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Bridgeport NT30 Spindle Swap and Ballscrew for the Knee.

Aaron-M

Aluminum
Joined
Jun 19, 2020
Hi All,

I have a Bridgeport Clone I'm going to retrofit. Not here to discuss the morals of that decision, but I have some question about swapping the spindle to a NT30 for repeatability. They have a lot of those for sale on ebay and I was wondering if anyone has heard anything about them. I was going to just but the bare spindle then get good bearings elsewhere.

Thoughts and opinions?

Also, for this conversion, I'm going to Z axis the knee and leave the quill manual. Any idea who makes a Z axis ballscrew kit?

Thanks
Aaron
 
I believe you'll find the ballscrew on the knee a bad idea. Ballscrews can backdrive- that is, just the weight of the knee can cause the screw to turn on it's own, and "fall".

Ballscrews are generally used as they have minimal or no lash. On a knee-mill conversion, the same weight of the knee itself takes up the "lash", so you can keep the original ACME screw.

You will find, however, that without powerful servos, the knee motion will not be particularly responsive. Generally, the quill is used for the Z, and the knee, if it's automated at all, is only used for gross height positioning.

Doc.
 
I agree with Doc regarding putting a ball screw on the knee of a “ Bridgeport “. It’s not a good idea.

Regards Tyrone.
I'd go a bit further and say it's a Very Bad Idea. Have you ever disassembled that area? All the crap that finds it's way into there is not an ideal thing to expose a ball screw to. (pardon ending a sentence with a preposition)
...lewie...
 
I'd go a bit further and say it's a Very Bad Idea. Have you ever disassembled that area? All the crap that finds it's way into there is not an ideal thing to expose a ball screw to. (pardon ending a sentence with a preposition)
...lewie...

It’s British understatement, it’s what we do. You’re right, it’s a bad idea for a couple of reasons.

Regards Tyrone.
 
Alright, Ballscrew bad for the knee.

What about the NT 30 Taper spindle?

Thanks

I would be on the lookout for a head from a worn out CNC knee type machine. Someone here probably has one they would part with. Already set up for quill drive and probably has a quick change of some variety. Adapters can be made.

Ed.
 
Alright, Ballscrew bad for the knee.

What about the NT 30 Taper spindle?

Thanks

I keep thinking of doing this myself so it's probably a bad idea...

Actually the main reasons I don't are, I already have a bunch of R8 tooling, 2 other mills with 40 taper spindles, little to no desire to own yet another set of tooling and I regard the B/port as a glorified drill press anyway. YMMV.

PDW
 
Back when I was working at the plastic company they had a cnc'd Bridgeport with some kind of quick change system. Pull the quill all the way up then push back a little more and it released. Great thing was the tool holders were short so it was easy to set multiple tools without using big endmill holders.
 
One of the problems with MMT in a B-Port and the clones that no one mentions when bashing R8 is the tool projection. R8 has very little compared to a MMT. So the only thing you gain is a positive drive which is not a big deal on an R8 machine since most are 3hp or less. In all the years I've use R8 mills I have only spun the tool a few times and that was with 6" saws in low range. I was asking for it so I use caution in low with large diameter tools. I have heard they will break a 3/4" end mill before they slip, maybe they slip then break the mill?
The distance from the tip of the tool to the center of the two lower bearings is very important when it come to eliminating chatter. The closer it is the more ridged it is.

BP's and clones all have a 2 bearing lower which is part of the R8 design,AFAIK. If they were designed for a MMT I'm sure they would have a more substancil quill and bearings.
I have a Wells Index at work and one at home (home mill has a Erickson QC 30). Wells have a 3 bearing lower and a 2 row upper. They would be a good candidate for a MMT.

A few years ago one of our members posted about putting 3 lower bearings in their mills. Don't know which kind or if all the BP's and clones will accept an extra bearing.
I have been thinking about pulling the quill on our Webb and seeing if that is possible.
IOW, if your work requires higher capacity than a BP then buy a mill designed for it.

The Wells are definatly smoother than the other R8 mills I have used.
 
Alright, Ballscrew bad for the knee.

What about the NT 30 Taper spindle?

Thanks
If you go with a ballscrew and drive on the Z you will want a counterbalance or air balance cylinders. This is common.
Advantage here is none of the quill slop in it's bore as it remains locked so a stiffer arrangement.
The NT 30? I see no gain over the R-8 even at 5 HP.
Bob
 
The NT 30? I see no gain over the R-8 even at 5 HP.
Bob

For power transmission, no.

For consistent insertion length so you can pre-set the tool stickout values, yes, IMO.

Is this worth the hassle on a machine without a tool changer?

Dunno. Probably not as far as I'm concerned.

PDW
 
I feel like if even 1% of these CNC conversion projects proposed online were completed there would be a substantially larger number of such modified machines seen in the wild. I have only ever seen working factory CNC knee mills like Prototrak, dead factory CNC knee mills like Bridgeport Boss crap and unfinished DIY projects sold at a loss. This is in 20 years of buying and selling machines.

To the OP- Not simply buying a working CNC machine tool is a mistake. A big one.
 
...pre-set the tool stickout values, yes, IMO.
PDW

This only really applies if every tool for a given operation doesn't require the knee to be moved. Once you have to manually reposition the knee during a tool change you introduce error into all your saved length offsets.

I've always wanted to add a servo to the knee on my mill but I don't know how you'd get CAM to output the correct code for making gross changes with the W axis and only use Z for drilling or quick jumps up and back down. One idea I'd had was to add extra limit switches near the top and bottom of the quill travel and use them to run the table power feed up and down with a summing box on the quill and knee scales so the control wouldn't get lost. But I just keep using it as is when I don't have a way to make the part on my other machines.

The CNC Boss head is a lot different than a quill with a ballscrew on the front, the entire OD of the quill is the ballscrew.
 
Moving the knee between tools doesn't matter - tool probing is done off table height, offset can be set off the first tool called. You touch off the first tool.
 
Thanks Everybody,

So the official answer is maybe. Couple hundred bucks, I probably buy the nt30 spindle. And considered putting an air spring on the bottom to counter balance, but will have to look at protecting the gear area, where the crank goes. Will look into it.

The machine is a 3hp Exacto, it's actually pretty good quality. It has integrated oiling system, so that's how it won the conversion idea. The kit is Microkinetics, with the ballscrews they offer. Board is a Centroid, the servos are all DMM. I've done a bunch of other CNC conversions so this shouldn't be a big deal. I bought everything last year, just didn't have the time, now I have some time.

Garwood, I agree 100% with what you are saying. This is going to be a bastard machine when I get done with it. But it has a very narrow range of expectations, that range does not have a machine you can buy at a price point that makes the task feasible.

Plus I enjoy this kind of stuff, so it makes it a lot less like work.

Thanks Everyone.
 








 
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