Spindles, bits, and tolerances

[sploo]

Hi all,

I'm not sure that this is quite the right forum, but I've can't think of a better one...

I'm still musing over buying one of K2's KT-3925 CNC routers, but I have a few questions:

1. Getting fixed base routers in the UK is difficult, and because I want to cut shapes from 3/4" MDF and softwoods I was wondering if I could just use a small Dremel type machine as a spindle (small, light, and relatively cheap)?

2. If I want to cut out a shape in 3/4" material, I presume I'll need a cutting bit that's at least 3/4" long. Obviously, cuts would be done in several passes, so the load on the bit shouldn't be high. Can you get suitable bits - i.e. 1" cutting length and 1/8" diameter?

3. The KT-3925 is specced as having 0.004" tolerance on all axes. What does that actually mean? For instance, if I were to cut a series of ever smaller circles (in 3/4" MDF), with slightly chamfered edges, and then stack them together to create a cone, how accurate could I expect the diameters to be (i.e. would I get a smooth external surface up the sides of the cone)?

Any info greatly appreciated!

[ger21]

1) Imo, not enough power.

2) Possibly, but hard to find. Also would be very easy to break, and chips can get packed pretty tight in a 1/8" slot 3/4" deep.

3) I can't tell you exactly what they are specing, but if there is no backlash, you should be able to do what you want no problem. A round nose tool would work best for that.

[sploo]

Not enough power... OK. The problem is that almost all the decent plunge routers here have fancy moulded cases that would be a pain to clamp, even once you've extracted all the plunge componentry. I know I can get some fairly small, cheap (reasonably round body) routers that are around 3/4HP. Would that be enough?

From using routers handheld, and in a table, I know it's harder to drag a wide bit through a workpiece than a thin bit. Of course, you don't tend to use long thin bits as, of course, they'd break.

Do I take it therefore that, on a CNC machine, if you need to cut through a thick piece of material you have to accept you're going to be dragging round a, say, 1/4" bit (thus adding load, and making lots of dust)?

Why would a round nose tool be better for chamfered edges? Is it just that it'll be producing smoother sides than a straight bit?

Thanks.

[ger21]

A round nose tool won't leave the steps. With a large size bit, and the right stepover, you will have a smooth surface with little sanding needed.

Look through Kong's build thread if you can find it, or PM him. He ordered a router from the US I think.

[buscht]

Originally Posted by sploo 1. Getting fixed base routers in the UK is difficult, and because I want to cut shapes from 3/4" MDF and softwoods I was wondering if I could just use a small Dremel type machine as a spindle (small, light, and relatively cheap)?

I agree with Gerry, a small Dremel will not work well. At least use a RotoZip, preferably a 2HP router. The smaller the router the more time it will take to cut.

Originally Posted by sploo 2. If I want to cut out a shape in 3/4" material, I presume I'll need a cutting bit that's at least 3/4" long. Obviously, cuts would be done in several passes, so the load on the bit shouldn't be high. Can you get suitable bits - i.e. 1" cutting length and 1/8" diameter? )?

I think that Rotozip has bits that might work. If you take it slow with lots of shallow passes. You have to have an upcut bit. Personally, I wouldn't use anything smaller than 1/4" diameter, preferably 3/8".

Originally Posted by sploo 3. The KT-3925 is specced as having 0.004" tolerance on all axes. What does that actually mean? For instance, if I were to cut a series of ever smaller circles (in 3/4" MDF), with slightly chamfered edges, and then stack them together to create a cone, how accurate could I expect the diameters to be (i.e. would I get a smooth external surface up the sides of the cone)? Any info greatly appreciated!

I think that the .004" tolerance is referring to a +-.004" tolerance per foot on the ball screw. Your real tolerance would proably be more like +-.010" based upon the screw tolerance, backlash, and any structural flex.

In order to cut a slightly chamfered edge you would need many shallow passes with each pass being slightly larger than the earlier pass. This would make the cone. A bullnose bit would make the steps smoother than a square bit. If you were to stack these pieces up to make a cone, you would probably have to do some sanding to make them perfect. But depending upon your definition of smooth, it might work OK with lots of small step down passes (.008" per pass) and cutting some alignment holes in the parts.

Another option here is to get a tapered router bit to cut the edge. You'd have a pretty good chance of getting a smooth surface stack this way. The problem is that custom tapered bits are relatively expensive.

[sploo]

Gerry,

Many thanks for the info again. The only issue is that I want to cut hollow rings: the inside being less important and therefore probably straight cut, the outside requiring some amount of chamfer or bevel. I then plan to glue them together to create hollow forms which will be painted (i.e. spheres, eggs, cones etc.)

Understanding that I'll need to do a bit of sanding and surface finishing, am I being realistic here?

I'll look up Kong, thanks.


buscht,

I wasn't really engaging brain when I wrote that. What I meant was one of the small (3/4-1hp) routers. There are a few, fairly cheap, examples of these routers in the UK, and they have relatively uncomplicated bodies. The problem is that we don't get fixed base units here, and most of the plunge models have fancy moulded bodies that would be difficult to clamp.

Using a 1/4" bit with plunge router, you are creating one hell of a lot of dust to just cut through something. Do people with CNC routers generally use 1/4" and greater bits for their work then?

I don't mind doing a bit of sanding, but I do want to be able to stack pieces together so that, with a bit of sanding, I could paint it and not see any joins. I can (just about) do this with some MDF panels cuts with shop made jigs and sanded.

Thanks for the info!

[ger21]

Originally Posted by sploo Using a 1/4" bit with plunge router, you are creating one hell of a lot of dust to just cut through something. Do people with CNC routers generally use 1/4" and greater bits for their work then?

It depends on what you're doing. Guys cutting balsa for models use 1/16" bits. If you're cutting 3/4", then yes, use 1/4" minimum. Yes, you'll get lots of dust.

I cut a lot of curved work the way you're talking about, with multiple passes each offset from the previous. I've found with a round nose tool, very little sanding is required. With a square tool, a lot of sanding is required.

[sploo]

Originally Posted by ger21 It depends on what you're doing. Guys cutting balsa for models use 1/16" bits. If you're cutting 3/4", then yes, use 1/4" minimum. Yes, you'll get lots of dust.

OK. I'm no stranger to dust extraction and decent filter masks (I spend most of my free time hacking MDF), so I can sort something out.

Originally Posted by ger21 I cut a lot of curved work the way you're talking about, with multiple passes each offset from the previous. I've found with a round nose tool, very little sanding is required. With a square tool, a lot of sanding is required.

OK. At least what I'm trying to do is feasible then

The only problem I foresee is that I'll be cutting hollow rings. Therefore I want to cut the inside in the most efficient manner (it won't be seen), but get a nice rounded external edge.

I suppose I could use a square bit, and get it to cut most of the way through the insides, then change the bit and mill the external edges. Or would a round nose bit happily dig through the MDF for the internal cuts?

[ger21]

Originally Posted by sploo Or would a round nose bit happily dig through the MDF for the internal cuts?

Not as happily as a straight bit, but somewhat happily. They get dull pretty fast, though.

[sploo]

Originally Posted by ger21 Not as happily as a straight bit, but somewhat happily. They get dull pretty fast, though.

Hmmm... OK . How about this...

I'd obviously try to cut several rings from a single sheet (each would be roughly 1" thick - i.e. the difference between the inside and outside diameters would be 1").

1. Get a sheet of 3/4" MDF, and screw/bolt to table top with two screws/bolts through each (as yet uncut) ring.

2. Use a straight bit to quickly (and fairly roughly) cut the rings.

3. Remove rubbish

4. Replace bit with a round nose bit and use to mill the outsides of the rings.

Would I right in thinking this should be OK, as the workpieces shouldn't move, and I'd be putting less load on the round nose bit when using it to shape the edges?

This does bring me on to something I've not yet sussed out with these machines - does your CAM system 'know' the dimensions of the material it has in front of it? I.e. if you were milling a shape out of a solid block, the controlling software would surely have to know what was solid, so it didn't try driving the bit sideways though 1" of material.

[ger21]

They don't get dull THAT fast.

Yes, the method you describe will work just fine.

Does your CAM system know?? Depends on your CAM system The lower end .dxf converters pretty much do what you tell them to do, they just follow the drawing you give them. Most if not all 3D CAM systems do need to know the stock size. But you should always verify the code they produce "just in case"

I'm not sure if you'll be able to do exactly what you want in a CAM package. It shouldn't be too hard with a .dxf converter, though.

[sploo]

Originally Posted by ger21 They don't get dull THAT fast.

I've worked out that I'll need to make LOTS of rings to make a large sphere, so the linear distance travelled by the bits will be quite high, so I'm not expecting them to last too long.

I seem to recall reading an article about router bit life. Quality cutting was measured in the hundreds of feet. Not much when you think about the distances travelled by these CNC machines' spindles.

Originally Posted by ger21 Yes, the method you describe will work just fine.

Excellent. Thanks.

Originally Posted by ger21 Does your CAM system know?? Depends on your CAM system The lower end .dxf converters pretty much do what you tell them to do, they just follow the drawing you give them. Most if not all 3D CAM systems do need to know the stock size. But you should always verify the code they produce "just in case" I'm not sure if you'll be able to do exactly what you want in a CAM package. It shouldn't be too hard with a .dxf converter, though.

OK. That all sounds logical. I guess it shouldn't be too hard to follow a 'cut lots of rings through this solid sheet' job with a 'you've got a solid ring, now round the edges' run!