Is this span ok for 20mm linear shafts? (see pic)

[RMS]

Hey Guys,

I am working on a cad design for a fixed gantry router, and was wondering what size dia. linear shafts to use? This will hold a small router or dremel nothing heavy. I started to design this and its actually showing 16mm shafts, cause originally the span was going to be only 16"

What do you guys feel the minimum requirements would be if the span went to 30"?

[RICHARD ZASTROW]

Depends on the force against the tool and the distance from the rods that the force is applied. Back to high school physics.LOL

Dick Z

[RMS]

Any physics students in the house?

[harryn]

If you want to stay with an unsupported rail approach, and your specs are not exceptionally tough, then about 1 inch outside diameter per foot of length.

[RICHARD ZASTROW]

RMS, sorry about the smart *ss response @#2. I really wasn't trying to be, but in retrospect, I was.

You'll have to describe the forces and physical construction. Is this router to cut deeply into hardwood or engrave lightly into balsa? How much force on a cutter that is what distance from the lower rod? The lower rod will act as a levers fulcrum, the upper will resist in the opposite direction.

Sagging, due to the weight of the suspended mechanicals and cutting forces, depends on how the ends of the rods are secured. The ends can be supported on 2 points on each end. Better yet, use supported rails. Much stronger than unsupported.

Forgiven?

Dick Z

[RMS]

So, I am thinking then based on what was stated that I would need min. of 25mm on each of those shafts at a max span of 24" for light duty work.


Dick:
My design was going to be for routing Mahogany for a guitar neck and that's a hard wood. I may have to re-think my design now. My cutting area needs to be 10" x 28", If I make that picture above 10" vs the 25" and call that the y-axis then I could get away with lighter shafts say 20mm, then use supported shafts on the x-axis.

[ger21]

You're probably going to need at least 18" shafts to get 10" of cutting area. You should be fine with 18" long 1"D shafts. I really wouldn't go much longer than that without going up to 1-1/4" shafts.

[harryn]

Hi, on a similar note:

http://www.cnczone.com/forums/linear...ease_help.html

BTW - I am a big fan of fixed gantry designs. Good luck.

Guitar work tends to be pretty precise work IMHO, esp. if you are trying to route frets and inlays.

[Joe21961]

Found a Great Source....Misumi USA has everything I was looking for! No min on an order and discounts on more than one peice! it was worth taking a look at I spoke to a Rep Robin at 800-681-7474 x221 She was very helpful and getting a catalog too for more parts.

[RMS]

You're probably going to need at least 18" shafts to get 10" of cutting area. You should be fine with 18" long 1"D shafts. I really wouldn't go much longer than that without going up to 1-1/4" shafts.

Thanks Gerry!

[neilw20]

The material acts like a spring.
Get a length, and hang it out of the vice, and tap the side of the end with a piece of hardwood. It will ring, and you will easily be able to deflect it visibly with your hand. The lighter the material is, the more chatter you will get for the same chip load -- read that as the same cutting force.
Once the deflection exceeds the the chip thickness you have chatter.
Unless you are using high speed machining, the inertia of the tool assembly changes everything, but you won't be using HSM.

[Brenck]

You can virtually have any diameter you want, it will deppend on how much flex you are can have on the axis, if a 1'' flex is ok then you could go with 8mm rods, but if 0.01'' is ok then you should consider a higher diameter. To machine wood a lot of flex is not something that will ruin everything up, but talking about metal things will change drastically

SKF linear bearing catalog shows how to calculate flex of linear shafts. Here is a copy. Page 36

Remember that you will be using 2 shafts, so you will have to divide the result by 2. On a real world its not perfectly twice as hard, there will be other factors involved, but for calculating this it will be more then enough.

The way that the shaft will be fixed at the structure makes a lot of difference on calculations. Check the formula that fits your situation

Result is given in microns


Well I tryed to upload the .pdf 5 times and it keeps sayng that upload failed. Strange because the file is just 1.7mb

Here is the link Media library - SKF*Linear*motion

Search for 6402 and the file is: Linear bearings and units technical handbook