. .
Page 2 of 5 FirstFirst 1234 ... LastLast
  1. #11
    Quote Originally Posted by joe.ninety View Post
    Do the external dimensions of BK/BF blocks vary by manufacturer or are they standard dimensions dependent only on rail diameter?

    Cheers
    Yes they can do slightly. The internal will be all the same as it's a standard but the externals can vary in size but in my experience they are very close in size.

  2. The Following User Says Thank You to JAZZCNC For This Useful Post:


  3. #12
    Quote Originally Posted by JAZZCNC View Post

    Whereas the rails on rear plate mean that the spindle can be lifted high up the rear plate so the tool clears the bottom of the gantry giving the full height from bed to underside of gantry even with a long tool. This means you can make your gantry lower and still have the same material clearance as the other method but it makes a much stiffer machine and better cutting so eliminates any advantage the rails on front plate offers.
    Does having rails on the rear plate also mean that you can have a smaller plate on which the spindle mounts? Thereby saving on material costs?

    Cheers
    Last edited by joe.ninety; 19-04-2020 at 12:51 PM.

  4. #13
    Hi Joe,
    Thanks for asking many of the questions I need answers to as well!. I am currently trying to start my own design from the Z axis out. You would think this would be the simplest axis with least design options, but still the choices come. I had also picked up the tit-bit about rails on Z plate for stiffness, so grateful that Jazz has spelled out the trade offs.

    It seems to come down to how to sandwich rails/bearings and ballscrew/ballnut between two plates neatly and compactly.
    A 16mmm ballscrew nut and holder is 40mm high, so that is the minimum distance between plates,
    However BK12 are 25mm from base to ballscrew centre, and ballscrew holder is 20mm, So using these simple components, plates must be 45mm+ apart
    If you are going to have to pack up rails or bearings to span this gap, then 20mm rails are far from too big, as at 30mm combined height, you will need a 15+mm space. FK bearings and clever design might make it possible to pack things a little tighter, but I am still looking around for a reference design that solves all this with minimal parts and machining.

    Good luck with your design

  5. The Following User Says Thank You to Andrewg For This Useful Post:


  6. #14
    My approach to this problem was to use 20mm rails, and 20mm Ecocast plate. This gave enough depth of material to be able to pocket the plates for clearance around the nut holder. The important point here is that the pocketing is for clearance and is not super-critical in terms of dimension. This means that you can use a woodworking router to do the job. I actually used my vertical mill, but if you search the forum for (from memory) Joeharris's posts he talks about how he used a conventional handheld router successfully to machine aluminium. Might be worth a look and a few back-of-emvelope sketches. I know that I spent a long time on the details in this area; apart from anything else you have to be able to assemble the Z axis and I ended up doing things like drilling access holes in the moving plate to be able to get to a few allen bolts going into the fixed plate. All good fun!

  7. #15
    Quote Originally Posted by Neale View Post
    My approach to this problem was to use 20mm rails, and 20mm Ecocast plate. This gave enough depth of material to be able to pocket the plates for clearance around the nut holder. The important point here is that the pocketing is for clearance and is not super-critical in terms of dimension.
    This is the easist and best way to build a Z axis. Packing rails off or bearings off is a fiddly bodge. 15mm rails while more than strong enough are fiddly things and make it awkward.
    You could go with 25mm rails and get away with no machining but then it gets tight with space and really requires a wider Z axis, plus there's more mass to deal with.

    Hand machining isn't difficult provided your comfortable using a large router. I did several of the parts for my first machine this way and was the one who talked Joe into giving it a go, however Joe cut far more parts than I did using this method and made a very nice job of it so I'd advise anyone thinking to do this to go check out how he did it.!

  8. #16
    Yes, as you say Joe Harris's videos show simple routing of Aluminum is doable if high precision not required. However looks from video that he actually used 15mm+ spacers under his Z bearings and flat plates.

    Neale, presumably you have to recess one plate for the ballnut holder, and the other for the BK/BK bearings....or go another way? What am I missing?

  9. #17
    Quote Originally Posted by Andrewg View Post
    Yes, as you say Joe Harris's videos show simple routing of Aluminum is doable if high precision not required. However looks from video that he actually used 15mm+ spacers under his Z bearings and flat plates.

    Neale, presumably you have to recess one plate for the ballnut holder, and the other for the BK/BK bearings....or go another way? What am I missing?
    Just because Joe did it that way doesn't mean it's a good way. He did it because it was the easiest way, I'm telling you the other way is better from experience. Those building have to make there own minds up which to route to take based on there own abilty's and equipment just like Joe did.!

    This might help with the confusion.

    Click image for larger version. 

Name:	IMAG1037.jpg 
Views:	204 
Size:	302.0 KB 
ID:	27879

  10. The Following User Says Thank You to JAZZCNC For This Useful Post:


  11. #18
    Quote Originally Posted by Andrewg View Post
    Neale, presumably you have to recess one plate for the ballnut holder, and the other for the BK/BK bearings....or go another way? What am I missing?
    As per Jazz's picture, you need a recess in both halves. As I remember (and I can't find a drawing of it to hand) the recess was wider somewhere around the middle of the plate to give space for the ballnut mounting bracket. As ever, make sure that your design allows adjustment to align the ballscrew and nut in two dimensions - well, I needed to as my machining ain't that accurate...

  12. The Following User Says Thank You to Neale For This Useful Post:


  13. #19
    Andrewg - I went through this same thinking, and am in the process of building it now. I'm using the spacer method, it's requiring 15mm spacers with the 20mm hiwin rails to get enough clearance with no pocketing. My thinking is that if I can get the CNC up and running I can then pocket out a new piece if needed, as I'm more likely to mess up the plate routing by hand (and no access to mill anytime soon). I'm offsetting the overhang by using 15mm thick tooling plate instead of 20mm. So saving 10mm but spacing 15mm.
    Mine is untested so far though! - so I'd go with Jazz's and others advice any day if you are happy to pocket them.

  14. #20
    I've been doing a lot of reading about PSU design but there are still a couple of issues I'd like help with.

    Firstly, I've seen some people say that that the current drawn by a stepper is per phase? So if a motor is rated at say 4A per phase its actual requirements might be as high as 8A? Presuming it is of course a 2 phase motor. Whilst I've seen lot's of drivers that can happily supply that amount of current, it does seem to have an enormous impact on PSU requirements, which brings me neatly to my second question...

    I've seen differing percentages quoted regarding exactly how much of that current a driver actually uses, AndyUK talks about it being 60-70% but I've read elsewhere that it can be as high as 80%. Obviously this has a significant impact on the design of the PSU, not just on the actual tranny itself but also on the size of the capacitors needed for the smoothing function.

    So can some kind soul clear this up for me... when calculating motor current draw, do we calculate the A per phase or not, and what is a good % figure to use for actual driver current draw? OR does this last point very much depend on what drivers you actually end up using? So some might use 60% whereas others will take 80%.

    On another note...

    I sunk a big chunk of my CNC budget into one of my other loves and bought one of these babies...

    https://www.brewuk.co.uk/grainfather-g70.html

    It does mean that my hopes for my CNC will be knocked back a bit further but I'm saving a fortune by not going to the pub so the CNC kitty should be back to pre-lockdown levels soon.

    Cheers

Page 2 of 5 FirstFirst 1234 ... LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. BUILD LOG: New Build - For Your Amusement - MK-2 build
    By Karl in forum DIY Router Build Logs
    Replies: 12
    Last Post: 08-02-2017, 08:03 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •