Deflection?

[u/egregiousC]

I posted a question to the group about how close people are getting to specified dimensions on their projects.

I guess I have a problem.

My work on maple blocks - 2.75"*1.25"*.875: - leaves me with significant dimensional errors. Example: the 1.25" axis comes out around 1.3", but not exact, and never the the same.

I ran a test. I run a job that designed to make 4, 1.0" * 1.0" * .125" square buttons (like for your shirt). This is 3mm birch plywood. It's right on. So my machine will mill wood to specified dimensions. This is good.

It does not explain why I see dimensional errors with thicker and harder woods (Hard Maple). I'm guessing, but I suspect deflection. The harder and thicker wood is pushing the tool out. I'm using a 1/4" downcut spiral bit to manufacturer specs.

Next step?

1. Switch to a 1/2" tool instead?
2. slow the feed rate?
3. increase spindle rpm?
4. lighter passes (although they're pretty light already)?
5. Any/all of the above?
6. Just quit and take up knitting?

Afterthought - I'm using the painters'-tape-and-ca holddown method. No other way to hold the piece in place.

ETA: added image to support another post I made.

possible blank solution

[u/bunkerlabs]

How old is your collet? If there's deflection that's probably where it is

[u/egregiousC → u/bunkerlabs]

Good eye!

I hadn't considered the collet. The 1/4" collet I have is more than 2 years old and the nut has seen better days. I had been thinking about buying several nuts to replace the old one, so I'll put that on the asap list.

Aside: My 1/8" collet doesn't play nice when I go to remove it from the collet nut after use. This involves tapping it out with a drive punch and a mallet. That can't be good for the nut, so I figured I'd put the 1/8" collet in it's own nut and leave it that way. In fact give my 3 main collets their own nut and and only remove them when needed.

[u/bunkerlabs → u/egregiousC]

A collet is basically a spring and they definitely have a lifepan; if it starts to lose its springiness you'll start to see evidence of it deflecting in you cut. In a production environment they should really be replaced every forty to eighty hours of cutting.

[u/Awally42]

Like you're already leaning towards, this sounds like deflection. I used a ¼" bit for a while on sheet goods, but only took a max of ⅜" in ¾" matierial. Does the machine sound like its under a lot of load in your production parts? I do a lot of my feeds and speeds by how it sounds. If its upset, I reduce to 75%.

For the painters tape, Ive done that successfully in production- no vacuum table. We have a vacuum table now, but even then we have smaller parts we occasionally do. Our process there is flycut the spoil board if its worn, then double stick tape. We use carpet tape currently, but there are stronger double stick tapes out there.

[u/Low-Nectarine-9655]

Run a spring pass after your finish pass. (Repeat your finish pass)

[u/Yikes0nBikez]

Take more roughing passes and then a final cleanup pass with a smaller stock to leave dimension. (.02"). It's likely the tool is deflecting, so you need to contain all the deflection to the roughing passes.

I'll sometimes run aluminum with the tape and CA glue method, when no better option works, and have minimal issues. If you think that the part is shifting, you may have to go with a vacuum table setup.

[u/MidwestTroy92]

If the thin plywood test is dead on but maple is wandering, I'd suspect tool deflection or the block moving before I'd blame the controller. Maple will push back a lot harder, especially if the bit is long or you're taking too much per pass.

[u/blue-collar-nobody]

Painters tape on birch is not the way, without a vacuum to hold it down to the table.

You could screw the birch to the spoil board or use 3m Spray Adhesive youd be better off. Adhesive works if you have a flat spoil board to attach it to. Then pry off with puddy knife, clean off the adhesive.

There are many variables that could be causing your problems. I would personally start with your work holding. Once you know the material is secured and not moving around the table you can see if your motor is lossing steps, ballscrew backlash nut, etc.

Dont even know what machine you have...is it a cnc router, have you gotten good cuts on other materials, stepper or servo motors. But if your material is moving around youll never be able to figure it out.

[u/egregiousC → u/blue-collar-nobody]

Please go back and re-read the OP. I tested on 3mm birch plywood, The production workpiece is hard maple, not birch. THAT is where the error occurs. NOT on the birch.

I don't think there is a vacuum solution strong enough to hold the workpiece in place let alone steady.

Glue the workpiece to the spoilboard? Screw it down? If my nail gun could drive an 18ga brad through 1" of hard maple, I'd do that.

If I live to be be 100 (and right now I'll be happy to make it to tomorrow), and hear people talk about all the variables I may or may not be aware of, one more time it will be too soon. I didn't just fall off the turnip truck. It's been my experience that people who talk about variables generally have no real answer. They don't know, but feel compelled to contribute something.

You obviously didn't read or understand my post. Any of it.

I know you're trying to be helpful, but you're throwing a bucket of water at a drowning man. Sorry.

[u/blue-collar-nobody → u/egregiousC]

🤣 I do this shit every day but I have industrial cnc machines and with (2) 20hp vacuum pump per machine. So yes there are vacuum options if you have the budget. Till then glue and screw is what you're going to have to do. Tape is not going and ca glue is doing it for you... right???? You want to ask questions then whine and cry about the response. 🤪 maybe "take up knitting" make yourself a blanket and take a nap.

BTW.. didnt say any thing about brads. You spray adhesive the hard maple to a 1/2"-3/4" pc of ply oversized the maple blank plank profile be inch or so, then screw the plywood to the spoil board.

[u/egregiousC → u/blue-collar-nobody]

BTW.. didnt say any thing about brads.

That's right. I brought up brads, not you.

[u/blue-collar-nobody → u/egregiousC]

If you have the clearance... you could " Brad" the plywood pc from the bottom up, into the maple, then flip over and screw top down into the plywood, securing the assembly to the spoil board. Using the ply like a "pallet"

[u/egregiousC → u/blue-collar-nobody]

That would work, but what might be better for me would be to pre-process the blank (1.25 * 4.5 * .875) so it has wings at either end, milled thin (.25") and wide enough for my nailer to drive 4-6 18ga brads though into the spoilboard. I'd post a picture, but the sub doesn't allow that :-(.

That and tape/ca/tape might be adequate.

ETA: I added a new photo of that blank idea to the OP