Feed rates and passes
  • Hi All

    I've only just ventured into the world of CNC and I thought I'd construct a Septimus. I only just assembled an Australian version of the X-carve. I have bought a Septimus kit from Dave to show me what me own products should look like; and I also purchased the DXF plans. I have purchased Cut2D based on favourable reviews.

    Questions:
    1. How many tabs and dimensions of tabs suggested for the profiles?
    2. How many passes for 4mm ply cutouts with a 2mm end mill? (I was thinking around 0.7mm??)
    3. What rpm and feed rates (ball park) should I be looking at?

    Many thanks for any advice. I understand this is a bit of an art and I need to experiment and there are many variables but any approximate figures (or tips for new players) would be very much appreciated.

    regards to all

    Pete
  • It really is the case that there is no definitive answer as far as CNC parameters are concerned, there are calculators out there but they seem to me over complicated and of course very much based upon theoretical ideals.

    First and most important factor is the CNC router itself, the more rigid it is the higher you can go with the feed rates, easiest test is to grab hold of your spindle/router and give it a good tug and push, if it moves like a leaf in the wind then you must keep your feed rates very low so that the bit does not push too much against the material causing flex in the machine which will lead to inaccuracy.

    Also your spindle/router plays an important role, does it maintain a constant speed under last, does it run perfectly round ? You do not want to pick a too fast feed rate where the router is not able to power the bit.

    I use these bits for all cog work - Finishing Mill
    And this one for frames etc - Double flute
    I use the same feed rate for both at 20 mm/sec and a spindle speed of about 20,000 rpm.
    I will go up to 3 mm depth on a single pass with the finishing mill and up to 6 mm with the double flute.

    But generally speaking if you are not cutting for commercial reasons then take it nice and easy, lower feed rates and shallower passes. So I would say for 4mm then cut in 3 passes to depth of 5mm ( you obviously need to cut through) at 1.7 mm each pass and pick a leisurely 10 mm/sec and see how that suits your machine, with of course the proviso that you can always step up any or all of the variables.
  • Regarding tabs, I find that you need a minimum of 3 tabs per piece for most objects. Anything bigger that 10cm might need more than 3. The tabs need to be large enough to prevent them from breaking while cutting. A tab size of 3mm long by 2mm tall would be very strong in 10 layer 5mm ply, but might be a bit weak in standard 5 layer Baltic birch. I think Dave uses tabs smaller than 3mm by 2mm.

    With Cut2D, tabs will often leave a small line where the router pauses to raise and lower the bit. Aspire has a feature where the tabs ramp up and back down so the side profile is much cleaner. I am not sure what products have that feature, but I have not seen it in Cut2D desktop. Therefore, assume that you will need to do some sanding for each tab. Try to position the tabs where it is easy to sand. Outside curved surfaces are easier to sand than inside curves (unless you have a spindle sander).

    Steve
  • Thanks Dave and Steve

    A bit of an update:

    Prior to receiving Dave's post above, I tried a feed of 700mm/min, spindle at 15000rpm. Tags were set to 2mm x 2mm - four tags for the test piece. Five passes of about .8mm.

    This was the very first CNC cut I have made, on a newly assembled hobby mill. I was pretty happy with the results. Image attached.

    The tabs were a little weak but allowed complete cut without incident - would beef the up. Also, I believe the depth of cut was too shallow for the average quality of plywood I was using (not Dave's ply!). It was splintering the first laminate if you look closely.

    I tried again with slightly deeper cuts for a 3 pass process and achieved an even better result. Checking the result with vernier cals, and they're what I would call "good enough for guv'mnt work" in terms of circle work.

    I used Cut2D desktop, which was a breeze to use as a novice. Hope to expand the knowledge on for advanced cutting (in time).

    I'm experimenting with different grades of ply - I have a kit in the mail from Dave so I can compare with local products.

    Will keep the community abreast of progress (or maybe lack thereof!).

    Thanks once again for the advice.

    Pete
    first cnc test.jpg
    1280 x 622 - 154K
  • Just a quick update. I have cut all the pieces from plywood using a x-carve equivalent CNC mill. I am reasonably happy with the results considering I have never done any milling before. The end results are not anywhere near as good as the products from Dave, but I think they will work. I found that my end mill got blunt before the end of the 4mm ply job set and so I had splintering. I'm wondering if I'm spinning the tool too much for the feedrate? Never mind - it's a cheap and cheerful carbide bit, so I can afford to replace it. Looked at getting the ones recommended by Dave (which look nice) but six mills would cost 30 euro to get to Australia - ouch.

    I also accidentally overlaid some jobs in the setup and so ruined a couple of pieces. The ply I'm using is not expensive so it's good to make these mistakes now and learn from them.

    See attached for the full glory.
    Pete
    4mm ply cuts.jpg
    4032 x 1960 - 2M