[Loadedagain]

I have a sharp clone of a bridgeport (http://www.sharpmachinetools.com/mil...achines03.html) which i will be doing a cnc conversion on. after a fair bit of research i've decided the "off the shelf" conversions just don't have all the components and features i'm after. so i'm going to do this the hard way! note - I'm not trying to do this the cheapest way... i'd like to use the best/most reliable components.

I have a good supplier of stepper motors locally who distributes danaher motion products, specifically superior. (http://www.danahermotion.com/product...p?parent_id=50 ) What motor/sizes do i need?

Driving the x and y with a 2:1 reduction, and I'd like to drive the knee for z using a 3:1.

is there a problem with over sizing the steppers?

Thanks in advance... This is just the beginning of a few thousand dumb questions.

Cheers, Colin :cheers:

[mxtras]

No - oversizing is not a problem - depending on your budget. If you are after speed then bigger is better. With larger motors come larger power requirements, etc but in my opinion it's worth the added cost up front.

If you are after 'cheap' then your choices are made for you.

Scott

[Loadedagain]

Thanks for the quick reply mate...

Although cost is a consideration it is not the #1 concern. I'd like to do this once the best way possible.

The motors i am looking at are the KML092F07. They are a nema 34 frame rated at 770 oz-in holding torque. spec sheet here: http://www.danahermotion.com/frame.p...orth%2BAmerica

I think this will be sufficient for the x and y (2:1 ratio)... but will they be enough for the knee? am i barking up the right tree?

cheers, colin

[mxtras]

There are some on this board that have had good success driving the knee - read around. You will find some good info on driving the knee. Some have had success and some aren't happy with their results due to decreased speed. Shooting from the hip I don't think the 770 oz-in will yield a satisfying result for the knee, but I could be wrong. Sorry for the lack of a definitive answer. I am planning to drive my quill but I have yet to get it accomplished.

Scott

[Hood]

I did a Bridgeport manual conversion and used 916 oz/in 2:1 for the X and Y and for the Z I used a 276oz/in 3:1. My Z was the quill rather than the knee which I think is probably the best way to go.
I wouldnt recommend anything less than 916oz/in but you may get away with the ones you describe above. For the Knee however I would say you should be looking at quite a bit more as you have to remember you are fighting gravity when raising the knee and also you will have the weight of vice and workpiece as well.
The bigger the motor you go the slower it will get, or it seems that way to me. I no longer have my manual mill conversion running as I have swapped motors etc onto a Bridgeport series 1 CNC, but I can tell you that I could spin the small motor a lot faster than I could the larger ones, possibly this was because of less weight to shove around.
Just out of curiosity, why are you wanting to do the knee rather than the quill?
Hood

[Loadedagain]

aha! now i start to see the drawbacks of larger steppers. loss in speed. !? .. not trying to do 100in/min... when you say speed... how fast?

i can easily bump up from a nema 34 to a nema 42 (KML111F05)which would then be 1444 oz/in hold torque. but what will the speed loss be? or i could stay with the nema 34 with a higher torque spec (KML093F07) 1155 oz/in.

my reasoning for driving the knee as opposed to the quill? mostly travel. although more difficult due to the weight i feel the gain in travel will be beneficial. i've seen elrods argument about quill vs knee, but... this is a brand new machine so i don't expect the z axis to have an excessive amount of misalignment. maybe i'm barking up the wrong tree.

I did a Bridgeport manual conversion and used 916 oz/in 2:1 for the X and Y and for the Z I used a 276oz/in 3:1. My Z was the quill rather than the knee which I think is probably the best way to go.
I wouldnt recommend anything less than 916oz/in but you may get away with the ones you describe above. For the Knee however I would say you should be looking at quite a bit more as you have to remember you are fighting gravity when raising the knee and also you will have the weight of vice and workpiece as well.
The bigger the motor you go the slower it will get, or it seems that way to me. I no longer have my manual mill conversion running as I have swapped motors etc onto a Bridgeport series 1 CNC, but I can tell you that I could spin the small motor a lot faster than I could the larger ones, possibly this was because of less weight to shove around.
Just out of curiosity, why are you wanting to do the knee rather than the quill?
Hood

[Hood]

aha! now i start to see the drawbacks of larger steppers. loss in speed. !? .. not trying to do 100in/min... when you say speed... how fast?

i can easily bump up from a nema 34 to a nema 42 (KML111F05)which would then be 1444 oz/in hold torque. but what will the speed loss be? or i could stay with the nema 34 with a higher torque spec (KML093F07) 1155 oz/in.

my reasoning for driving the knee as opposed to the quill? mostly travel. although more difficult due to the weight i feel the gain in travel will be beneficial. i've seen elrods argument about quill vs knee, but... this is a brand new machine so i don't expect the z axis to have an excessive amount of misalignment. maybe i'm barking up the wrong tree.

Cant say for sure as I dont enough about steppers etc but as I said there was quite a difference between the smaller motor and the larger ones.
916oz/in with 2.5:1 reduction with 5mm pitch ballscrews on my new mill gives me around 2500mm/min (100IPM), on the last mill I had less reduction and 10mm pitch and I got 4000mm/min.
I never had any issues with lack of power with the 916oz/in on my last mill but I never pushed it too hard as it was a well worn machine. The new mill has the same motors but the pitch is reduced and reduction is greater and there is certainly plenty torque. 1000 oz/in would be more than enough in my experience but others may disagree.
As for the knee/quill debate I opted for the quill as it was quicker and easier to do and manually moving the knee to allow for clearance is no great problem.

Hood

[Loadedagain]

well... i am going to order up the nema 34 (KML093F07) 1155 oz/in. motors for the x and y. once the x and y are up and running i will try one on the z to see how well it runs the knee.

[Hood]

Are you going to use ballscrews for X and Y and also what about Z?
I still think the quill is the better option for the Z but look forward to your findings. I think the Bridgeport series 2 CNC used the knee for the Z but they had air assist which basically balanced the knee so that there was less stress on the motor when raising, kind of like the clock spring does on the quill of a manual bridgeport, might be worth looking into doing something like that.

Hood

[Loadedagain]

now this sounds interesting... my main concern is speed loss due to the weight of the knee, although its weight also removes the need for a z ballscrew. regulating air pressure could easily find a happy medium... time for more research here.

Are you going to use ballscrews for X and Y and also what about Z?
I still think the quill is the better option for the Z but look forward to your findings. I think the Bridgeport series 2 CNC used the knee for the Z but they had air assist which basically balanced the knee so that there was less stress on the motor when raising, kind of like the clock spring does on the quill of a manual bridgeport, might be worth looking into doing something like that.

Hood

[Kerry Harrison]

I see this thread is over a year old but I am curious if you converted your mill?

I am learning how to convert my Lagun mill which looks exactly like your Sharp and could sure use some advice from an experienced CNC convertor

Would you be willing to share how your mill came out and any suggestions on the best motors, drivers, power supply and other parts to use?

Thanks,
Kerry

[mxtras]

I am with Kerry - we need an update here!

Kerry - several members here have converted these machines. Hood, myself, Jdelaney, Demea are just a few. Do some wandering through the Bridgeport forum and start a thread - we will chime in. Especially Hood - he always helps folks out and seems to have an appropriate photo to help answer every question!

Scott

[rogerbsstt]

Hello all,
first time posting.
I have converted my bridgeport eztrak mill to steppers and Mach3 due to cost of replacement parts and the obsolete computer system.
I have used the knee for the Z, I have fitted a 1700 oz/in motor to the knee at 2-1 at the shaft plus the original 2-1 on the knee screw. I have fitted two 100 kg gas struts in line with the screw, to support the knee, and added two counterweights acting from the knee slide end to even out the lift. I can only get 30" per minute rapid on the Z before the stepper starts to lose steps. Fast enough for me. So far so good, but the knee acme thread screw has to much friction and backlash, so I want to fit a ball screw later. Its possible if I reduce the drive ratio on the Z to 3 - 1 instead of 4 -1 I might get a faster rapid. I use this machine for 3D modelling work and am reasonably satisfied with the results.
rogerbsstt

[SCzEngrgGroup]

Hello all,
first time posting.
I have converted my bridgeport eztrak mill to steppers and Mach3 due to cost of replacement parts and the obsolete computer system.
I have used the knee for the Z, I have fitted a 1700 oz/in motor to the knee at 2-1 at the shaft plus the original 2-1 on the knee screw. I have fitted two 100 kg gas struts in line with the screw, to support the knee, and added two counterweights acting from the knee slide end to even out the lift. I can only get 30" per minute rapid on the Z before the stepper starts to lose steps. Fast enough for me. So far so good, but the knee acme thread screw has to much friction and backlash, so I want to fit a ball screw later. Its possible if I reduce the drive ratio on the Z to 3 - 1 instead of 4 -1 I might get a faster rapid. I use this machine for 3D modelling work and am reasonably satisfied with the results.
rogerbsstt

"Fast" and "knee" don't belong in the same sentence. With a stepper, I expect you're getting killed by the torque fall-off of the stepper, as that puts you up at 600 RPM on the motor, and large steppers don't do high RPM. If you want to go faster, I think you have little choice but to go to a servo. My setup is similar to yours (5-pitch leadscrews, 2:1 bevel gears, 2X200# gas springs), except I have a servo motor and 4.8:1 belt reducer. This gives me 75 IPM.

I'm in the process of CNC'ing the quill, at which point the knee will be used only for tool length compensation, so speed won't matter at all.

Regards,
Ray L.