Pattern bit versus straight bit

[Niku]

I just finished my first router project: an adjustable router jig. Its quality is what you might expect from a first project, but it does the job. Nevertheless, I am going to build another using my experience from building the first one. Before starting on the remake; however,  I decided to see what other jig patterns I might use. I found one that interests me, but it says that it should only be used with a pattern bit. Why do you suppose that is? Is there a reason why an ordinary straight bit couldn't or shouldn't be used for any particular jig?

[Lifesaver2000]

Some jigs are designed so that the pattern bit bearing rides along on the jig to guide the router.  For example, this is one way to make an adjustable width dado jig, instead of using a guide bushing.  On those designs, a straight bit would simply cut into the jig if there were no bearing.

[Eric.]

Flush trim bit: bearing on bottom (opposite of shank)

Pattern bit: bearing on top (in between cutters and shank)

Straight bit: no bearing

 

If you use a straight bit with a template, you'll chew right through the template, hence the bearing.  You can use both flush trim and pattern bits with templates...I often use both with the same template to avoid routing against the grain.

[Tom King]

The best thing about a pattern bit is that you can make the jig to exactly the size you want the finished cutout.  It makes making the jig a lot easier.  If you use a bushing, you have to add the dimension for the space from the cutting edge of the bit to the outside of the bushing, and it adds all sorts of extra possibilities to screw up.

[Guest]

Eric and Tom pretty much sum it up.  Like Eric I often use both the top and bottom bearing bits so I don't go against the grain, or I will will cut half with through with the top bit, flip my stock over, and cut the other half with my bottom bearing bit riding against the freshly cut part.  It does pay to have them in various lengths.  Sometimes I will use the bushing and straight bit (actually, usually a spiral bit) with the template first, then just clean it up with the bearing bits.  With all of these bits you only want to be taking off a small amount at a time.   Template routing looks simple, but sometimes I wear an adult diaper just in case.....

[Niku]

Once again you people prove that if you have any questions about woodworking, this is the place to come. Not only do you get clear answers here, but you get them fast. Thanks again for giving me the information that I needed.  With people like you to rely on, I now know that I SHALL become proficient in using my router. 

[wdwerker]

I used a pattern bit and a quick simple jig to route the angled dados for the stair treads on my staircase project. I used an actual tread to space the guide bars apart and screwed them down. The bearing rides against the guide bars and cuts the exact size I needed. You route up against one side and then pull back against the other side.