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Okay you have your lump of metal, the internal and external cuts are all coded...
... and I think this is the BIG question...
...How do YOU fix the metal to the bed to get maximum access?
Bit at a time with a vice?
Bolts?
Tabs you saw off afterwards?
Shuffle your clamps half way through?
Glue?
Something obvious that I haven't tried?
Something really sneaky?
I know what I do, but what do you do? :naughty:
Sorry thats classified information.
No it's not, you're just embarrassed because you think everyone else is doing it better than you.Quote:
Originally Posted by Lee Roberts
I know the feeling :heehee:
I always thought it was just magic? Isn't it?
Just weld it.
I suspect that machinists find it easier to talk about their haemorrhoids and catamites than about their hold fasts :rolleyes:
Robin, I think the question is too generic to answer correctly.
I often hold down anyway I can onto sacrificial plates.
I then program the part to cut all the entities but the first few lines drill thru the part on programmed holes with a tapping size drill into the sacrifical plate.
I then stop the program, remove the part, drill the part holes out from tapping to size, tap the holes in the plate and bolt the part back to the plate.
I then rewind the program and block out / delete the first drilling cycle and carry on with the parts. Any subsequent parts are drilled using the first one as a pattern so I know they will fit the sacrificial plate.
Doesn't apply to all parts as it need some holes in the part or holes into places that don't matter.
.
I am also a big fan of bolts, almost invariably cap heads in the M3 to M8 range.Quote:
Originally Posted by John S
Another favorite of mine is clamping a bar to the bed, machine the end, then clamp the free end while parting it off. (Although I do tend to leave a tab, Nearly Headless Nick style, don't want to risk it twisting and gouging).
I have heard stories of thermal release adhesives but don't see them for sale.
No im just embarrassed, im just fat !Quote:
Originally Posted by Robin Hewitt
I dont attully have a working machine up and running to practise hold fasts on yet Robin ;-).
Not long to go tho, got a complete z axis done and some other bits just need to get two more plates made and we should be away....
Hi Lee
I think it's an important subject so to encourage you lot I'm going to post pics of how I do it and show what went wrong. Lots of explanation for the noobs :naughty:
This part was secured with 2 bolts on 3/4" steel standoffs. I used 3/4" because I couldn't find 8mm capheads between 40 and 65mm. A good selection of bolts helps minimise overhangs.
The wonderful things about bolts is you can set the tool to lift above them for the G0's without ridiculous quill extension. In this case 10mm to clear 8mm. No need to watch the G0 paths to make sure they don't hit a clamp.
I didn't dare run it right down in to the standoffs because they would cause the tool to dig in as it cut the extra metal, so I set it shy of the top surface and got a .06mm lip left around the bottom.
If I'd bothered to machine the stand offs so they didn't overhang the botttom I could have set the final cut deeper than the material thickness and lost the lip.
I used Gsimple to produce the G code because it was easy. Gsimple insists on cutting outlines up hill, internals down hill. I didn't notice the tool tip had snapped. See the pic at the bottom and notice the difference in cut quality inside to out. With a missing tip this long tool was bending like a banana and went past final depth on the externals before the finishing pass. Have to make it again :sad:
It never rains but it pours, my camera got on the wrong setting and all the pics came out black, but I did take a brief video that shows the setup. Notice sheets of acrylic in front and rear tee slots to catch the splash.
Bright mild steel, 12mm, 2 flute cutter (1.5 flute really) cutting 2.5mm deep, 2mm/s feed at 335rpm. A tadge slow perhaps but don't you just hate it when the suds gets diverted by a fixture and everything goes red hot in a trice leaving you stuffed mid cut :rolleyes:
Robin
http://www.youtube.com/watch?v=pwyHC-9nd7M
I tend to use the same method as John, drill & tap, although I always do it just before cutting the outer perimeter. I like Johns idea more as it allows for quicker production of multiple items.
But sometimes you don't want holes in the piece your cutting, so then I use tabs but can only do it in artcam, not figured it out with soldiworks yet.
I never seem to use the vice on the cnc mill, but use it all the time on the manual mill.
I also use the rotary table to hold square stock if it does not need tight tolerances and is quite short, just because is so quick to clamp etc.