heavy gantry

[drafterman]

I have a table design that I am working out which uses a steel gantry structure (mainly since I have much more experience with steel than wood). My CAD model tells me this will weigh around 90 lbs. without the router (36" x 48" table). My question involves the weight of the gantry and how accelerations will affect the machine. In other words, will it be too heavy for my 425 oz-in motors to control during higher speed operations? Also, do you think I am exceeding the skate bearing rating with this much weight? Please advise.

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[Al_The_Man]

You've made a Cad Model, How about a Motion Model? see the Kollmorgen link.
http://cnczone.com/forums/showthread...=inertia+ratio
e.g. Do you have steppers or servo's? Gearing? Accel/Decel? etc
Al.

[drafterman]

Thanks Al,
I couldn't get the inertia program off the website...it looks like they have moved it. Please excuse my memory of high school physics...it is limited:

At any rate, the two long axis motors are 425 oz-in steppers; coupled directly to a 1/2-10 ACME thread (1 start). I was hoping for around 60 ipm (motors may not turn that fast). What I did as far as a motion layout thus far is to say: if F=MA, then a 120 lb. (3.75 slugs) gantry with router travelling at 60 ipm (.08 ft/sec) will decelrate and accelerate in .1 sec (so 1.6 ft/sec/sec); thus, F=6 lbs. (seems very low to me...am I doing this right?)

Then, if F (oz)=2pi(motor oz-in)(screw eff %)/screw lead....using 35% to 40%
efficiency for my leadscrew, I get 9341 oz per motor or 583 lbs. per motor (seems way high...am I doing this right?)

Please advise

[bearwen]

I know this is an old post but if the info is still needed or anyone else reads this I have 260 oz/in motors with 30 vdc running them I have an unmodified JGRO the only thing I changed was I did not cut out the back on the gantry mine is solid so more weight and I am using Acme 1/2 -10 1 starts and running in 1/2 step mode and I cut at 45-55 ipm depending on bit size depth and what I am cutting hope this helps someone.

[drafterman]

Thanks Bearwen,
I can't quite get 50 ipm out of my machine with a similar set-up. The motors start to "break-up" at close to 50 ipm. I am not exactly sure if this is just my motor limitations or if I have some mechanical binding on certain axes. What is your max. rapid speed?

[cpsheets]

I have to tell you that I have had to adjust speed and feed according to the material that I am working with and I don't work with normal materials, I work with 15,000 year old mammoth tusk that I have to adjust speed and feed to the sound of the material to the load on the spindle, sometimes it does not work with normal feed speeds for metal or wood but somewhere in between. You have to rely on your senses of how much load you are putting on your spndle to determine your most efficient operating speed. No charts or advice can give you the actual operating speed better than just plain experience of listening to the sound of your machine when it operates under load...sorry if this is redundtant of other posts but it is experience that makes a better user.

[bearwen]

drafterman,
Sorry I have been away anyways if your still reading this I have Gecko 203 with 40 volts DC and I am running 1/2-10 acme this thing will rapid consistant in the 80ipm range but I cut in the 40 ipm range there are formulas to figure it all out I did it once for the 269 ozin motors and the 1/2-10 acmes at like 800 rpm or close to it and the numbers were something like 1000lbs of force if you really want the figures do a search here on the forum for mariss he is one of geckos engineers really sharp but anyways do a search for him in the forum he has posted the formulas as a matter of fact I have replied to a post that had mariss posting formulas in anyways good luck and the best thing I can say is when I changed from the DIY cards to the geckos man it was worth the cost of the geckos and then I had the ability to add more voltage which yep you gueseed it allowed me to really make this thing fly the only real limitation I have is that the screws spin so fast that they generate a lot of heat and eventually swell and start to bind but I can actually with geckos and 60v make my JGRO move at over 120ipm.