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View Full Version : Which end mill to level my acrylic bed?



HankMcSpank
05-11-2011, 09:16 PM
My CNC has an aluminium bed, but it's not completel;y flat...don';t fancy levelling that, so intend bolting some 5mm acrylic sheet onto it.

The largest internal diameter collet my spindle accepts is 6mm....there's a bewildering array of endmill, materials, flutes etc.....could someone recommend one that would be a good candidate to level acrylic?

Thanks.

Jonathan
06-11-2011, 10:55 AM
I'm guessing you've chosen acrylic over MDF as you're intending to use coolant? If not I'd go for MDF as you can screw things into it much more easily and it's cheap.

The 6mm since flute carbide cutters I keep mentioning work well on polycarbonate / nylon so I should think they'll be effective with acrylic. For levelling the bed having clearance for the chips isn't a problem so you may be better off with 2 flutes ... then again the former is much more useful in general. Shame you can't use 1/4" (?) router cutters, as a big one of those would be much faster and probably sufficient.

The little router at school used a white plastic bed, not acrylic...not sure what it was precisely but it was easier to cut and we did level it with 4mm 2fl cutter, which took forever, as that's the biggest the router accepted.

blackburn mark
06-11-2011, 11:06 AM
i bought a cheap fly cutter (ebay) and turned the shank down

http://www.ebay.co.uk/itm/Fly-Cutter-Emco-Unimat-and-Compact-5-Milling-units-/270795943069?pt=LH_DefaultDomain_3&hash=item3f0cb22c9d

this one is a bit pricey but its the same as the one i bought... youll have to have a hunt around, RDG might be better (i ended up putting a dcmt insert holder in mine)

4817

i can skim alli with mine at 60mm (ish) diameter if im carful (my machine is quite rigid though and my spindle will cut at low rpm)
cuts acatal quite well up to 0.5mm without to much trouble

might be worth a ponder :smile:

m_c
06-11-2011, 12:12 PM
I was just going to suggest if a flycutter would work aswell.

I wonder if a disc style with a single vertically mounted cutter may be better for this, as it'll be more balanced?

HankMcSpank
06-11-2011, 01:51 PM
Thanks guys.

i'm using acrylic cos I have a ready free supply of it! (which normally helps sway the argument in my books). Re screwing stuff to it....I just have my cnc pepper it with clamping screw holes.

It's very flat....it doesn't melt (it's cast acrylic not extruded...the latter gums up the cutting bit).

EDIT: Having just watched a couple of youtube videos, woaah fly cutters rock - I want one! Mark what diameter is the business end not the shaft but the end that holds the cutter)...I've a mini lathe collet that holds up to 20mm diameter - & do I need the dcmt insert holder you mention?

My bed is reasonably flat, so when I bolt the acrylic sheet down to it, I'm only going to be skimming something like 0.2mm off it. (also my bed area is small so could go with two runs of 0.1mm)

blackburn mark
06-11-2011, 05:45 PM
i'm using acrylic cos I have a ready free supply of it! (which normally helps sway the argument in my books).

yep.... that'll about do it for me to :)


woaah fly cutters rock

to right they do :)



what diameter is the business end not the shaft but the end that holds the cutter

19.8mm (ish)


do I need the dcmt insert holder you mention?

not at all... thats just me being lazy...just try to put a "small" radius on your tip when your sharpening your tool


I wonder if a disc style with a single vertically mounted cutter may be better for this, as it'll be more balanced?

the theory is good but iv had no problems getting lovely finishes with this one at 1500/2000rpm

Jonathan
06-11-2011, 05:54 PM
the theory is good but iv had no problems getting lovely finishes with this one at 1500/2000rpm

Good luck getting that spindle speed with a normal router!

I've got these:
http://www.ebay.co.uk/itm/RDGTOOLS-3PC-FLYCUTTER-SET-TOOLSTEEL-SUIT-MYFORD-/370497278536?pt=UK_Home_Garden_PowerTools_SM&hash=item56435bd248#ht_1050wt_1270

You're welcome to the smallest one if you want ... the shank is 1/2" but I can probably turn it down if required.
Don't worry about the tip - though carbide would last longer flycutters are dead easy to grind so it's not really an issue. Assuming you have a bench grinder that is.

HankMcSpank
31-01-2012, 07:35 PM
Old thread I know, but I've just got my fly cutter!

(I went for the one blackburn_mark suggested .... http://www.ebay.co.uk/itm/Fly-Cutter-Emco-Unimat-and-Compact-5-Milling-units-/270795943069?pt=LH_DefaultDomain_3&hash=item3f0cb22c9d )

I now need some guidance ....

So the thing is the fly cutter has an 8mm shank, but my modest spindle (200W) only goes up to 6mm collet size. So I used my mini lathe to turn the shaft down, but while doing so the lathe control board blows, to get the remaining 0.5mm off the shank, would you believe I attached a benchtop test psu directly to the lathe's dc motor (needs must!)) ...the PSU only goes up to 60V (vs 180V the motor expects) so the motor turned slower than normal but I got there in the end.

So I slaps the the fly cutter in my CNC spindle & turn it on ...."woah there fella!" ..... Vibration City, Arizona!! (btw: this particular tool scares the life out of me -a sharp bit of metal hurtling around that I can't even see..... god forbid if that cutting tool ever flies out of its holder).

So I backs the revs right off (fortunately my spindle speed goes down to about 800rpm) & sets the thing off cutting. I've got my CNC aluminium bed topped with 5mm acrylic. Sure enough it's well out of true...the right side of the bed is higher than the left & the front of the bed is lower than the rear! (that'll be why I'm getting such crap result wrt pcb milling)

ok, to my question, the fly cutter is only cutting on one side of it travel as the blade spins around from about 1 o' clock to 5 'o' clock (in other words looking at the spindle from the front, the actual fly cutter is only cutting the surface on the right hand side) ....presumably this means my Z axis is not bang perpendicular (the cutting radius is about 48mm) ....is such an assumption correct or could it be something else causing this (fly cutter shaft, spindle shaft etc). Does it even matter - wrt fly cutters - if the cutter doesn't cut equally around it's full travel? Yeah, I know to you guys it'll be obvious...it's only when I hit issues like this - no matter what I mighnt sometimes con myself - that I realise what a totally un-engineering mind I really have.

Jonathan
31-01-2012, 07:56 PM
Which mini lathe do you have? Anything obvious burnt on the driver board, picture? Might be able to help...

For HSS 800rpm sounds about right for that diameter. As you found the fly cutter being inherently imbalanced causes a lot of vibration which will very quickly wear out the spindle bearings at high speed. Even if you did balance the fly cutter it still needs to be operated at low speed as for the speed of the tip of the tool to be correct you have to lower the rpm as the diameter is increased.

What you're describing is the spindle being out of 'tram', so like you say the axis of rotation of the spindle is not orthogonal to the plane defined by the X and Y axis motion. Does it matter? If you're using a large diameter cutter and trying to get a very good finish then yes it does since the cut will be ever so slightly concave. Depends how far out it is...to measure it on a mill you would put a dial indicator in the spindle, facing down to touch the bed, mounted on the end of a reasonably long bar attached radially. Swing the indicator round and use it to measure the height of the bed at 4 points of the circle (12,3,6 and 9 o'clock). By comparing the readings you can find the gradient and thus how far out the spindle is and adjust accordingly (shims etc). That gets the spindle orthogonal to the machine bed. That's great on a mill but it's not a lot of use on the router unless the bed is already surfaced to get it parallel to X and Y. So you need to surface the bed first then indicate off it... but it will never be perfect due to the initial surfacing being slightly uneven due to the problem you're trying to correct! I'm sure it will be plenty near enough though as I bet most people don't bother with this at all. Unless it's out by a lot you only notice it on very large diameter cutters.

Edit: Also if you haven't already then grinding a reasonable radius on the fly cutter tip should improve the finish. Not too great though as it will increase the cutting force.

HankMcSpank
31-01-2012, 08:43 PM
Thanks Jonathan....all good info.


Edit: Also if you haven't already then grinding a reasonable radius on the fly cutter tip should improve the finish. Not too great though as it will increase the cutting force.

I have no idea what to do here...but have a grinder & this interest me....have you got a url/piccy re what you're suggesting?

Web Goblin
01-02-2012, 07:53 AM
Hank,
I might be able to get the board repaired for you if Jonathan cant help you out. Drop me an email if you cant get it sorted.

Ian

HankMcSpank
01-02-2012, 09:35 AM
Hank,
I might be able to get the board repaired for you if Jonathan cant help you out. Drop me an email if you cant get it sorted.

Ian


Hi Ian,

That's useful to know ....alas, at the moment I'm unemployed, so it''ll stay bust until....

1. I find a job
2. I find a schematic & fix it myself!

or more likely....

3. "For sale, one mini lathe in need of repair"

rgds,
Hank.

Web Goblin
01-02-2012, 01:31 PM
Hank,
if you cover the postage costs I'll have a look at it for free. Its always good to have a favour in reserve:naughty:

Ian

HankMcSpank
01-02-2012, 07:37 PM
Hank,
if you cover the postage costs I'll have a look at it for free. Its always good to have a favour in reserve:naughty:

Ian

Hi Ian,

That's very gracious of you ...when it went bang, I went to change the fuse but my lathe's fuse holder disintegrated (so I can't even try it since the large bang) ...let me at least order a replacement 32mm chassis fuse holder up & fit it, dry the control box out (very damp in my workshop) , visually inspect the pcb (it's too parky to go up in my loft & do all this right now!), if still no go, then I'll gladly pay the P&P for you to look at the board....many thanks.


Meanwhile, back to my "getting some FR4 copper board to mill evenly will be the death of me" saga ......so after spending SEVENTEEN QUID on a fly cutter, turning it down to fit in my CNC spindle, then setting it off on a bed levelling exercise...imagine my excitement/anticipationat mill my first 'bed levelled' pcb .....sigh, EPIC FAIL!

I have the V cutter cut depth (final depth) set to about 0.2mm ....I saw perfect isolation as the cutter did it's thing on the left hand side of the board...then alas as the spindle traversed slowly 80mm over to the right...I saw the tell tale sign of the isolation route getting thinner & thinner....then the track disappeared altogether! Of course what this means is that my machine's bed (thankfully acrylic atop of the ali bed & therefore expendable) is about 0.3mm lower on the right hand side of the bed vs the left hand side.

So then the penny dropped (I think)....if my fly cutter is only cutting on one side of it's travel 9the right hand side) ....then I need to take the cutter all the way past my bed's left hand extremity ...and then some, to ensure the fly cutter cuts it ....my bed levelling g-code obviously assumes that the cutter cuts evenly around its full rotation.

So now I have an unlevel bed - the only way I can think to rescue this situation is to use something like 6mm endmill & run that on a levelling routine ....at least the Z plane being out of tram shouldn't impact that too much....once my bed is level with the 6mm endmill, then the fly cutter back on to try & get my Z plane into tram....sound like a plan? (if so, can anyone lend me a 6mm endmill?!!!)

re pcb milling, thankfully, I now have a USB-CNC hardware card on its way to me, which will at last (hopefully) allow me to mill even pcbs with an uneven bed, this just in from the program's author (who I pestered to death to get this implemented).....

http://www.youtube.com/watch?v=0jGY92S8bxM (thankfully, my bed isn't as warped as his demo!)

Web Goblin
01-02-2012, 08:32 PM
Hank,
no problem. Let me know how you get on.

Ian

Jonathan
02-02-2012, 06:21 PM
I was going to offer to have a look at the board too :lol:. Lets see some photos first...


So now I have an unlevel bed - the only way I can think to rescue this situation is to use something like 6mm endmill & run that on a levelling routine ....at least the Z plane being out of tram shouldn't impact that too much....once my bed is level with the 6mm endmill, then the fly cutter back on to try & get my Z plane into tram....sound like a plan? (if so, can anyone lend me a 6mm endmill?!!!)

Sounds sensible to me. I'll post you a 6mm single flute carbide cutter if you like... they've useful for everything anyway.

Theoretically if you're just using the surfacing from the 6mm cutter as a reference to dial in the Z-axis you could just make the 6mm cutter cut a ring to save time. In practice it might be best to surface the whole lot anyway.

HankMcSpank
02-02-2012, 10:47 PM
Sounds sensible to me. I'll post you a 6mm single flute carbide cutter if you like... they've useful for everything anyway.

Many thanks Jonathan...I'll PM you.

Jonathan
03-02-2012, 01:48 PM
Posted ... along with something else I think you'll find useful :wink:

HankMcSpank
03-02-2012, 01:57 PM
Posted ... along with something else I think you'll find useful :wink: ...ooh the anticipation, it's like December all over again!

This is a fantastic forum full of great people who are always willing to help....thanks to you all.