I’m looking at cutting out a negative of a car design to form some plastic in it. I need to carve it out 65mm deep.
How do I (or can I) cut this deep when the bit isn’t that long?
Do I need to get a bit that is longer than what i need to cut?
I did a quick conversion of metric to inch and 65MM is 2.559". So that will mean a very long bit. I have a Melin 4" OAL 1/4" bit that I use in my SO3 Dewalt router. It works but has come loose a few times due to vibration. When I install that bit I need to spin up my router to hear and see if it is spinning out of round. I usually just have to reseat it and when it runs smoothly it is good to go. The longer the bit the more runout you get. Because it has come loose before about every 5 minutes I pause and check the tightness. Regular bits never come loose but that extra long one will.
The consideration of how deep you can cut must include the router/spindle collet not hitting the top edges. Plus most bits have a maximum cutting depth. After you exceed that maximum cutting length you are spinning the solid part of the shaft against the project. Not sure any bits can cut 2.6" deep without rubbing.
Using a 1/8" bit would likely not be likely because the smaller the bit the more deflection it would get and would likely break off if it has a 2.6" cutting depth.
Another alternative would be to cut your model in half and machine them in two halfs and then join them back together. That way you are not cutting so deep but would still need a longer bit than most of the stock tools.
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machine the positive and cast a mold to get the negative?
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I never even thought of that. It’d still be cool to know how to do this though. If I can’t get it to work any other way I’ll probably do that.
I was thinking I may have to cut it up but it would be good to know how to make this work.
It appears your carve has a lot of vertical wall all the way around. I would divide the carve into two pieces, and attach them together later. Slice the model at the lowest point where you have vertical wall all the way around. Even then you might need a longer bit, but it would be considerable shorter than the full depth.
The top could then just be a pocket or contour cut. You could add matching holes in the remaining material, and use dowels to align things.
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Where is the parting/trim line? Does the molded part come up the vertical walls? Could those walls be designed with a taper? (Tapered tools are inherently stronger). What is the smallest radius of curvature in the model? (This will determine the smallest tool needed)
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I’m assuming this is a mold for a casting??? If I had the challenge, I’d reduce the thickness of my stock to the highest point of the cut. The walls will be shorter, but then add rectangular stock to build up the walls to the height needed for casting.
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Based on the description, I guessed a vacuum form for plastic. ???
I think everything above the trim line could be tapered out, leaving more room for the tool & spindle nut. Using a larger tool or a tapered tool would also increase rigidity a lot.
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Great discussion here, I would take the 2 piece route to gain the vertical depth needed. Set the two pieces the height needed stacked with dowels. Lock the bottom piece down and perform the 3D carve, do not remove when complete. Place the second on top on the dowels, I would glue it on or secure it with tape around the seam. Then cut the top vertical walls out to get your full depth as mentioned by Tod 1D and mhotchin suggested above.
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If it were a vacuum-form mold, I’d do it as a male. It’s easier to pull plastic over a male form unless the female mold is porous, and forming over a male form puts the non-contact (thus smoother) surface on the outside and the tool-marked surface on the inside… Just some experience from a few decades ago…
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Thanks Everyone. I’ll take the suggestion of cutting it down into 2 parts and put some dowels in to align the 2 parts.
I really appreciate your help.
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