Seeking feedback on large bench mill design

Hi All,

Soon I'll be undertaking a new project to replace my CNC'D BF20 which I've outgrown.

Proposed specs:

• 520x, 280y, 550z travels
  
• X and Y motors will be 600W AC servos (Chinese)
• Z motor is yet to be decided on.
• Possibly a counterweight/gas spring to help lift the Z axis (the head alone will weigh over 80Kg)
• 2505 double-nut ballscrews on all axes
• HIWIN 25 rails on all axes
• 2.2kW (3.7kW peak) Servo-spindle (10.5N.m from 0-2000RPM, top speed of 8000rpm, Chinese product)
• BT30 spindle cartridge, 12000RPM max speed. (Chinese product)
• Pneumatic/hydraulic tool release mechanism (comes with the spindle).
• Table will be donated from an old RF45 - that will have the dovetails milled off for the linear rail bearing blocks.
• Machine will be controlled by a KFLOP/Kanalog combo with Mach3/Kmotion

Machine will primarily be constructed of thick-wall tubular steel, welded together and stress relieved before being machined.
I'm thinking about epoxy-sand filling all the hollow sections to avoid the dreaded ringing that's almost certain to occur on a steel structure. Will see what the community says...
Solidworks estimates the weight to be about 400Kg at this stage.


Anyway, half of the components listed are already in my possession, so it's not so much of a fantasy.


You will notice a lot of detail in the photos is absent - namely how the axes are slaved together and other detail like that.

I'm looking for any input that could help.

Pretty nice design. I especially like the wide spacing between the Y rails.

Does your version of SolidWorks do FEA? If it does, you might want to investigate the effect of increasing the cross section of the column vs the braces you are adding. Using a square tube instead of the rectangular tube you have modelled would greatly increase the stiffness of the column (stiffness being a cubic function of the size).

You might also consider mounting the X rails and X servo to the table and mounting the blocks to the saddle. By doing so, the rails can contribute to the rigidity of the table, your saddle can be shorter and you can pocket the blocks into the saddle (both X and Y) to reduce the base to table distance and increase stiffness. As well your rails will have some protection from chips, coolant etc...

If you get the column right, I don't think the weight of the head will be an issue. You will likely not need the extra complexity of the counterbalance though it is a good idea to build it into the design so you can add it later if you need it. Will you be using the head from the RF45 as well as the table?

Good luck!
bob

Quote:

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Originally Posted by rowbare

You might also consider mounting the X rails and X servo to the table and mounting the blocks to the saddle. By doing so, the rails can contribute to the rigidity of the table, your saddle can be shorter and you can pocket the blocks into the saddle (both X and Y) to reduce the base to table distance and increase stiffness. As well your rails will have some protection from chips, coolant etc...

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Good point.
You can get linear rail trucks that mount from the rail side to make mounting easier. Hiwin call them HGW I think (HGH mount from the top on the bearings)

Thanks for the feedback.

I might increase the column from 200x100x9 hollow section (what it is right now) to 250x250x9 hollow section. This would also give me significantly more volume to put the epoxy/sand mixture into.

I was actually thinking that having the very wide (1000mm) X axis saddle would help when machining things at the extent of the table's travel, as this would reduce sagging. I do think it is a good idea to pocket the rails or blocks into the table to reduce the height though. And as for coolant and chip protection, I will probably use one of the products on this page: Machine covers

I probably won't be using the head from an RF45. I fear that attempting to put these items:
Spindle
Servo spindle into an existing head would be very, very difficult to achieve. I would rather not design a head, but there doesn't seem to be a viable alternative right now.
What are your thoughts on this?


And thank you pippin for the Hiwin part definition. I'll be needing the HGW in that case.

I've increased the column size to 250x250x9 hollow section, and have also added a few components to the top of the column to allow a counterweight to be added.Attachment 250610

That is a nice looking spindle cartridge. I am assuming you have a donor mill. If you pull the quill out of the head you should be to open up the quill bore to insert the spindle. Of course that also depends on the height of the head vs the height of the spindle.

I have suggested to a few people that they simply buy a PCNC 1100 mill head from Tormach. They are about USD 1000, come with a motor and spindle, and all of the engineering is done... Of course that is easier for people based in North America. For a few minutes I thought that you could have gone that route and replaced the spindle cartridge with your own (recovering some of your cost by reselling the original R8 cartridge) but the bolt pattern on the Tormach is 103 mm vs 106 mm on the spindle you linked to. This isn't insurmountable but might be too much trouble.

nateman_doo has a couple builds, one using a Tormach PCNC 770 head and another using a head from an Industrial Hobbies mill.

http://www.cnczone.com/forums/vertic...ach-build.html
http://www.cnczone.com/forums/vertic...d-charter.html

As well someone built an e/g head for a 45 series mill:
http://www.cnczone.com/forums/genera...head-hm45.html

Make sure you get a spec sheet from the seller for the specific motor you are looking. What is listed there doesn't make a whole lot of sense to be honest. It looks more like specs for a range of motors and a specific motor will fall in somewhere in there.

You should also look into having two speed ranges. You will need lower speeds for drilling and tapping but with a 12K top speed, there won't be any ooomph left once you slow things down.

bob

Thanks for those links, and especially the one for the RF45 head.
I had considered going down the tormach head route, but given that I'm putting a different spindle, motor and automatic tool release mechanism, i think it would just be simpler to design my own solution than retrofit it to the tormach head.

As for the motor and spindle, the links I provided were similar products: not the same price or same seller that I am dealing with. I also have the manuals and specifications of all the items.

And with the motor, I would prefer to keep it a single speed just for simplicity. I don't intend on doing a whole lot of stuff that would require an absurd amount of torque - but if i did, the motor should help in that regard as you'll see from the fantastic torque it's capable of producing, even from <50rpm.


The detailed specs for the motor are in this PDF:



Thanks for the tips.

250mm column will be vastly stiffer and make a nice improvement.

I'm looking at a cartridge spindle for a lathe build. They seem great, just bore a hole in the head to fit and drill and tap a couple of mounting holes and you are in action.

Did you ever follow through on this? I have been designing my second cnc, and i too have been looking at the atc chinese spindles.

Never did proceed.

I ended up buying a chinese 6012 machine made to my spec (3.2kw ISO25 spindle, 250mm Z clearance, C5 TBI ballscrews and Yaskawa Sigma 7 servos, MQL system)
Retrofitted with a KFLOP motion controller. Can position to about 20um repeatably.

Not exactly in the same realm as the machine I had intended to make here... but it does more of the work I actually want.
Attachment 294192