The situation: a large sheet of (thin-ish) plywood only held at the edges which could possibly be lifted by the cutting action of an endmill, especially in the center of the stock.
I am curious if anyone has ever figured out the net force of a compression bit on stock. We know a down-cut bit pushes the stock down and an up-cut mill tends to lift stock up but what does the compression bit do and does it maybe depend on the amount of engagement of the mill.
I could use double sided tape to make the question irrelevant but would prefer to not use tape.
Hi @Jens!
That sounds like a really interesting question. I think the answer is, it depends, on two factors - the engagement of the up and down cut parts of the bit, and the slope of the up and down cut portions.
Giving it an approximation, I would expect the portion of the bit with more engagement to exert more force, if the pitch / slope of the up and down cut portions are equal.
From what I have seen, a typical compression bit is mostly down cut, with only a portion of up cut at the tip, so the stock thickness would be the determining factor. Looking at the 1/4 inch compression bit that Sienci sells, the bit has 4mm of up cut, and 21 mm of down cut. I canāt really tell if the slope (helix angle) is the same for both, but they only list a single angle, so I will make that assumption.
So, if you were cutting 12mm (or 1/2 inch) stock, and cutting to exactly full depth, you would have 4mm of engaged up cut, and 8mm of engaged down cut, netting the equivalent force of 4mm of down cut (so, net downwards force).
Given your stock thickness and how far through the stock you intend to cut (i.e. how much you cut into your wasteboard) I think you could contrive to always have a net downward force.
Does that make sense? Anyone else have any thoughts?
Quick edit to add - during any plunge or spiral in, you would pass through a time of net upwards force. I would rough with all downcut, and only finish with the compression bit, if upwards force is a concern.
Hey Jens,
The tip of a cmp endmill is upcut, so during the plunge it will pull on the material, after that tne downcut part will work against that, as long as you have it cut deeper than the first upcut part.
I have not used any cmp bits on plywood and always stay on the scene, pressing down the ply by hand. Butā¦ I donāt use ply much and am not planning to either. Itā¦ wait for itā¦
Sucks!
@Spamming_Eddie I agree plywood sucks - but sometimes you have to embrace the suck!
I have done a few signs for friends with random veneered plywood, just rolling the dice that the voids / imperfections / etc revealed would be interesting - and did get very lucky that way.
The other option is going for something like Baltic Birch - which is worth it when it suits your uses, if you find the real thing. I do have to say though - if you are using a face veneered ply, the faces are so thin you really do have to be very careful about up cut / down cut, the tear out is abysmal.
Thanks everyone. In my case I am doing cabinets/drawers so voids arenāt a problem and yes, the veneer is less than paper thin. So far I have stayed away from Baltic Birch because of cost but it could be that I get (slightly) thicker veneer and I might try a sheet to see if the increase in cost is worth it.
I had also speculated that as long as the down cut portion that is engaged in the cut is longer than the up cut portion then the net force is downward but I wanted to double check if that was a reasonable assumption. I will have to make sure that the last pass is always sufficiently deep to result in a net down force.
As it turns out, I am not in a position to use a compression bit at the moment ā¦
I am using mostly SPE Tools mills and their 1/8 cutting diameter compression bit has 5 mm of up cut at the tip. The stock I am cutting is 12mm. For neutral cutting forces in the vertical direction I would have to cut 10 mm deep or deeper. At this point in my CNC journey I am limiting myself to a maximum cut depth of the diameter of the mill which in this case would be roughly 3 mm. I will have to grow a lot more hair on my chest before I attempt to cut anything at 4 times the diameter of the endmill ā¦
The speed/feed tables for SPE list speeds at a depth of cut of one diameter and have a note on it to reduce the feed by 50% when cutting at 3 times the tool diameter. There is no mention of being able to cut at 4 times the mill diameter and I donāt know if that is even possible. The bit I was thinking of using has a suggested feed rate of 125 in/min when the DOC is one diameter of the endmill. I would think if I reduce feed speeds too much, I would get into ārubbingā territory which would kill the endmill in short order.
Despite all of this, the mills of extended flute lengths seem to be offered primarily in compression versions.
@Jens can you rough everything using a down cut bit, leaving a significant stock to leave, to be cleaned up after a bit change using the compression bit? That has been the way I have transitioned to doing it.
Donāt rough with an upcut though - that has the force issue, and the tearout issue.
I try to avoid tool changes if at all possible. That is why I currently use a 1/8 bit because it makes dogbone mortise and tenon joints viable (for me). What I really should do is use a 1/4 bit (down cut) and do 90% of the job and then change to a 1/8" bit to deal with the inside corners. Then, as a final step, change to an up cut mill (or compression) and do the last tiny little bit.
To be honest though, that will not be happening until I get a tool changer going. I was hoping that using a 1/8 compression bit would allow me to do everything with a single tool. I donāt care if a particular setup takes me a bit longer because I have to go slower with a smaller mill - I am not in a production environment.
You could also run almost your full job on a 1/8 downcut bit, leaving just a final depth step-over and maybe a light skim for the compression bit. You would get the engagement of the up and down portions of the bit giving you a really nice finishing pass (especially if you are using adaptive clearing with Fusion 360) and pretty much eliminate tear out while requiring only a single bit change.
I do find that with a 1/8 downcut bit I can get minimal tearout against a fresh wasteboard - but that becomes an issue if cutting across old scars in the wasteboard or t-tracks.
What kind of volume are you doing? Iām in the early stage of sorting some cabinet designs for my kitchen, and definitely understand minimizing bit changes. I really like being able to just hit go and have all my drawer runner holes drilled, without needing any bit change.
That is what I currently do (mostly) - I run with a 1/8 down cut bit and things come out pretty much ok after about a half dozen (+) attempts. I am having a heck of a problem with little tiny errors. My latest was not catching āstock to leaveā in one of the operations (fusion360 defaults to enabling stock to leave). Previous to that I had a logic error in my design and before that yet another logic error. This design is under fusion360 and is fully parametric (when everything works).
Unfortunately, even with all my twisting and turning, all 4 sides of a drawer require double sided operations as well as one bit change. Only the drawer bottom is done as a single sided operation. Yes, under ideal circumstances it would be a āpress the button, cut out all the parts and glue them togetherā operation.
I do not have any volume as such. There are many many drawers I am intending to build for my wood shop and my electronics shop. Probably in the neighbourhood of 80 or more mostly very shallow drawers that will contain small hardware in many many Gridfinity boxes. I have built some of these the old fashioned way but truth be told, even with meticulous attention to details, I can not produce the level of precision I wish to have. The CNC router can, at least in theory, produce the accuracy I am looking for.
Commercial cabinets really add up quickly in cost and you canāt always get exactly what you need. My thinking/hoping is that I can do all the cabinets and drawers that I want, pay for the mill, and still have money left over.
@Jens Iām glad to find out what youāre up to! I have been taking a similar approach to putting together a kitchen - the cost of the machine, more expensive materials, and even my mistakes will still be far less than the cost of custom cabinetry.
Iām curious about your design and how it is you require double sided cutting? I have managed (through a few creative rabbets and careful choices about where to locate my machine zeros) to get a parametric cabinet and drawer design that doesnāt need any double sided milling.
My philosophy on it is to build in features that ensure squareness - rabbets and dadoes - that basically ensure the boxes build up squarely and precisely. Additionally, I ensure that Iām only ever working off of reference surfaces with known dimensions - so stock thickness variability doesnāt affect critical dimensions or fit.
I would really enjoy connecting more on this - seems both our projects and our tools have a lot in common!
Rather than clogging up this thread, I sent you a direct message.
Got it, will send you some pictures and details tomorrow.
Nooooooo!! Iām Dutch! I love clogs!
I am more into flip-flops myself. The concept of putting chunks of wood onto my feet seems rather strange. I am however partial to a good windmill ā¦
If you put it like that, yes, it sounds strange. Butā¦ consider thisā¦ clogs are certified for workfloor use. Drop a hammer on your flipflops while in them, and its off to the er. Drop one on your trusted clog, sand out the scratch, kick the hammer into its place, and yer back at it.
Hereās a good breakdown on clogs in general and as a bonus, they show off an amazing design on Dutch chip-chops you can make yourself for cheap. Theyāre a kinda flip-flops so durable, youāll never need another pair again.
Disclaimer: this video is a sketch.
Now this is how you clog up a thread!
I am impressed that the guy managed to keep a straight face during the entire clip!