Need Help to Insulate Metal Motor Case from Aluminum Motor Mount

Thread Starter

TSmith

Joined Sep 21, 2010
63
I need help to find a way to insulate a few hundred 50mm spindle motors from the 6061 aluminum motor mounts they will go in. I can anodize the motor mounts with something like a type III hard finish that can build up a .002 to .003" nonconductive layer. But I'm a little worried that in the process of mounting and remounting the motors the anodizing can get damaged and allow a current to pass. The motor mount also serves as a heatsink so I don't want to add a layer between the surfaces unless absolutely necessary. The motor mount halves will also be tightened up quite firmly against the motor.

The red area in the image is steel and conductive. Blue motor mounts shown in blue come in raw 6061 aluminum.

I 3D printed a spacer out of PETG to test but it softened up and caused issues. The motor temp running is about 85C/158F.

Spindle Motor.jpg
 

gerty

Joined Aug 30, 2007
1,298
Are you saying the motor housing is live with respect to ground ? If it's line voltage that would be a code violation here in the US.. What voltage are you using?
 

MaxHeadRoom

Joined Jul 18, 2013
21,962
Why do you need to insulate the motor from ground?
Is this to isolate the frame electrical Ground connection? As It is normal to earth ground the frame.
These motor are typically restricted to a minimum RPM due to excessive heating at lower RPM levels, typically 6kRPM.
Max..
 

jpanhalt

Joined Jan 18, 2008
10,913
TO-220 transistors (etc.) are frequently mounted to heat sinks with electrically insulating washers. Silicone rubber and mica are used. You could attach the motor mount to the frame the same way, Non-conductive screws could be used or you can use metal screws, more clearance (maybe even line the holes with a sleeve), and insulating hat washers under the screw heads.

No reason to use low-melting 3D printed stuff. Nylon and other insulators can withstand whatever temperature the motor mount may attain. Mica and silicone are decent heat conductors with an appropriate "grease."

I once built a low-voltage supply that depended on insulation by an anodized coating. It's doable, but I would not depend on that for the motor to motor mount insulation.
 

ronsimpson

Joined Oct 7, 2019
1,121
Kapton Tapes are made from Kapton ® polyimide film with silicone adhesive. They are compatible with a wide temperature range as low as -269°C (-452°F) and as high as 260°C (500°F).
 

Thread Starter

TSmith

Joined Sep 21, 2010
63
@gerty - The voltage of 50v DC. Not a safety issue. Insulation is just to be able to monitor when the tool on the spindle contacts a surface by making a ground.
@MaxHeadRoom - I need to be able to detect when the tool in the spindle makes contact with certain features in the machine to complete the circuit. Similar to probing in a CNC machine.
@jpanhalt - I thought about isolating the motor mount from the machine early on in the project and totally forgot about that until you mentioned it now. I'll have to revisit that option with your tips. Thanks!
@ronsimpson - Ooooh, I have a roll of Kapton tape here. Perhaps that along with the anodized finish on the motor mounts can provide the extra protection needed.

Thanks for the suggestions so far. This will keep me busy for a couple of days trying out a few things.
 

jpanhalt

Joined Jan 18, 2008
10,913
I presumed the user is more likely to move the motor in its mount rather than move the mount on its frame. Hence, insulating the mount would be a lesser source of problems.

As for detecting when a tool touches a workpiece, that is common practice. On the devices I have seen, the touching part is insulated from ground. When it touches, a ground connection is completed and the signal (e.g., an LED) is activated. Of course, you cannot use the working tool for that purpose, but once you determine accurately where the workpiece and tool are located relative to each other, it is no big deal to change the tool.

Although 50V is considered fairly low voltage, electrocution at far lower voltages (e.g., 30V) have been documented with welders. The low voltage devices just mentioned avoid that potential risk.
 

MisterBill2

Joined Jan 23, 2018
7,686
Definitely insulating the motor mount from what it is attached to is the better choice, because that can easily be done. There are a lot of makers of thermal coupling systems for high powered semiconductor devices, and one of them may also be able to help. But really, insulating the mounting block is a better choice.
 

MaxHeadRoom

Joined Jul 18, 2013
21,962
@MaxHeadRoom - I need to be able to detect when the tool in the spindle makes contact with certain features in the machine to complete the circuit. Similar to probing in a CNC machine.
So presumably if sensing using the tool, you will be completing the circuit through the chuck and bearings to frame, this may be touch-and-go to get a satisfactory path through the spindle bearings, especially if ceramic bearings are used?
Either way, this bearing path is not the ideal.lubricant - grease could be an issue.
Max.
 
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MisterBill2

Joined Jan 23, 2018
7,686
The other scheme for detecting tool contact is looking at motor current, but that may not work if the approach is rapid. This has been a very long-time concern in CNC systems and so there ought to be a number of published solutions, It has been over 40 years since I heard an engineer commenting that he had to solve the problem, and it was not new at that time. So there must be some solutions around by now.
For reliable connection with the motor shaft there are mechanisms made exactly for that purpose, often used to avoid bearing damage from circulating currents. So products are available in that area also.
 

jpanhalt

Joined Jan 18, 2008
10,913
So presumably if sensing using the tool, you will be completing the circuit through the chuck and bearings to frame, this may be touch-and-go to get a satisfactory path through the spindle bearings, especially if ceramic bearings are used?
Either way, this bearing path is not the ideal.lubricant - grease could be an issue.
Max.
The electronic ones that complete the circuit through the machine are presumably pretty good. I have one (circa 1986), but never use it. I use my Starrett wobbler, when I want reasonable accuracy, or a piece carefully calibrated legal pad paper, which is about 0.0015 to 0.0020 thick when I feel drag.
 

shortbus

Joined Sep 30, 2009
8,169
Funny all of the different ways of doing this. Where I worked they preset each tool to a known height above the table and assigned that number to that tool holder, so when the machine had tool "# whatever" in it, it knew where zero was on that holder. And each holder did it's job.
 

Thread Starter

TSmith

Joined Sep 21, 2010
63
I've been tasked to do this fast, cheap, and with good quality when normally you have to pick two of the above. If cheap was not a requirement, there would be more readily established means to do some of this. Your input is helping! I have not given up yet.
 

MaxHeadRoom

Joined Jul 18, 2013
21,962
Funny all of the different ways of doing this. Where I worked they preset each tool to a known height above the table and assigned that number to that tool holder, so when the machine had tool "# whatever" in it, it knew where zero was on that holder. And each holder did it's job.
Also it is not so much tool height above the table but typically the tool length and diameter is registered.
Then the work position is detected via the touch probe sensor.
Max.
 

MrSalts

Joined Apr 2, 2020
105
How about a tiny 3mm x 3mm microphone or piezo sensor on the cutting head. The machine vibration will be background noise but I'm guessing it should be pretty easy once the cutter contacts the stock. Even if it is encapsulated or on the tool holder, the sound should travel though pretty well.
 
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