Little Help with starting a retro

[klxrcr]

Hello all,

My first post but I've been studying up on the retrofit projects others have been doing. Just a couple of questions/opinions I'd like to ask.

First off I'm starting out with a Bridgeport Series I J-head a 1960 model. Seems to be in decent shape ways aren't too worn, the spindle is a bit noisy more of a loud whirring but I think it just needs some lubrication, I haven't got it home yet, but its $800 so I think I could do worse.

I Have a 3hp Yaskawa VFD for it and (Heres where the opinions come in) I want to drive the knee, Because I like lots of Z travel and hate cranking the knee up. Does anyone have expirence driving the knee? These guys seem to like it http://www.cncauto.com/Advantages-to...Knee-Mill.aspx. Plus I like the quill free for tapping and such and i think building a accurate quill drive that can be disconnected would be difficult. If I was to drive the knee how much power do you think it would take? I did a little math (Possibly way off) and came up with about 420 watts to drive the knee at 80ipm. And that calc. is with using the acme screw, I would be using a ball screw but really don't know how much that would reduce the required force.

Whats with the rolled ballscrews? Is the lead really +\- .004 per foot? And a better question is the lead constant through the length of the screw? AKA If the lead is .253 per rotation at one end will it be .253 at the other end? the only other question is the X Y servo sizing I want to run around 120ipm rapid would a 250w servo be correctly sized for that. i Run an EZ-trak at work much of the day and its 100ipm seems to lack urgency so I'd like to run a bit faster, whats about the limit you can run a bridgeport?

I know I can think of more questions but I need to go to sleep,

oh and, thanks for taking the time to read this.
Adam

[wcarrothers1]

Personally I don't think driving the knee is a good idea. After pulling my s1 table off I'd hate to see how slow Z axis travel would be if I were waiting for the entire knee to move the part to the bit. Sounds crazy and sounds like it would really limit some otherwise speedy stuff you would/could otherwise do moving the quil.

I'd agree about a power knee for possition adjustments but not for much else.

I run around 50ipm max on my round pole. my boss 10 machine rapids around 200+ ipm give or take. To me for such a big thing that seemed a little on the to fast side..hehe

b.

[protman16]

Adam,

There have been lots of retrofits done with a moving knee and/or quill. What you may not realize is most use a counter balance of some sort on the knee. Without it your knee servo drive and motor need a lot of power to move the weight of the knee up and down at your goal of 80ipm. Even bed type mills use a counter balance. Along with a counter balance you should replace the Acme type screws with ballscrews. This will help reduce the loads on the drives and motors. As for a source for ballscrews try Hiwin, they make a kit that is sized specifically for a Bridgeport/clone knee mill. Whomever you get your screws from be sure they include a screw for the knee (same as the y axis).
As far as speed goes, with good ground screws, a counter balance properly matched to its mass, and some 1.2 - 2.0KW motors and drives you should be able to achieve 500ipm all day long. Provided you have very good way lube system in place.

Regards,

Trevor

[wcarrothers1]

aaa counter balance... that makes sense..hehe

Never seen one in person that was like that.. Putting the table and Knee on my s1 last week the weight of that table is fresh in my mind..hehe

b.

[klxrcr]

Yeah I am planning to use a pair of (at least) 200lb nitrogen springs, I forgot to add that in. something else I wanted to know, all of the knee drive models I've seen drive it through the hand crank, should I worry about the backlash in the gears or will it not be a problem with the weight of the unlocked knee (always having force in a single direction) on it?
80ipm I think will be an ok speed on the knee, most of the time the Z will be operating in less than a 6" work envelope so speed shouldent be a problem. Just for hobby use anyway, no deadlines to meet.

I checked on the hiwin screws they seem to be a very high quality product, maybe a little "too high quality" $$$$ but I haven't got a quote yet and they also have some pretty good rolled screws.

protman,
So youre saying that I should use a Y axis screw for the knee, just wanted to clarify that, I haven't seen any kits that come with the screw for the knee.

Also I'll be picking up a set of 3 servos tomorrow, found them on eBay for 149.99, and the guy lives 15mi. from me so I'll save $60 shipping, .5kw all Identical W/ encoders. I wont be running this thing at 500ipm as that would scare the crap out of me. I wince a little when hitting GO on a new program in VMC's and thats with the door shut . I definitely don't want to be standing in front of it.

I checked out the mill a little more today, I think the spindle may take a crap on me. I checked it with a test indicator and could not find any play in it, but it sounds a bit like a skateboard going 60mph down a paved road. any Ideas? (other than replace them of course ).

Where do most people source their timing belts and pulleys from?

Thanks for the input!

[FranH]

Some questions needing answers.....lead screws need changing. Will the +/- .004 keep you happy?

Do you have the time to search out different diy parts for the retro?

Have checked with with Southwestern Industries? They have a diy package complete with ballscrews, motors, and controls.

Not sure about the 200 rapids.

PM me your progress (I'm doing one now)

FranH

[protman16]

Originally Posted by klxrcr protman, So youre saying that I should use a Y axis screw for the knee, just wanted to clarify that, I haven't seen any kits that come with the screw for the knee. Also I'll be picking up a set of 3 servos tomorrow, found them on eBay for 149.99, and the guy lives 15mi. from me so I'll save $60 shipping, .5kw all Identical W/ encoders. I wont be running this thing at 500ipm as that would scare the crap out of me. I wince a little when hitting GO on a new program in VMC's and thats with the door shut . I definitely don't want to be standing in front of it. I checked out the mill a little more today, I think the spindle may take a crap on me. I checked it with a test indicator and could not find any play in it, but it sounds a bit like a skateboard going 60mph down a paved road. any Ideas? (other than replace them of course ). Where do most people source their timing belts and pulleys from? Thanks for the input!

Adam,

You won't find any ballscrew kits that include a screw for the knee. Since the knee and y axis are both left hand (usually), just order a second y axis screw for the knee.
Try running your spindle when it's in neutral, this should help determine the source of the noise.
Most conversions use "L" size timing belts and pulleys. They are low cost and work very well, although there are a couple of down sides to them. First it’s hard to find aluminium pulleys that are low cost. Aluminium helps to lower the inertia that the motor has to overcome. Second the "L" type pulleys tend to cause a torque ripple when the belt is tight. Loosening the belt can induce lash into the drive train. HTD type belts are better because the have a rounded tooth profile.

Regards,

Trevor

[titchener]

I have a Mach3 controlled 3 axis CNC converted knee mill with Z on the quill and use a "floating" tapping head to do power tapping on it and it works great. This requires a VFD with a voltage speed input that Mach can set to control the spindle speed. Search "tapping" on the Mach forum for more info on this. I bought an ETM floating tap holder but then found cheaper ones from www.maritool.com .

I'm in the process of powering the knee also as an auxiliary axis, but I'll still use the quill for machining, the power on the knee will just be to make setups faster (I'm tired of turning the crank).

I also have a manual BP clone mill that I thought I would still use for quicky tapping jobs but I hardly ever turn it on anymore, its just easier on the CNC mill.

Paul T.

[klxrcr]

I don't think I'd be happy with +\- .004 if thats the best rolled screws can do. Thats not anywhere close enough for dowel pins, and I'll be making some motorcycle engine components from time to time so it needs to be pretty good (not sissy harley parts, 15000 rpm turbocharged parts ). I run a 2 axis Ez-trak at work pretty often and can put it within .0005 if I have to (usually I don't, thank god). So if I could approach that it would be awesome.

The servos I got Are in excellent condition they apparently used to drive some monstrosity of a ram edm. So they've never really been beat on and edm's are never in much of a hurry. The only problem is they have 17 pin sumtak opt. encoders which I doubt will work with a gecko. So I guess I'll get some new encoders unless someone knows how to narrow these down to 5 pins.

The servos are rated 65v, 10.5amps, .5kw, and continuous rated. this is within the capabilities of a gecko 320 isent it? is the rated current Its max?

I'll be making all the motor mounts and stuff myself to keep costs down, I'm also trying to buy a house right now. Does any one know where to get a PSU to power these servos? I have 3 of the ones mentioned above.

The noise definitely seems to come from the spindle, I can max out the rpm in Low gear and its quiet as can be. I'll probably disassemble it and see how the bearings are, they just sound really dry. And to simplify the job I'm planning on throwing all the power down feed stuff in a box and putting it on a shelf. less parts to figure out.
I need to learn how to stop typing so much.

Thanks!

[protman16]

Remember that most motion controllers have screw mapping. Which means if you have a rolled screw with a preloaded nut, you can easily tune it in to +/-0.0005".
The servos you have will match up well to a G3x0 drive. Max volts/amps a gecko can handle are 80Vdc @ 20A. The encoders on your motors may still be useful. You will probably end up only using 5 of the 17 pins for the encoders. The thing to remember is to look for the supply connections and the A and B channels. Don't use any inverted outputs from the encoder and it should work.
For a power supply, you should be able to find one online (google, ebay, etc.) for a good price. Conversely you could build a PS from one of the many free schematic sources available online.

Regards,

Trevor

[klxrcr]

thanks for the help, I dident know the software could map the screw so accurately, I'll have to research the encoder some more and see what I can figure out, I'll attach a pic of the encoder connections if any one happens to have some knowledge of them. They are 200mA so I'll have to use a separate power supply, the gecko's won't take it.

Thanks!

[Al_The_Man]

One source for encoder supply is the PC 5v (spare HD plug).
If that is a TTL differential encoder then it looks like A is the A ch, F is B and D is the marker J & K are the 5v supply and H is Gnd (shield).
It also appears they have hall effect outputs (R&S) If so, they are either BLDC or AC servo's, Do the motors have brushes?
Al.