you are using and wiggle it back and forth without turning either the nut or the rod - that's backlash. The longer the nut the less backlash you are likely to have as any errors tend to average out.
For cutting Nipkow disks you need virtually zero backlash.
There are all sorts of error sources in these machines - another source of error is "runout" that is the amount of movement in the spindle as it turns. Another can be "racking" where one side of the axis moves less or more than the other side. Yet another is "flex" - especially when cutting harder materials that puts a strain on the machine.
Of all of them, in my experience, "runout" is the worst.
Sorry Harry I don't think I answered this very good question. Actually it covers two areas - the first is trivial - most people use a sacrificial board on top of the bed to protect it. Our American friends tend to use a scrap piece of Mahogany or Walnut, or Cherry, or Birch, and sometimes they even deign to use plywood or MDF. I try to explain to them that we don't have "scrap" in Australia, that every tiny piece of natural or man made wood has to be cherished and wrapped in cotton wool in case it gets scratched - they don't care... (but I digress )
So yes, use your least expensive material as a sacrificial bed - you will get quite a lot of use out of it before it needs to be replaced (or skimmed).
Clamping your work is a bigger question and there are many solutions - none of them, in my mind, ideal.
[/quote]For acrylic (perspex) cutting I generally use double sided tape from the $2 shop. For wood I use a homemade cam clamp, for PCBs I generally screw them to the bed.
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