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View Full Version : Building a hybrid CNC mill. Need advice and recommendations



oldmam4m80s
13-08-2013, 11:07 AM
Hello All,

i am in the process of building a Cnc Mill. I have a rough table built in Sketchup and am in the process of buying the material for the table. i am using 4"x8".25" tubing A500 B steel for the table build. I would like a working area of about 5'x10' I know this is rather large but the table will have its purpose with these dimensions.

I am also looking into some railing for the axese and this is where I have little knowledge and experience. I am looking into some used railing and I am unfamiliar with manufacturers and quality. I am currently looking at rexroth (Bosch) rails. They have some discoloration on one of the faces of the rails. Looks like it may be rust or something else. I dont believe that the surface is rough in the areas the bearings are going to make contact with but any advice on how to choose these rails would be greatly appreciated. If these dont pan out for some reason there is a used set of THK rails that may work but they only have three. I need four for my build so I dont know exactly what to do here. The location of the rails are about two hours away form where I live so i can go and see them and test them out. Is there a way that I can measure the rails with precision equipment to ensure they are worth buying? Should I take some calipers or other devices to help ensure these are still within machining specs? Any advice on how to choose the rails is greatly appreciated.

For the movement of the axese I am using DMM Technologies AC Servo kit. The motors are 750 watt and specs can be found on this page: http://dmm-tech.com/Files/dmmmaxs14a_p.pdf

I have not yet decided on Ballscrews. I would like something that is better for precision and accuracy more than speed. I dont know if I need a low pitch in the ball screw in combination with the motor kit I am using etc... I need advice in this area as well.

My current budget and design are in choosing the rails at this very moment so I need help most in this are. Motors, ball screws, and spindle can hold off but if you have information and would like to contribute in any area it is appreciated. Criticism positive and negative are all appreciated as it can help me strengthen weak points in this build. Thank you all and I look forward to hearing your thoughts.

Robin Hewitt
13-08-2013, 01:31 PM
I would start out by taking a piece of this here 4" x 8". Support it on two blocks separated by the width of the bed. Put a DTI underneath it in the middle. Estimate the cutting load. Put an equivalent weight on top and see how far it moves.

That would be a good start. Get some numbers to work with.

mekanik
13-08-2013, 02:26 PM
Hello from across the pond & welcome to the forum, best bet is to include a drawing of your intended machine/what you will be intending to machine with it.
That way the more experienced members can point you in the right direction.

JAZZCNC
13-08-2013, 05:33 PM
Regards the rails then it's virtually impossible to tell you if they are ok without seeing and feeling the actual components. Even if you post good quality pictures describing the feel of preload etc is very hard to describe.

If you haven't any experience with them I suggest you try to find someone local to you who does and ask them to go look with you or look/feel at some they have.

oldmam4m80s
15-08-2013, 03:32 AM
Thank you gentlemen for your responses. I can try to run the motion test recommended on first response but I am unable to do it easily. Location is a good distance away. I understand not being able to test the material unless tangible but I was wishfully thinking it were possible.

I would like to machine aluminum alloys, some mild steel and maybe even some harder steel alloys. The motors are 750 watt DMM Tech ac servos. I am unsure what these are capable of machining combined with the right spindle. I had been reading people having good and bad luck with the China made spindles on ebay but i am looking into something like those only with reliable quality. What general spindle information do I need in order to figure out what material I can machine. i figured 2.2 kw water cooled spindle could machine quiet a range of materials with speed and accuracy beuing the determining factor? Am I wrong with this train of thought?

I need to try and find some ball screws. I have looked into NSK, BSA, NOOK, THK... These all seem to be reputable manufacturers so I thought I would be ok going with one of these depending on price. Do all ball screws for the 3 axese have to be the same pitch? Can I go with a #10 pitch x axis, #6 pitch y axis and #14 pitch z Axis?? I am assuming higher pitch ball screws have better accuracy with diminished torque ratings just an observation. If anyone can help a lending hand on the understanding of ball screw selection? Experience and or knowledge of the topic is greatly appreciated. Or just copy and paste a quick link and I will read it. Gotta run for now be right back...

oldmam4m80s
15-08-2013, 05:22 AM
ok, I'm back for a moment. I apologize for for abruptly leaving. Had something to attend to. Thank you all for the warm warm welcome and friendly advice thus far. I am feeling a lot less confident in purchasing the used equipment i was talking about in the first post. I need this build to be accurate and precise. I looked into the Chinese ball screws and railing and they have something like a .05 inch loss per per foot in ball screws and rails im sure are machined to similar standards as the ball screws. Not that i'm bagging a machine with these specs its just not what I am aiming for with this build. I will continue to search around. I looked into NOOK and they have ball screws at about .005 accuracy every foot so that is very desirable, im sure costs reflect that but I will see if this can be something I can attain. Thank you for the advice thus far every one please provide as much of your insight as you wish. I hope to someday soon have a very nice machine working with me.

as far as pics go I will get some off the computer where I have them and post them.
Numbers... Which would you like to know?? I dont have a set speed rating for machining. Dimensions are goinf to be around 4.5xx9.5 ft machining area. Z axis is going to have 14 inches of movement.
I have not quiet figured out a cooling solution for the tool but its in the works.

Mainly now I need to determine the rails to use, ball screws and how I am going to make these work together to get the machine performing a modern miracle.

Thank you all again. Hope to read more from the community.

JAZZCNC
15-08-2013, 06:45 PM
I would like to machine aluminum alloys, some mild steel and maybe even some harder steel alloys. The motors are 750 watt DMM Tech ac servos. I am unsure what these are capable of machining combined with the right spindle. I had been reading people having good and bad luck with the China made spindles on ebay but i am looking into something like those only with reliable quality. What general spindle information do I need in order to figure out what material I can machine. i figured 2.2 kw water cooled spindle could machine quiet a range of materials with speed and accuracy beuing the determining factor? Am I wrong with this train of thought?

Forget the 2.2Kw WC spindle for cutting steel or hard alloys properly it's far too fast and with very little torque at low speeds.
Getting a spindle that is capable of cutting all ranges of materials accurately is harder than you may realise and chances are much more expensive than you realise.? Most likely you'll end up with spindle cartridge type spindle and separate DC Motor with speed control.
The other options are normal AC motor and VFD for speed control or if you really want to splash out servo motor.

In any case it's hard to do/find at sensible money and something many many new optimistic builders fall foul of and end up greatly compromising on.!!

oldmam4m80s
16-08-2013, 07:53 AM
Thank you so much JazzCNC, I needed to hear that. I had not researched that too much. I thought that WC spindle was what i was looking for. i actually have a 2 hp motor with a 4hp VFD just laying around. I bought it for another project in the past and never let it go due to this exact scenario. I have seen this one used in youtube videos for cutting wood but I'm unsure if it would be ok for steel and other materials similar. Do you have any guidance on spindle information for cutting varying materials? I will do my due diligence in the meantine but will look into a recommendation with extra care. Thank you all for taking your time to respond I really do appreciate it. Please keep the info coming for the optimistic new guys trying to further themselves in CNC.

oldmam4m80s
16-08-2013, 08:11 AM
For Thos like me in need of information to make better decisions, i found this site regarding spindle selection. Hope it helps some folks out.

Machining Center Spindles: What You Need to Know : MoldMaking Technology (http://www.moldmakingtechnology.com/articles/machining-center-spindles-what-you-need-to-know)