Proper tools

[richards]

After cutting aluminum with the Shopbot and with a vertical mill (which is totally designed to cut metal), I have to conclude that the Shopbot excels at cutting wood, but metal is best cut with a mill.

Yesterday I did some fairly serious cutting to see what I could expect with a vertical mill. Unfortunately, the mill is non-CNC, so parts cut with it aren't as 'pretty' as cuts made with the Shopbot. However, cutting at 0.25 deep per pass with a 1/2-inch cutter and using 0.450-inch overlaps sure beats making cuts at 0.05 inch depths, using 1/4-inch cutters and being able to only use 0.0075 inch overlaps on the Shopbot.

I guess what I'm really trying to say is that each tool in the shop can do certain things extremely well and other things only with a certain degree of difficulty. Knowing which tool to use to do a job is just as important as knowing how to use a tool. Using the Shopbot to cut 1/2-inch thick aluminum is totally impractical. On the other hand, using the mill to cut wood would be just as impractical.

Sometimes I read something on the forum where someone is trying to make a cut with the Shopbot that is best made on a router table, or with a drill press, or with some other tool. Just because a cut can be made with the Shopbot doesn't mean that it should be made with the shopbot.

[mikejohn]

The other Mike
Although I agree with "Just because a cut can be made with the Shopbot doesn't mean that it should be made with the shopbot." absolutely, there is one occasion when I will use the Shopbot. It's when the shopbot can be quickly set up in a few minutes to carry out some simple cutting that may have taken me an hour or more to do by hand. Put another way, making a one off would be way quicker using another tool, but if I have a lot to cut, set the Shopbot going and I can be doing something else more useful whilst the Shopbot cuts for me.
I need to drill alignment holes and countersunk screw holes in precise positions on many pieces, for which the shopbot is ideal. Whilst it is doing this, I also let the Shopbot lightly cut the guide line for the bandsaw.
The time of this cut is less than the time it used to take to place the pattern on the alignment holes and draw around with a pencil, it takes just 30 seconds.

...........Mike

[knight_toolworks]

actually I mill wood all the time it works really well. if you have to make a through cut crank up the rpms. otehrwise you can use metal cutting speeds.

[Brady Watson]

Absolutely...BUT...If you have no other means to cut the part out, then the ShopBot often times is the ONLY choice for people (myself included until Bridget is online...)

It's nice to know that the ShopBot CAN do things like cut thick AL when you have no other means...Even if it means taking a very long time & cutting is inefficient...You're still gonna get your part when all is said & done


-B

[richards]

Steve,
You're right about being able to mill wood, but I have really hard time knowing just how to fold up a sheet of 48X96 plywood to fit the 33X12 table of my mill. Again, it's not whether it is possible, but whether it is the best tool. And as Brady has pointed out, any tool that can do the job, if it is the only tool that you have, becomes the proper tool by default. However, if I had to make the choice again of whether to spend an entire day cutting a small amount of aluminum and then spending the better part of the next day washing down the Shopbot, walls, floors and ceilings from coolant spray, or whether it would be better to farm out the work to someone who had the proper tool, I would be better off farming out the work.

Often, it's a great feeling to do something new - once - just to prove that it can be done, but when it comes time to make a tool earn its keep, I think that it's better to not ask it to do too much outside the task that it was designed to do best.

The motive for starting this thread is to put a little bit of a caution out to those who saw some images of aluminum that I cut on the Shopbot. Even though those images were posted more than a month ago, I still get daily emails asking about the technique that I used. In almost everyway, cutting that aluminum was worth the experience; but like so many other experiences in life, I paid dearly for the priviledge. I burned up almost $200 worth of cutters before turning to the coolant (even though I was making light cuts with the RPM on the spindle turned down to the knee point on the torque curve). The coolant saved the day, but cost me an extra day to clean the shop before I could cut wood again without worrying about staining the wood from coolant. Since I'm curious, by nature, I enjoyed most of the experience - but I wouldn't recommend it as a 'rite of passage' on the path to becomming a 'real' Shopbotter.

[Brady Watson]

Mike,
Good point...a 4X8 won't fit on my Bridgeport mill either


Question: Why are you using liquid coolant on the ShopBot? I've had great success with dry machining AL cooling it with 25psi of air from the compressor.

-B

[richards]

Brady,
You posted while I was editing my post. Maybe I was doing something wrong, but I tried cutting at 0.25-ips to 0.40-ips with the RPM between 8,000 and 15,000 (finally settling on 12,000, which is the knee part of the torque curve - anything less than that looses torque in porporttion to the RPM) and cutting at a depth of 0.005 inch with a high helix 3/8-inch 2-flute cutter designed for aluminum. (I also tried several Onsrud 2-flute up spiral cutters.)

I tried blasting away with air, but I didn't have my machine jigged up to supply constant air at the cutter and I value my fingers too much to risk getting trapped. Then I tried spritzing the cut with WD-40, which helped a lot on non-pocket cuts. The only thing that worked for me when cutting large pockets (5 X 5 inches, 1/4-inch deep) was to flood the pocket with green tapping fluid that was made for tapping aluminum. When I did that, the cutters stopped burning up. I was able to increase the depth of cut to 0.0075 inches and the speed to 0.40-ips. AND I had a real mess to clean up.

Now compare that to cutting aluminum on the mill. I run at 0.25-ips (15 inches per minute) at 1,600 RPM (top speed without a VFD and 3-phase motor) at 0.25 inch depth with a 1/2-inch 2-flute high-helix HSS cutter. The mill can easily cut to 1/2-inch depth at that speed and with that cutter. It can also be run faster, but I have to count revolutions as I turn the crank by hand (and I'm a slow counter). Those cuts are made totally dry. The chips come off hot, but the cutter stays cool because the chips are removing the heat.

It's really the same situation that I had when I tried to cut 3-inch thick oak on my Sears contractor's table saw. It wasn't designed to do that - even though it could eventually burn its way through the oak. When I sold that saw and bought a Delta Unisaw (cabinet model), cutting thick oak was as easy as cutting plywood.

The Shopbot excels at cutting wood product sheet goods. It can cut plastic with good results (although I've really had to play with cutters and speeds to find the magic spot between melting and chipping the pastic). Many people make a good living cutting nothing but plastic with the Shopbot. It can also cut aluminum, but if I were going to buy a machine to primarily cut aluminum, I would seek expert advice (from experts that make their living from cutting aluminum with a CNC router) before deciding that the Shopbot was the best tool for the job.

[Brady Watson]

Mike,
I was doing between .0625 & .125 stepdown with a standard carbide endmill (even though I bought a HSS hi-helix AL specific cutter) and was running between .8 & 1.2 IPS @ 13,000. Z speeds were reduced on toolpaths that didn't have a ramp into the cut. I used my cobbled together air line & 25 PSI and was cutting .75" thick 6061 AL using both 2D & 3D strategies.

When the air nozzle was blocked by deep pockets, I spritzed quick-evaporating isopropyl alky on it. That REALLY cools things down & won't swell the spoilboard unless you go overboard with it. I found that aiming a hand-held blow gun was totally ineffective. I only added a few drops of WD on the finish passes because it seemed to make a small difference.

The manual mill is better in every way to the ShopBot in terms of rigidity and cutting efficiency on SQUARE parts. As soon as you need to put a radius or cut circular or oval parts, it's efficiency goes way down. I bet you are really good at turning those handles...but nothing beats CNC. It would be nice to have to time to finish getting my Bridgeport CNC running...It moves, but I haven't done any cutting on it yet. It is going to be SB3 powered since I don't feel like learning Mach3...

Question: When you cut AL on your mill, how do you deliver the coolant? When cutting steel, do you use coolant & lubricant or ??? & how do you deliver it?

-B

[richards]

Brady,
You're running 12X to 25X deeper than I was and 2X - 4X faster at about the same RPM. Your chip load is about where I wanted to be - somewhere close to 0.005. However, when I tried faster feed speeds, my cut wouldn't follow a scribed line within 0.020-inch. I am using a 3hp spindle, but the Robb-Jack calculator says that I would only need 1.8 hp using the data that you posted. Anyway, cutting aluminum with my Alpha gave me new respect for the tool. (However, as I posted above, next time I need to cut aluminum parts that require CNC, I'll first talk to the local machine shops, unless I have an extra two days idle time on my Alpha.)

I agree that SQUARE and rectangular parts are easy on a manual mill, but so are CIRCLES and arcs with a $250 rotary table. Mine is the little 8-inch model from Phase II. With it, I can do almost anything manually that I can with CNC controls, but at the cost of lots of clamping and fixturing. CNC is the only practical way to make a mill productive - for anything except the most elementary parts, BUT, I don't want to add CNC to the mill until I can see how the machine performs manually. That way, I won't be expecting the CNCed mill to do more than it can.

I cut aluminum dry on the mill. At 1,600 RPM and 15-ipm feedrate, I'm getting big flakey chips that pull the heat away from the aluminum. I've rigged up a small shopvac near the cutter to pull the chips away from the cutter so that I'm not recutting chips when pocketing. On finish cuts, I brush on some WD-40 with an acid brush before starting the cut. (I buy WD-40 by the gallon instead of buying the more common spray cans.)

I've heard of steel, but after President Buchanan sent Johnson's Army out to keep us rowdy Mormons in line, in the mid 1850's, we're not allowed to buy metals than could be turned into weapons.

Seriously, I don't cut much steel. Other than squaring up pieces that I buy precut just barely oversize and drilling bolt holes for tapping, I don't mess with steel. Those kinds of cuts can easily be handled by spreading on a little cutting oil with an acid brush.

When I do add CNC controls to the mill, I'll have to build an enclosure so that I can use flood coolant. My mill is one that Aaron Moss sells at Industrial Hobbies. His site has a lot of information and photos that would be of interest to anyone thinking about adding CNC to a vertical mill. (And Aaron is a really nice guy. When I dropped in on him unexpectedly a few weeks ago, he dropped everything to give me a demonstration of his mills and the CNC controls that he sells for vertical mills. He's only a few blocks away from Gabe Parl's shop. Gabe is another really nice guy who cuts parts for trailers with his Shopbot. He is also the 'botter who showed me how to combine climb cuts with conventional cuts to get better edge finish without offsetting the tool path.)

[mikejohn]

Surely Rowdy Mormon is an oxymoron?


.........Mike

[Brady Watson]

Thanks Mike. I haven't cut steel on a mill yet, and to be frank, it scares me a bit! (I really like my organs being intact and all...thoughts of sharp steel parts spinning off into my abdomen keeps me VERY respectful of the mill...)


-B