Mounting bracket for router

[ctoddh]

While recently trying to plane a glue up of laminated boards I noticed that the rectangle I cut resembled a stair step patern. Upon further inspection of the router and mounting bracket I discovered that the aluminum U channel that the router was sitting in had been twisted slightly. This caused the tip of the router bit to be off in the Y direction. The machine that I work on sat for 2 years virtually unused (PR-96) before I came to work for this company. I have been running it pretty heavy for the last 5 months and don't know if I was the cause of the damage or if something before my time did it. When I removed the bracket from the machine and set it on the base of the unmounted router I noticed that when the bracket is slid towards the bottom of the cylinder on the router the one side of the bracket is flush and the other side is almost 1/8" higher. Is this something that other people have had problems with? I have a second Z or A axis and when I inspected this bracket there was only a minimal bend noticed, but there was still one there. I am going on Monday to the local metal machine shop to have brackets made from steel but I wanted to see if I was the only one to have this problem. I tend to keep my feed rates slow for finish purposes so I dont think I have overstressed the brackets, but like I said I dont know what was done with the machine before I started working with it. Thanks to all those who reply.

[Ted Hall, ShopBot]

Hi Todd,

We beefed the bracket up on the PRT models, and more recently have begun shipping a bracket that is a cast and machined ring. Unfortunately, the new brackets will not fit on the smaller footprint (tubing) of the PR model Z car. However, we'd be happy to send a new bracket of the type you have, let us know.

[ctoddh]

Thanks Ted, I had a pair of brackets made out of stainless steel by a friend that owns the local machine shop and everything works fine again.

[kaaboom_99]

Great forum. I have a PRT96. Can someone suggest the best approach to adjust my Z axis for a "heavy" tool. When I turn power off to the control box, my tooling pulls my Z axis down until it bottoms out or my tooling strikes the table. This is a new addition to the Z axis and I am guessing it weighs about 30#. I only just put this tooling on, manually jogged my axis with no problem. When I powered down, my tooling was 1/2" from my work surface, within seconds it was in full contact. Now I know if this had been sitting at 3 - 6", it problably would have backfed power to the drive board and blown it. I have looked at the counterbalance spring and am unsure as to the right approach to adjust this. Also, I have read in the Users Guide that you can disconnect the Z axis when adjusting the counterbalance spring to get the right tension. How do you pull this motor and gearbox off, or do you split the two?
Thanks in advance. Perry

[kaaboom_99]

As a follow up to my previous post: I have since mounted additional springs to help support the tooling and presently am working on a closed pnuematic counterbalance system with a cylinder that travels 8". After some research I feel this is probably the best solution that requires the least effort out of the drive. Anyone have any other thoughts?

[sheldon@dingwallguitars.com]

Are you talking about an air spring? Pnuematic springs have a more pronounced rising rate than coil springs which are more linear. 8" is a lot of stroke. I don't know if that will cause you any problems, but it's something to consider.

You might look for a cylinder with more volume to keep things as linear as possible.

[kaaboom_99]

The cylinder I was looking to install is a straight pnuematic cylinder piped to a 40 cu ft tank. Typical pnuematic counter balance arrangement. At present, the springs on the SB are normal, typical coil springs just to get a couple of projects off the table so I can work on the pnuematics. The volume you speak off is made up of the lines and the 40 cu ft tank (off to the side of the table). The pressure in the system I calculate is going to relatively low (20 - 30 psi).

[gerald_d]

Perry, a cylinder and tank will make a deluxe counter-balance system. But a 40 cu ft tank sounds like a HUGE overkill!

If I am guessing correctly, you are going to mount a cylinder on your carriage with 8" of stroke. The bottom end of the cylinder is connected to the tank (standing in the corner of the shop) via a flexible pipe routed along the flexing cables. Top end of the cylinder free to breathe to atmosphere via a dust filter.

The tank must be pumped to the pressure to counter-balance the tool, and this pressure must be accurately maintained - zero leaks, or constant recharging.

My gut feel tells me that the tool weight is under 50 lbs and that your shop air pressure is over 50 lbs/in2. Therefore the cylinder need be no bigger than 1" in diameter if it is pushing up. (If it is pulling up then the rod must not be too thick in relation to the piston diameter) A pushing cylinder will have much less leakage than a pulling cylinder. So, lets assume that you are considering a 8" long by 1" piston diameter, by 3/8" rod diameter cylinder as a "pusher".

The air volume needed to fully extend this cylinder is 0.0023 cu.ft. If you want a very good counter balance, that only gives 1% difference from the top to the bottom of the stroke, then you need a tank that is 100 times bigger than the volume inside the cylinder. 100 X .0023 = 0.22 cu.ft This is only about the size of a smallish fire extinguisher.

If the pipe between the tank and cylinder is too thin or too long, then the z-motor will have to work harder to overcome this resistance. Can you mount a small fire extinguisher tank directly next to the cylinder? (or put the tank on the y-gantry and use only 8' of pipe) And maybe charge it with a bicycle pump?


Bigger cylinders have more friction, leakage and mass - keep the cylinder as small as possible.

There seems to be no need to go better than a 1% system because the friction and "stiction" in the system would probably be greater than 1%. Plus your temporary tension springs are doing the job, and even if they were 6' long, you are already getting more than a 10% difference in force between top and bottom of stroke.

[kaaboom_99]

Hi Gerald.
The reason for the tank is cost (free). The cylinder I am looking at has a bore of 1 1/2". Repair kits are free to me. The tubing is 1/4" x approx 20' long, routed as you describe. I will have a regulator on the system for charging it. The springs I currently have on my SB are only 12" and are not linear enough for the whole travel of the z axis. As far as adding the tank onto the y axis, I am concerned about the present weight that it is moving now and having to adjust ramps in the software to ensure accuracy. It hasn't been an issue yet, but as I said, I am concerned as to how much farther I can push this drive. I have already made it a policy not to jog my x or y axis faster than 2 ips. Maybe I am concerned over nothing, but I would rather err on the side of caution.

I appreciate your knowledge and input. Thank you very much.
Perry

[gerald_d]

Perry, I was just having some fun with my rusty calculations this morning


If you say that all the parts are free then you have hit on the deciding factor!

The regulator for charging may cause you a problem. If it does not close off completely tight, then the pressure will creep up over the set value. Normally, regulators for spray guns are not so exact because the air is being consumed and there is a steady flow through the regulator.

If you have this monster tank at a very low pressure, you can seriously consider a car tire air connection valve and a once-a-day top up.