3imoh
10-24-2005, 11:33 PM
Howdy All-
Just had an odd experience I wanted to relate...maybe someone has run across it before.
I am cutting a bunch of parts out of Baltic birch. There are thru holes throughout the parts with the remainder of the operations being profiling and sawing. The bot I am using does not have a vac setup, so instead of doing the drywall screw thing over and over I figured I would be really slick and drill holes in a sheet of MDF that matched where the holes in the parts were and thread some inserts into it. I screwed the MDF to the table, and used brass 1/4 inch machine screws to hold down the BB sheets to the board. My holes in the BB were slightly oversize to deal with any problems hitting the threaded insert. It worked great. I did a couple of runs with no problem.
I returned a week later to finish the remainder of the sheets. I drilled all the holes in the BB...When I went to thread the insert into a hole in the far end of the sheet, it would not hit the insert...WTF? I took a flashlight and looked down, and the insert was a good 1/8" or more off in the x and y direction. As I looked at all the other holes, I saw that they got worse along the y and the x, with the far diagonal corner being the worst. At first I thought I had screwed up my file, as I had done some "tweaking" on it earlier that week. But I confirmed that the hole locations had not changed. Thought there may be some accumulated error happening...we had been having some issues with the SB software throughout the day, so I thought there might be some problem there. Did some test holes and measured with a tape...bot seemed to be fine.
Then I took one the parts I cut out fine from a week before and put it on the MDF...the holes did not line up well either! Started hearing twilight zone music in my head until I came to the only logical conclusion...the MDF had expanded (a lot). We did have some rain earlier that week (we are in So. Cal), and I figure high humidity coupled with relieving stress by cutting into the board was enough to get all my perfectly drilled holes out of wack...
I never even considered expansion/contraction when making the jig as I did not think it could be that much. Unless anyone else has any thoughts, I am thinking that is the culprit. Back to drywall screws, I guess.
-Howie
Just had an odd experience I wanted to relate...maybe someone has run across it before.
I am cutting a bunch of parts out of Baltic birch. There are thru holes throughout the parts with the remainder of the operations being profiling and sawing. The bot I am using does not have a vac setup, so instead of doing the drywall screw thing over and over I figured I would be really slick and drill holes in a sheet of MDF that matched where the holes in the parts were and thread some inserts into it. I screwed the MDF to the table, and used brass 1/4 inch machine screws to hold down the BB sheets to the board. My holes in the BB were slightly oversize to deal with any problems hitting the threaded insert. It worked great. I did a couple of runs with no problem.
I returned a week later to finish the remainder of the sheets. I drilled all the holes in the BB...When I went to thread the insert into a hole in the far end of the sheet, it would not hit the insert...WTF? I took a flashlight and looked down, and the insert was a good 1/8" or more off in the x and y direction. As I looked at all the other holes, I saw that they got worse along the y and the x, with the far diagonal corner being the worst. At first I thought I had screwed up my file, as I had done some "tweaking" on it earlier that week. But I confirmed that the hole locations had not changed. Thought there may be some accumulated error happening...we had been having some issues with the SB software throughout the day, so I thought there might be some problem there. Did some test holes and measured with a tape...bot seemed to be fine.
Then I took one the parts I cut out fine from a week before and put it on the MDF...the holes did not line up well either! Started hearing twilight zone music in my head until I came to the only logical conclusion...the MDF had expanded (a lot). We did have some rain earlier that week (we are in So. Cal), and I figure high humidity coupled with relieving stress by cutting into the board was enough to get all my perfectly drilled holes out of wack...
I never even considered expansion/contraction when making the jig as I did not think it could be that much. Unless anyone else has any thoughts, I am thinking that is the culprit. Back to drywall screws, I guess.
-Howie