[Levant]

I've been looking for the right mill to buy to do a CNC conversion. It seems the Grizzly 0704 is the most popular for home shops with the RF45 a close second. Based on the success folks have had building the things they want to build with those mills, I'm sure I'd be happy with either. But what I would like to know is why not buy an older but much heavier and more powerful used mill from eBay for about the same money or not too much more. Can they be converted for CNC use? Or an old non-working dedicated CNC machine, could I just rip out the electronics and use the motors with new electronics?

What are the risks of buying an older or heavily used mill on eBay? Would wear and tear result in backlash that would make the machine inaccurate? Can you replace bearings and other critical alignment parts to restore or maintain the accuracy on an older mill like that?

Many of the listings say they were pulled from service working but there's no guarantee.

As I look at what it would take to rebuild an older mill like that I imagine I might want a mill, perhaps the 0704, just to have for using on the restoration of an older, larger, mill.

I'd really appreciate the thoughts and any advice from others who have done conversions and restorations on older mills or just from those of you who have more experience and thoughts on the topic.

Thanks.

Dale

[keebler303]

I'd not recommend converting a machine that was not designed for CNC. The parts just aren't suitable.

I'd not recommend converting a big old industrial beast. Even mechanical parts can be quite expensive and things are heavy and hard to work on alone.

My recommendation would be to buy an old CNC knee mill and then freshen up the electronics with something you find suitable. Something like this (I have a similar one): Bridgeport Series 1 CNC Machine Sold as Is for Parts with Boss Control | eBay

It is a "real CNC machine", but not so huge that you can't handle most of the parts yourself. It already has ballscrews, accomodations for motor mounts (you can probably reuse the motors with new drives), parts are available (H&W machine repair), etc.

Good Luck
Matt

[whateg01]

A friend of mine converted an IH, RF45 clone, to CNC. There were some hurdles, and every now and then, he still finds something strange happening, but it has been serving him fine for several years now. He's currently building a belt drive, so he can have better spindle control for rigid tapping, and the like. He's also going after faster spindle speeds than the gearbox will allow. So, IMHO, it depends on how much you want to work on it and how much you want to spend, vs. write a check and make parts.

Hope that gives you something to think about.

Dave

I might add that he did convert to ballscrews on all axes, running servos and Mach3. He made his own limit switches and other brackets.

[Levant]

Thanks for the feedback. That style of Bridgeport seems like a great candidate. I still would like to hear about any experiences in making or finding all the right parts to make it accurate. I assume bearings wear and screws wear and things get loose. I suppose with the right tools I could make my own replacements but without a highend machine could I make them accurate enough?

[keebler303]

Thanks for the feedback. That style of Bridgeport seems like a great candidate. I still would like to hear about any experiences in making or finding all the right parts to make it accurate. I assume bearings wear and screws wear and things get loose. I suppose with the right tools I could make my own replacements but without a highend machine could I make them accurate enough?

Bearings are pricey. I've replaced the Z ballnut bearings. List price was about $700 for 2 bearings. I got them off of ebay brand new for $150 or something. Ballscrews can be reworked for a few hundred bucks. A big part of it is buying a machine that has all the major parts in good shape. A $2k "steal of a deal" may quickly become a $10k boat anchor after you find out about all the parts that need replaced. Even if you do all that, you still have a 30 year old machine.

[Bob La Londe]

I think it more boils down to what you plan to do with the machine. If you need to do heavy hogging out on steel all day long a big heavy mill is the ticket, but if you plan to do small cutter detail work in aluminum a smaller mill with a very high speed spindle will eat the big machine's lunch.

I know for which speak. I've got a Hurco KMB1 4HP (motor is actually 5HP) 3500 RPM machine, and a Taig with a 28000 rpm spindle. Those are about as far apart as you can get in mills, and yet they both do things well that the other one doesn't do very well.

... And then there is cost and time. I had my Taig up and running and cutting parts in a couple days. Mostly getting it figured out. It took me almost three years to retrofit the Hurco.

[Levant]

None of the things I imagine I would use a mill for are aluminum. Gunsmithing and large scale trains both include a lot of steel. But I have a day job so my mill would only be run, not just when I'm off work but when I am off work and not too tired from work and not too busy taking care of chickens, ducks, geese, and pigs..

I guess my point is that I really don't expect to wear one out; I just want one that will make really accurate parts in relatively hard steel. Do you think I'm over-analyzing? Should I just get the G0704?

[Bob La Londe]

The 0704 will do it, but I think if you have the budget an RF45 class machine would make you happier. Either will do fair manual milling. Its up to you where you go from there. I think off the shelf a Novakon Torus or Tormach 1100 is a nice mid morning price CNC if you just want to CNC parts without doing your own conversion. Not sure what type of gun smithing you want to do, but I am pretty sure AR lowers are 7075 aluminum.

[Levant]

I might do an AR lower but am more interested in 1911 frames and slides and in tuning 1911s. But the first piece I would do is a new carrier for my Puma 92 in .357. The Winchester part is a piece of beauty but the Puma part is a cruddy piece of MIM. But my biggest reason for looking into a mill is to do live steam railroading. My second biggest is just to be able to make whatever it is that needs making. I'm constantly needing a part I can't buy and wishing I could just make something.

The three things that made me question about getting a larger older machine are: 1. Weight for rock solid stability. 2. Horsepower - but I noticed some of the big older machines are 1 to 2 horsepower even though they have 408 3-phase motors. And 3. Spindle speed. Is it true that higher spindle speeds are required for good clean cuts? Is it as important on steel as on aluminum? I know that with my hand tools I cut harder materials slower, not faster. But the RF45 has a top spindle speed of about 1500 RPM. Isn't faster better?

I've looked at the ready-made CNC machines in the home/hobby/gunsmith class of machine and just can't see myself spending the money. I usually don't buy stuff ready to go, I buy tools so I can make stuff. But this would be my first foray into buying a tool to make a tool so I can make stuff... But I'm pretty sure I'll have to assemble my own CNC in order to be happy. It's the only way I know it's done how I want it. Usually that works out for me pretty well. Sometimes I get in over my head and end up with expensive piles of junk...

Besides, I would like to at least get some proficiency manually (if a power mill can truly be called manual) and then turn that knowledge into programming CNC. My dreams of putting a block of steel on the mill table, clicking a button, and taking off a completed 1911 frame have long been dashed by reading reality on this forum .

[keebler303]

The heavier the better in a machine, almost without exception. The one exception is when you have to move it.
2 HP is a lot of power, you can do some serious work with that. You need a big face mill pushing pretty hard to exceed it.
Here is some info on RPM: Milling: Cutting Speeds & RPM Calculations
You need more speed in aluminum than steel and more speed with small cutters than larger ones. In steel you will rarely be speed limited with a max of 3-5k RPM.
A spindle with a higher speed can do different things. A 2HP 20k RPM spindle cannot run a 3" face mill in steel. A 2HP 3k spindle will run that face mill like a champ and will still handle the small tools in aluminum, it will just be much slower going due to the slower spindle speed.

Matt

[Bob La Londe]

The heavier the better in a machine, almost without exception. The one exception is when you have to move it.
2 HP is a lot of power, you can do some serious work with that. You need a big face mill pushing pretty hard to exceed it.
Here is some info on RPM: Milling: Cutting Speeds & RPM Calculations
You need more speed in aluminum than steel and more speed with small cutters than larger ones. In steel you will rarely be speed limited with a max of 3-5k RPM.
A spindle with a higher speed can do different things. A 2HP 20k RPM spindle cannot run a 3" face mill in steel. A 2HP 3k spindle will run that face mill like a champ and will still handle the small tools in aluminum, it will just be much slower going due to the slower spindle speed.

Matt

What he said. I would more emphasis to the differences though.