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Awesome New Mill- Is it worth it?
- Joined
- Dec 20, 2012
- Messages
- 9,422
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.
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.
- Joined
- Mar 26, 2016
- Messages
- 58
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?
A
Alan H.
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No response to emails and phone are strong signals! What happens when you need customer service after the purchase?
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
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
- Joined
- Mar 26, 2016
- Messages
- 58
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.
- Joined
- Mar 26, 2016
- Messages
- 58
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!"
- Joined
- Mar 26, 2016
- Messages
- 58
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. . .
- Joined
- Jul 2, 2014
- Messages
- 7,594
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...
- Joined
- Jun 12, 2014
- Messages
- 5,071
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.
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.