# CNC PM-728V-T



## Toolmaker335xi (Jan 23, 2021)

I have completed the first phase of my CNC  conversion. I had a couple questions in another thread, but didn’t receive any feedback so I started a new thread to hopefully get a few answers and some feedback.

First, a little bit about my setup. It’s a pm-728vt with PM’s CNC kit. G540 running Mach3 on an old desktop using the parallel port. Right now it’s only 2axis with 570oz dual shaft steppers with a sepraty DRO.

I’m happy with the CNC kit and can say the cast iron mounts were worth the 8 month wait. Everything bolted up as it should with no issues so far. I have about .0009 backlash in X and .0026 in Y. X is acceptable to me but Y seems like a lot. I tightened the lock nut till I had maybe .0001 axial play on the shaft end. So my question is can I or should I try to shim in between the double ball nut? Is it possible to have that much play in the shaft coupler? What else can I do to improve this?... as I’m typing, I realized I can swap the couplers to check there... 

Also, in Mach 3 is there a way to use my homing switch as a limit switch or do I need to have two limits and a home switch for my x and y.

Overall I’m pleased with the machine, the only complaint I have is the Y axis ballscrew is not left hand thread. So since I tried to keep my hand wheels the table moves opposite as it normally would cranking it. And hopefully I can improve the backlash of the Y axis some.

Here’s a couple pics. Short term changes are going to be upgrading to Mach4. Installing the Z axis. I struggle to get fusion360 to post toolpaths with zero Z moves. ( I kind of wanted to have it like the 2 axis mill power setup I use at work and since I mainly have been running wire edm for the last 10 years. Z moves cause me anxiety. Lol.) Also need to clean up the wiring and start planning a partial enclosure. 

Thanks for any feedback in advance.


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## rwm (Jan 23, 2021)

That looks slick! I have a 728 that will eventually be CNC so I am watching with interest. Those limit switches look pretty large and beefy? Is that just overkill? I see you went with double ended steppers and kept the handles. I was advised not to do this. How do the handles turn when using it manually? Does it require a lot of force or could you use it manually? Those are funky sleeves you made to fit the handles. Integrated key? Any info on how you made those? What size stepper do you plan for the Z? Sorry but I have more questions than advice!
Robert


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## macardoso (Jan 23, 2021)

Toolmaker335xi said:


> I’m happy with the CNC kit and can say the cast iron mounts were worth the 8 month wait. Everything bolted up as it should with no issues so far. I have about .0009 backlash in X and .0026 in Y. X is acceptable to me but Y seems like a lot. I tightened the lock nut till I had maybe .0001 axial play on the shaft end. So my question is can I or should I try to shim in between the double ball nut? Is it possible to have that much play in the shaft coupler? What else can I do to improve this?... as I’m typing, I realized I can swap the couplers to check there...



You should be using shaft couplers made for CNC. These would be the "Lovejoy" style jaw couplers with an elastomer spider (must be the zero backlash kind). Even better would be a disc or bellows type coupling. The helical spring coupling are garbage for this application.

Eliminate the play in the screw mounting, then any remaining backlash must be in the ballnut. If you have double nuts, shimming between them is exactly how you should reduce backlash, although I'd be talking with the screw manufacturer or kit seller to explain why it is so bad. I bet you spent good money for the kit and they should make it right.



Toolmaker335xi said:


> Also, in Mach 3 is there a way to use my homing switch as a limit switch or do I need to have two limits and a home switch for my x and y.



Yep, that is allowed and most people do just that. Assign them the same setting in ports and pins.



Toolmaker335xi said:


> Here’s a couple pics. Short term changes are going to be upgrading to Mach4. Installing the Z axis. I struggle to get fusion360 to post toolpaths with zero Z moves. ( I kind of wanted to have it like the 2 axis mill power setup I use at work and since I mainly have been running wire edm for the last 10 years. Z moves cause me anxiety. Lol.) Also need to clean up the wiring and start planning a partial enclosure.



I highly recommend Mach 4. I'm a big user of it and happy to help you out if you need. I also highly recommend a motion controller like the Ethernet Smoothstepper (ESS) for Mach 4. You can use the parallel port with a paid plug-in, but it is like towing your new Ferrari behind some horses.


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## rwm (Jan 23, 2021)

What is wrong with the helical couplers? 





I assume they have zero backlash? No?

Robert


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## macardoso (Jan 23, 2021)

Helical (beam) couplings have zero backlash but poor torsional compliance. When they experience load, they "wind up" which creates error between the motor shaft and the ballscrew position. This acts similar backlash when you are machining and represents error in the positioning that you can't be certain of.

In fact all couplings have torsional compliance, but they can be orders of magnitude less than the helical coupling. These designs will have better performance for your machine, but also may require the shafts to be aligned more closely from the get-go and may cost more. Bellows are probably the best for precision motion control applications (hobby CNC probably does not fall in this category )






This one below is a disc type.


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## rwm (Jan 23, 2021)

Thanks for clarifying. I had no idea about that. That info will be quite useful!
Robert


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## macardoso (Jan 23, 2021)

Not to go off topic, but shaft couplers are a very interesting topic. Different applications require very different couplings. In precision applications, zero backlash and ultra low compliance are key which necessitates precision couplings. These tolerate very little misalignment (although any misalignment is intolerable because it can cause the universal joint surging of motion, even at small scales, which is usually undesirable).

Other applications care not about backlash or compliance, but require large misalignment tolerance or vibration damping characteristics. Non-motion control rated jaw couplings are great for this as the "spider" durometer can be tuned to dampen the load. 

Other applications (like pumps) require over torque protection and can rely on clutch or magnetic couplings that slip on an overload condition.

Finally some applications require fail safe couplings (rips apart when over torqued, separating the shafts) or fail functional (like jaw couplings which will ride metal to metal if the spider gets ripped apart).


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## GB21 (Jan 23, 2021)

Hello new guy here.
I thought the couplers might be contributing to the backlash too, I tried a few haha. Mine also had alot of wobble on the x&y axis. I ended up going with the disk type and it fixed the wobble and cleared up about .0005 backlash. It must be in the nuts/screws. My Z is pretty bad...


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## currinh (Jan 23, 2021)

Toolmaker:

Yours does look like a nice setup. I have a PM728 in the shed, and a conversion kit sitting under the bench. The "manual" for the conversion kit is just an assembly drawing though. Since you've done the conversion I'm hoping you can shed a little more light on the process. How much did you need to disassemble to install the kit? Any tips would be greatly appreciated as I start on this journey.

I can't help much with the controller. I'll be using steppers from Automation Technology (425 oz-in X/Y, 920 oz-in Z) all using Gecko G201X drivers. The controller will be LinuxCNC with Mesa 5i25 and 7i76 cards. I mocked up one axis and was able to get stepper motion (woo hoo). So I'm now wiring the controller. I just discovered I need some wire I don't have, shopping Monday. SO, I thought I'd tackle the hardware side.

Any tips would be greatly appreciated. Thanks.

Stay safe.

Hugh



Toolmaker335xi said:


> First, a little bit about my setup. It’s a pm-728vt with PM’s CNC kit. G540 running Mach3 on an old desktop using the parallel port. Right now it’s only 2axis with 570oz dual shaft steppers with a sepraty DRO.


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## currinh (Jan 23, 2021)

Well, I went ahead with the PM728 CNC conversion installation. All the parts were in the package, and they fit the kit. I've assembled the X and Y axes, at least have the ball screws and end plates in place. Nice kit.

However, I found the assembly drawing (all the direction that came with the kit) shows M6-1x16 cap screws (part #911) holding the Y front plate to the machine. These screws came with the kit and the counter sinks in the Y plate fit these M6 cap screws. Now the kicker. The original screws, and thus threads in the base casting, are M8! Has anyone else run into a similar problem?

I suppose I'll re-drill/bore the kit's Y plate for the original M8 cap screws. May be a trick to align to the existing holes as the mill doesn't have a functional Y lead screw. Would have been easy if I'd known before taking the mill apart.

Thanks. Stay safe.

Hugh


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## rwm (Jan 23, 2021)

Definitely ask PM about that one before you modify anything. (I assume you have a PM conversion kit?)
Robert


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## Toolmaker335xi (Jan 23, 2021)

rwm said:


> That looks slick! I have a 728 that will eventually be CNC so I am watching with interest. Those limit switches look pretty large and beefy? Is that just overkill? I see you went with double ended steppers and kept the handles. I was advised not to do this. How do the handles turn when using it manually? Does it require a lot of force or could you use it manually? Those are funky sleeves you made to fit the handles. Integrated key? Any info on how you made those? What size stepper do you plan for the Z? Sorry but I have more questions than advice!
> Robert


Thanks, sorry for the late replies. I work 3rd shift and sleep during the days...

The limits are probably overkill but the price was cheap so I decided to use them. The hand wheels aren’t really hard to crank at all. The only draw back I have is that it’s a little jerky and wants to rest at increments of a few thousands but they make it nice to back it off if a limit or if I want to just face something off real quick with out firing up the computer. They slip right in and off no problem. I wire EDM’d the adapters. Just a solid piece of aluminum.

I have ( I think, need to double check) 565oz nema 34’s.


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## Toolmaker335xi (Jan 23, 2021)

currinh said:


> Well, I went ahead with the PM728 CNC conversion installation. All the parts were in the package, and they fit the kit. I've assembled the X and Y axes, at least have the ball screws and end plates in place. Nice kit.
> 
> However, I found the assembly drawing (all the direction that came with the kit) shows M6-1x16 cap screws (part #911) holding the Y front plate to the machine. These screws came with the kit and the counter sinks in the Y plate fit these M6 cap screws. Now the kicker. The original screws, and thus threads in the base casting, are M8! Has anyone else run into a similar problem?
> 
> ...


I did not have this issue. It may be possible they altered your machine with bigger threads to fix a damaged thread? Just a guess. Mine were M6 and so far required no modifications.


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## Toolmaker335xi (Jan 23, 2021)

macardoso said:


> You should be using shaft couplers made for CNC. These would be the "Lovejoy" style jaw couplers with an elastomer spider (must be the zero backlash kind). Even better would be a disc or bellows type coupling. The helical spring coupling are garbage for this application.
> 
> Eliminate the play in the screw mounting, then any remaining backlash must be in the ballnut. If you have double nuts, shimming between them is exactly how you should reduce backlash, although I'd be talking with the screw manufacturer or kit seller to explain why it is so bad. I bet you spent good money for the kit and they should make it right.
> 
> ...


I’m using the couplers with the elastomer spider that came with the kit. I’ve considered the disc type, but I’ll be waiting till I get new motors. Right now just have 40$ steppers. I was just reusing the G540 I already had to get it up and functional.

I did get the home switches to function as limits too, thank you. Probably going to go ahead with getting mach4. This is just the first phase of this conversion, I will be upgrading to a centroid controller but haven’t decided yet if I want to spend the extra on the Oak or just get the acorn and have more of a budget for better motors and other upgrades.

I’ll be contacting PM to see what they have to say about the backlash in Y... I did notice the x ballnuts had a +8 etched onto it and I have .0008-.0009 back lash on x but didn’t notice anything in the y. But also wasn’t looking for it either... I just went and checked the z since it’s not installed and it has a +4 may be a 42 hard to tell if it’s just a squiggly line or a 2. 

Thanks for the info


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## Toolmaker335xi (Jan 23, 2021)

GB21 said:


> Hello new guy here.
> I thought the couplers might be contributing to the backlash too, I tried a few haha. Mine also had alot of wobble on the x&y axis. I ended up going with the disk type and it fixed the wobble and cleared up about .0005 backlash. It must be in the nuts/screws. My Z is pretty bad...
> View attachment 352156
> View attachment 352157
> ...


Thanks for sharing. The two I installed were tight. I couldn’t feel any noticeable wobble in them.

I noticed etching on the ballnuts. Did you? The x had a +8 I didn’t pay attention to any before I put y in. Z is marked +4 or 42 hard to tell... I’m really hoping I don’t end up with .0042 in z.


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## GB21 (Jan 24, 2021)

Toolmaker335xi said:


> Thanks for sharing. The two I installed were tight. I couldn’t feel any noticeable wobble in them.
> 
> I noticed etching on the ballnuts. Did you? The x had a +8 I didn’t pay attention to any before I put y in. Z is marked +4 or 42 hard to tell... I’m really hoping I don’t end up with .0042 in z.


I did notice the etching but I thought they might be production numbers or something. I was going to take a picture now I wish I would have....
If my memory serves me right 2 of them had 2 digit numbers and one had a 3 digit number like 127


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## macardoso (Jan 24, 2021)

Toolmaker335xi said:


> I did get the home switches to function as limits too, thank you. Probably going to go ahead with getting mach4. This is just the first phase of this conversion, I will be upgrading to a centroid controller but haven’t decided yet if I want to spend the extra on the Oak or just get the acorn and have more of a budget for better motors and other upgrades.


AFAIK, the centroid Oak and Acorn require use of their CNC12 software and are not compatible with Mach 4. 

The Oak serves a different customer base than the Acorn. Make sure you need the features it has rather than getting it because it is a "better" board. The Oak is meant for you to use their drives with AC or DC servos. It is not appropriate for steppers, Clearpath servos, or servos with standalone drives.


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## currinh (Jan 24, 2021)

Toolmaker335xi said:


> I did not have this issue. It may be possible they altered your machine with bigger threads to fix a damaged thread? Just a guess. Mine were M6 and so far required no modifications.


I contacted PM Saturday PM expecting to hear back maybe Monday. They responded (by the time stamps) in 4 minutes saying one other customer had the same thread size problem. Their suggestion was to just drill out the casting for an M8 cap screw (and offered to if I wasn't comfortable). I did it today and the plate fits well now. I suspect it was a change in the PM728 manufacturing, it was too well done to be an error. I received the machine last spring so it was an early one.

My plan is to install home switches but no limit switches. With LinuxCNC you can set soft (software) limits which stop motion outside given offsets from home. Works well for a personal machine but a little sketchy if made for sale?

Thanks. Stay safe.

Hugh


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## macardoso (Jan 24, 2021)

currinh said:


> I contacted PM Saturday PM expecting to hear back maybe Monday. They responded (by the time stamps) in 4 minutes saying one other customer had the same thread size problem. Their suggestion was to just drill out the casting for an M8 cap screw (and offered to if I wasn't comfortable). I did it today and the plate fits well now. I suspect it was a change in the PM728 manufacturing, it was too well done to be an error. I received the machine last spring so it was an early one.
> 
> My plan is to install home switches but no limit switches. With LinuxCNC you can set soft (software) limits which stop motion outside given offsets from home. Works well for a personal machine but a little sketchy if made for sale?
> 
> ...


Most people double one limit switch as home on each axis. I’d recommend adding the second switch on each axis and doing both hard and soft limits. 

Machines can mess up and the hard limits have saved me a few times. Also soft limits don’t work if jogging around prior to homing. I have soft limits set up and I’ve still hit the hard limits a few times.

If you just have steppers, they’d probably stall out, but I have servos that would definitely bend a screw or crack a casting before they faulted out.

it isn’t difficult or expensive to add them.


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## Toolmaker335xi (Jan 25, 2021)

currinh said:


> I contacted PM Saturday PM expecting to hear back maybe Monday. They responded (by the time stamps) in 4 minutes saying one other customer had the same thread size problem. Their suggestion was to just drill out the casting for an M8 cap screw (and offered to if I wasn't comfortable). I did it today and the plate fits well now. I suspect it was a change in the PM728 manufacturing, it was too well done to be an error. I received the machine last spring so it was an early one.
> 
> My plan is to install home switches but no limit switches. With LinuxCNC you can set soft (software) limits which stop motion outside given offsets from home. Works well for a personal machine but a little sketchy if made for sale?
> 
> ...


I purchased mine back in March and it had m6 threads... I wasn’t suggesting it was an error, but an alteration or rework. Some of the m6 threads in my machine are a little loose/sloppy.


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## Toolmaker335xi (Jan 25, 2021)

macardoso said:


> AFAIK, the centroid Oak and Acorn require use of their CNC12 software and are not compatible with Mach 4.
> 
> The Oak serves a different customer base than the Acorn. Make sure you need the features it has rather than getting it because it is a "better" board. The Oak is meant for you to use their drives with AC or DC servos. It is not appropriate for steppers, Clearpath servos, or servos with standalone drives.


Yes, I have looked into the Oak because it is a true closed loop system. It can support more than 4 axis. Also the Acorn only supports one encoder input. Regardless of what controller I upgrade to, I am definitely going to be upgrading motors in the future.

I used the G540 because I already had it and was going to upgrade to mach4 to reuse it on a plasma table build. I am going to use the centroid on a lathe conversion so I might as well get familiar with their CNC12 software.

I have plans to lease some industrial space once the current occupant retires in the next couple years. Then I will be able to purchase larger machines. I bought the pm-728vt because it was easy to move into the basement. I want to learn more than just stepper motors and have a better understanding of a more industrial type controller and setup. Yes the Oak would be overkill for the 728 but it would be a learning experience for me.

I’m thankful for communities in forums like this and others. Without them I probably wouldn’t attempt to take on projects.

Thanks for the advice and input from everyone


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## Toolmaker335xi (Jan 26, 2021)

Made an adapter to use 1/2” shaft stepper with 14mm ID coupler. It collapsed a little but fits nice. Will check it for runout once I solder the cable to the stepper. 

Probably going to install the ballscrew and motor for the z axis today.


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## Toolmaker335xi (Jan 26, 2021)

I decided to take a closer look at the Z axis double ball nut before installing and noticed that there is a gap between the spacer.

The gap of light that is in the first pic is at the bottom left side of the spacer.

Has anyone else noticed this?


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## macardoso (Jan 26, 2021)

Toolmaker335xi said:


> I decided to take a closer look at the Z axis double ball nut before installing and noticed that there is a gap between the spacer.
> 
> The gap of light that is in the first pic is at the bottom left side of the spacer.
> 
> Has anyone else noticed this?



That is no good. You have zero preload on the ballnut and therefore the double nut is doing absolutely nothing more than a single nut.

Making a new spacer is trivial on a surface grinder, but a bit trickier on a lathe. A tenth or two of difference in height makes a big difference in the preload. Also the faces must be very parallel or you will tip the nuts relative to the screw.


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## Toolmaker335xi (Jan 26, 2021)

macardoso said:


> That is no good. You have zero preload on the ballnut and therefore the double nut is doing absolutely nothing more than a single nut.
> 
> Making a new spacer is trivial on a surface grinder, but a bit trickier on a lathe. A tenth or two of difference in height makes a big difference in the preload. Also the faces must be very parallel or you will tip the nuts relative to the screw.


That’s what I was thinking but wasn’t sure. I sent some picks of it to PM just a little bit ago.

I researched a little bit about shimming the ballnuts. The faces being parallel  was definitely something that was emphasized. Worst case scenario, do you think it would be possible to wire edm the ID and OD, then slice it off and lap off the cutoff tang and then split it in half. ( I have way more axis to edm rather than a lathe or surface grinder

I did come across someone working on a larger machine and was able to loosen the key that locks them and tighten the ballnuts  snug to the spacer. This gave him desirable results... one thing I noticed is the key only had one Philips head screw in it as you can see in the pic


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## macardoso (Jan 26, 2021)

Toolmaker335xi said:


> That’s what I was thinking but wasn’t sure. I sent some picks of it to PM just a little bit ago.
> 
> I researched a little bit about shimming the ballnuts. The faces being parallel  was definitely something that was emphasized. Worst case scenario, do you think it would be possible to wire edm the ID and OD, then slice it off and lap off the cutoff tang and then split it in half. ( I have way more axis to edm rather than a lathe or surface grinder
> 
> I did come across someone working on a larger machine and was able to loosen the key that locks them and tighten the ballnuts  snug to the spacer. This gave him desirable results... one thing I noticed is the key only had one Philips head screw in it as you can see in the pic



Sure you could do that. The ID and OD are not particularly precision, so why not buy some steel tubing and use the EDM to finish the flat faces?

The key should keep the ballnuts from rotating relative to eachother. I would definitely expect to see two screws holding it together. The spacer thickness should be such that when the ballnuts are clocked to eachother such that the key fits in the slot, the spacer is firm against both faces and under a few tenths of preload. This is probably a trial and error deal to get it right.


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## rwm (Jan 26, 2021)

Why would you not just return that? 
Robert


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## macardoso (Jan 26, 2021)

Toolmaker335xi said:


> Made an adapter to use 1/2” shaft stepper with 14mm ID coupler. It collapsed a little but fits nice. Will check it for runout once I solder the cable to the stepper.
> 
> Probably going to install the ballscrew and motor for the z axis today.



I'm intrigued... How did you machine the internal D?

EDIT: Forgot you said you had EDM equipment, that is sweet. That would would be a complete headache for me to make with the equipment I have. Maybe on the CNC, but it wouldn't look that sharp.


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## Toolmaker335xi (Jan 26, 2021)

rwm said:


> Why would you not just return that?
> Robert


I contacted PM about it and sent some pictures. They responded quickly as usual but have not said anything other than they will be looking into it since it’s a fairly new product.

If it can be tightened up more against the spacer I’ll just do that. But waiting to see what the have to PM has to say about a resolution before I touch it.


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## Toolmaker335xi (Jan 26, 2021)

macardoso said:


> Sure you could do that. The ID and OD are not particularly precision, so why not buy some steel tubing and use the EDM to finish the flat faces?
> 
> The key should keep the ballnuts from rotating relative to eachother. I would definitely expect to see two screws holding it together. The spacer thickness should be such that when the ballnuts are clocked to eachother such that the key fits in the slot, the spacer is firm against both faces and under a few tenths of preload. This is probably a trial and error deal to get it right.


Yes tubing would be easier. I was mainly thinking about slicing the thickness and then cutting the ring in half. Just use a rough single pass to make the OD and ID since I have flat stock on hand

I was confused that there is only one screw in the key and it appears the opposite hole that doesn’t have a screw in it, and is threaded?  Yet there is no hole or threads on the second half of the ball nut?You can it if you zoom in on the pic with the shim.

That’s why I was hesitant to try and tighten it up onto the spacer... I’ll wait to see what PM says about this. But I feel like I’m going to be pulling X and Y back out too.


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## Toolmaker335xi (Jan 27, 2021)

After researching through the night. I realize most double ball nuts have only one screw securing the key. My mistake, I was under the impression they had screws going to both halves

I was hoping I could just tighten up the ball nut to the spacer. I guess theoretically I could if there is clearance on the sides of the key.( don’t know if or how much it’s got)

Still don’t know why the ball nuts were not preloaded before being shipped to me. Since the spacer is just floating in there as it is now. I don’t know if one half is crooked to the screw or not so I’m going to try and record all the gaps in each axis before loosening the key and shim it in .0005 increments over what gap I measured, tighten them up and make sure they still turn freely the length of the screw. Maybe work the shim thickness up a few times till it becomes tight on the screw at any point. Then back it off a little and then plan on making new spacers based on what I had to shim each one the get a nice preload.

Assembling and disassembling multiple times is not what I wanted to be doing. Oh well, it seems to be what needs to be done.


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## Toolmaker335xi (Jan 27, 2021)

macardoso said:


> I'm intrigued... How did you machine the internal D?
> 
> EDIT: Forgot you said you had EDM equipment, that is sweet. That would would be a complete headache for me to make with the equipment I have. Maybe on the CNC, but it wouldn't look that sharp.


I don’t have my own EDM machines, but I work in a primarily EDM shop... well I don’t have my own yet. Planning on it once I get some industrial space to lease.


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## macardoso (Jan 27, 2021)

Toolmaker335xi said:


> Yes tubing would be easier. I was mainly thinking about slicing the thickness and then cutting the ring in half. Just use a rough single pass to make the OD and ID since I have flat stock on hand
> 
> I was confused that there is only one screw in the key and it appears the opposite hole that doesn’t have a screw in it, and is threaded?  Yet there is no hole or threads on the second half of the ball nut?You can it if you zoom in on the pic with the shim.
> 
> That’s why I was hesitant to try and tighten it up onto the spacer... I’ll wait to see what PM says about this. But I feel like I’m going to be pulling X and Y back out too.



Weird.. I started looking online at a lot of the Chinese double nut ballscrews and they only have one screw on the key. All it needs to do is oppose motion of the nuts relative to each other. Shimming or adjusting the spacer thickness should work regardless if the screws are well locked with that key.


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## Toolmaker335xi (Jan 28, 2021)

I was busy yesterday morning! PM did get back to me with a response about the ball nuts not being preloaded. They said that their supplier said it wasn’t right and they are contacting the factory. Not sure what that means for me though.

I couldn’t wait anymore since the CNC KIT came 5 months after the initial estimated delivery time I was given. So I pulled the X and Y ballscrews yesterday morning. Shimmed all three and then got them all back in the machine.

I used heavy duty and regular aluminum foil since it could give me increments of .0003.  It wasn’t to bad to do. I printed out a quick template to cut the profile for the shims out. Worked out ok. I was able to get .0002 over each gap I measured for each one.

Z was .0027
X was .0042
Y was .0033

All felt smooth after reassembled. No tight spots thru out the length of the screw. Although once back installed Y still felt a little loose. I’ll be getting the CNC hooked back up in the morning and actually see how much backlash decreased.

One tip I can give to anyone who has to do this would be to use just a very little dab of grease to hold all of the shims together. Set the ball nut up in a v block to align the two halves together when putting the key back in.

Overall wasn’t too bad of a job... I won’t regret shimming them if PM says to send them back to be preloaded. I couldn’t wait, I’ve had projects on hold for too long... spacers were a little burned too. Wonder how flat the actually were?


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## GB21 (Feb 1, 2021)

Any update on the results? My backlash is worse than yours I am considering pulling the machine back apart


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## Toolmaker335xi (Feb 2, 2021)

GB21 said:


> Any update on the results? My backlash is worse than yours I am considering pulling the machine back apart


Yes... sorry I was a little busy the last few days.

I only added .0002-.0003 shim to the gaps measured before taking apart. I now have .0020 backlash on all 3. ( my x actually increased some. The original value of .0009 I stated above was right after install, never actually cutting. Ended up having .0014 on X axis before adding the shim.) My Y axis did decrease some. And didn’t install z before adding shims.

I didn’t have the results I was hoping for, but I’m ok with it being even across XYZ. Now that I have an idea on what thickness the shims need to be. I will eventually make new ones to the required thickness instead of using any kind of shim stock. But for now it is what it is.

I was informed after I used the aluminum foil shim material that it probably won’t hold up under a load.

As for the reply from PM, their response was “Supplier says it actually won't cause any harm, so you're ok as is.”... doesn’t explain why they didn’t come preloaded. 

Good luck GB21, I’m curious to hear what size gap there is between your spacers, how much you shim it, and what your results are.


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## macardoso (Feb 2, 2021)

Toolmaker335xi said:


> Yes... sorry I was a little busy the last few days.
> 
> I only added .0002-.0003 shim to the gaps measured before taking apart. I now have .0020 backlash on all 3. ( my x actually increased some. The original value of .0009 I stated above was right after install, never actually cutting. Ended up having .0014 on X axis before adding the shim.) My Y axis did decrease some. And didn’t install z before adding shims.
> 
> ...



I mean, yeah, it doesn't cause harm, but I'd want them correctly preloaded.


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## currinh (Feb 2, 2021)

I didn't pay a lot of attention as I put mine together, but the ball screws seemed tight. I have the CNC kit installed on my PM728 and the controller mocked together. The cnc kit installed with little trouble. I'll need to put the controller in a box and add plugs, but it's now functional. Will be much easier to install plugs etc. in a controller box with the cnc functioning. It's using steppers with Gecko G201X drivers (Mesa 5i25/7i76 boards with LinuxCNC).

I did a quick check on backlash. Just a quick check so could be off some. But I measured about 0.001" on both X and Y. Z came in about 0.0025". I'm real happy with this for X and Y, and Z is workable. The head has quite an offset weight so the Z ways may be giving noticeable stiction. No tests yet as to speeds I can get, or other tests.

Toolmaker335xi: Some of the backlash could be from the bearings. Hard to get a hold on the ballscrew to tighten the bearing retaining nut. I installed the couplers and holding these by hand I tightened the retaining nuts as I could. Could be as your X was worse after re-assembly?

Thanks.

Hugh


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## currinh (Feb 2, 2021)

I also had a couple pictures I forgot to attach.


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## Toolmaker335xi (Feb 3, 2021)

macardoso said:


> I mean, yeah, it doesn't cause harm, but I'd want them correctly preloaded.


My thoughts exactly!


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## Toolmaker335xi (Feb 3, 2021)

currinh said:


> I didn't pay a lot of attention as I put mine together, but the ball screws seemed tight. I have the CNC kit installed on my PM728 and the controller mocked together. The cnc kit installed with little trouble. I'll need to put the controller in a box and add plugs, but it's now functional. Will be much easier to install plugs etc. in a controller box with the cnc functioning. It's using steppers with Gecko G201X drivers (Mesa 5i25/7i76 boards with LinuxCNC).
> 
> I did a quick check on backlash. Just a quick check so could be off some. But I measured about 0.001" on both X and Y. Z came in about 0.0025". I'm real happy with this for X and Y, and Z is workable. The head has quite an offset weight so the Z ways may be giving noticeable stiction. No tests yet as to speeds I can get, or other tests.
> 
> ...


Hi Hugh, nice setup

I really didn’t have any trouble tightening the lock nuts on the screw. I just tighten the coupler on to the shaft end and kept my Allen wrench in the bolt of the coupler so it was perpendicular to the screw and printed a wrench to fit the locknut. Only had about .0001 axial play in the end of the screw. And of course there could be a few tenths deflection on the mounts... I would be satisfied with only .0010 backlash in x and y. I think x gained some backlash from swapping the couplers from each axis. I don’t think I added enough shim to preload. I only went .0002 over the measured gaps on the spacer. And if you figure a tenth or two gap from what I measured and only have increments of .0005 to measure... I don’t think I actually got any preload still. 

Once I get time to pull the screws out again I’ll make new spacers and make sure I actually get some preload. But .0020 on each axis is ok for now. Backlash comp seems to be working out ok and have been within .001 in some test cuts.


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## GB21 (Oct 31, 2021)

After 9 months I finally decided to address the backlash and started taking the mill back apart to check the ballscrews. They are worse than I thought here is the Y axis screw.   I measured 0.0059 gap. I messaged PM we will see what they say. These are supposed to have a 5 year warranty so I suspect its not too late... but if they wont help me out I will have to try and shim them like you did.


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## sakumar (Dec 17, 2021)

Toolmaker335xi said:


> Once I get time to pull the screws out again I’ll make new spacers and make sure I actually get some preload. But .0020 on each axis is ok for now. Backlash comp seems to be working out ok and have been within .001 in some test cuts.


Hello Toolmaker335xi,

I know, old thread -- but did Precision Matthews provide any solution?

On reading through this thread, the simple solution for them would be to provide spacer(s) of the correct thickness that you could then install yourself. The idea being that the thickness of the spacer is such that at just the right torque, the two halves of the keyway align perfectly.

Of course, the trick is to get the thickness right but you had gone through the process of figuring that out yourself and could provide PM the specs.


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