Shopbot never cut flat

[ClayM325]

Clay,
Get yourself a dial indicator. They are like $25-30 with a magnetic base from any number of places. Stick the indicator base on the YZ car and indicate the spoilboard. See if you can measure the deviation reliably in the areas you describe.

I would also put the indicator on the YZ car and measure along the bottom edge of the gantry beam and makes sure it isn't going up & down. I would also indicate/measure the length of your table and see if there are any dips etc.

There is no other tool more useful than a dial indicator for times when you can't find where the problem is.

Keep in mind that when your bleeder board starts to get thin, it can warp internally and pop up in the middle - plus other weird things. I'd say once it gets below 5/16" thick, it's game over. Titebond as good as it is can still fail on MDF if there is a significant amount of movement from atmospheric conditions. The glue itself doesn't fail, the MDF does - it will literally pull away and leave jagged break points.

-B

Awesome I wil try that thank you. I have an indicator I used to measure case runout on ammo casings, im sure I can rig that up somehow

Do you have large humidity swings in your area over the course of the day? (your info doesn't indicate where you are) This can cause the spoil board to swell and shrink. Gary Campbell described to me some of the issues he dealt with when his shop was in FL.

I am in Raleigh, NC. We have large humidity swings but not quickly. This seems to happen immediately. I could resurface then immediately cut and it will do this.

Are you using vac hold down

Yes, I have 2 vacuubes, each piped to 2 quadrants of the table (4 total quadrants)

[Chuck Keysor]

Here are the images for my PRT. I have never seen a PRS in person but it must have some clear similarities...…. Chuck

[guitarwes]

I have the same problem, hadn't figured it out yet. I have a 2006 PRT Alpha. My bit runs deeper into the freshly surfaced spoilboard the futher you go down the X axis.
I contributed it to a very worn thin spoilboard and we put a new spoilboard on there and does the same thing. Very aggravating when you have Vcarvings on an entire sheet of material, the carvings on the X0 end are perfect but the X96 end are really deep.

[Brady Watson]

I have the same problem, hadn't figured it out yet. I have a 2006 PRT Alpha. My bit runs deeper into the freshly surfaced spoilboard the futher you go down the X axis.
I contributed it to a very worn thin spoilboard and we put a new spoilboard on there and does the same thing. Very aggravating when you have Vcarvings on an entire sheet of material, the carvings on the X0 end are perfect but the X96 end are really deep.

Do you have a car pump under the table at the 96 end?

-B

[bill.young]

When you bring it back to the starting point does it come back to the right height, right at the surface? That should tell you if your z-axis is drifting down or if something wonky is happening with your table or spoilboard?

[ClayM325]

When you bring it back to the starting point does it come back to the right height, right at the surface? That should tell you if your z-axis is drifting down or if something wonky is happening with your table or spoilboard?

yes it does

[Brady Watson]

Do you have a ̶c̶a̶r̶ vacuum pump under the table at the 96 end?

That should have read vacuum pump...They generate a lot of heat and do nasty things to your table glue up from the heat.

-B

[jTr]

While I'm not qualified to comment on the other mechanical variables, I wholeheartedly agree with Brady - Vac pump certainly seems to make a difference. New shop configuration = relocated vac box under table for space savings. I also have gone from a 208V in old location to 243V. Average temp in box was 120-130 degrees F. Now pushing 150. Table moves even more than it used to. General trend for the ply/mdf sandwich is to flex downward in the center as it heats. Again, even more pronounced now. If this is similar to your experience, you may want to investigate using a portable fan under table for extended cutting sessions.

Sidebar for others using the "black box" vac system /FWIW:
Ordered new pair of primary Lighthouse vac motors based on age of originals, sudden jump in temps and rated voltage. Though separated on the web page into 220, 230 and 240 volt ratings, Mr. Bartch says this is irrelevant, and these motors can run any voltages and up to 180-190 degrees and still be okay. Good news is system now is pulling a bit stronger, so I'm still happy with this "temporary" vac source at close to 6 years use.
( I'd like to have a real vac pump when I grow up)

jeff

[dlcw]

I set my black box vacuum under my CNC table. To help with the heat issues, I have a box fan that blows air over the vacuum box towards a window where it is exhausted outside. It's not hot under my table at all.

[guitarwes]

That should have read vacuum pump...They generate a lot of heat and do nasty things to your table glue up from the heat.

-B

I do not run a vacuum (or car pump) at all.