Mill question

[Amberly Brown]

Hi all,

 I know nothing about mills or milling. What does a mill do that is cool for maker spaces? What is the benefit of a bridgeport over other brands? Is newer better (ie: a 1986 computer is a paperweight) or is older name brand better  (a 1986 craftsman drill press could be good quality) ?

I've seen a mill at TechShop, but I don't understand the appeal. Help me out?

Thanks!

Amberly

[Myles Farrell]

So milling machines are used primarily for machining planer surfaces, precision drilling holes, cutting profiles on parts. Common uses would involve making flat mating surfaces of parts, drilling hole patterns where the coordination between holes is vital, and machining pockets into larger stock. Basically any machining operation not done on a lathe would be done on a milling machine.

http://zyci.com/wp-content/uploads/2015/09/BE-Aerospace.jpg

Pretty much all the operations used for that part would be done on a mill. This one was obviously done on a CNC mill as the curves would be insanely had to do on a manual machine, but the concept is the same.

Bridgeport J-2 vertical mills have built an incredible and well earned reputation for quality and durability. The motors are strong and they are built like tanks, so they are forgiving of newbies and last forever. To the point where "Bridgeport" has basically become synonymous will all vertical mills and is used interchangeably. Kind of like most people use "Vise grips"  instead of locking pliers. Case in point most of the ones out there that are in our price range could be 30-60 years old. Depending on how they have been used and maintained some of these could be perfectly serviceable.

All that being said there are some other brands that make decent machines. Enco is another big brand that I have used that was perfectly adequate, even if they only seem to last 30-40 years under the same abuse. I have read Jet's new stuff is good as well, which is what I believe Techshop has. I have never heard of KBC before and from the pictures it looks like a lighter weight machine. However this probably won't be seeing the continuous use it would in a production shop and probably not for 40 years either so it probably doesn't need to be as beefy. Plus the price is quite good especially with the digital read outs and all the associated tooling.

Newer is usually better with tools, but new ones are way out of our price range at 7-10k. Since they are built like tanks used ones will be fine for our needs. The previous owner might be replacing it because they are a production shop and need to replace it because it can't hold the .001" tolerance they require, but we do not. Or maybe they are upgrading to a CNC machine. Two common issues with buying a used mill are the spindle bearings and the ways. The spindle bearings see very high loading and sometimes wear out. This is why you shouldn't used a drill press for milling as the bearings won't withstand  the lateral forces. Checking the bearings when buying a machine is important, but if they are shot you can replace them for about $800 bucks on a bridgeport and you just haggle that cost off the price. The real killer of these machines is wear on the ways. The ways are the tracks that the tables slide on and over the years these will get worn spots on them. Usually in the middle from people always setting up their work there. This can be an issue if you are doing a long part the table will actually sink in the middle and rise at the ends giving you a slightly dished surface. Worn ways is basically the death nail of the machine as there is no way to replace them. However, it all depends on you definition of "worn" and we have mitigating factors. We will probably not need as high as tolerance as most commercial shops and we mostly like will not be doing long parts. If the ways have a .005"-.010" run out to them over the course of the ways, does that matter to our members who are use to making parts by hand with the calibrated eyeball? Or if the run out is even more drastic and has a .050" dish over the course of it's 36" ways, that scaled down to the 6-8" parts most of us would be doing would be only a couple thousands. Either way checking the ways is stupid simple and would be done before we bought any mill.

Anyway there is the small book and and intro into milling machines.

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[Morgan Miransky]

What Myles said.   Short version is that it is like a 3d printer in reverse. Same code,  but uses a drill bit to cut your shape out of a block instead of using plastic filament to build it up.

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[Amberly Brown]

Thank you, that explanation makes very good sense. I've been bugging Mike to explain this stuff to me, and I think he was starting to wonder if I had lost my mind :). Physics and geometry was my weakest subject, so figuring out why we needed this machine for the space was hard for me to wrap my head around. I like the idea of a Bridgeport if it is truly "built like a tank". If we don't get this (off brand) one off of craigslist, is there a formal plan for getting a mill?

Amberly

P.S. It's been a few years (ok, more like a decade) since I did material donation requests for a non-profit, but i have done it in the past. Running/working with a committee that collects the donations from members, reaches out to bridgport or to machining companies for a matching donation/in kind donation, etc is something I can do, if we go that route

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[Myles Farrell]

Yeah, bridgeport became synonymous for a reason and they are good machines. Down side is they usually cost more. Even used you are looking at 2.5-3k without tooling (very expensive itself). Also while built like a tank is nice, there is one downside, the weight of a tank. A bridgeport is usually in the neighborhood of 2200 lbs. It will be a fun move day.

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[EschewObfuscation]

Think of it as a drill press that's so beefy it can drag a spinning bit sideways through metal, and you have the concept. If you have a decent mill and lathe, you have the core of a machine shop, and can build anything possible through WW2 tech (though extra operator skill and patience may be required to compensate for the lack of computer assistance). Add some silicon intelligence, and you can build pretty much anything big enough to be seen by the naked eye, if the materials aren't too exotic.

Machinists will tell you the american built machines of the 50's and 60's are pretty much the most solid and durable ones ever built, though some of the euro companies have done as well, at vastly higher prices. (Taiwan is a small step down from that level, china a big step down from there, measured both by solidity and precision.) Solid is important in an organization that will let students or incompetents use the equipment, as it *will* be abused. Machines "just strong enough" will take far more damage from an operator blunder than will a "built like a tank" US industrial model. Vintage Bridgeport mills and South Bend, Monarch, LeBlond etc lathes etc are built on massive cast iron bases that are more stable and goof proof than lighter castings or piece-assembled frames you will see on modern asian machines.

The US probably now has 10-20% the number of machine tools it did 30 years ago; vast numbers were either shipped to china during the industrial exodus, or simply scrapped and melted down by liquidators uninterested in looking for a buyer for "that old thing". The exodus/liquidation is pretty much complete now, and what companies still survive have largely switched over from manually operated machines to CNC, so the heyday of "hey, honey, look what followed me home!" is over and we're now back to more steady state. Which means machines still appear at intervals, but that's not as frequent as it was a few years ago.

Used Bridgeports, if you don't get a sweetheart deal, currently run around $2500 in average condition. Even worn ones can be rebuilt, the toughest part consisting of a hand scraping procedure to straighten up the long guide surfaces, called "ways". That operation costs essentially nothing in materials, but a lot if you have to pay skilled labor by the hour to do it for you. (Scrapers charge around $200 per hour for their time, as they don't generate anything like 40 hours billable a week. If you have to pay one to fly to your city for the job... ouch.)

Given those prices, you can see why the light weight asian machines are popular with hobbyists; can't fault people for that. OTOH, if you look around at the high competence hackerspaces, you will see... Bridgeports. For the other reasons listed above. Which is a reason we would be well advised, if we're interested in a mill, to put together a fund, either pre-funded or at least pre-pledged, so we can move quickly when an opportunity arises, and arrive at a consensus whether we're willing to go the extra mile for classic US machinery, or go the asian/hobbyist route. You could make an argument for either decision, given limited budget.

[Jim S]

As a point of reference I was looking at a Grizzly G0704 for home shop but various things have put that off - time, space for it, other higher priorities...  But this is the machine:

http://www.grizzly.com/products/Drill-Mill-with-Stand/G0704?utm_campaign=zPage

One of the reasons for looking at this machine was that it was much more easily moved than a Bridgeport.  But the down side is that the work area is much smaller.  And it's not US made and less precise and less "like a tank".  Still good for milling steel and such.

The G0704 is a popular mill to convert to CNC.  Basic machine is $1225.  It would take about that same amount to add CNC with ball screws - that would cover the basic parts: step motors, ball screws and perhaps motor drivers but not the PC and some other odds and ends.  And tooling can easily be as much as the machine - cutters, vise, etc, etc.

I ended up getting a small desktop CNC mill off Craigslist that looked a little rough but was in decent condition (made by Servo Products in the 1980's).  I can mill steel but only small pieces...  A good start but bigger would be nice.

[EschewObfuscation]

Beggars can't be choosers, ofc, but I'll toss out a datum that may not be obvious: There is a difference between a 1 phase motor and an apparently equivalent 3 phase motor plus phase controller. If we do the trig and add the torque contribution of each of 3 phases, we find the torque remains constant throughout the full 360 degree cycle. So a 3ph with converter doesn't display the 120 Hz pulsing torque a 1ph motor does. One could argue that's a minor thing, and people do get by with 1ph machines, but it contributes to smoothness of operation, better precision, and less stress over time on all the various bits in the mechanical chain from floor up through tool contact point. Just tossing that out as an educational note...