Why is it so?

[craigb]

Last night, I was routing four 19mm wide 10mm deep stopped dados in spotted gum. I was using a new carbide tipped bit and I cut the dados in three passes.

Now two of the dados had bad tearout on the edge of the timber and the other two were fine.

Can one of you experts explain this to me 'cause I can't igure it out

:confused:

Craig

[Wayne Davy]

I'm not an expert but I'll try to help.

Were the two dados that had tearout cut across the grain? If so, then that is the reason. When you rout (cut) across the grain, you cut the timber fibres which will pull/splinter out. You can minimise this by using a Blade Knife to cut the top and bottom edges of the dado (ie your markout lines) before routing (or cutting).

[Dean]

Hmm

1. Was the exit speed on the tearout trench too fast?
2. Was their some kind of grain deformity at the point of tearout on that trench?

[DaveInOz]

Its the atmospheric pressure that does the work.
See the egg pops right into the bottle :)

Given it is the same timber, same router, same cutting direction (relative to the timber grain) the only likely variance would be the feed rate. You may have been a bit to enthusiastic on the two with tear out.

[craigb]

Thanks Professor:)

Actually I should have been more specific.

It was two pieces of spotted gum, 2 dados in each piece.
The dados run across the grain.
The tear out was at the entry to the dado and on the edge of the timber.
Both tearouts were on the same piece of timber.
So could it just be the peculiarity of that piece of timber?

I thought the feed rate was about the same but I guess that it's a subjective judgement.
Next time I'll try to start the cut really sloooowly. (Then I'll probably end up with burning
:rolleyes: )

Craig

[Dean]

Only way to get rid of tearout is to use a sacrificial piece of timber butted up against the entry and exit pieces... this provides support for the good piece and avoids tearout in most cases.

[DaveInOz]

Sacraficial wood won't work with stopped trenches.

Brace yourself for a big guess ....

When you plane a piece of timber and cut against the grain (not across) it splinters and tears out.
As the cutters on the router bit are striking the wood at <90 degrees could the same thing occur, that is raising splinters. The explanation would then be that on one piece the grain ran left to right, and on the other right to left ??? easily tested too

okay, okay, I said its a guess, don't laugh. :o

[derekcohen]

Dave

I think that you are probably right.

Regards from Perth

Derek

[soundman]

Why is it so?
Many people ask me this question & I tell them" I don't know & if I did I wouldn't tell you".!!!!


I will however give this one a go.

I am assuming you ripped the trench to full depth straight off.

I would sugest Taking the bit down till it just cuts the surface by a puffteenth. I run the length.
Perhaps take one more light pass then hog out the remainder.

Or just take multiple cuts.

Be aware that if you are trenching the width of a bit one side will be cutting fore hand the other will be back hand there is therfore a standing risk of tear out or chatter in the groove.

ensure the router is positively retained in both directions and or take light cuts.

cheers.