Need Help!- Re-grinding spindle for new collet system?

[Stuff-Builder]

I first posted this under the Knee Mills category, but thought I might get more suggestions here.

I recently bought an old Gorton 8D knee mill. It has a 300 DA collet system that will accept a 1/4" dia cutter.

I'm planning on having the spindle re-ground to accept some other collet system that will have a larger capacity. I'm leaning towards a #3 Morse Taper at this point. There's plenty of adaptors and tool holders for this size, and I figure it can handle the 1 hp spindle motor. If I went this way would I need to have the "tang" feature milled into the spindle as well?

Is there a different collet system that would be better? The machine does not have a draw-bar or the capacity to accept one, so systems that tighten at the spindle nose are better (ER perhaps?) Any advice would be appreciated.

Thanks in advance,
Scott

[RICHARD ZASTROW]

Scott, Morse tapers or any other tapers have a tendency to fall out of milling spindles if they're not held in mechanically. Also, spindles are usually case hardened. Re-grinding might break through the case or allow distortion in the thinner case hardened wall.

Dick Z

[Stuff-Builder]

Dick, thanks very much for the response.

I know MT3 toolholders are often used on drill presses, and a few of the small milling machines. I thought that might be an option here. Perhaps they're better suited to drilling and other straight-line operations, rather than side-milling.

My problem is that what I have now is broken and needs to be redone - one way or another.

Could I take a straight-shank ER tool-holder and press-fit it into a new reamed straight hole in the spindle? I really like the ER collets, since they can cover a wide range of cutters, are easy to use, and are held to tight tolerances.

What do you think would work here?

Thanks, Scott

[jmkasunich]

As Dick Z already mentioned, Morse is not a good choice unless you can fit a drawbar. In a drillpress, there is always axial load to keep the taper seated. In a mill, you get side load, and its only a matter of time before the tool comes out. If you are cutting with the side of an endmill, the helix adds axial force in the wrong direction - trying to pull the tool out of the taper.

If you absolutely can't fit a drawbar, then something like ER is probably the best bet.

Regards,

John Kasunich

[RICHARD ZASTROW]

Scott, Is it possible for you to reproduce the existing spindle with a more appropriate business end?

If the existing spindle can accept an insert without serious weakening, that could be your solution. Very careful and fussy work. Shrink fit (interference fit) and finish bore (grind) after assembly etc.

Dick Z

[jmkasunich]

Heh, this thread sounds familiar.... I'm in the middle of "re-tapering" a spindle myself. See this thread:

http://www.cnczone.com/forums/showthread.php?t=64191

[Stuff-Builder]

Richard and John, thanks for your input! I need all the help I can get!

John, that's some nice analysis you're doing on those surplus spindles. Good luck with the end results.

Richard, do you think press-fitting a straight-shank ER tool-holder into a reamed hole would give acceptable results, or would it likely be too far off for decent work?

I've pondered having it machined to fit a morse taper and then hitting it with loctite. That might not have good results, though.

What's considered "ok" for TIR on a manual mill spindle? Before it broke, the 300 DA nose was good to within 0.001" (as best I could measure), while the ER40 adapter was at best 0.005". What's the starting point as far as decent surface finishes and tolerances. With my capabilities I'm happy to get anything within a thou or two, but maybe with the right setup that would be different.

I don't have any of the capabilities to do this work myself, so whatever I do will have to be through paying a machine shop or bribing my crusty-old-machinist friend to help. I still need to call him and see what he thinks.

Thanks again for the replies.
Scott

[jmkasunich]

A drilled and then reamed hole isn't likely to be concentric with the spindle axis. Better to drill, then bore, and if possible grind.

If you could come up with a dimensioned sketch of the existing spindle it would be easier to discuss alternatives.

It seems odd that the original spindle could only take a 1/4" cutter. If you radically increase the bore, you might regret it. What kind of tooling do you want to use on the mill? What spindle speed range does it have? Spindle speed and diameter determines the cutting speed - fast spindles want small tools, slow ones want large tools.

Regards,

John Kasunich

[Stuff-Builder]

John,

I snapped a few quick photos of the spindle. You can see the brilliant job I did destroying the 300 DA nose (don't ask, I'll just feel even stupider...). To give you an idea of the scale, the thread on the 300 DA is 3/4"-16, and the OD of the nose is 5/16". The portion of the spindle protruding through the lower bearings is about 1-3/8" OD.

These mills were originally made for high-speed machining with small cutters in tool steel. Tool and die machines, basically. I want to use it for general machining, mostly in mild steel and aluminum. It's got a 1720 rpm 1 hp motor right now, but I hoping to get a hold of a 1 hp 3 ph motor and VFD from a former co-worker. The original motor pulleys will turn the spindle quite fast, but the PO put a smaller pulley on to reduce the speeds.

Realistically, I'd like to run at least 1/2" shank tooling, be able to use a fly cutter for light passes, etc. I don't need to push things to the max - this is just prototypes and weekend messing-about. If I had an ER system that'd probably do everything I'd need.

Best regards,
Scott

[TOTALLYRC]

From the looks of the spindle where it broke off it looks pretty hard to me all the way thru but that is just a quick opinion based on what I can see in the picture. The OD of the spindle looks to be a lot larger than the diameter of the da threads. It looks like you could use an er style without any trouble.

I think a straight shank adapter would work but I would say a 3 to 4 thou interference fit to keep it tight and that means a heat shrink fit by heating the spindle or chilling the adapter with liquid nitrogen or something. Either way I am not sure about affecting the hardness of either item. As long as the hole that was put into the spindle is dead on the adapter should be very close as well when it is all said and done.
I would shoot for as little run out as possible, on the order of less than .001. The more runout the faster the cutter wears out because one flute is doing more of the work than the rest. If you can find a spindle reginding shop and ask them what they would charge and what they would recomend that you do in the first place. There is one guy that was on ebay but I haven't looked for anything like that in a while.

A good spindle shop just might be able to put and er 32 or something similar right on the stub that is left.

Just my 2 cents, Mike

[Stuff-Builder]

Thanks to everyone who chimed in.

I've been pondering a different direction this morning. I could buy a large straight-shank collet-holder - like an ER40 or a DA 180 - and have bearing races ground right on to that. I'd just pitch the center of the existing, and put the new piece in instead.

What I like about this idea is the straight-shank collet-holder would already be precision ground and hardened. Adding some bearing surfaces with an OD grinder seems simpler and less expensive than doing an ID grind to fit a taper or straight shank. Also, the tool holder would be by nature much more rigid than smaller fare.

What do you think?
Scott

[TOTALLYRC]

Originally Posted by Stuff-Builder Thanks to everyone who chimed in. I've been pondering a different direction this morning. I could buy a large straight-shank collet-holder - like an ER40 or a DA 180 - and have bearing races ground right on to that. I'd just pitch the center of the existing, and put the new piece in instead. What I like about this idea is the straight-shank collet-holder would already be precision ground and hardened. Adding some bearing surfaces with an OD grinder seems simpler and less expensive than doing an ID grind to fit a taper or straight shank. Also, the tool holder would be by nature much more rigid than smaller fare. What do you think? Scott

If you can get one with enough shank to do the job and if it can be ground properly you should end up with a couple of tenths runout.
I like it.
Mike