We do a blind tenon for out cabinet backs and are having an issue where the outline cut breaks the shoulder on the mortise. We are running a Thermwood CS45 router and a .5" cutter at 16,000 rpm and 600 ipm.

If anyone has any suggestions on how we could resolve this I would appreciate your input. We can do outline cuts in 2 passes 1 being climb and the last conventional but that would cost a lot of time.

TIA

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Is this something you've done successfully in the past and is now suddenly not working? If so have there been any changes made to procedure, materials, tooling etc.?

Or is it a new process you are trying to incorporate?

BH Davis

BH,

Yes we have been running our cabinets like this for some time and would have this problem rarely. The last small run we did more than 1/2 of the mortise shoulders were broken.

The only thing that has changed is we moved the router to a new shop. I suspect the the gantry may have been taken out of square in the move but don't know how that would cause this problem. I am going to check it for square tomorrow.

Thanks,
Gerry

Gerry,

If this had been a new process for you my response would have been "what do you expect.....it's particle board." Obviously now though that isn't the problem.

Unless that is you somehow have a bad batch of material with weaker than normal glues.

However the fact that the machine has been moved is a huge red flag. I don't see how out of square could cause this to happen but do see other potential issues from the move.

I'd think things to take a second look at would be machine leveling, machine vibration potential (more an issue I'd think on lighter duty machines vs. heavy iron equipment), z-axis squareness to the table, vacuum hold down strength (material vibration potential) and probably even more issues that don't come immediately to mind.

You could also run some stand alone test dovetails in a scrap piece of material while closely studying all components including material to see if anything out of the ordinary can be seen or felt.

Probably checking for squareness as you suggest is a good place to start but also a thorough go over of the machine install to see if anything pops up.

Good luck,
BH Davis

BH,

I agree, I am going to give the machine a good going over. The material is from a new supplier so that could be an issue but I think checking the machine is a good place to start.

I don't think we are the only shop cutting our back in like this so I was hoping someone else who does this might have some input. Or maybe we are the only shop trying to do this ;)

Thanks,
Gerry

How about cutting your grooves after doing the cut out pass, assuming the parts will not shift.

Tom,

I have not found a way to do that in the software yet but I think it would be a good fix. We are using the the Thermwood control nesting software.

Thanks,
Gerry

Gerry,

Another thought. Are you climb or conventional cutting that perimeter cut out pass? Whichever it is you could try reversing the cut direction. Maybe not a long term solution but it might help you figure out what is happening.

BH Davis

BH,

If I do a climb cut it leaves a rough surface on the PB and I need to come back with a conventional pass to clean it up. That is time consuming and I would like to avoid it if possible.

Thanks,
Gerry

We inset the backs an 1/8" to leave a little more meat on the shoulder

Bob,

That is a good idea. I will give that a try.

Thanks,
Gerry

Gerry, I got the same machine. I use a 3/8 compression and also set my back in and have this happen very rarely. We also use 3/4 backs and rebate those out, so 3/8 slot in ends,top,bottom and the back gets 3/8 rebate.

Mike,

Good to hear I am not the only one who does their backs like this. I shortened the tenon on the top and deck and that closes the mortise on the end but leaves a little daylight in the corners which I don't care for.

I will play with some of these suggestions when I I get a little time.

Thanks,
Gerry

Are the top panels in the picture the backs or the ends?

Trying to follow along.

With the backs in an 1/8th I rarely get the shoulder breaking. When a job requires flush backs, I do get some breakage.

Pat,

The pieces with the broken mortise are the top and deck of the cabinet.

Bob,

When you install do you shim the middle of the back to prevent it bowing?

Thanks,
Gerry

Never needed to shim. The inset back allows the cabinet to have a better chance of sitting flat on a wall that is not flat.
Does that make sense? lol

Hard to tell from here but... It looks like the PBd broke very uniformly near the bottom of the cut. That would seem to indicate particularly weak internal structure of the board. Make some trial cuts in different board and see if it makes a difference. Try MDF and some high quality PBd.

Bob,

Yes that makes sense to me, I do lot renovation work and the walls are lumpy bumpy and never plumb or square ;) I am sure it would help the cabinets sit flatter on the wall.

Larry,

I think the core is too soft on this board. We have quite a bit of it in stock so I will need to work around it for the short term.

Thanks,
Gerry