New project: HiWin or Comtop rails?

[nash]

Hi All,

After lurking for a while, this is my first real-post.

I'm designing a gantry mill with a 2000mm x 1700mm x 900mm of work envelope. I'm not a CNC machinist but am hoping to learn from this experience and from your wisdom. What an excellent site!!!

The machine frame (under design) is currently in 120mmx80mm-8mm thick rectangular CRS tubes, welded and stress relieved. The stress relieving part still needs to be taken care off. The base frame is 2500mmx2000mmx400mm.

The gantry is also in the same section CRS frame as the base.

The Z axis is built out of a 220mmx220mm-8mm thick CRS tube and welded to a 120mmx80mm CRS tube.

All the axis have linear rails with 2 carriages each. The design is not yet totally done. I'm in the process of ordering the rails+carriages and precision ground (C5 grade is all I can afford) ball screws. The two makes of ballscrews and rails that fall in my budget are:

www.hiwin.com lead time 5-6months by a local supplier
www.comtop.com.tw lead time 3-4months by a local supplier

I'm looking for advice from someone who has seen/tried/has-experience with these products from them as far as rails+carriages and ballscrews+nuts go.

Please respond.
thanks
N

[nash]

Has anyone heard of comtop? Has anyone used comtop? Any experience you'd be willing to share? I really need some input to order my stuff.... Please HELP!

N

[Mat-C]

No direct experience... but a few suggestions:

2 carriages per rail seems odd, don't you need 4 to get any kind of stiffness at all?

Will the CRS tube be stiff enough? (Depends on your design of course) Have you calculated deflections/stiffness? Any plans for damping?

Those leadtimes seem excessive for what should be stock parts I would think?
These guys have online prices, which may help: http://www.technico.com/lm_guides.aspx (never used them myself).

How do you plan to do the stress relieving?

[ViperTX]

Are you making a wood router?

[tivoidethuong]

i choose Hiwin rail

[nash]

Hi All, thanks for your responses:

You can look at this thread for a sketch of the mill.

Matt, there are two carriages per rail and one pair of rails per axis. So there's no degree of freedom other than the linear motion along the axis of the rail-pair. I've done the calculations for deflection and stiffness. It meets my design expectations. The machine weight is around 800kgs, for the size its not heavy, but its structural steel which with the high stiffness will definitely need some damping. I've not yet looked at damping and will do so when all the elements have been defined. I am actively searching for a stress relieving location in the vicinity of Parma, Italy. Does anyone know of a place here that could do the thermal treatment of the frame?

ViperTX, this is for milling (not just routing), wood, foam, modeling clay (like the epoxy compound used in making master models), in rare cases for aluminum at low feed rates (1-3mm/s).

Tivoidethuong, good that you've made your choice. I have some factors to think of in the decision. Mainly, the ball screw supplier has told me that comtop quality is better than hiwin (they are also a wee bit more expensive) and that all the local companies use them as replacement for more expensive screws on used machines. I have no other way to check these other than this forum. The comptop on its site shows all the certifications for their products and they don't look any different... any experience?

[Mat-C]

Sorry, that's 4 carriages per axis, now I understand.
Damping... just in case you haven't seen it, here it is again: http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf
No idea at all on stress relieving.. presumably it is heated and cooled slowly? Sounds tricky for something this big, although $1000 sounds rather expensive.

[nash]

Mat-C, Yeah, Thanks a lot for the link, luckily, I've read that thesis. Very interesting. Actually, I'd love to redesign the structure in sheet metal filled with dense material that could absorb most of the high frequency vibration that would come from such a stiff structure, however, that would make the machine bulkier and not that easy to transport (risk of losing all accuracy as well). There's always a trade-off. I need to think of some other means that could help in damping.

Good news found, a heat treatment facility. Just posted about it on the other thread.

Is there no one with comptop ball screw experience?

[RICHARD ZASTROW]

nash
When I Googled comptop their ad was displaying THK linear ways. This is a high quality but expensive choice here in the USA. I believe comptop to be a dealer rather than a manufacturer, but I could be wrong.

[nash]

Richard, in Italy, the comtop (the one I mean is www.comtop.com.tw) is a tiny bit more expensive than hiwin. I think thats still way under THK. I don't know more than a couple of conversations, a price quote from the dealer here and the site that he pointed me to. I don't know anything about thk, maybe one of you who knows that could look up comtops site and see if they are indeed talking about identical part-numbers or suspiciously similar. I'm leaning towards buying them, mainly due to their relatively shorter delivery time for all the screws and nuts from one place, and their price.

[RICHARD ZASTROW]

nash, I looked at several sites with comptop in their name. One was from India selling ballscrews and linear ways (indiabearings) and another selling the same thing from China-Taiwan The one from England, as previously mentioned, showed THK in their ad. It begs the question, does THK, Hiwin and comtop all have their products manufactured at the least expensive location and then market them under different names in the marketplace that will pay the most? I still do not understand the long delivery time whith all the fast delivery companies in the world.

[nash]

Richard, my $0.02, the precision ground ball screws sound expensive and rolled ones cheap. Today most of these companies manufacture supposedly C5 rolled ball screws and are willing to deliver rather quickly. Rolled ball screws are easier to manufacture as well. Thus they should have a larger margin in a rolled product and maybe they're trying to dissuade us from ordering precision rolled vs. precision ground by unreasonable lead times. BTW it is quite likely that comptop is a contract manufacturer for some big names. I donno and don't care so long as my screw is within tolerances and is supplied in the time they agree to. BTW the delivery will be via ship (which takes 2 weeks). I was told that it take around 5-6 weeks to get the grinding and lapping done at the factory after the part is premachined, and heat treated. I've also asked them about standard lengths and have yet to get a response. However, I found the guys at the Italian dealer are very knowledgeable and helpful.