Trimming Tenons

      Woodworkers bring massive brainpower to bear on a miniature problem. August 21, 2006

Question
I am jumping into child rocker manufacturing because the opportunity fell into my lap and it seemed like a good way to jump start my career change at age 60. I bought pre-cut parts for 375 rockers from the estate of a retired and recently deceased owner of a furniture factory.

My background is Harvard, computers (programming, teaching, consulting, managing, 14 years), my own auto repair business (w/ staff of 5, 11 years), selling auto repair management software (12 years). Woodworking has always attracted me but I did not get seriously involved. I am smart, I've studied lots, and, of course, I know virtually nothing practical and I stumble repeatedly! So I search for answers. I really need the expertise of people who are in the business - I think my immediate problem is I don't know enough about production wood manufacturing.

I have roughly 2,000 wood pieces (shown in the "related link") that have the round tenon circumference pre-cut but the shoulders remain uncut. Related link:
One of the stacks to be trimmed

That's 4,000 shoulders to cut. So I want to be efficient. Since they must have been ordered pre-cut this way, there must be a reason. And there must be a way to easily cut the shoulders. But it sure baffles me!

Notes:
1) there are two types of stock - square stock and rectangular stock (the round tenons are the same size)
2) the depth of cut required varies too much to simply rotate the piece over a table saw pushing against a shop block. 3) I could make a jig that uses the tenon as a pivot, suspending it over the saw blade - that seems doable but too complicated - like there must be a simpler answer.

Here are my questions:
1) Any suggestions for this problem?
2) Why would the pieces have been ordered without having the shoulders cut?

Forum Responses
(Furniture Making Forum)
From contributor A:
It looks like they were made on a drill press with a tenon cutter. If I had to use them I would clean them up the same way using the next size larger tenon cutter to remove the waste.



From the original questioner:
Two hours and I've got a completely doable solution! The picture isn't very clear, but it looks like the MLCS tenon cutters have sufficient meat on the outer edges that they will work just great.


From contributor B:
Another way, since you do have volume, is to buy a lathe, put live center in tailstock, and just clean them up. You are going to wind up buying one anyway since you are working with turned parts all the time. Plus, it is safer on a lathe, also faster in long run I think.


From the original questioner:
Thank you for the idea. I do have a lathe, although I haven't used it yet. But I would think it simply must be pretty slow - I have several thousand tenons to cut, meaning several thousand sequences of:
1) mount the wood on the lathe
2) turn on the lathe and wait for it to come up to speed
3) trim
4) turn off the lathe and wait for it to stop
5) unmount

It seems to me the need to "turn on" and "turn off" would force this process to take a lot of time.



From contributor B:
How about a drill press? Build a special jig for each size, and try to get a setup that is exactly centered vertically and horizontally. Then load a leg - drill maybe centered or not. Is shoulder square to tenon? If not, rework. Next leg - oops different size so a new jig, new setup. If your lathe has a stock tube large enough you can stick every size leg thru and just chuck the tenon end, square cut and youre done. That way I could do 10 to your 1 on the drill press. Check the square sizes they are probably the same size.

The other choice, again because of the volume, is to sub it out to a larger shop that has automated equipment. Ill bet there are some right here in this forum who could buzz thru your pile in no time at all. A big advantage on the lathe is the ability to undercut the shoulder cut.



From the original questioner:
To contributor A: Your suggestion did spark an idea a horizontal drill with a tenon cutter, a fixture that holds a tenon against a fence aligned with the cutter, a jig to push the piece the correct distance into the spinning cutter and then pull it back release - flip end for end, push and pull, and toss in the done pile

I bought this stuff from a furniture maker that had 50 employees. The parts had been manufactured in-house or subbed out. Whoever did these parts probably used industrial tools. All the parts have 6-digit part numbers, for goodness sakes! It really seems to me that this was a professional operation using professional workers and techniques. If trimming these tenon shoulders was going to be complicated, it would have been easy to add an extra piece to the tenon cutter(s) so it (they) cut the shoulders on the same cut.



From contributor C:
There is a machine that is sold by Woodworkers Supply Co. that is called the matchmaker. That machine cut tenons and mortices and once set up it can do the same cut over and over. It costs about $800 and you supply the router.


From the original questioner:
I sought a solution here and I found several. But productivity matters a lot and any plan requiring a setup for each piece seems like it will take a long time when multiplied by thousands. And I am still a novice, so I also am considering safety (I have some nervousness about several thousand DP cuts with a tenon cutter, or chucking up a lathe on the fly).

1) I will do a cut using a TS at the shoulder on however many fit comfortably on my sled.
2) I will rotate the square pieces one flat and repeat the cut (and again twice, cutting all 4 sides)
3) I will rotate the rectangular pieces two flats and repeat the cut
4) Then I can build a TS sled that can hold 20 pieces (or whatever) at a 45 degree angle, raise the blade, and cut the remaining small triangular stubs that didn't get cut above.



From contributor D:
I agree with contributor B - I'd use a lathe. With 4000 to do, I'd make an adapter for both stocks so that there was a tube or pipe with an ID slightly less than the size of your existing tenon, and chamfer the inner edge to get a fast, self centering clamp. Slip it in the head stock, pull up the tail, and clean up both ends. If you cannot turn, build a jig and put in a small router with a bowl bit in it and run the lathe on its slowest speed. I'd guess about 30-45 seconds each once you got going.


From contributor E:
How about a raised sliding jig and band saw. Encase part of the precut round tenon. Position stops. It would be fast.


From contributor F:
If I had to shoulder all those spindles I would use a saw blade and spin them. I am picturing a tablesaw sled with a "V" block and stop. Slide pipe sections or bushings with ID's that fit the OD's of the two different spindle sizes over each end of the spindles (one each, two or three inch long "bushing" per end). Lay the "bushed spindle" on the "V" block and against the stop (stop should contact tenon only). Slide the table saw sled mounted "V" block forward to a "stopped" cutting position. Rotate the "bushed" spindle 360 degrees. Pull the sled back, flip the spindle end for end and repeat. If you wanted to get really fancy and spend a few more bucks you could even have "fixed bearings" with an ID to match the OD of the spindles instead of bushings and a "V" block.

Note that the spindles would need to be uniform in square and/or rectangular dimensions for this to work. If they differ it could be made correct by sending them all through a planer beforehand and taking equal amounts from all four sides assuming the tenons are perfectly centered.

I have a lathe and I know that this method will net identical and consistent results and be faster and require much less skill than a lathe and hand held tools. I think you will have to face the fact that there will be much handling of parts and repetitive motion involved no matter what method you use. I think we all wish we had a machine with a hopper that you load full of spindles, press a button and the shouldered tenon parts are delivered out the back on a conveyer belt. Some one could build such a machine but you will sell a lot of rockers before you have it paid off.



From the original questioner:
I experimented with the square spindles on the table saw. I experimented with the blade height and fence on my Ryobi BT3000 which has a sliding table that I use like a sled.

Here are the results:
1) the tenons are not cut perfectly - there are slight differences in centering and depth, but not a lot.
2) without a jig, doing a bunch at one time doesn't work well - I need to keep downward pressure on every piece as it goes over the blade and end pressure pushing each piece toward the fence.
3) it works pretty well if I set the fence distance a tad tight it is better to have a groove around the tenon than shoulders that aren't flush; and do one at a time, rotating 1 flat before each pass of the blade - it feels solid cutting on the backward pass. So - it's forward / back / forward / back, flip end for end, and repeat
4) some tenons that aren't perfectly centered may get a nick on the tenon - I call them my engineered excess glue reservoirs!
5) it takes 30 seconds per piece (both ends)

I haven't done rectangular tenons yet - I think the same process will work, but I will need to do two passes with the blade at the same height as above and then two passes at a "hair's breadth" height. Fortunately most of the pieces I need to do are square!

Somebody must have screwed up when they made these every solution, including mine, requires more work than was required originally. I think some apprentice forgot to attach the "shoulder cutter" flange to the "tenon cutter" when he setup the machine that manufactured these parts. It was probably the same apprentice who spent the day boxing up the tenons.



From contributor B:
Remember I said same thing? These pieces were probably scrap. Now if you need pieces that are shorter than these, simply cut off the bad ends and re-machine into the shorter pieces - a whole lot faster than trying to save these. In fact you may want to setup to make new ones and see if it is faster than trying to save any of these. In time you might find a use for them.


From contributor G:
Regarding contributor F's response: "I think we all wish we had a machine with a "hopper" that you load full of spindles, press a button..."

Such a machine is called a back knife lathe. A hydraulic copy lathe will also do this operation easily, but is not typically fitted with a hopper. Even a hand lathe can be loaded and unloaded on the fly by a skilled turner (bodger for those with a historic bent). The automated stuff can also turn profiles on the length of the part. Most production turning is turned along its length, and square sections and square shoulders are undesirable since no joints on chairs are truly square.

The problem with a hand lathe is the splitting/tearout of the shoulders. The problem with the automated equipment is the cost and set up. You can see what these things (auto lathes) look like if Ex-Factory has any on their site.



From contributor H:
Heres one you can use. Its very cheap, accurate, safe, and best of all quick. Use a tablesaw sled, two 3" blocks, two twenty penny nails to make a small lathe atop the tablesaw sled and put the legs into it. Rotate the leg over the blade in the location and depth at which you cut the shoulders and then lower the blade below the table or move the sled back so as to change the leg or spindle, and keep the right height for the cut every time. Hope you can understand what Im trying to explain but I figure you could do about 50 or so an hour.


From contributor I:
Try using the table saw and a V groove jig (this jig will hold your spindle on its pointed corners) together with a cross cut sled. By merely making a pass - rotate the spindle - make a pass x 4 loping the corners each time - the meatiest part of your spindles - flip it four times with stop blocks for tenon depth - this should take about 15 seconds per spindle with no tearout .

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