Controlling down pressure

[djreiswig]

I have an air scribe mounted to the side of my Z. I had issues with the extension speed of my air slide, so I changed the post to make it like the one in WMF's videos where the z is used to raise and lower the scribe and the air just extends the slide and provides some down pressure. The air for the slide doesn't shut off until the scribing is finished.

I have had some problems getting the down pressure set to where the scribing looks good, but the scribe point doesn't stick in the material. It looks like I need to have it somewhere around 5psi. I have the Z forcing the scribe into the material slightly to account for the variations in the table slats. If I back this off too much the scribe tries to float on the material. I see this in changes in the depth of the pattern.
I have a regulator close to the slide to reduce the air pressure to around 5-6psi. Sometimes I can get it to work pretty good, and other times it seems to want to stick and drag.
I'm not sure if the regulator I have is a relieving type or not (it's from Harbor Freight), so that could be some of the issue. If the material tries to put more pressure on the slide and the air can't escape then it would force the scribe into the material more. I'm wondering if it would help if I found a regulator that was a smaller range. Something more precise.

[BTA Plasma]

Down pressure is a dance between cylinder size and volume, return spring or return pressure and the weight of the scribing assembly.


If you cylinder is too small a single acting cylinder with a return spring wont have enough return spring pressure to lift the assembly
If your cylinder is too big it will take a lot of pressure to get it to activate and will smash the scribe into the material making it not scribe at all and have a slow retract.


What you need is an appropriately engineered scribe for your machine. The Jeweler for the Star Lab is a great example of one. https://www.starlabcnc.com/the-jeweler-scribe-add-on

[djreiswig]

I researched air regulators and determined that I needed a precision regulator. I found one for a pretty good price on ebay. It is a SMC IR1000-N01B-X3. I tested it out this weekend and my scribe now works wonderfully. The pressure stays constant no matter what happens to the slide. I can literally push it up by hand and the needle on my gauge never moves.
I set the regulator to 12psi, as the force on the scribe seemed to feel about right. It is higher than the setting on my old regulator by quite a bit, but with the constant pressure it didn't stop or drag at all. I'm happy and it only cost me $17.50.

[acourtjester]

Have one ordered thanks for the tip, should come in handy for air brushing

[djreiswig]

Glad to help.
Mine hisses a tiny amount of air out the relief port when it has pressure. I took it apart and couldn't determine why. I'm thinking that is how it is supposed to work. When you get yours, let me know if it does the same.

[ggeh]

Im buliding an air scribe and Im having that down pressure problem with my design.I have a 4x4 bulltear table, using sheetcam, and mach3 . the control box is a candcnc espII-A. I have a floating head with a magnetic mount for the torch and a the scribe is on a air slide that is on a rigid mount on the Z plate. I'm not sure if I built it right cant seem to find a lot of pictures or descriptions on this. I also ordered that SMCIR1000 regulator and I'm going to re do my scribe assemble with some V-TRACK STEEL DRILLED CNC actuators guide rail. The one I made I think it is way to heavy to work right. Do you have any recommendations on the air cylinder I should be using. I seen some of the post on here and your designs look better than what I came up with.

Thanks gary

[tcaudle]

I made a revised POST for SheetCAM /MACH that lets you set a delay on the motion after the M7 is called to turn on the air. It lets the scribe descend slowly and delays motion for whatever time you set in the "Start Delay" of the tool. Contact us for the updated POST if you need that feature. Only works when using the Plate Marker tool.

[ggeh]

How do i contact you for that revised post?
Thanks Gary

[djreiswig]

I copied WMF's idea for mine. The scribe and air slide air stay on through the entire operation and the z raises and lowers. I think it is more precise using the motor instead of relying on the timing of an air cylinder.

ggeh
Your setup looks like it should work. I just have an SMC air slide with a sheet metal bracket similar to yours. Don't have the part number with me. One thing I would do is move your downpressure regulator closer to the cylinder. I run full air pressure up to my gantry and then cut it down to 12psi about 2 feet from my slide. Less line loss. Although with my slide staying down it probably doesn't matter.

[tcaudle]

How do i contact you for that revised post?
Thanks Gary

tom(AT) {the website address]

[ggeh]

djreiswig

I ordered the smc precision regs and a smaller air cylinder will move the D.P. reg closer to the cylinder as you suggested.

Thanks for your help, Gary