Question on Z-axis retrofitting

[machintek]

Just a reminder that the original NC machines by Bridgeport had the Z axis as the knee using a stepping motor but also making its work lighter by the air assist feature. The quill used the air over oil Spindle Wizard. Consider 4 axis with the 3rd being the quill and the 4th being the knee.

Conversly, I have seen PROTO TRAKS and MILLTRONICS with sliding heads to overcome the quill travel limitation. Maybe, price out one of those to see if a purchase of a used machine would be cost effective. I have seen new MILLTRONICS, enclosed with a tool changer selling for what my R2E4 cost new 20 years ago.
Still, none of these have your spindle speed requirement.

I installed about 14 EZTRAKS at Boeing in Oak Ridge, TN. They had a requirement that each pass a acceptance/positioning test. With a step guage and some effort, I could get these tuned to where they would position/repeat within a few tenths. But, rapid generated heat, and since these ball screws are not under tension, with the ball screw securing bearing on one end, they did grow and it could be measured very easily. A used machine is always a gamble as to what shape it is in.

Some great answers above!

George

[Neil_J]

Originally Posted by machintek Still, none of these have your spindle speed requirement.

Would there be any way to mount on of these to the spindle? It does 30K RPM and has a pneumatic tool release....

[jdelaney44]

There's probably a way to rig it off to the side, but that will introduce a lot of flex in my estimation. But for high frequency milling this might be OK?? Not sure.

Have you considered scratch building a gantry machine like the wood routers but for aluminum? I think the router forums probably have folks doing this. You may just be trying to get a Bridgy to go too fast.

[NC Cams]

Follow-up to Neil J: You can put any high speed motor you want on the BPT head BUT, you are still going to be speed limited by the speed rating of the spindle bearings.

My original post cited some specs of the OEM p/n bearings as I have some real familiarity with them. They just won't run/live long at 10K rpm as the system wasn't designed for that type of service.

As far as comparing the capabilities/expectations of a Sherline to a BPT, it really isn't a fair or reasonable comparison. One is a hobby, table top system designed for light duty, small part machining.

The other will cut 0.100 of 0.375 steel plate material (shuddering while doing so) but cut it it will and never raise a sweat doing so and remain straight and true after doing so.

I'd strongly suggest some research into the manufacturer's perfomance specs and qualifications for any machine tool you have your eyes on. The OEM's are generally conservative and realistic for liability and warranty reasons.

However, when it comes to cutting and travers speeds, you can bet that they have been tested and proved capable at the published speeds and a bit more perhaps.

If you insist on trying to turn your BPT into a 10k high speed system, you may find it to be an expensive and difficult if not impossible task - especially if you do not have access to the necessary engineering assistance.

Having formerly been a machine tool bearing engineer, I"ve done some "hot rodding" of my machine using prior knowledge of MT's and bearings used therein. There are places where you can tune up the maching by replacing bearings. But, due to space and design limitations of the OEM spindle design, this is one place where it is difficult and quite costly to turn a Ford into a Ferrari.

My prior suggestion remains the same: try to find a machine that, in factory trim, meets 90% or so of your realistic expectations. For the small percentage of time that you may need 10K or some other minor feature/capability cited on your "spec list", it may simply be often cheaper and faster to outsource the work to someone with a Haas or Mazak or.......

I wish you well with your endeavors....

[Neil_J]

Originally Posted by NC Cams Follow-up to Neil J: You can put any high speed motor you want on the BPT head BUT, you are still going to be speed limited by the speed rating of the spindle bearings. My original post cited some specs of the OEM p/n bearings as I have some real familiarity with them. They just won't run/live long at 10K rpm as the system wasn't designed for that type of service. As far as comparing the capabilities/expectations of a Sherline to a BPT, it really isn't a fair or reasonable comparison. One is a hobby, table top system designed for light duty, small part machining. The other will cut 0.100 of 0.375 steel plate material (shuddering while doing so) but cut it it will and never raise a sweat doing so and remain straight and true after doing so. I'd strongly suggest some research into the manufacturer's perfomance specs and qualifications for any machine tool you have your eyes on. The OEM's are generally conservative and realistic for liability and warranty reasons. However, when it comes to cutting and travers speeds, you can bet that they have been tested and proved capable at the published speeds and a bit more perhaps. If you insist on trying to turn your BPT into a 10k high speed system, you may find it to be an expensive and difficult if not impossible task - especially if you do not have access to the necessary engineering assistance. Having formerly been a machine tool bearing engineer, I"ve done some "hot rodding" of my machine using prior knowledge of MT's and bearings used therein. There are places where you can tune up the maching by replacing bearings. But, due to space and design limitations of the OEM spindle design, this is one place where it is difficult and quite costly to turn a Ford into a Ferrari. My prior suggestion remains the same: try to find a machine that, in factory trim, meets 90% or so of your realistic expectations. For the small percentage of time that you may need 10K or some other minor feature/capability cited on your "spec list", it may simply be often cheaper and faster to outsource the work to someone with a Haas or Mazak or....... I wish you well with your endeavors....

The KaVo spindle I was asking about attaching to the spindle is actually a small-ish high-speed/low-torque type. I was wondering if there was a way to fit it into the taper or attach it to the side and use X/Y offsets. After talking to people, both in the forum and our machinists at work, I'm not sure that's the way to go.

It looks like I will be buying a knee mill of some sort in the next six months, retrofitting it, and use it to start building a decent Gantry/mini-VMC with it... I doubt it would ever be sturdy enough to accurately mill steel, but it would handle my engraving / panel making / PCB manufacturing / aluminum cutting needs.

[jdelaney44]

OH, and on the cost thing, Hood's right, a big driver is the cost of the machine. I could have saved $1.5K right there if I'd have waited and looked around more. Next time I will search every used machinery place and auction I can find in the local area. Being that I live in the L.A. area, that's a lot. Not as much as it used to be.

Getting a CNC machine to begin with is a big cost savings. You get screws, motors, motor mounts, the power supply, and cabinets at the very least and probably don't have to pay too much of a premium for the privledge.

The other cost driver is over engineering. I bet I could save another $1.5K on the next go 'round just cause I know what can be made lighter and how to do things differently.

[Cold Fusion]

I've actually had Z axis issues on my mind, so this is the perfect thread. Some Series II Bridgeports have their Z axis on the knee. While the 15" travel is nice, it's a LOT of weight to move around. The air piston counterbalance helps, but don't expect much speed in your rapids. What I am considering right now is keeping a powered servo on the knee, and adding a gearhead servo on the quill. The 5" travel on the quill will be my live Z axis, with the powered knee as my A axis. During tool changes I'll be able to lower the knee to accommodate longer tools, then lock it during actual machining.


Speed and accuracy don't really go hand in hand on a retrofit. I've got about a thou backlash in my ballscrews, but with compensation this isn't a problem, under 30ipm. After that, things start to stutter and evidence of overshoot is seen. I can hold .001 if I really try, but I have to kick the feed way back. Normally I run near the limits of my 3/8 HSS endmills and can hold .002-.003 without a problem.

[Neil_J]

Originally Posted by Cold Fusion I've actually had Z axis issues on my mind, so this is the perfect thread. Some Series II Bridgeports have their Z axis on the knee. While the 15" travel is nice, it's a LOT of weight to move around. The air piston counterbalance helps, but don't expect much speed in your rapids. What I am considering right now is keeping a powered servo on the knee, and adding a gearhead servo on the quill. The 5" travel on the quill will be my live Z axis, with the powered knee as my A axis. During tool changes I'll be able to lower the knee to accommodate longer tools, then lock it during actual machining.

yep, I've been convinced that buying myself a knee mill and motorizing the knee is the way to go. Definately post back to the forum if you make any progress... I'll be posting my stuff in a month or so....

[NC Cams]

If you are trying to hold PRECISE position tolerance, moving the knee is not necessarily the way to go.

In my case, I have to do precise concentric milling with concentricity held to a ground center hole on a fixture.

THus, I first lock the knee and center on the hole w/precision plug gage (0.0015 clearance). Fast and accurate enough for set-up. If you then loosen the table and move up or down, you'll find that the plug gage won't re-index. When you combine gib clearance, machine deflection, thermal growth, etc, this should not be surprising. Any way, in our case, we can't simply move and then re-index as the tool path precludes that as an option.

Grante, the BPT wasn't designed to offer jig groud positionlin quality BUT one has to have reasonable expectations. For MOST work, moving the knee up and down to adjust Z capability will work just fine. HOWEVER, one should not assume that things will always end up where you expect them to as you start to move heavy iron up, down and around.

At some point, you just may have to go to a machining center in order to get the Z axis travel you want/need.