Compression - Flush Trim Router bit leaving step? Am I the problem?

[HoosierBuddy]

Hey guys,

I'm building a set of card catalog drawers and the drawer sides feature a curved cutout area that was best made (at least with equipment I own) by cutting to rough shape in the band saw and then finishing using a template and flush-trim router bit.

I ordered a Spetool Compression Router bit with guide bearings on the top and bottom from a ultra-large vendor of all things named after a river in South America. Looks like this:



In using this bit, with a template, and my vintage Craftsman router/router table, to cut 1/2" hard maple drawer sides, I'm getting a step about mid-line on the routed edge. This is causing me to have to sand every one to remove the step and (as it's on a curve) and I have to make tons of these drawer sides, it's pretty annoying.

Questions:

Is there something I could be doing incorrectly that is causing this?

If "no", is this just the way "compression" router bits are?

If "no" and "no", should I send the router bit back and ask for a replacement?

Thanks guys, I appreciate any wisdom you can supply!

 

[The Cobbler]

sounds to me that it's something in the way the cutter is ground. I would try another bit
is the step where the grind stops 2/3rds of the way up in your picture?

 

[cgrutt]

Not sure but will a second pass with bit raised (or lowered) resolve problem?

 

[jar944]

If its in the middle of the cut, and not towards either bearing its the bit. Get a different/ better bit. You don't need a compression bit for solid wood.

 

[PCustoms]

Post a pic of the setup and the issue?

 

[cgrutt]

Could it be caused by interference from chip ejection? The chips will accumulate in center of cut with upspiral sending chips up and downspiral portion of bit sending them down concurrently. Shouldn't be issue on outside cut but you may be getting some interference with inside cut if chips have no room to eject. Did you try moving more slowly? Do you have any dust collection attached to router to help pull chips away from the cut?

 

[HoosierBuddy]

Could it be caused by interference from chip ejection?

I don't think so? This router/table is the most basic setup you can imagine. There is no dust collection, but I didn't leave much stock on there to remove after the band saw.

My technique has been to use double-sided tape to fasten the template to the rough cut piece. Then gently route along the top of the drawer side (the other 3 sides are square, and were cut on the table saw) taking most of the 1/8" to 3/16" wood away, then make a final pass by pressing the template firmly into the bearing on the compression bit (the top bearing as I have the template taped to the top of the workpiece). The final cut is just a very light skim....maybe 10 to 20 thousandths of an inch.

The step I'm complaining about is uniform on the entire cut and appears to be exactly the same on every piece I've finished.

Reviewing the responses above, I think my next move is to adjust the bit slightly (probably have to go a little higher) and see if the step moves from the center-line the same amount as the bit was adjusted.

That would be definitive.

I guess no one is saying "that's the way these bits are SUPPOSED to be"?

 

[MoonRise]

If the step/ridge is right at the location of the cutter edge helix 'shift', it's most likely a problem with the bit. Do you have a picture of the step/ridge on your workpiece?

Shifting the bit location a little should enable the bit to shave off the wood that it missed the first time if the step/ridge is just a 'bump' and not a 'ledge'.

Unless the 'step' is not a line and is instead an offset from the two cutter diameters being slightly different between the up-cut and the down-cut helix sections. In which case send the bit back and get a replacement.

 

[HoosierBuddy]

If the step/ridge is right at the location of the cutter edge helix 'shift', it's most likely a problem with the bit. Do you have a picture of the step/ridge on your workpiece?

Shifting the bit location a little should enable the bit to shave off the wood that it missed the first time if the step/ridge is just a 'bump' and not a 'ledge'.

Unless the 'step' is not a line and is instead an offset from the two cutter diameters being slightly different between the up-cut and the down-cut helix sections. In which case send the bit back and get a replacement.

Good info. I will check diameters (top vs bottom) of the bit with my digital micrometer.

 

[MoonRise]

For a problem with the bit, it could be a diameter difference between the two helix sections or it could be an axis offset between the two helix sections (effectively giving you a cut diameter difference even though the two helix diameters could be the same).

Is the cut problem a 'ridge' at the location of junction of the two cuter helixes, or is it a 'step' at that location?

Got a pic of the problem cut?

 

[HoosierBuddy]

Thanks for all the info guys. Here is a pic of a sample piece of 1/2” hard maple I just cut.




So, based on ideas tossed around above I did some measuring with a digital micrometer. I measure the step at -.007 on the router table side. I measured the max OD of the cutter near the top bearing and near the table. I measure the diameter as 0.004 larger on the router table end of the bit.

So my measurements don’t exactly add up, but it’s possible there is also an eccentricity issue. I could check for it with a dial indicator, but I’m confident my problem is the cutter was made wrong…so I’m just going to send it back.

 

[The Cobbler]

that's with one pass, not a pass from each side>?
that's the cutter doing that

 

[PCustoms]

Thanks for all the info guys. Here is a pic of a sample piece of 1/2” hard maple I just cut.




So, based on ideas tossed around above I did some measuring with a digital micrometer. I measure the step at -.007 on the router table side. I measured the max OD of the cutter near the top bearing and near the table. I measure the diameter as 0.004 larger on the router table end of the bit.

So my measurements don’t exactly add up, but it’s possible there is also an eccentricity issue. I could check for it with a dial indicator, but I’m confident my problem is the cutter was made wrong…so I’m just going to send it back.

A flush trim would clean that right up!

 

[jar944]

Thanks for all the info guys. Here is a pic of a sample piece of 1/2” hard maple I just cut.




So, based on ideas tossed around above I did some measuring with a digital micrometer. I measure the step at -.007 on the router table side. I measured the max OD of the cutter near the top bearing and near the table. I measure the diameter as 0.004 larger on the router table end of the bit.

So my measurements don’t exactly add up, but it’s possible there is also an eccentricity issue. I could check for it with a dial indicator, but I’m confident my problem is the cutter was made wrong…so I’m just going to send it back.

That surface finish is awful.

This is a single knife carbide insert bit.

If you plan on much pattern work, insert bits are the way to go.

 

[PCustoms]

@HoosierBuddy jokes aside, what/how are you using a pattern bit making drawer sides?

 

[MoonRise]

Yeah, bad bit. 4 thou diameter difference and a 1.5 thou axis offset/mismatch (×2 for the diameter) and you have the 7 thou rabbet.

That's not a line bump, that's an offset between the two helixes.

You didn't order a ledge bit or mini-rabbet bit by accident?

Send it back.

And that surface finish is horrible.

Skip the fancy compression double-bearing bit and just get a single bearing pattern trim bit. You pick top or bottom bearing, whatever works for your setup.

I've usually been satisfied with Freud or Bosch bits, but I also have some other brands too that have been fine.