New member and project introduction

[ahorton]

After lurking here for months, I'd like to introduce myself and my project.


I've got a small (hobby) business making high-end head-torches and a large part of my costs come from getting a few aluminium parts machined. Throwing thousands of dollars at machinists every few months is getting a bit tiring, so it seems like a good time to buy the ultimate shed toy; a CNC mill.

Most of the parts I make are about 50x50x15mm and I'm hoping to make about 15 at a time by putting down something like a 400x200x15mm flat bar, milling out one side, flipping it over, and milling out the other side. We're not talking about big production runs here, but being able to produce 15 a week would be excellent.

My budget for the setup is about $4000, so that puts me in the Chinese mill + homebrew CNC category.

The machine I think I'll be using is a RF31 clone that I can get from H&F here in Perth, Australia. The RF45 clone (with its X/Y-stable dovetail and longer Z-travel) was also a temptation, but I think I actually prefer the RF31 for a few reasons:

1. Bigger Motor - 1.5kW & 2080rpm vs 1.1kW & 1600rpm
2. Bigger table - 840x240mm vs 730x210mm
3. Bigger X-travel - 540mm vs 475mm
4. Belt drive (seems a bit more beginner-friendly and I don't expect to be changing speeds often)
5. Cheaper - $1485 vs $1969


I understand that with the round column I'll loose X/Y position everytime I raise it, but I think that for my low profile parts (under 20mm) it shouldn't be a problem. In fact I actually prefer the idea of controlling the quill rather than the whole head. Seems a lot lighter and maybe I can get away with a smaller motor.

As for the CNC gear, I'll probably buy a whole package like kelinginc's NEMA 34 Servo Motors 3 Axis CNC Kit for $1080 and of course a ballscrew kit from someone like linearmotionbearings2008.

Still lots of work and research to be done. I know it probably won't be finished this year, but I'm looking forward to the adventure!

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[109jb]

As for the round column mill, they can work fine, but with the limited Z-axis travel, you will wind up wishing for a dovetail column machine. Consider that the Z-axis travel has to be sufficient for the part height, and the tooling height differences, and for clearance for tool changes. Say you go from a drill in a chuck (long) to an end mill in a very short holder. The Z-axis travel has to soak up the difference in tool height before it can start milling with that end mill. Also, you need height to r&r tools from the spindle. With an R8 toolholder you are going to need several inches clearance to do that. On a dovetail machine it isn't a problem as you just raise the whole head, but a round column you may not have enough room. This can be alleviated to a point with tooling choice. Something like the Tormach TTS system reduces the room needed for tool removal, and you could use a collet system to keep all of the tools close to the same length. It can be made to work fine, just wanted to add some things to consider.

For parts 50x50x15mm, have you considered a BF20/G0704 size machine? Cheaper to CNC, lots of support for them on here, dovetail column, etc.

[691175002]

I have a rf31 CNCed. There are advantages to the belt drive but the quill is so bad that I am converting a g0704 just so I can ditch the round column.

The quill has something like 5" of travel but you really don't want to extend that quill more than 3" or so unless you are taking very light cuts. The quill is like a noodle, and you cannot lock it when running CNC which makes things a thousand times worse.

If you are using endmills and drill bits on the same part you are screwed without getting creative with tooling. I use endmill holders and a 1/4" drill chuck with machine screw length drills.

[SCzEngrgGroup]

"I think that for my low profile parts (under 20mm) it shouldn't be a problem" - You seem to think you can avoid moving the head because your parts are not very tall. The part height has almost nothing to do with it. I don't see that. First, you always need some clearance above the part, to clear bolts, clamps, etc. I find, in general, this amount to at least an inch to an inch and a half. So, with a 1" tall part, you've already used up 2-1/2" of your ~4" quill travel. Now, you'll have small tools, in short holders (think small endmills), and long tools in longer holders (think a drill chuck and drill bit). The length difference between these can EASILY be 4-5". Sometime, you can minimize this using screw-machine drills, but not always (they will BARELY go to 1" depth). Now you need room to get your tools in and out. For an R8 tool, this takes about 4-5" of clearance. For TTS, at least 1.5". To make things worse, the quill fit on these machines is usually poor at best, and the longer the quill is extended, the more slop you'll have, which will reduce precision, ruin surface finish, and be very hard on tools.

Round column mills for CNC are just not a great idea.

Regards,
Ray L.

[ahorton]

Thanks for the comments guys.

I knew I'd need to reposition the X/Y each time I raised the head, but my bigger concern was the rigidity of the quill. When I had a play with one, it didn't appear to be very sturdy, but many others have CNC'd it so I thought it might be ok. Now I hear it described as a 'noodle'!

So that's probably enough to push me back to the HM46 (RF45 clone) from H&F.

The smaller table won't be a big issue and the smaller motor probably won't matter much. I am a little nervous about the gearbox since people seem to have trouble with them. Maybe I'll just need to give it a good clean out regularly and catch any bearing problems as they develop.



The G0704/BF20 seem like a fine mill, but I was hoping to go a bit larger to be able to produce several parts at a time (plus all the other benefits of weight, power and rigidity).

[109jb]

...
The G0704/BF20 seem like a fine mill, but I was hoping to go a bit larger to be able to produce several parts at a time (plus all the other benefits of weight, power and rigidity).

You also have to consider spindle speed. Since most of your work will be aluminum, you need RPM's. Since you are talking about 50mm x 50mm parts, I assume that you are going to need to use small diameter tools which means even higher rpms. Guessing, I don't think 10,000 rpm would be out of line. You can do it with lower, but the higher the better. The bigger machines have more power but less spindle speed. Also, they are more difficult to modify to get more rpm's. Machines like the G0704/BF20 have higher speed spindles to begin with and getting more speed out of them is relatively easy. The RF45 size machine tops out about 1500 rpm where the G0704 gets 2200 rpm and mods to get up to about 5,000 are pretty cheap and easy.

If you do decide to go with the RF45 size machine you may be well off to put an auxiliary spindle on. Some guys make a bracket and attach a router or similar high speed machine to the side of the head that can be used as a high speed spindle. You could use a variable speed router so you can tailor the speed better.

As for table size, With 50 x 50 parts, you should still be able to do several at a time on a G0704 if you fixture them right. Probably 10 to 12 at a time or so.

Don't get me wrong, I eventually want to have a RF45 size machine in my shop. The budget doesn't allow it right now though. So, I just bought a G0704 as an interim machine. That decision was based on acquisition cost, CNC conversion cost, table size, table travels, dovetail column, variable speed drive, etc. It looks like a fairly capable machine. It certainly won't hog like a RF45, but for 50mm x 50mm aluminum parts I don't see that it would need to.

Good luck whichever way you decide to go.

[ahorton]

The more I look at the total parts cost, the more I consider a BF20/G0704 equivalent. It's not too much smaller. Certainly an X2 is too small for my needs.

One of the challenges is finding a BF20/G0704 for a good price in Oz.

As for the auxiliary spindle; it's something I had in the back of my mind. Seems relatively simple to get a good rotary tool and bolt it on the side of the head.

[DRock]

I went with the G0704 and it is quite an amazing machine.

I ended up finding reasons to go from an X1 to an X2 and then when I found out about the G0704 that switch was a no brainer. The column is a lot better and you can increase the work area pretty easily, plus it comes with a stand.