I'm a novice with an odd problem

Reply

  #1  
Old 12-16-20, 06:56 PM
S
Member
Thread Starter
Join Date: May 2020
Posts: 12
Received 0 Votes on 0 Posts
I'm a novice with an odd problem

Hi! I'm trying to screw a piece of aluminum to a section of plywood, and I want to make the aluminum perfectly flat against the plywood. However, no matter how careful I am to smooth the aluminum, by the time I get to the final screws there are several portions of the aluminum that are annoyingly separated from the wood. When I did this on the second piece of plywood, I was even more meticulous in affixing the aluminum but I got the same result. There's always a bulge, or, really, several bulges. Tomorrow I have to affix aluminum to three more pieces of plywood. Any tips on how I can do it so that the aluminum is perfectly flat against the plywood in all places with no unsightly bulges?
 
  #2  
Old 12-16-20, 07:06 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 60,786
Received 1,324 Votes on 1,224 Posts
The holes in the aluminum need to be bigger then the screws you use to install the aluminum to the plywood. In other words..... the screws should not thread into the aluminum..... only the plywood.
 
  #3  
Old 12-16-20, 07:21 PM
S
Member
Thread Starter
Join Date: May 2020
Posts: 12
Received 0 Votes on 0 Posts
Thanks for replying Pjmax.

After drilling the holes, I tested the adequacy of the size by putting a screw in-- and it went through the hole without a problem. But if, in your considered opinion, that's the only possible reason for the poor results I got, then tomorrow I'll drill the holes with a larger drill bit. If that works, I thank you profusely in advance for the suggestion. If it doesn't, I thank you in advance for your efforts on my behalf (even if not quite as profusely!
 
  #4  
Old 12-16-20, 07:24 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 60,786
Received 1,324 Votes on 1,224 Posts
You could post a picture to clarify what you are seeing but it sounds like the screws are binding on the aluminum.

How-to-insert-pictures.
 
  #5  
Old 12-17-20, 05:51 AM
Norm201's Avatar
Member
Join Date: Sep 2013
Location: United States
Posts: 9,451
Received 289 Votes on 259 Posts
A lot dep[ends on the size and thickness of the aluminum and the screw size. If it's aluminum angle it should stay flat, but if it's a thin piece of aluminum strip it will deflect with very little pressure
Aluminum is soft and will easily bend or dimple at any point of compression. Using a large fender washer might reduce that concentrated point of compression and spread out the pressure.

edit...You could try using an adhesive first to hold the total underside surface to the plywood. Then apply screws. But you will not want to tighten to hard on those screws.
 
  #6  
Old 12-17-20, 06:03 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,426
Received 747 Votes on 686 Posts
Screws in wood are inaccurate. As accurate as you think you are being getting your screws exactly in the center of the aluminum holes is almost impossible. The screw will always be slightly off which will push on the aluminum causing it to buckle. Drilling the holes in your aluminum oversize will allow for this inaccuracy.

Flat washers underneath your screw heads will help. Thin aluminum can be deformed, stretched or pushed by the torque of the screw head twisting downward. A washer can stay flat and stationary against the aluminum while the screw head comes down and clamps it tightly in place.

You can clamp another board on top of your entire aluminum piece to hold it perfectly flat while you install your screws. This will also help hold the aluminum in position so you can put all your focus into getting the screws as accurate as possible.
 
  #7  
Old 12-18-20, 05:20 AM
B
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,902
Received 101 Votes on 88 Posts
What size is the piece of plywood you are covering with aluminum and what is the thickness of the aluminum?
 
  #8  
Old 12-18-20, 08:14 AM
W
Member
Join Date: Feb 2006
Location: USA
Posts: 6,740
Received 22 Votes on 21 Posts
Where are the bulges located in relation to the screws? If near the screws, the screws are too tight.
 
  #9  
Old 12-18-20, 06:56 PM
Z
Forum Topic Moderator
Join Date: Sep 2005
Location: USA
Posts: 5,548
Received 156 Votes on 139 Posts
My first thought is aligned with Hal's. A contact cement or adhesive is really the only way to keep the aluminum completely flat. Contact cement is preferred as it can be painted or rolled on flat (as opposed to construction adhesive).

Also, some type of different attachment method might be worth exploring. A flat bar or something to spread the pressure away from one location.
 
  #10  
Old 12-19-20, 05:01 AM
Norm201's Avatar
Member
Join Date: Sep 2013
Location: United States
Posts: 9,451
Received 289 Votes on 259 Posts
My first thought is aligned with Hal's.
Gee I didn't even see even see Hal make comment on this thread. Maybe he telepathed this idea to me when I posted!
 
2john02458 voted this post useful.
  #11  
Old 12-19-20, 11:38 AM
T
Member
Join Date: Jul 2003
Location: California
Posts: 1,917
Received 14 Votes on 12 Posts
In addition to large enough holes maybe you want screws that are perfectly flat on the bottom of the head. For sure don't use drywall screws. The suggestions to use washers will give you the same or better effect than flat bottomed screws but that might be unsighlty for your application. Also try to only hand tighten the screws so you can stop before they draw too far into the wood. Leave them all just short of tight then go down the line and give each one a little more. Or maybe start in the center and work your way toward each end alternately.
 
  #12  
Old 12-19-20, 03:19 PM
Z
Forum Topic Moderator
Join Date: Sep 2005
Location: USA
Posts: 5,548
Received 156 Votes on 139 Posts
Bah! Sorry Norm - your suggestion. Apparently being stuck in the house is catching up with me.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: