# is it safe to use a straight bit like a flush trim bit?



## achoox4 (Nov 9, 2012)

I've got some routing to do where top-bearing straight bits with a variety of cutter-to-bearing diameter offsets might work very well in place of a plain straight bit or bits and guide bushing(s), if I could afford such a thing.

Can I instead use plain straight bits - I've got 1/4", 3/8", and 1/2", all with 1/2" shafts - where I'm running the last part of the full-diameter shaft next to the cutter itself against a template or part of the workpiece that amounts to a template?

I'm thinking of the way bits like 5118 in the Flush and Bevel router section, second section from the bottom, on this page
MLCS solid carbide router bits
are supposed to be run against a template or edge without a bearing.

What I've got is this kit
MLCS 15 Piece Router Bit Sets in 1/2"
and would be using the 1/4" and 3/8" straight and 1/2" straight or mortise (because that cut happens to be quite shallow) bit.

If it's just a problem of wearing the shaft or the template, this is for doing two ~3" dia. circles per specific diameter and thus per bit size, so each bit has to do about 20" of linear cutting in this manner (with total material of 1" MDF), and a single template would do 6 circles.

Is this an utter no-no, normal, or somewhere in between?

thanks.


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## Phil P (Jul 25, 2010)

Hi Benedikt










That type of bit was designed to use for trimming laminate. In order to use them you normally dip the solid pilot at the _bottom_ in Vaseline (petroleum jelly) to prevent it from scorching the material. Doing it your way you'll need to "grease" the shank between the collet nut and the cutting zone. Apart from the slight risk of scorching (because they will probably get very hot, very quickly) or possibly finishing problems caused by the petroleum jelly being absorbed by the MDF (which may not be an issue) you might well find that the bit shanks are just too short for the task you have in mind. They really do have very short shanks. Also not sure how good the quality of cut would be on MDF because they aren't really designed for it. They are certainly cheap enough to give it a whirl, though. I am struggling a little bit to understand why you'd need to 

Regards

Phil


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## achoox4 (Nov 9, 2012)

Thanks. I didn't realize that operation of those bits in the intended manner was already marginal (if it depends on vaseline being present, you obviously can't go very far without reapplying it without scorching).
That pretty much answers my question - besides the definite problem of very little height to work with, I could basically do if I had to, but it's not recommended.

My reason for pondering this is my problem described here, of needing to do circle cutouts with multiple stepped diameters for difficult-shaped speaker chassis in a box that's already assembled, on top of smaller existing holes - and my template guide/base set (that I'd normally use to generate the multiple diameters from one circle template) doesn't seem to fit the router (MRC23 = Bosch GMF1600)...and I'm in a bit of a hurry to finish this, and cost is an object. Otherwise, I wouldn't be looking at iffy workarounds.


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## jw2170 (Jan 24, 2008)

Hi Benedikt.

Phil is much more experienced in commercial routing than I will ever be, but, personally, I would not use a straight cutter without a fence, a bearing, a jig or a guide bush.

To use a normal straight cutter in a router with enough of the shank exposed, to rub against the workpiece would mean that very little of the shank is in the collet. A big no no as far as I am concerned.

Can you make a simple, one use circle jig to make the cut outs? All it needs is a length of 1/4" mdf screwed to the router base with pins set at the required radii.

As Phil said, the bits shown are designed for laminate trimming with no real depth of cut.


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## Semipro (Mar 22, 2013)

Benedikt
Why don't you buy a 1/2 pattern bit bearing on top then buy different size bearings for the top ( like 3/4" 1" 1 1/4"


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## achoox4 (Nov 9, 2012)

Semipro said:


> Benedikt
> Why don't you buy a 1/2 pattern bit bearing on top then buy different size bearings for the top ( like 3/4" 1" 1 1/4"


Money, time, and money.


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## achoox4 (Nov 9, 2012)

jw2170 said:


> Can you make a simple, one use circle jig to make the cut outs? All it needs is a length of 1/4" mdf screwed to the router base with pins set at the required radii.


Yes, all in all, that is probably just about the lowest-effort (even compared to trying to affix the existing Milescraft base) method, and with maximal stability/reliability - 
IF I can manage to properly mount something to hold the center pin:

Where I'm cutting the two difficult holes (in addition to an easier larger one between them), I've got a total of 1" of MDF, with ~1.75" dia very rough-cut holes which are to be replaced by the nice larger holes I'm trying to make, and the box's side wall is attached less than 1/2" away from the existing hole. This means I'm unable to reach the back of the 1" of MDF with the router, which would be convenient for the main hole diameter and really helpful for tapering the opening outward towards the back as recommended for these speakers (but rabbetting a bit from the front will likely (have to) suffice).

More importantly, I have no center material to attach the circle pin to. Once I screw something onto the back of the 1" material, I have a 1" distance between it and the router base to bridge. How do you do that? Just screw a longer bolt really tightly into the non-sacrificial back plate? 
Put as large a piece as will fit of 1" of MDF onto the back plate, and put the pin into it, or a longer bolt through it all? 
Perhaps a bit of 1xX, even pine, would be stiffer and thus better for this than MDF, and not quite reaching the front plane leaves room for a washer and nut?


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## jw2170 (Jan 24, 2008)

Use the circle jig to make a male template? No centre pin required for the template, but you will need a bearing guided cutter or guide bush.

Sometimes, you do need the right tool for the job.


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## achoox4 (Nov 9, 2012)

No, that puts me right where I started from (though it has now been pointed out to me that the Milescraft base and guides are as ill-fitting and weak as I thought, and I'll return it and get something proper, but that's too much time and money for right now).

Also, securing a male template would be even more difficult than a female one in this case (particularly once I've cut around it - it seems like all the worst aspects of a circle jig and a regular template in one handy package), whereas building up for the center pin with 3/4" wood should not be so bad.


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## jw2170 (Jan 24, 2008)

Either of those ideas should work,



> _More importantly, I have no center material to attach the circle pin to. Once I screw something onto the back of the 1" material, I have a 1" distance between it and the router base to bridge. How do you do that? Just screw a longer bolt really tightly into the non-sacrificial back plate?_





> _Put as large a piece as will fit of 1" of MDF onto the back plate, and put the pin into it, or a longer bolt through it all?
> Perhaps a bit of 1xX, even pine, would be stiffer and thus better for this than MDF, and not quite reaching the front plane leaves room for a washer and nut?_


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## achoox4 (Nov 9, 2012)

Excellent. I built the quick-and-dirty circle guide yesterday, with both all the currently needed mm holes drilled and precise inch marks for later. When I went to do a test hole to fit the more difficult speaker into, I realized that I'd measured diameter sizes instead of radii, and decided it must be past time for bed.
I'll try again today with correct hole placement, and with close numbers placed on different radius lines (too many similar holes of a distance that puts them under the router casting, placed too close together, are quite prone to picking the wrong hole).

I can see why it's a better idea to have a sliding pin holder that you measure and set, rather than depend on drilling a hole in the exactly correct spot.

Also, for the imperial vs. metric debate: I couldn't find any long metric bolts, so it's #10 for the pivot. That's not terribly well matched by the metric drill set that's the only one I have currently in reach.


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## steamingbill (Jan 13, 2013)

achoox4 said:


> I can see why it's a better idea to have a sliding pin holder that you measure and set, rather than depend on drilling a hole in the exactly correct spot.
> 
> .


There are a few variations on circle jigs out there.

Some of them look better than others for fine tuning a radius distance.

Some of the links in this post may be helpful

http://www.routerforums.com/jigs-fixtures/40902-what-does-jig-do-3.html#post331890

...... and I have seen some that use a threaded rod to minutely adjust the radius

If you use Google Pictures as a search engine you will see some of the ideas there

Biil

Check out


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## achoox4 (Nov 9, 2012)

I hadn't thought of using a threaded rod, that would probably the utmost in stable precision.

It reminds me of something I thought of and forgot months ago (but it can't do the tiny radii I need to do right now): A minimalist circle jig for routers with rod mounts, using a single rod (or a U or W-with-low-center shape if you want to use two rods for stability) with a block on the end that holds the pin or pin hole.

Of course the radius is nonlinear to the rod extension, but if your measure the triangle formed by the rod and cutter-to-rod distance and calculate a table, you don't have to measure each one and could actually make alignment marks on the rod.

Of course one could combine this with threaded rod for adjustment, e.g. with major distance marks for quick coarse adjustment at the base, and then a captive knurled nut for fine adjustment at the pin holder. And perhaps a stop against which the fine adjustment starts, to avoid needing marks on the thread in that area of the rod.


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