I am worried about cutting this mdf part out.

[knight_toolworks]

this job is going to be a pain for sure. three separate parts out of mdf. the largest one has me the most worried because it is two sided and one side is almost all pocketed. I am worried about it warping as soon as it is cut.the main pocket is .52 deep and the rest are a bit more.
the pic is out of 3/4" mdf and it is only one piece thats two sided. I have not gotten it setup to cut but just to know approx how long it will take to cut. three bit changes too. it is about 60"x28"
plus I will have to get it painted. it will have to be very accurate with very accurate hole sizes. one small piece wants the one hole to be between. .003 and .006 accurate.



7555

[benchmark]

Steve,

It will be difficult to keep the sort of accuracies you require on a wood based product. The only advice I can give is to do all the machining in one hit and not leave it on the table over night part machined.


Paul

[dray]

Heya Steve.. Regular Mdf may warp. If you can find a trupan type of lightweight MDF that has the same consistancy all the way through it will not warp.

When I have to mill both sides of a piece. I surface the whole table. Then do .01" - .02" outline cuts for reference marks on the spoil board. Then block the piece in with more pieces of 3/4" MDF and 18G brad nails. Has not failed me yet.

Hope this helps.

[knight_toolworks]

After looking all of the drawings have .005 tolerance. they are only painted mostly on the inside of the cuts. Just thinking the mdf may vary more then is tolerated.

[stevem]

I think you would have a hard time attaining .005" tolerance in MDF, even on a high dollar CNC router.

One coat of paint in going to be close to .005" and the wood will change size by more than that amount in the time from starting your cut to finishing.

[knight_toolworks]

I found the only paint for the most part will be on the inside of the cuts and the flats.
but I agree. what tolerance do you think I should be quoting?

[stevem]

You should be quoting tolerance that can be measured with a tape measure, not a micrometer.

[Brady Watson]

I would talk to the customer & recommend a material that is a bit more stable, such as Renboard or similar prototyping material. Butterboard is probably the cheapest, but you need to take care when machining it as it can be a bit rubbery (not gummy...just flexible) when doing 2-sided parts.

-B

[bearcat]

Steve,

You might verify that the ShopBot X and Y axes are straight enough to do <= 0.005" deviation for the job. From my experience, this spec is beyond current capability.

Ed

[knight_toolworks]

who carries Renboard? I could not find anything online.
but I see that I need to tell them about .01 or so in accuracy is about the best I could do.
right now I was looking at 40 sheets of mdf. the engineering company that is working with me has no idea what the customer is doing with the parts.

[Brady Watson]

Steve,
I would have a conversation with them regarding the fact that MDF moves ALL OVER THE PLACE before & after machining...to make sure that nobody wastes their time...

I have no doubt that you can get very accurate cuts on a well maintained machine (probably in the .003-.005" range) if you carefully tune your ramps, use sharp tools & do lots of tests with climb vs conventional finish & overall dimensions. If nobody can give you the straight answers you need, then the price has to go way up, or you should just turn it down (unless you are starving...)

-B

[jhicks]

Steve, I find that generally speaking designers put tolerances on drawings based on some standard template or previous work rather than a true specification based on needs. May not be true here but if you question them, I'll bet they dont have that tight of a tolerance on the parts fitting into this and could afford as much as .062 to .125 in some areas.
Based on your location and what I see here, I have a pretty good idea about what these may be for. My guess is a table top display or Kiosk type computer or flat screen unit.
If that is even close to correct, I doubt tolerances would be even close to the info you received.
On the subject of stability, I'm not sure I agree on the trupan suggestion as its even more porous, 80% of the weight of MDF, and not as strong as MDF. You might want to consider either Medex or Extira. Moisture resistant or water proof, more density, weigth and stability.
More $ too but not too much of a premium considering the assignment.
Is it possible to make it in 2 layers? One for the top at 3/8" and one for the bottom at 3/8" and lay them up? Not sure if that would make it easier or not but a thought to consider.
Sand and prime well with epoxy or polyurethane to truly seal and be avoid sharp corners/edges to prevent paint failures on corners.
Material manufacturers have guidelines on line to be mindful of.

[ljdm]

MDF manufacturer specs say it can move laterally approx .01-.02%. I made some custom pegboard, the sheets expanded almost 1/16" within a few hours, from opening up the board to moisture absorbtion/expansion. Even painting will make it swell to a degree. (My limited experience with MDF)

[knight_toolworks]

well Brady I am pretty broke since I am a terrible salesman (G) and I bid this at over 9k. but if I can't do it it would be pointless. I will tell them about .05 is the best they can expect.

[jhicks]

Steve, if you are uncertain of your tolerances, why not run a test file on a piece of scrap? One sheet of material for test is far better than many after a commitment when you may struggle holding a tolerance. Especially if it rains on some days, not on others and they measure 2 weeks after you are finished.
Be careful out there but most important, question what they really need before offering .005 and you may find .031 is quite acceptable.
Good luck

[knight_toolworks]

if they ok I would do a test part to confirm the part. this is way too complex to commit without doing a test. but I have had to spend 2 hours just on the drawings so I could make a quote no more free work.