Misc rambling on CNC, machines, motors, and such
Posted by
timgoldstein
on 2010-12-31 10:06:21 UTC
Got a fairly common e-mail from a potential customer. He asked questions that are pretty normal when someone is getting started and learning. Since the questions are so common and it took so long to reply in detail, I thought it might be helpful for others on this group.
Not looking to debate my opinion, as these are the conculsions I have reached after playing and working in this CNC hobby for 12+ years. If you agree, wonderful. If you disagree, I will respect your opinions if you take the time to write them up.
Comments below in []
There is nothing special about Sherline factory stepper motors and you pay considerably more for the name. You did not ask, but their driver electronics are far from the highest performance available. Specific brand is really not a concern, but you do want the modern style "square" motors. What is far more important is low impedance and proper voltage range and general match for the drive you choose. In that vein as torque will have a relationship to impedance, more power is not better. As the torque increases, so does the impedance. Impedance is an issue because it limits the motors speed and where you hit the torque knee and the running torque drops like a rock. Conventional/Hobbyist thinking is if my motor stalls I need a bigger motor. The reality is the bigger motor can likely stall at a lower RPM as the impedance limits the ability of the driver to fully energize the coil once you hit the knee on the torque curve and bigger motors have bigger coils with more impedance. Additionally, oversized motors produce larger shock pulses on every single step and when running in the fat part of the torque band the extra power can break things if you make a mistake. My experience is there is absolutely no upside to a motor that is larger than needed and lots of down. This is not the urban myth story that 99% of the hobby boards will support, but it is the facts based on how drive stepper drive systems work and are designed from my understanding.
Material you are machining is irrelevant in regard to the drive system. Either material can be programmed to run at full spindle load and it is the spindle load that will determine the required thrust. Now the one thing that does enter in, is you have to run plastics at much higher feed rates to avoid melting and to load the spindle if you are looking for maximum removal rates. This is where our more coarse than conventional screws come in. Our precision leadscrew system (available separately or the same as put on our Monster Mill) with the coarse screws is designed specifically for stepper driven CNC mill. The pitch is far too coarse to do any precision machining by hand. Fine to knock a step down on a part or something like that, but virtually useless to try machining anything to spec. That is what the CNC system is for. We frequently will use either MDI mode and put in simple cuts one line at a time or just use the jog. This is true on all the machines we use from the Monster mill to our Haas equipment. But this is answering your next question before you ask.
Back to the coarse pitch. As I have already explained, stepper motors stink at high RPM due to the torque knee that is created by the coil inductance and the manner steppers are driven. Most mill out there are designed for hand use where you need fine pitch to have accuracy. Virtually all modern drives are microstepping. So when you combine a micro step drive with a fine pitch screw, you end up with step increments that are impressive on paper, but the reality is the machine is incapable of moving at that microscopically small increment. But the combination does require relatively high drive motor RPM to achieve the required feed rates for things like plastic and aluminum. So put together the high motor speeds, the torque knee, and if using the most popular controller software Mach3 with parallel port step output which is known to occasionally throw a mis-spaced pulse when Windows demands CPU cycles and you get motor stalls and scrapped parts. By going to a coarse pitch screw the motor RPM is reduced drastically. That put the motor in the fat part of the torque curve where it works best. Additionally at these lower pulse rates the CPU generated pulses are more evenly spaced as the computer is not working as hard and the system is more tolerant of a mis-spaced pulse as it is not trying to work at the ragged edge. The on paper downside is your step increment is increased. The reality is, it is still in the range that everything but $25K plus machines can't follow due to slip stick in non linear guide ways and not having fully ground double nutted true zero lash ball screws. So final outcome is a greatly improved rapid and cut speed range, virtual elimination of the frustrating random motor stalls, and a CNC machine that you can rely on with the same precision to the final part as finer screws.
Just should mention, we do offer the same 20 tpi screws that is the norm and from real world experience when using steppers they work far worse than the 4 tpi or 5 mm pitch screws. Additionally, If you go with Mach3, you can eliminate the pulse spacing issue by using one of the outboard pulse generators that are available. Or, you can look at EMC which has a realtime kernel and avoids some of the resource contention issued of Mach3 on the windows platform.
Oh, BTW stainless depending on the grade can be nasty stuff to machine. Use the correct tools, speeds, and feeds and in the right grade it is like cutting butter. Pick the wrong grade and the incorrect tools, speeds, and feeds and you will be cursing for days. Right now we have a product where we are drilling on the Monster mill. It is .035" holes through 1/8" 303 stainless shafts. Carbide bit, proper feed, lots of pecks, and mist coolant. We can drill hundreds on the same bit. Mess up anything and you will not get a single part per bit. Try it manually and your chance of success is very slim. So just wanted to point out that machining involves far more than a machine and everything else is as or more important than the machine. It really is an art and to learn it costs $$ in time, scrap, and tooling.]
Just so you are fully informed, the issue with the factory Sherline gibs is they are injection molded and the molds were not designed and/or run to compensate for mold shrinkage. So the faces that should be flat and sit in full contact with the ways are concave and only touch on the extreme edges. This makes them incapable of holding an oil film as is proper for a way guide system. Plus it puts so much pressure on the little edges and the little edges have so little support that the way is not rigid until you crank it down so tight it acts as a lock partially on. Super Gibs are manufactured on a RP 3D printer and the faces are flat. The improvement is unbelievable.
Super Gibs are suitable for both stock Sherline factory machines and also for all our Sherline machine upgrades. As I have pointed out, a Monster mill is just all the upgrades without buying a Sherline to put them on. Super Gibs are 1 per axis and a Monster mill has 3 of them installed. To understand where they are installed either look at our XY Precision leadscrew instructions on the literature page at www.a2zcorp.us or look at the parts breakdown for a Sherline mill on their site.]
BTW, we work with 2 partners that do the motion control systems and can sell you a complete CNC Monster mill with all the electronics ready to go. Both at vastly different price points. The first is www.soigeneris.com who is a long term dealer, but a new partner in offering CNC Monster mills. The other is www.FlashcutCNC.com who sells a custom anodized version of our Monster mill as the 3000 mill. ]
I hope you find this useful and that it illuminates many of the half truths, misconceptions, and urban legends that are propagated on many of the forums and groups.
Tim Goldstein
A2Z Corp
A2Z CNC division
3955 S Mariposa St
Englewood CO 80110
720 833-9300
Toll Free 877 754-7465
www.A2ZCorp.us/store
USA made accessories for desktop mills & lathes.
Specialized tools for the jewelry industry.
Not looking to debate my opinion, as these are the conculsions I have reached after playing and working in this CNC hobby for 12+ years. If you agree, wonderful. If you disagree, I will respect your opinions if you take the time to write them up.
Comments below in []
> Hello Sirs,[First let me say, we do not sell or support the motion control portion of a mill, just the machine hardware side. With that said, I will reply with both no real vested interest in what you get, and 12+ years of working with hobby CNC systems.
>
> I'm planning to build a CNC Mill for myself and I came across your Monster
> Mill in my research.
> I'm new to this so naturally I have a few questions. :)
>
> 1. What steppers do you recommend?
> I noticed Sherline uses Nema 23 but there are several versions with
> different torque. Do you
> recommend Sherline standard steppers or do you have other preferences? I
> will mill some parts
> in stainless steel but mostly I will work on Aluminium and Plastics.
There is nothing special about Sherline factory stepper motors and you pay considerably more for the name. You did not ask, but their driver electronics are far from the highest performance available. Specific brand is really not a concern, but you do want the modern style "square" motors. What is far more important is low impedance and proper voltage range and general match for the drive you choose. In that vein as torque will have a relationship to impedance, more power is not better. As the torque increases, so does the impedance. Impedance is an issue because it limits the motors speed and where you hit the torque knee and the running torque drops like a rock. Conventional/Hobbyist thinking is if my motor stalls I need a bigger motor. The reality is the bigger motor can likely stall at a lower RPM as the impedance limits the ability of the driver to fully energize the coil once you hit the knee on the torque curve and bigger motors have bigger coils with more impedance. Additionally, oversized motors produce larger shock pulses on every single step and when running in the fat part of the torque band the extra power can break things if you make a mistake. My experience is there is absolutely no upside to a motor that is larger than needed and lots of down. This is not the urban myth story that 99% of the hobby boards will support, but it is the facts based on how drive stepper drive systems work and are designed from my understanding.
Material you are machining is irrelevant in regard to the drive system. Either material can be programmed to run at full spindle load and it is the spindle load that will determine the required thrust. Now the one thing that does enter in, is you have to run plastics at much higher feed rates to avoid melting and to load the spindle if you are looking for maximum removal rates. This is where our more coarse than conventional screws come in. Our precision leadscrew system (available separately or the same as put on our Monster Mill) with the coarse screws is designed specifically for stepper driven CNC mill. The pitch is far too coarse to do any precision machining by hand. Fine to knock a step down on a part or something like that, but virtually useless to try machining anything to spec. That is what the CNC system is for. We frequently will use either MDI mode and put in simple cuts one line at a time or just use the jog. This is true on all the machines we use from the Monster mill to our Haas equipment. But this is answering your next question before you ask.
Back to the coarse pitch. As I have already explained, stepper motors stink at high RPM due to the torque knee that is created by the coil inductance and the manner steppers are driven. Most mill out there are designed for hand use where you need fine pitch to have accuracy. Virtually all modern drives are microstepping. So when you combine a micro step drive with a fine pitch screw, you end up with step increments that are impressive on paper, but the reality is the machine is incapable of moving at that microscopically small increment. But the combination does require relatively high drive motor RPM to achieve the required feed rates for things like plastic and aluminum. So put together the high motor speeds, the torque knee, and if using the most popular controller software Mach3 with parallel port step output which is known to occasionally throw a mis-spaced pulse when Windows demands CPU cycles and you get motor stalls and scrapped parts. By going to a coarse pitch screw the motor RPM is reduced drastically. That put the motor in the fat part of the torque curve where it works best. Additionally at these lower pulse rates the CPU generated pulses are more evenly spaced as the computer is not working as hard and the system is more tolerant of a mis-spaced pulse as it is not trying to work at the ragged edge. The on paper downside is your step increment is increased. The reality is, it is still in the range that everything but $25K plus machines can't follow due to slip stick in non linear guide ways and not having fully ground double nutted true zero lash ball screws. So final outcome is a greatly improved rapid and cut speed range, virtual elimination of the frustrating random motor stalls, and a CNC machine that you can rely on with the same precision to the final part as finer screws.
Just should mention, we do offer the same 20 tpi screws that is the norm and from real world experience when using steppers they work far worse than the 4 tpi or 5 mm pitch screws. Additionally, If you go with Mach3, you can eliminate the pulse spacing issue by using one of the outboard pulse generators that are available. Or, you can look at EMC which has a realtime kernel and avoids some of the resource contention issued of Mach3 on the windows platform.
Oh, BTW stainless depending on the grade can be nasty stuff to machine. Use the correct tools, speeds, and feeds and in the right grade it is like cutting butter. Pick the wrong grade and the incorrect tools, speeds, and feeds and you will be cursing for days. Right now we have a product where we are drilling on the Monster mill. It is .035" holes through 1/8" 303 stainless shafts. Carbide bit, proper feed, lots of pecks, and mist coolant. We can drill hundreds on the same bit. Mess up anything and you will not get a single part per bit. Try it manually and your chance of success is very slim. So just wanted to point out that machining involves far more than a machine and everything else is as or more important than the machine. It really is an art and to learn it costs $$ in time, scrap, and tooling.]
> 2. Can you supply hand-wheels which has correct gradation for the 5 mm[See above. It is really meaningless as it is designed to be a CNC machine from the ground up and the hand wheels are just there for peoples comfort and the most crude of work. So rather than imply what offering the wheels would be saying, we are just up front that it is not a manual machine. Different story if you put on the 20 tpi screws, but then the gradations are correct and the CNC performance with stepper stinks.]
> precision lead-screw PLSS1-5MM?
> How much would they cost (metric)?
> (When providing upgrades like the Monster you REALLY should be able to ;)
> 3. Will it be possible to add Sherlines DRO#8160-240 to the CNC Mill in the[Yes and no. Yes you can put it on as the entire motor mount and drive system is a reproduction of what Sherline does. No in that it is useless for the above reasons and the reading will be off as they do not allow you to recalibrate to a different screw pitch.]
> same way Sherline
> does to their mills?
> 5. The SG3DS Super Gib for Sherline with Turcite. Is it made for use with[The Super Gib was designed to overcome some serious short coming in the Sherline factory gib. Then we added the Turcite to make our Z column upgrade for Sherline and our XY Precision leadscrew system a 100% Turcite way system. These components are what get assembled into a Monster mill. On the Z column we ship it with the Super Gib installed. On the XY precision leadscrew upgrade, we offer it as an option. But when we turn the parts into a Monster mill we use the Super Gib on all 3 axes.
> the Monster Mill or only for Sherline?
> I noticed it was not listed on your Monster page. You claim better
> performance and we all want that. I haven't
> figured out where it would mount. Can you explain, please? How many are
> required, one or more?
Just so you are fully informed, the issue with the factory Sherline gibs is they are injection molded and the molds were not designed and/or run to compensate for mold shrinkage. So the faces that should be flat and sit in full contact with the ways are concave and only touch on the extreme edges. This makes them incapable of holding an oil film as is proper for a way guide system. Plus it puts so much pressure on the little edges and the little edges have so little support that the way is not rigid until you crank it down so tight it acts as a lock partially on. Super Gibs are manufactured on a RP 3D printer and the faces are flat. The improvement is unbelievable.
Super Gibs are suitable for both stock Sherline factory machines and also for all our Sherline machine upgrades. As I have pointed out, a Monster mill is just all the upgrades without buying a Sherline to put them on. Super Gibs are 1 per axis and a Monster mill has 3 of them installed. To understand where they are installed either look at our XY Precision leadscrew instructions on the literature page at www.a2zcorp.us or look at the parts breakdown for a Sherline mill on their site.]
> Sorry for the long questions but these mills are expensive so due diligence[I understand and that is why I just invested an hour out of my life to give you probably the straightest answer you will receive.
> is mandatory. ;)
BTW, we work with 2 partners that do the motion control systems and can sell you a complete CNC Monster mill with all the electronics ready to go. Both at vastly different price points. The first is www.soigeneris.com who is a long term dealer, but a new partner in offering CNC Monster mills. The other is www.FlashcutCNC.com who sells a custom anodized version of our Monster mill as the 3000 mill. ]
I hope you find this useful and that it illuminates many of the half truths, misconceptions, and urban legends that are propagated on many of the forums and groups.
Tim Goldstein
A2Z Corp
A2Z CNC division
3955 S Mariposa St
Englewood CO 80110
720 833-9300
Toll Free 877 754-7465
www.A2ZCorp.us/store
USA made accessories for desktop mills & lathes.
Specialized tools for the jewelry industry.