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Awesome New Mill- Is it worth it?

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by JetMech, Jun 12, 2017.

  1. JetMech

    JetMech United States Active Member Active Member

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    Hi guys,

    Another post in the stream of noobs asking "which machine?" I have an introduction post here if you wnat to know more about my situation.

    In this case, I have a pretty narrow question:

    I have an opportunity to buy a mill, very similar to a G0704, for example. My plan is to use it for small hobby work, primarily in aluminum.


    Issue is, I have a tough choice:

    I can get the variable speed model, with speeds from 40-3000 RPM, or I can get the non-variable version, with 6 speeds from 115 to 1700 RPM.

    If it was a matter of a small premium, then it's a no-brainer and you get the variable speed!

    However, there is a catch-

    I can get a discount on the 6-speed. A big, big discount. Think 2/3 price.

    So, my question is, if I'm working aluminum with small bits, am I goin to miss the higher speed capapbility?

    Is 115 to 1700 rpm sufficient range for a mill?

    What price-premium would you put on the variable speed and wider range?

    What capability, if any, do I lose from choosing the much cheaper 6 speed?

    Any thoughts appreciated.
     
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  2. higgite

    higgite General Manger - Proofreading Dept. Active Member

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    Doesn't matter what price-premium anybody else would put on it. You can obviously afford either one or we wouldn't be having this conversation. IMHO, get whichever one you want or you will regret it later.

    Tom
     
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  3. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    Don't know how much torque it would deliver, but I like the lower speed even more than the higher speed.
     
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  4. Randall Marx

    Randall Marx United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Not only do you loose the higher range in speeds, but also the lower range. The lower speeds is what I would miss, but I often need to put somewhat large holes in steel without burning up my twist drills. Using small bits in aluminum, you might miss the high end of the range more. Look at the speed-feed charts for the materials you want to cut and the size of cutters you plan to use, then consider how close you can come to the desired surface speed with the ranges you might have. Another option would be to get the 6-speed model and add a VFD and 3-phase motor to gain a much larger speed range than the variable speed model. Depending upon the price difference, the upgrade may or may not make sense.
     
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  5. Reeltor

    Reeltor United States Active User Active Member

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    I don't know how to advise you but my mill has speeds from 50 to 1,400. There were times when I would have liked to go even slower than 50 rpm but I don't recall ever regretting not being able to go faster than 1,400 rpm
     
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  6. JetMech

    JetMech United States Active Member Active Member

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    Good advice, thanks.
     
  7. JetMech

    JetMech United States Active Member Active Member

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    Cool, one vote for low speeds being desirable- very good, this is the kind of info that helps!
     
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  8. JetMech

    JetMech United States Active Member Active Member

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    Awesome, another vote for low speeds and a wonderful explanation and recommendation besides. I appreciate the idea to look up feeds/speeds of anticipated cuts, will do so. Some definite info about what I should expect would come of that. Thank you, Sir.
     
  9. Eddyde

    Eddyde Active User H-M Supporter-Premium

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    If you are going to be milling aluminum with smaller tools, I recommend you get the variable speed as you will want the higher rpm.
    This hobby always costs more...
     
  10. mikey

    mikey Active User H-M Supporter-Premium

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    The way to look at speed is to consider the material and cutter size and then look at the recommended cutting speeds for that material. For example, aluminum is rather soft and requires pretty high cutting speeds. A 1/4" finishing end mill taking a shallow profiling cut of 0.02" deep would have a cutting speed of 800 sfm. This translates to a speed of about 12,000 rpm.

    To make this useful, remember that you calculate the rpm required by finding the listed cutting speed of the material and using this formula: RPM = SFM X 3.82 / D, where SFM is the recommended cutting speed for the material in sfm and D = diameter of the cutter. You can look up cutting speed charts on the net.

    You will need lower speeds when cutting hard materials with larger diameter cutters. If you plan to do smaller work primarily in aluminum then lower speeds is not as important as the higher speed range. This is especially true if you ever use carbide tooling that requires higher speeds to cut well.

    Personally, I would go for the variable speed with the higher range.
     
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  11. T Bredehoft

    T Bredehoft Active User H-M Supporter-Premium

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    I agree with Eddyde, above, I do a lot with aluminum, I miss being able to tap below 200 rpm, I can and do mill above 2200, 1/8 four flute, feeding at 8 inches a minute. I lube with rubbing alcohol, it lubes the flutes and leaves no residue.
     
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  12. brino

    brino Active User H-M Supporter-Premium

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    What a great discussion and variety of opinions!
    I am glad the question was asked.

    Thanks all!
    -brino
     
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  13. JetMech

    JetMech United States Active Member Active Member

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    Well, it's clear that the low end is important and is more important than high end.

    I appreciate the responses, all of you.

    Since I posted this, I talked with our machinist at work, and he basically said the same and described feeds and speeds of various types of work. He made it sound like it would not be a particular challenge to work within the 6 speeds offered. This doesn't replace me looking it up and learning feeds/speeds myself, however. . . And he operates a full-size knee mill, so imay need to take his advice with a grain of salt.

    What occurs to me now, though, is that the real answer may be to buy the non-variable mill and convert it, thus, for the same or possibly less money, having the same or possibly better variable speed setup.
     
  14. JetMech

    JetMech United States Active Member Active Member

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    Right, I'm seeing the high end is important also- it's clear there is a good case for the variable speed. I agree with those who recommend the variable speed and wider range, that is clearly preferable.

    My issue now is that I can get the regular mill much, much cheaper and maybe should just convert that myself. But do I want the project?
     
  15. JetMech

    JetMech United States Active Member Active Member

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    What an awesomely insightful post, thank you.

    I admit I hoped you'd made a mistake with your math- 12,000 rpm is well outside the range of either the normal or variable speed mill, which tops out at 3000 rpm!

    I checked an online calculator and indeed, to get 800 sfm out of 1/4 diameter you'd need that rpm. So what now? Use larger mills for finishing? Feed slower?

    This continues to push me towards getting the regular mill and doing my own conversion, so I could perhaps get more speed out of it. But then, I have to ask- how can I determine how fast I can run the gearbox? How would one determine a safe rpm limit for such a thing? Even if I had the motor and drive for it, could the gearbox take 12,000 rpm?

    Questions, Questions. . .
     
  16. JetMech

    JetMech United States Active Member Active Member

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    Cool, thanks for the input. It's clear that even if I can't get 12,000 rpm, it would still be much better to have even 3,000 at my disposal.

    Good tip with the rubbing alcohol, will be sure to try that. Hear what you're saying on the tapping thing.
     
  17. JetMech

    JetMech United States Active Member Active Member

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    These guys are amazing! I'm in the final stages of settling my deal with the vendor, and when it goes down I'll be sure to post details.

    I'm leaning towards getting the cheaper mill and doing my own conversion. Talks with the vendor have indicated that the variable speed model is merely the same model of mill exactly, just with the motor swapped for what I'd guess is DC with a controller, but I suppose could be AC with a VFD, but I doubt it.


    I can get the non-variable mill for around $500 less, and normally the difference between the two is $150.

    So, that implies that the parts to convert cost the vendor about $150 or so. With a $500 discount, I could probably upgrade it myself with possibly a better motor and drive, and get better results. I've done that sort of conversion before.

    Ahh, decisions, decisions.
     
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  18. brino

    brino Active User H-M Supporter-Premium

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    That's probably true.

    If you haven't see the conversions done using DC treadmill motors, dig around here for some.
    I bet you could find a suitable treadmill for the price of hauling it away in your local Craig's List ads.
    Though you may need to replace the controller.

    Please do share what ever way you go!
    -brino
     
  19. JetMech

    JetMech United States Active Member Active Member

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    I have done a few treadmill conversions, it works great. For this size motor and my desired results, I'd probably buy the components but the cool thing is that i have some experience with it now from my treadmills and know what to get.

    Right now I'm researching whether a 3 phase motor and VFD might be the better choice. You could find success either way I'd bet.

    I expect if my mill purchase goes through, I will check out the motor, observe the ratings, and then shop from there, to see if DC/Controller or AC/VFD is the way to go.
     
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  20. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    Variable speed over a wide range is a wonderful thing, but the dirty little secret is that a wide RPM range by itself is not always so useful. There also needs to be substantial torque at the lower speeds, and indeed good torque across the entire range of speeds, and there needs to be adequate motor cooling, and the motor, spindle, and all the drive components need to not fly apart or otherwise fail at higher speeds. A lot of amateur and plenty of commercial variable speed setups do not pass all those tests, and some suffer from mechanical and/or electronic problems and failures. Make sure to do your homework. Find a bunch of favorable reviews of any system you are contemplating going with, especially long term. Reviews given the day of first motor run are not all that useful...
     
    Last edited: Jun 13, 2017
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  21. mudnducs

    mudnducs United States Swarf Registered Member

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    As Eddy said...small parts...in aluminum mean small sharp tools. You'll want the fastest speed you can get.
     
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  22. mikey

    mikey Active User H-M Supporter-Premium

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    JetMech, cutting metal is not just about the speeds. It is also about feeds and depths of cut, and they are intimately related to each other. For any given material, there is an ideal set of cutting conditions (feed, speed and depth of cut are known as your cutting conditions) and how well the cut proceeds depends on how close you get to this ideal. The finer your control over your cutting conditions, the easier it is to dial in a cut. Being able to do this on the fly is not a small thing.

    The benefit of variable speed is not just the range; its the instant and near infinite control you have over that speed - on the fly. Let's say you've set your depth of cut and your mill has variable power feed. You estimate the speed you need and start the cut. As it cuts, you will see/hear/feel how the cut is going and, over time, you will learn to adjust the speed of your spindle and power feed to obtain the best results for the depth of cut you set.

    The ability to control speed and feed rates on the fly is most beneficial on the lathe, where you can literally dial in a cut. On the mill, it is almost as useful.

    Most mills that come with variable speed will have a useful torque range in the speeds the maker lists. At lower speeds, the mill may not be as powerful but you can compensate for this with a smaller depth of cut and slower feed. But you cannot make up for the lack of speed at the top end and that is where you will spend most of your time if you really do stick to smaller cutters in softer materials. The top speed you have is what you have; you learn to cut at that speed and adjust your depth of cut and feed to compensate to the degree possible.

    Cutting metal is a complicated business, I think. Cutting conditions matter but there are many other variables we haven't even touched on here that have an impact on which mill you choose. In general, it is wise to buy the biggest, heaviest mill that will fit your budget and space and when possible, get variable speeds and feeds. Look hard at how robust the spindle and bearing support is because this has a big impact on accuracy. And if at all possible, get a knee mill.
     
  23. JetMech

    JetMech United States Active Member Active Member

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    Having trouble making the multi-quote work right on this forum . . .

    All good advice, certainly. The plan formulating in my mind now is to buy the discount mill and later upgrade it with better stuff than what they put on the factory conversion.

    I have a really, really nice Pacific Scientific motor that came out of a high-dollar gym treadmill, it's a beast. My plan was to mate that with a high-quality DC drive such as a KB pwm type drive. Those run around 150 for a cased model, closer to a bill for just the board and I can add the swtiches, potentiometer, and enclosure.

    That would get me variable speed with conceivably a better controller, more replaceable certainly, and only the cost of the controller in it. If I don't like it, I can always put the original AC motor back on . . .

    Basically, I'd rather have a larger mill that struggles on the tiniest stuff than a little mill that's got high RPM but essentially no ability to work anything of reasonable size. If it's really that small I can hand finish it.

    Also, from the figures above and looking at calculators, I can get the SFM I need on aluminum at 3000 rpms by using a 1" end mill, which is doable. So if I want a nice finish on the edge of some 1/4" aluminum plate, that's my process. On small interior cutouts, I'll either need to find more speed, live with the finish, or hand-finish it, Clickspring-style. Well, I'll never be him, but I can use a file and sandpaper!

    I am totally getting that, from all the great advice here!

    Like I said above, I can get most of the "good finish" I'm going to need with workarounds, but my real plan is to get the mill, make some measurements and observations of it's power and speeds. (For example, if the motor is a 1750 RPM job, then I'll need to hook the motor I have up to a controller and see what it really runs at RPM wise, and see if it is a suitable alternative. If not, I'll find one that does.

    In the mean time, I will try to figure out what the highest practical speed for that gearbox is. I mean, I can replicate the specs from the factory version, but I bet if the gearbox can take more speed, I could find a power train that offers more top end, since apparently I'll need it.

    As Bob rightly states above, I'll need to resaerch carefully to make sure I can get useable torque at all speeds while covering my range.

    The truth is, from what I've seen those KB motor controllers do while messing with them, that low-end torque is very unlikely to be a problem for a DC motor witha good controller. I am awaree that some considerations need be made, for example, an auxiliary cooling fan must be employed when running these slowly under load, because the internal fan isn't moving enough air at that slow speed.

    Now, if I could get the sales guys to respond to me. . . This is just my opinion, but when an email conversation leads to the customer saying "ok I'll take it, please send me the details for payment. . ." I would make that email a priority, instead of suddenly ceasing to respond once I'm ready to buy. I've heard good things about this company, but thier sales service is pretty lacking. Never answer the phone their either, and I guess I'd say that would hurt business, except that they are basically always out of 2 of 3 types of machine they sell.

    I guess, if you always sell all your pieces and don't need more money, then you can afford to be lackadaisical when it comes to sales. Personally I'd be ordering twice as many machines and answering all sales emails promptly, especially the ones that say "Ready to buy!" What do I know, though?
     
  24. Alan H

    Alan H United States H-M Supporter - Premium Content H-M Supporter-Premium

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    No response to emails and phone are strong signals! What happens when you need customer service after the purchase?
     
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  25. tq60

    tq60 United States Active Member Active Member

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    Not familiar with the machines in question but we have some input.

    We have what once wss the variable speed bp that the po converted to vfd.

    He installed a 1 hp motor with a single fixed ratio pulley and it works great as speed is set by a pot.

    Flip switch and is slows down then reverses.

    Issue is torque in the bottom end rather lack of it.

    To use our she'll mill we need to use back gear and crank up motor rpm.

    If your mill does not have back gear then look at how the speed is chsnged.

    If the variable speed is a set of variable size pulleys then you will have good torque at bottom end.

    If it is just motor speed control then not so much.

    If variable pulleys you can add a vfd to get higher and lower speed than design and may be best option as you can increase motor speed to over speed the spindle for the aluminum if the bearings can handle it.

    At the other extreme you can go below bottom rpm for tapping.



    Sent from my SAMSUNG-SGH-I337Z using Tapatalk
     
  26. JetMech

    JetMech United States Active Member Active Member

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    Wow, what a great, comprehensive answer.

    I may have left people with the impression that I have not used a mill before, or a lathe. If so, it's entirely my fault. I've used a few at work and I have the general idea about feeds and speeds, I'm actually a professional mechanic by trade so I've done a little metal work.

    I'm completely convinced I need the speed if not the infinite variablity, and I"m convinced the infinfite variability is pretty high on the list of "nice to have." So I appreciate you guys pointing me the right way.

    I've ultimately decided if I can get the "discount" mill, I will, and do the variable conversion myself. Then I can Tim Taylor it and put on whatever amount of torque I want. . .

    Your last paragraph echoes my thoughts on the matter exactly. I agree a knee mill is the way to go. For my part, it's a space limitation and even more relevant is the weight. I came to the same conclusion while lathe shopping as well- I don't want to buy more machine than I can move by myself, which pretty much limits me to "benchtop" level stuff.

    Even so, I'd have to disassemble this mill to move it wihtout an engine crane, which I don't have and don't wish to store. Same with the PM1030V I will eventually buy to replace my HF craptastic lathe.

    The mill in question here, a PM727M, is still over 400 lbs as it is. I think it's a good compromise between usefulness and size, it's much, much heavier than a "mini mill" although bridgeport owners will never give it any credit. For the 3d printer parts, RC stuff, and general slot-and-groove making I plan to do, this mill should have the power and weight to spare, and will be a nice addition even if it isn't the mill I die with.

    Maybe when I convince my wife to let me move to a house that allows the shop of my dreams, then I can have the space to load up on "old 'arn" mills and lathes.
     
  27. JetMech

    JetMech United States Active Member Active Member

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    Indeed. However, this outfit has a good reputation, and while people praise the sevice they get, every review mentions that you get response within a "reasonable time" meaning it could be quicker.

    The vendor is Precision Matthews. The reason that it's taking more attention and wrangling is I'm trying to buy a "return" off of them at a discount. Service after the sale isn't really an issue, as there is no warranty if I buy this mill. I don't look for much service after the sale on Chinese tools anyway, and if something breaks, I typically repalce it with better stuff.

    I'm going to email them again right now, and turn up the pressure, and I will also email their "regular" sales people. But hey, if they don't want to take my money, I'll take it somewhere else, many people make version of this mill.

    To tell you the truth, I can't figure outhow they run that business at all, given that they are nearly always out of everything they sell, just look on their site! I kind of think that this is some side-biz and they run it super-understaffed to save money.

    However, PM consistently gets better reviews for QC and for being the "best" version of these import machines, plus, the white and blue is so cool!

    But then, Grizz Green will be acceptable too, if they won't "shut up and take my money!"
     
  28. JetMech

    JetMech United States Active Member Active Member

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    The machine in question is a PM727M benchtop mill.

    It has a geared head, with 3 speeds and high/low gear for a total of 6 speeds. The stated range is 115 to 1700 rpm. The upcoming (this week) PM727V variable speed model is the exact same mill with a DC motor and speed control, and advertises 40-3000 rpm, which would come from a combo of gearing and motor control.

    I believe that with the proper DC motor, and controller, I can use the gearbox and the speed to get torque I need. Since it's a gearbox, the practical limit to speed is probably the bearings, can't see what else would be affected except noise level.

    I basically need the mill in hand to really check it out, see what it can torque its way through, then research and purchase the proper motor and control to really make it rip. If I do it right I might be able to prodeuce a mill with more torque than it needs and more range than PM can offer.

    Here's to hoping.

    Now, if they'd just sell me that mill. . .
     
  29. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    Matt, owner of Precision Matthews, is a member and often poster here on H-M. He seems to be mostly a one man show at PM, perhaps with some helpers. A lot of people on this site have PM equipment. I suspect you will get a response from Matt soon, either here, privately, or both. Matt has a very good reputation on this site from the posts I have seen. Disclaimer: I do not know Matt, and have never inquired PM or done any business with them. From what I have seen on H-M, Matt seems like a straight shooter...
     
  30. mksj

    mksj Active User H-M Supporter-Premium

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    Everything is at a price point, but I would not buy a mill just because it is a special deal but you will quickly grow out of it. If you want a mill to just play around with occasionally that is fine, but do not expect a 2 speed variable speed mill with 1Hp to be boring large holes with it.

    You also need to think beyond the speeds and what you are going to be putting on the table, i.e. travel/size. If you look at most purposely built variable speed mills and lathes, in many cases they are 2 or sometimes have 3 speed ranges, the motors are usually oversized to compensate for torque/Hp drop fall off across the operating range. If you are going to do just high speed work, then the PM727 variable speed would be a good choice, if you want/need low speed capability then you I would recommend looking into something like the PM-30MV, which has a 2Hp motor and a slightly bigger working envelope. The PM-930MV runs off of 220V, so you need to factor if that is available, but the difference in performance will be significant at a small increase in cost (about $250 over the PM-927M). Personally I feel 6 speeds is somewhat limiting getting the sweet spot for different materials, and you are limited by both the low and high speed range. Getting a fixed gear mill and adding variable speed will not be cost effective, you are limited by the Hp of the motor. Gear heads are also very noisy and there are other high speed limitations, most variable speed "conversions" entail a two speed belt drive and removing the gear drive.
     
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