Fear and trepedation

[mikejohn]

I have been asked to cut an eliptical table top out of solid walnut, 2m x 1m for someone. He has a woodworking shop, and assures me that the walnut will be sanded to an exact 20mm before going on the shopbot.
I intend to take small cuts with a straight 2 flute cutter, then using a round over bit to get the edge shape he wants. Ive done a bit of this in MDF, but thats slightly less expensive than solid walnut.
My question is hold down and tabbing. Where do I put the tabs, how long and how much material do I leave? Would you do a trial cut in MDF (or similar) first?
The table is for his mother -in-law, so I dont want to do anything to get him in hot water with her (think Atilla the Hun meets Maggie Thatcher
)
Another small question, where in physics does it say the volume of dust produced by cutting MDF excedes the total volume of the original sheet?
..........Mike

[bleeth]

Mike: go 4-6 tabs evenly spaced. I would make them about 25mm long and 6mm high. I would also leave a thin skin for a final pass and also offset out until the final pass. I would not use a straight bit but would use a spiral bit that is made to plunge. I assume that you plan on doing the roundover with a hand router. For that kind of work it is better than the bot.

Or play it safe and cut a template on the bot out of mdf and use traditional means to cut the walnut.

Dave

[daniel]

Mike John,
Just a few thoughts, I've purchased pointed round over bits that you can plunge with. I find I get a smoother round over if I do the round over first, then profile it with the end mill.

I've only had the bot for about a year now, Until recently I've always used straight flutes. They look so much stronger than a spiral cut bit. Well the other day I needed to cut some Ipe which is very very hard wood. All I had was the spiral upcut bit. I used it and it went through like butter compared to the straight flute bit.

[paco]

Hi Mike,

the first thing that would come to my mind is: how much more material there is as for the hold down... say your customer come with a blank that is about only one or two cm more than the required/intended size??? Ask for more (if possible) so for you to have a good hold down. I'd suggest to use as much tabs as need in a $uch case; and remove 'em with a flush trimer bit after(easier than to replace the blank).

Bit; as Dave I've observed the spiral to do a better fininish on hard wood... but a straight ones do the job; just more sanding of the edge (where it machine against the grain).

It sure a "little" stressful to machine $uch blank but the Bot can do this... the template is sure safer... but a little boring!

Remember that the hold down here is the key... mostly at the end of the process. Put the tabs a the end of the blank/part; perpendicular to the wood grain where they won't break and size 'em thick enought. Try to included a finishing pass to remove the stepdown marks...

Good luck!

[Brady Watson]

Mike,
If you are in doubt...then by all means, cut a sample in foam or cheap material before you get into the walnut. It pays back in spades knowing exactly what the tool is going to do...plus gives you an opportunity to make changes.

Don't sweat it! (yeah right...we've all been there before!!!)

-Brady

PS- How's the European camp shaping up?

[gerald_d]

Mike, in your situation, I would go the template and hand-router route. Why put yourself through the fear and trepidation? You have only had your SB for about 2 months and you are still de-bugging it - that's okay for low-cost MDF, but do you know for sure that you won't strike a bug in that hard walnut?

If your rounding-over cutter must meet the top surface tangentially, then that part is best done by hand-router in any case.

drawing ellipses for SB (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=31&post=8616#POST8616)

[robert_cheal]

Mike,

I seem to be hearing echos " make a mdf template".
Just yesterday I was cutting a multi layered MDF oval casing to be used for fiberglass molded parts. Due to the numerous files etc. I forgot to set up my "z" movements properly. Lucky it was only MDF. Imagine your errant cutter plowing down through the middle of the walnut blank and then suddenly your shop door is beat down and sword bearing invaders on horses come charging in.

[mikejohn]

With rocking horses, if you go a bit deep, you just say the horse was in very fit condition.

How did I get into table making?
.....Mike

[beacon14]

dont make it more complicated than it has to be. If you will be following with a roundover cutter by hand, mount the blank upside down on the table, and leave a 1mm skin all the way around. Flip it (carefully), rout the skin away with a flush trim bit, then when you do the roundover it will eliminate any waterline mark from the skin.
I would use a 1/2" spiral downcut bit, 1/2" deep passes offset .02, followed by a full depth (less 1mm) finish pass.

If you are going to make a template and rout this by hand, why did you buy the ShopBot? Once you've made a template, you now know if the toolpath is correct, so go ahead and rout the walnut on the ShopBot as well.

[mikejohn]

Water marks.
Are they caused by the bit being ever so slightly off perpendicular?
..........Mike

[bleeth]

That was the part about offsetting slightly and doing a final clean-up pass. This is part of the deal and can be caused by slightly off z axis, bid deflection, tool runout miniscule shifting of part or xy 0 and a combination of any of the above. Sand em off, have a beer (I know what time it is where you are), and go watch Roddick advance after the rain stops.

[richards]

When a question is asked, it implies that the person asking the question hasn't performed that particular task often enough to be comfortable to proceed without a little hand holding. To me, that means that it would be prudent to follow the safest suggestion offered - that being to make a template and then hand-route the walnut. Total time difference between using a template and running the entire job on the Shopbot is probably less than fifteen minutes. I would rather lose fifteen minutes than mess up a fine piece of walnut.

[bleeth]

Gerald: I only guarentee that every day the rest of our life is one day shorter than the day before, hence my suggestion above regarding playing it safe. One of my regular customers runs arched moldings regularly. I make templates for him out of particle board. He knows that it would save a step to give me the glued up blanks and let me swing them that way but one failed cut and it's a lot of work down the tubes. It costs next to nothing to recut a template.
On the other hand, if Mike wishes to take the more direct path he did ask for info in that area.