Small Milling Machine and Accessories recommendation needed

[mmagliaro]

The Sherline high-speed kit is just 2 pulleys and a belt.  I wonder if something similar can be made for Max's mill? The motor pulley needs to be larger and spindle pulley smaller. With small bits and high rpms the torque needed will be minimal (something the original motor should handle).

Sherline also warns about the spindle bearings getting hot during high speed operation (because of the preloaded spindle bearings). They give a procedure to decrease the preload slightly to reduce the friction.

This is exactly what the belt-drive speed upgrade kit is (LittleMachineShop sells it, among a few others).  It removes the gear drive that the mill comes with and replaces it with pulleys and a belt.  The pulleys are sized to get the speed increase and the belt drive supposedly runs a lot quieter than the gears (and I'm sure it does).
But it is designed to get you 4600, not 10,000 (or 20,000).   Yes, I could probably get different pulleys and get the speed way up, and yes, with such small cutters, I am not worried about the torque. With a two-step pulley, I could give it a high/low range (which the upgrade kit has anyway) so I could use it for micro cutters and "normal" ones.

But hmmm... what about the bearings at 20,000 rpm?  Maybe it wouldn't matter if I am not putting a lot of sideways load on it with such small cutters.

[narrowminded]

The Sherline high-speed kit is just 2 pulleys and a belt.  I wonder if something similar can be made for Max's mill? The motor pulley needs to be larger and spindle pulley smaller. With small bits and high rpms the torque needed will be minimal (something the original motor should handle).

Sherline also warns about the spindle bearings getting hot during high speed operation (because of the preloaded spindle bearings). They give a procedure to decrease the preload slightly to reduce the friction.

That's exactly the problem and reason that Bridgeports and any large mill doesn't have a higher spindle speed.  It's not because they don't know better or can't figure out the gearing to do it.  It's totally about bearing speeds and heat and lubrication.   They have a spindle that's BIG for rigidity running bigger cutters.  In most machine shops, that's where you're working.  And generally speaking, you don't build big machines to make small parts.  UNLESS it's a big machine to mount up and make LOTS of small parts per setup.  That's exactly the machine that was just put in at one of my customer's shops and because he makes LOTS of smaller parts.  (If I produce that chassis that's the machine it will be produced in.)  Here's a link to that machine and I'm pretty sure it is literally the fastest small parts and fastest tapping machine in the world (rigid taps, even little 4-40 and #0 at as much as 6,000 RPM) .  It's pretty big but for its size, has a relatively small spindle.  Still pretty big when compared to what we're talking about here but still, relatively small for THAT physical size machine.  BUT it comes standard with a 10,000 RPM spindle and has an optional (that's what was bought) 16,000 RPM spindle.  One of the reasons that it's optional is because it costs several thousand more and THAT'S in part because it has a water cooled spindle as part of the hardware to address the heat problem.  The reason a Proxxon or equivalent can run 20,000 RPM reliably is because it has a 1/8" capacity and MUCH smaller diameter bearings, exponentially reducing the surface feet per minute, therefore friction, therefore heat.  But THAT comes at a tremendous loss of load bearing capability but it works because a tiny cutter can't TAKE much load.  It's all appropriate for the conditions.  And that pretty much is the full circle on the topic.   Bottom line, don't just change the pulleys on your Bridgeport or your lathe to double or triple the speed of your  spindle.  There's a reason it doesn't spin faster.

BTW, for those really interested, that's all pretty standard bearing application stuff and the design guidelines are available from manufacturers that will graph these things.  They are interesting as the graphs are virtually 90 degree drops.  They start at a load rating and as the speed goes up there's a gradual drop but not much.  Then at some point out the graph, it turns a virtual 90 degree and drops off almost vertically.  That's where you hit the limits of the lube.  On a typical graph the first drop is a grease packed bearing.  The next example will be recirculating oil lubrication and the graph will be virtually the same, a very slight, continual slope downward, except that the 90 degree drop will be extended further out on the speed range before it does the same thing.  The recirculating oil acts as a coolant as well as lubrication.  It's all basic stuff.

http://www.brother-usa.com/MachineTool/Default.aspx?src=s1000x1_overview

[mmagliaro]

I am in contact with a fellow named: Murat Bilgin Bilici  who makes belt drive pulley conversion kits for
various incarnations of the Sieg X2 (aka Micromark, aka Grizzly, etc).  I asked him about making different pulleys to get the speed up to 10,000 or more instead of just 4700.

He is more concerned about belt wear and motor stress than bearing failure.  But I'll ask him again.
I think that as long as I am using tiny cutters with such light loads, I don't have to worry about the motor.

And even the belt is probably not an issue unless I am running this thing for a lot of hours, which I won't be.

My main concern, and narrowminded is scaring me about this, is the bearing failure.
Consider a little electric motor running free in your hand on the workbench.  Even with no load on it, if the rpms go too high, the bearings and lubrication are going to fail and they are going to heat up and burn.

[peteski]

I just mentioned the spindle bearings because that is what Sherline mentions in the high-speed pulley installation instructions. But (at least in my machine) that has not been a problem. I did not reduce the preload on my spindle bearings and I have operated my mill at high speeds for 10-15 minutes at a time (not continuous, but probably about 80% duty cycle). The head-stock did get slightly warm but nothing that would raise a flag for me.  The head-stock is a chunk of aluminum and it acts as a heat-sink for the bearings.  I would say that in a hobby workshop environment (not in 8-hour/day production environment) there is not much worry that we will be overheating the spindle bearings when running high speed milling operations.  Plus, you can easily monitor the head-stock temperature (with your hand).

I'm not sure why Murat is worried about motor stress: isn't the motor designed for whatever full speed it runs at?  It is not like you running the motor at higher than design speeds.  And a load created by let's say a 0.020" milling bit will be almost non-existent.  As for the belt - I don't know, but Sherline uses a belt for their 10,000 rpm pulley set and it works fine.

[mmagliaro]

Well, he's gotten back to me.  He says he does not know if the bearings would suffer damage at high RPMs.
He is worried about damage to the motor or belt when it first starts up with a high pulley ratio.  But he admits that it's just a theoretical worry and he really hasn't any idea that anything would be harmed.

I think he just wants to cover himself in case the thing fails at these high speeds.

So now... if I jump in and do this... how high do I push it?  You and narrowminded have run these tiny cutters.
Is 10,000 RPM enough?    I know from my experiences making the parts I like to make that I'd really like to get down to a 1/64" cutter (about .015").  Do you think I can do that in alumnium and brass, operating by hand, at 10,000 rpm?
 

[peteski]

Well, he's gotten back to me.  He says he does not know if the bearings would suffer damage at high RPMs.
He is worried about damage to the motor or belt when it first starts up with a high pulley ratio.  But he admits that it's just a theoretical worry and he really hasn't any idea that anything would be harmed.

I think he just wants to cover himself in case the thing fails at these high speeds.

So now... if I jump in and do this... how high do I push it?  You and narrowminded have run these tiny cutters.
Is 10,000 RPM enough?    I know from my experiences making the parts I like to make that I'd really like to get down to a 1/64" cutter (about .015").  Do you think I can do that in alumnium and brass, operating by hand, at 10,000 rpm?

Your mill as a variable speed brushless motor, right (the Micro-Mark Mill)?  You if there was a worry about motor damage, you can always just turn down the speed before turning the motor on so ti starts at slow speed. Very little stress on the motor and the belt.  I usually do that on my Sherline.

10,000 should be plenty fast if you go easy with the cut depth and feed rate.  I have not done extensive work with small mills (just some small jobs). It seems that narrowminded has more experience than me.

[mmagliaro]

More data.

The Sieg X2/Micromark with the MT3 spindle uses  6206-2RS bearings.
These are sealed and internally lubricated, which takes care of a lot of headaches because metal contaminants won't get into the spindle bearings and you don't need to grease them.

However, sealed bearings have limited speed because they heat up more.   The confusing thing is that reading the specs on a variety of different brands of 6206-2RS, I see maximum rated speeds anywhere from 6500 up to 11,000.
That's a mighty broad range.    But I suspect that I can definitely go higher than 4600.  The question is still whether I could push it to 10,000.

The bearings are, of course, replaceable.  EDIT.  Yes, they would have to be pressed off and on the spindle.  So not trivial with out the press tools.

[peteski]

More data.

The Sieg X2/Micromark with the MT3 spindle uses  6206-2RS bearings.
These are sealed and internally lubricated, which takes care of a lot of headaches because metal contaminants won't get into the spindle bearings and you don't need to grease them.

However, sealed bearings have limited speed because they heat up more.   The confusing thing is that reading the specs on a variety of different brands of 6206-2RS, I see maximum rated speeds anywhere from 6500 up to 11,000.
That's a mighty broad range.    But I suspect that I can definitely go higher than 4600.  The question is still whether I could push it to 10,000.

The bearings are, of course, replaceable.  EDIT.  Yes, they would have to be pressed off and on the spindle.  So not trivial with out the press tools.

I suspect that the ratings you are looking are for continuous (long-duration) speeds. Your use is what I would consider intermittent use.

[narrowminded]

I would not be worried about the motor because the speeds you would be running will also be at low torque load due to the small cutter diameter, small lever arm.  By style it's a self correcting torque/ horsepower situation.  The bearings are a reflexive concern to me because I've been there in machinery designs.  You smell the situation developing without even taking a pencil to paper.  At some point balance may creep into the equation too but if it's a lot of concentric parts at smaller diameters it may not be so much of a concern.  By getting the bearing number and checking its ratings you are doing the proper preliminary investigation.  If it seems to be in spec it should work but it still sounds like it's coming up pretty short even when running at its limits, something most machinery designers don't like to do which further makes sense of the rating and max speed being what it is.  My own rule of thumb was AT LEAST a two times safety factor and then up from there.  Sounds like they did too as it's a very common engineering design practice and not without legitimate reason.

That's where my own inclination would be to add another spindle as was shown in that video.  The bracket could be made in your mill and the 20,000 RPM tool might be no more than the cost of special pulleys and belts leaving your machine basically stock and with no risk to the spindle.  The one consolation is, evidence the video and repeated specs, the endeavor isn't without cause or merit.  And you've read the charts and experienced the conditions that those recommendations address. 

[mmagliaro]

Anecdotally, I did find a post in a machinst forum by a guy who had one of the Taig mills with changed-out pulleys that he ran up to 11,000 rpm.  He said that after only 15 minutes, he could smell it burning and the bearings were ruined.
He did not say what he was cutting or how much load he was putting on the spindle.  But still, 15 minutes isn't a long time.

Nah.   I don't think it is worth the risk.  I also investigated other bearing types that would fit (same ID, OD and width), and they do not have better speed ratings. 

The thing about the side-bracket idea with a Dremel clamped in there is that the Dremel's run-out is terrible compared to a mill.  I don't know how anyone does precision milling with a small hand tool like that.

[narrowminded]

Anecdotally, I did find a post in a machinst forum by a guy who had one of the Taig mills with changed-out pulleys that he ran up to 11,000 rpm.  He said that after only 15 minutes, he could smell it burning and the bearings were ruined.
He did not say what he was cutting or how much load he was putting on the spindle.  But still, 15 minutes isn't a long time.

Nah.   I don't think it is worth the risk.  I also investigated other bearing types that would fit (same ID, OD and width), and they do not have better speed ratings. 

They don't have different speed ratings because they're all facing and dealing with the same physics.  You can't defy gravity.

The thing about the side-bracket idea with a Dremel clamped in there is that the Dremel's run-out is terrible compared to a mill.  I don't know how anyone does precision milling with a small hand tool like that.

You might be surprised at the difference between a Dremel and a Proxxon tool (I have both) which is why that brand was always mentioned but the Dremel was not (other than reference).  The Proxxon has an aluminum housing, steel collets, and a better supported and truer running ground spindle.  The aluminum housing is not threaded but has a 20mm diameter hub machined true to the spindle bore which you can pinch clamp in a machined, true bore.  Then the steel collets are just better made, truer running.  It's still a low $100 tool so it's not the very best you could ever have but I have found it to be a viable and capable hobby alternative.  There are better industrial production use and even higher speed spindles available but they carry a price tag to fit.  Plus, in addition to potentially having a high speed spindle for your small cutters you will also have a really nice "Dremel" tool anytime you aren't using it in the mill.  And an extra Dremel can be downright handy.  Here's a link to the unit I'm talking about.
http://www.proxxon.com/us/micromot/38481.php?list

[mmagliaro]

Now that's impressive.  I guess I never thought about the Proxxon being a precision version of a Dremel.  Ha ha!
I will seriously consider this.

[rodsup9000]

  Max,

 Here is my belt setup on my X2 that I'm converting to CNC. With it the spindle rpm is maxed out at about 8000 rpm.  I read somewhere that there is a bearing set available for the X2 that is good for 16000 rpm.  Just need to go back and find it now.




 

[kondor]

From the factory the Taig CR mill will do 10,000 RPM.  I have used mine at that speeds routing circuit boards and the spindle housing does get warm. 

http://www.taigtools.com/mmill.html

Here's some chatter on spindle bearings for the Taig Mill
http://taigownersclub.forumotion.net/t99-taig-bearings