Which mill for specific job

[fatboy55]

Hello everyone,

I have a HF mini mill and love the thing but its time for a new job that is out of its range.

From 2 inch wide stock, 6160 Alum, I'll need to take off metal thats 1/8 inch deep and almost the full width of the metal, 1 7/8 leaving a 1/8 square edge above the surface at the edge. Thats the easy part, the piece of 1x2 that I'll be milling is 7 feet long. I'll have a few of these to do over the next year and the Co. wants a deadicated machine in house.

I know on any machine I'll need to recirculate the table to do a piece that long so its really about hogging out some metal more so than table travel. A big knee mill will give me 30 inches or so and a benchtop will give 20 or so. Either way it'll take a few runs to finish the 7 feet.

Of course we want the mill to do other things but this is its main job. What do you all think? And what about tooling? What do you think would be the best way to cut this job.

Thanks.

Jeff

[HuFlungDung]

Jeff,
I imagine that you want the part to look nice?

It would be tempting to get a 2" or 2.5" diameter shear hog mill and cut it in one pass. But, you might find that to get a nice finish, you would need to climb mill. So with that in mind, the mill you look for needs to have some anti-backlash nuts, or be in very good condition so that the backlash is quite small, all the way from one end to the other, for the full travel of the table. That could be a tall order to find in a used machine.

Or, you can conventionally rough mill the part almost to finish, then take a final cut in climb mill mode to put a nice shine on the part. A small air blast is almost essential when conventional milling to prevent the chips from being pulled in and welded onto the part, which really roughs it up and looks like hell.

[Runner4404spd]

i would look into a gantry type mill. build it on an 8x4 scale and that way you can do the whole part in one shot. you should also be able to setup multiple parts and program the cnc to do multiple passes on each at one time.

[widgitmaster]

Hello,
Back in the early 80's I had a similar task, and we made a fixture out of mild steel that mounted on a bridgeport table. The fixture would hold the entire length of bar stock, and fed it by the end mill using a seperate lead screw and power feed mechanism!
I realize the fixture is costly, and takes time to make, but it may increase the quality and quantity enough to make it worth some investigation!

Good luck, and keep us informed!
Eric

[Ron111]

Mr. FatBoy55,

Can't advise any thing differently, that the other guys have. But I can bring to your attention that there is one benchtop mill that has about 32 or so inches of X travel, it you choose to do your job in steps. Anyway, Industrial Hobbies make this large tabletop, so maybe that will give you an other option (you can find their link in the advertiser's section).

Ron

[fatboy55]

Originally Posted by Ron111 Mr. FatBoy55, Can't advise any thing differently, that the other guys have. But I can bring to your attention that there is one benchtop mill that has about 32 or so inches of X travel, it you choose to do your job in steps. Anyway, Industrial Hobbies make this large tabletop, so maybe that will give you an other option (you can find their link in the advertiser's section). Ron

Thanks Ron, I'll check that out. I also like the idea of making a seperate longer table for the full lengh of the bar. I need this to look VERY nice. The problem is I don't see that being an easy machine to make due to the stability needed.

Not too much of a difference between 2 or 3 passes. Its really about doing it in a single pass or not. Once you have the multipass technique down for the specific job and machine combo 2, 3 whatever. So with that in mind, I guess theres no reason to go nuts, 3 phase, all that stuff for a knee mill.

No time to build the big table rig right now but maybe later. The job with go on for a year or two with a few dozen of these made per year. With that in mind, I forget about cnc. This is a manual job. The lengh of the bars will change a bit with each made, not a lot but enough to keep them from being identical.

Theres going to be some welding and grinding of the welds involved in this job so I'll have the polisher do the edge too to smooth out any seams in the work where it was reset.

Does anybody have a good way to machine this with the least marks left where the cut was handed off to the next. When I tried this in the past I got a bit of a mark due to a new cut trying to match a cut where the tool had been deflected during cutting. I'm thinking a big insert face mill that won't bend as it cuts??? What do you all think?

Thanks for the help everyone.

Jeff

[widgitmaster]

In order for a 7-foot bar to pass under a cutter, the fixture needs to be 15-feet long! This is easily done with round bar stock and a 15-foot long plate! the carrage would travel on the rails just like one of the DIY CNC tables that everybody on this site has been building! Just longer!

The only problem I see is finding a threaded rod longer than 6-feet!

[widgitmaster]

Actually, I found a 3/8-16 threaded ron thats 12-feet long on McMaster Carr