Alternate profile molding and tight mitres

K

Kburke

Grumbler in Training
Joined
Jan 28, 2006
Posts
8
From
Columbia, SC
Dear Grumblers,

I hope all is well. First day on and I have another question already. Most of my frames that I've made (probably around 100) have been moldings I've made myself on my router and most are fairly simple profiles. I purchased a few sticks of premade moldings that have a much fancier profile than I've used in the past (they are about 3 inches wide and are the gold variety that have become vogue to use in framing mirrors). I can't for the life of me get the mitres perfect. On my other profiles I work them until they are perfect. On these new profiles (which I will not be able to sand post joining) I can get 'em close, but that isn't good enough. I checked to see if my mitre saw (10 " compound mitre saw) was at 45 degrees and it was. I checked to see if my fences were perfectly straight and everything seemed to check out. I did a few test cuts and checked those against a carpenter's square. Everything looked good, but I still can't get the mitres on the wild profile molding exact...any thoughts?

Thanks so much,

Kevin
Columbia, SC
 
Assuming the moulding isn't warped or twisted, there are two places you can run into problems: cutting and joining. The wider the moulding, the less forgiving in both areas.

What are you using to join the frames?
 
What are you using to cut the frames?
 
There can be a number of problems contributing to a "loose" miter. The moulding you have described sounds like it may have some compo on it or a bunch of that fancy stuff all over it and that will sometimes chip or cut poorly particularly if your saw blades are getting dull.

Speaking of saw blades, what type saw blade do you use and how many teeth are on the blade?? Is the rake of the teeth such that it is made for ultra smooth cuts in wood? You can't expect to get good miter cuts with a combination blade or rip blade or a $9.95 Home Depot special. You can do a search on saw blades and find more information than you can handle on the cheapie blades on up to the top of the line models. Generally speaking, for a 10" blade, 80 to 100 teeth of a good quality carbide is preferable with about a 5 degree positive rake.

You say that your miter saw is set at 45 degrees. Is the blade also cutting in a dead vertical position?? IF it isn't it can appear that the top or bottom of the miter isn't coming together properly when laid on a flat surface for joining. If the cuts aren't vertical it will open either top or bottom of the miter along its length.

You mentioned that you "work your homemade mouldings (miters) until they are perfect." You shouldn't have to do that on either type of moulding if your cuts are true. There is a fine line between a "good" miter and one that fits together perfectly with no "massaging" or filling of the joint. It all has to do with the way your equipment is calibrated and how accurately it functions for the job it is meant to do.

So we have the wood mitered now. Poor joining can cause bad miter joints just as poor cuts. As Ron mentioned, how are you joining your miters? Are you checking to ensure that the legs of your frames are lying perfectly flat when joining? There are so many little things that can mess up a mitered joint that it is difficult to hit them all unless we have additional information about your cutting/joining techniques and your equipment used, and your general habits when handling the moulding.

Framerguy
 
Does your cutter have rebate supports? If not the blades may be pushing the moulding down as it cuts, affecting the vertical cut. Invest in a morso - this will also do away with sawdust.
 
Grumblers,


Thanks for taking the time to write. Before I cut the molding, I bought an 80 tooth frued, I can't remember what the rake is though. I did a few test cuts to see if the cuts were "dead verticle" but, really, was unsure of how to test. I did check it with a square and everything seemed to be o.k. I thought that possibly Tim was on the right track with the supports on my mitre saw (as I don't have any) but I make rough cuts to size the legs and then make final cuts. The legs are totally supported between the fence and the "bed" of the mitre saw on the final cuts. The mouldings do have some compo in them. How could I tell if they are twisted? Although I doubt that, I had trouble with two seperate molding profiles...

It is defintely the top of the mitre that's loose...the bottom is fine. And actually, the only part of the mitre that is loose is the portion of where the rabbet is. The main section of the molding is tight.

When I said that I work my homemade moldings until the are perfect...what I mean is, I make my first leg to my measurements and then I am able to stack the 1st leg onto the next piece of molding and clamp it. When I cut the second leg I barely trim the original so that I know I have two exact pieces...it works like a charm and I never have trouble with those mitres. I've never used filler or puddy on a mitre.

When I am ready to join I use a bessie strap clamp. I have been totally happy with them in the past. I fit everything together and check for clearances...in the rare occasion that there is something off I will switch two of the legs and restrap/reclamp everything and see how that looks (usually one of the two is perfect and I'll proceed). I then loosen everything, glue (I tried both regular yellow wood glue and Gorilla to no avail) the mitres and let set over night. On moldings that will allow, I shoot 1 1/4 brads,fill, let dry, sand, paint/stain.

These new profiles have really about broken my spirit. Any new ideas?

Thanks again for your time,

Kevin
 
I would guess that you either are suffering from a little runout of your blade or you have a slight variation in your angle of cut. It can happen that, if you are using a single saw to miter your moulding, when you change from one side miter to the other side miter you could have some play in the detent of the setting and get a minute or two of variation in each succeeding angle of cut.

I would check the vertical cut with an engineer's right angle and buy as accurate a 45&#186 drafting angle as you can find to check your angle of cut. They can be off by a minute or two of a degree and not show it visibly except in the mating of the adjacent legs of the frame.

good luck.

FGII
 
Sounds more like "China-Dog joints"....

This happens occationally with 80,100, and 120 tooth blades. More importantly it is a huge factor that has to do with the carbide tips themselves.

Over time the bits travel at about 650 mph, [which is why the blade "sings", it is flirting with breaking the sound barrier.] As each of the very hard carbide tips slam into the wood, time and time again there is a micro wearing that occures.

The cure for this is, of course:

Sharpai knees


[sharpenings]

As your dull blade makes first contact, it skews to the outside of the cut and then due to the ridged nature of the blade, continues through the cut in that new relation to the wood. Resulting in a nice tight fit except the top [where it moved]

A sanding wheel would "fix" the problem . . . but would not fix the "problem".

Saw blades are tricky... at least with chopper blades you can take them off and with a magnifing glass see the divet at the tip.... or it just won't shave your arm easily...

Hope this helps.
 
Another situation that could have this affect is the manner in which the moulding is held during cutting. At the point of first entry the teeth from the blade may be rolling the moulding forward. This only has to be a small amount to get the affect you have described. Once the blade moves beyond the initial entry point the moulding is coming to rest flat. This rolling would cause the top of the mitre to be shorter (center point of bottom measures 12" but center of top may only be 11 7/8"). If this is the case you need to look at your method of holding it in place and possibly ad clamps.

Hope that makes sense to you.
 
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