What are you guys getting in terms of stiffness?

Hey guys, first post here. I built a joe2006 last semester for a mechanical engineering project and am pretty happy with the results. I have spent a good year lurking around on this amazing site. Such great info.

I was wondering if anybody has or could measure the stiffness at the tool for comparison. I am measuring a deflection .043" in the y (short axis) and .052 in the x (long axit) with a 20 lb load which equates to 465 lb/in and 384 lb/in stiffness respectively. The load is attached to a 1/2 endmill about 1.75 inches below the spindle.

These numbers seem pretty low compared to what I have read online. What are you guys getting?

Haha, I am assuming the 90 views so far are people who don't yet have a machine. I'm kinda surprised this most awesome site doesn't have more stiffness measurements. Seems to me like one of the most important machine factors. Anybody?

Congratulations on having a machine! I'm one of the ones without one yet. I'm working for the last year on a (probably crazy) plan for a 3' x 4' table with a 12" tall x 6" deep gantry. The gantry sits right down on raised X rails, so it will have a tiny sideways wiggle. Z clearance is 6", total Z travel is 12" (!). My calculations so far show a .001" deflection with a 100 lb load at the tool tip (adding up several .0003 deflections). That stiffness will probably cost me $1000 more than otherwise (Big gantry, HIWIN rails instead of Glacern, heavier gantry needs larger steppers with better drivers and bigger power supply). My 1st quote on just the metal was $1500. I then decided to actually do the deflection calculations and lighten it up a bit where I could. I ended up adding T-beam stiffeners to the Z. With lots of sweat and luck we will see what stiffness my virtual machine actually has in about a year. Enjoy your real machine!

Cheers,

Steve

Thanks. Now that is a stiff machine. 100k lb/in. According to this guy you should be able to cut steel. Is that a goal of yours?

My plan is to be able to cut aluminum and hard wood with a clean finish. I also want to be able to do 3D work on 6" thick wood (so very big Z). These two things work against each other. I really don't want to have to start over after making this machine to get a machine that will do this.

I think it will be able to cut mild steel with a very small cutter and very slow progress as some others have done. I am really looking forward to trying it! One of the main problems with cutting steel is that any router's slowest speed is about 8000 rpm (that's what my Makita router can do). I read that you really should have around 1000 rpm for steel. I haven't figured out what kind of spindle/motor you could swap in to do that.

I'm slowly working right now on replacing the plastic parts with baltic birch parts. I'm also incorporating some dust collection and sharpie mounting. My ultimate goal is to increase the stiffness in this area. I will post my results. Would still love to hear some numbers from other joe guys.

I am so pleased to see people address weight and geometric rigidity and the effects of mass properties on stepper size and life span.

IMO, weight that does not do any work is bad engineering and the domino effect elsewhere in the system is never positive.

I am about to build a Joes2006 but have tossed the MDF aside, choosing instead to build the first iteration entirely out of Apple Plywood (Baltic specs-lower costs and available in 4' by 8' sheets)

Were I not in so much of a rush to start cutting parts for my prototype designs, I'd reverse engineer the gantry and Z-axis fixture around composites of vacuum bagged low density structural foam cores and multiple plies of bidirectional fiberglass.

Since this is my first attempt at building and using a CNC router, I'll stick to the basic design and gain some experience first.

Still, on the basis of 30 years as a mechanical designer, I think the questions being addressed here are valid and pleased to see the responses.

L.J.

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Originally Posted by steve123

One of the main problems with cutting steel is that any router's slowest speed is about 8000 rpm (that's what my Makita router can do). I read that you really should have around 1000 rpm for steel. I haven't figured out what kind of spindle/motor you could swap in to do that.

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I have several machines in my shop that require variable spindle speeds and the ability to change speeds on the fly without having to stop the machine and change drive belts & pulleys.

I've used two solutions:

1. I used a high-torque 28VDC motor and a solid state variable frequency circuit.
2. I used a 3 phase 220VAC motor with a single to 3 phase variable frequency inverter.

Of the two, the 3 phase motor offers the greatest utility because the inverter allows me to select soft start and soft stop and has a torque compensation algorithm that applies more power at slow speeds.

You can regulate the spindle speed, right down to zero rpm if need be or you can even over-drive the motor with 50% higher frequencies than the standard 60CPS provided it has good shaft bearings. This option may not be needed, however, if you have alternate sheaves on the quill shaft and the ability to move the belt to a different ratio when cutting harder materials.

The downside in the "Z" axis fixture is the weight of even the smallest 3 phase motor. It isn't difficult to mount the motor in a fixed mount in the back of the gantry and have a sliding quill such as we use in vertical milling machines.

Properly designed and built, the "Z" axis stepper should enjoy the diet. Just remember that the added weight of a 3 phase motor and related hardware will add to the mass properties of the gantry. You may want to stiffen it accordingly and add some muscle to the X & Y steppers too.

Like the Man said: "There ain't no free lunch!"

L.J.


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