Trouble chamfering parts

[EricSchimel]

Hey all, I'm having a heck of a time adding a chamfer on some parts. I've been making these black HDPE parts by cutting them out of full sheets of HDPE. They need to have a chamfer on both sides. I do the chamfer on one side by running a V bit around the perimeter of the first side, doing a bit change and then cutting everything else out.

To do the chamfer on the other side I made this jig:

28809

It's a pretty basic jig. I milled the jig in place and drilled holes into my table so I could pin it in place. I never even removed it from my table or powered off the machine.

I clamped my parts in place and ran my chamfer path. The chamfer path is a line that is offset .02" from the actual perimeter of the part that cuts .02" deep. No matter what I do I can't get it to line up. The toolpath is not lined up, it's shifted and ever so slightly at an angle.

I'm baffled because I cut the jig with the tool and pinned it in place without ever moving it or powering off the machine.

The part is about 5" by 10"... Is adding detail like this too intricate for a 4x8 machine?

My machine is an older PRS, but it's all tuned up really well... Has anyone tried something like what I'm attempting to do here?

[tlempicke]

Two things here.

First, how did you set the machine at the starting point when you cut out the original piece and then the jig? that is probably where the problem arises.

But there is better news. What you are trying to do is really easy.

First cut all of your through holes and the chamfer for that side. When you do it cut two, or three, dowel holes outside of the parts that you are making. 1/2 inch dowels work well but you can use whatever comes to hand in your shop.

Now in Aspire select the entire job and flip it using the mirror function. Flip the entire job including the dowel holes and you must use "Flip about job center".

Remove the work piece and use the new drawing to cut just the dowel holes in your spoilboard. Put short pieces of dowel in and mount the work piece on it.

Now cut first the chamfer and then cut out the pieces.

Remember that the spindle must start from exactly the same position. So the first time you cut do a C3 and then move the spindle doing a Move X. a Move Y and then Z2. This way you can always return to that spot with great accuracy.

You can hold the edges within about .002 this way. I use this all the time to register stuff and it works great.

[EricSchimel]

I put 4 holes in the spoil board and recorded their location. I also put the same 4 holes in the jig. I've got metal dowels that I use to align it.

I came in today, powered up and zeroed. I put the tool in each of the holes in the spoil board and they line up perfectly.

I then mounted my jig, and just to be sure I re-machined the pockets just a little wider.

I mounted a part in the jig and did my chamfers and this is what I got":

28815

Everything is zeroed and lined up, I'm totally baffled about what is going on.

I do like the dowel idea but I can't do it here because the holes are a little larger than .25" and I don't have any dowels that are .265 in diameter. Also, I am making hundreds and hundreds of these parts so I need a reliable repeatable jig, not just a one or two off one...

[Kyle Stapleton]

Might be faster to just use a router table and a countersink for the holes??

[jerry_stanek]

when you made your jig gig you cut it out mirrored so if the part is not centered right it will be the same as the jig when you flip it

[Bob Eustace]

Eric - you will hear it many times on this forum that some jobs are best done on a router table and in particular second ops. Amana have round over and chamfer bits with a 3/16 ball bearing. These are fantastic! You didnt mention your hole size but if it fits it will do a fantastic job. If it doesnt Amana have amazing "no marr" countersink bits at $32.00.

[EricSchimel]

So I figured out the problem... Somehow I must have scaled the vectors I was using as the toolpath. A total stupid move that took me several parts to realize.

I have the same thought process as a lot of you folks, I'd love to be able to take these parts off the machine and just use a router table to chamfer the back side, the problem is that I've got a lot of tiny radii I've got to do. The only router bit I've found with an 1/8" guide bearing is a little Dremel one... Honestly I don't have a lot of faith in that bit lasting any length of time. These parts I have to make will be in the hundreds per month... I'd rather depend on the CNC and some nice heavy duty bits.

The idea with the jig is to scale it up so I can do 15-20 parts at a time...

[scottp55]

Seal the heck out of the jig to reduce humidity changes.
scott

[EricSchimel]

Yeah, I had a thought about that. What do you normally use for that? Shellac?

[bobmoore]

In my opinion you need a much stiffer jig if you are going to make multiple runs of this part. I would take your print to a machine shop and have one made from aluminum at least .375 thick. It will be well worth the price.

[scottp55]

I've tried about everything on MDF and solid wood,and wasn't totally happy until we went HDO and then thin lacquered that 6 times, but we still have to finish what we start the same day or button hole CS is off and very visible(to me).
Like Bob's suggestion if it's a repeat/quantity type product.
Maybe look into those copper cap/self centering jigs/separate Z-3 routines I've seen people use?
scott

[EricSchimel]

Yeah, I'm wondering if that's going to happen to this jig. At least I put enough holes in it that I can check it before I use it. I was thinking of making another out of Baltic Birch and sending it to my finish shop to be sealed up really well. The other thing I thought of was actually making it out of HDPE since I now have a lot of it around....

[bill.young]

Moisture expansion and contraction is certainly an issue, but be aware that plastics in general are much more thermally reactive to temperature changes than wood...particularly plywood. If you have wide temperature swings in your shop, plastics including hdpe will move a lot!

\http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

[Burkhardt]

I really like Garolite XX or Garolite LE (a.k.a Bakelite) for durable fixtures and mechanical parts. Much easier to machine than aluminum and less flexible, humidity and temperature dependent than simple plastics or wood. The glass-epoxy Garolite G10 is also great but nasty to machine with the glass fibers. McMaster has all that.

[scottp55]

I got a lot of recommendations to go Phenolic when I was first looking at repeatable jigs?

[EricSchimel]

Hey all, I got this all worked out:

Normally when I make jigs like this they are one time use... So I'll just mill them on scrap, use them right away and then toss them. In that context things like this work great. Like I said earlier I need to chamfer/break the edges of hundreds of parts over and over. I did get this jig to work (after I fixed my file) but like a lot of you pointed it out... It will wear out/move over time.

I ended up going to stupid simple route:

https://www.amazon.com/SHAVIV-29249-Deburring-Handle-Pieces/dp/B003JY7LXI

These little guys perfectly debur/break the edges and are faster than using the machine. So now what I do is chamfer one size and then cut the part off, and while I'm running the next batch I debur. Simple, effective and reliable.

[scottp55]

KISS principle:)

[EricSchimel]

Definitely! I was really skeptical about the de-burring tool. They're pretty great and indispensable if you're cutting plastic like this.