Acceptable Frame Stiffness

[Aethernet]

As I have just built my 4" x 22" x 44" router and don't have motors & screws hooked up, I was a little disappointed that I can easily push the router head +/- .003 with my finger, with all axis locked & using an indicator.


For cutting wood, is this tolerable, and if not, what amount of structural frame bending is?

[Oldmanandhistoy]

Originally Posted by Aethernet As I have just built my 4" x 22" x 44" router and don't have motors & screws hooked up, I was a little disappointed that I can easily push the router head +/- .003 with my finger, with all axis locked & using an indicator.

Hi,

This really isn’t accurate enough information to get a usable response.

Do you have a fishing scale or similar to measure the force against the amount of deflection? If not I would say try the machine and see as 0.003” does not sound that bad to me (for a wood machine).

John

[Geof]

That does not sound too bad but it is possible a finger cannot apply enough force. Have you ever used a wood router by hand so you know what kind of force you need to exert to push it through a piece of wood at a decent speed? My guess is that it can be up to twenty or thirty pounds, or more in some cases. I doubt whether you are exerting thirty pounds with one finger.

That said your stiffness may be quite okay for wood working where +/-0.01" can be quite adequate.

[harryn]

I think I read on the mechmate forum, that they attempt to put holding forces of 80 lbs in all directions on the router body itself. I am not sure what that comes out to in actual movement.

It might be interesting to see how much of your movement is from the frame flexing, and how much is just movement of the router spindle.

As noted above, it is surprising how much frame stiffness and precision it takes to stay below +/- 0.010 - 0.020 actual results when cutting a circle. The bearing guides we all use for hand held router work take a lot of load.

If you post a few pictures of your system, I am sure people will be happy to make suggestions. In the end, stiffness comes from the depth of the resisting frame part in that particular direction as well as the fundamental stiffness of your construction materials.

[fatal-exception]

If I put all my weight to the front of my milling machine, I can get it to deflect over 0.01" Normally, this won't happen when actually cutting since that amount of force will snap a 1/4" end mill.

You can either add more bracing to your machine (assuming you know where the deflection is coming from) or use the roughing/finish pass strategy, where your finishing pass is enough to accurately finish the edges, making up for the heavy cuts taken originally.

I've been meaning to measure the force my machine is capable of producing. I have a strain gauge loadcell and software to record the results. I will let you know.

Paul