Work holding Discussion

General
,
1

As I’ve been getting more comfortable and confident with my machine (Shapeoko 5.1 Pro, PwnCNC conversion, 2.2kW spindle) I’ve started running into situations where my current work holding isn’t doing the job.

Current Work Holding

Currently, I flatten one side of the work piece either with a planer or with the CNC itself. Once one side is flattened, I double check for tram, hit it with a sander if needed, and then completely cover the flat side with XFasten Double Sided Tape. I then stick the work piece to the spoilboard and sit on it. Or put a big lump of lead on it.

The Problem

I make things for a living, so I’m usually trying to make the robot go a bit faster than the last time. This is probably a huge part of the issue.

When machining some cherry today at a 0.25" depth of cut, 0.3" step over, 12,000 RPM and 200 inches per minute with IDC Woodcraft’s Deep Ripper, I ended up with a huge amount of tear out in one or two places and then the part detached from the spoil board.

(Settings rationale: I would like to go slower, but to avoid other issues I should keep the chip load the same by adjusting other settings.)

Current Thinking

  1. This is my fault.
  2. Probably because I’m using a ridiculous up cut end mill that pulls the work piece off the bed.

Where Do I Go From Here?

Some thoughts I’ve had are:

  • Keep tuning the tool path by:
    • Decreasing step over,
    • Increasing feed rate,
    • Increasing RPM,
    • Sacrificing an end mill to some kind of deity.
  • Stop using aggressive tool paths without putting the work piece in a vise that is secured to the CNC.

Why am I Posting This?

I’d like to check in with other people, validate my own ideas, and understand what other folks do to keep work held down on the table.

So, my CNC pals, what are y’all doing to keep your robot working quickly?

2

I have t-tracks, and use hold down clamps. I’ve really only had problems with white oak, so when it comes to that material i go overboard on clamps

Biggest disadvantage, i don’t use dust collection, and it gets down in the cracks, also some of my t-nuts have stripped threads.

More likely than not you don’t need a prefectly flat material surface as the clamps will hold them down, I’m assuming at least it’s not too potatoe chippy.

These are the specific ones i use and i have 4 orders of them

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I used to have the clamps from carbide that would come from the side and thread in t-nuts but potatoe chips were hard to deal with, and my task list is 90% customer provided material. Also i couldn’t get a good enough layout… not to mention all those t-nuts that you have to put in 1 at a time because the second you do 2 and hit it with the hammer the second one would fly out.

I did try the ca glue and tape, but my problem was that my tool paths would take 5 min and applying the tape would take 10 min meaning the first one would take 15, and sequent ones took 10… then i still needed to flush trim themv away. Just not working in my workflow

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3

I just saw this.
Cherry is pretty soft, so it might be a material issue too. Looks cool though, i might pick me up one, I’m a big fan of the hog and the beast. Tear out will occur with a roughing bit, and it’s largely unpredictable where (aside from along grain)

4

Aye, I think so (about material). I’ll switch to a different end mill that’s less aggressive on material removal. It’s GREAT for hogging out huge cavities/pockets.

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5

You could still use it, you would just need to undersize your pocket your roughing with that monster, then go in with another bit to get it at dimension. Also cutting against the grain should help but that’s assuming your pockets are all in the inside of the workpiece

This of course depends on the size of the pocket your hogging out.

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6

Tramming also plays a huge role because, you know, geometry and stuff. Definitely need to leave a lot more stock than I thought (and I was being relatively conservative initially on stock to leave), but I’m dialing in tool paths. \

Will post more findings later.

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7

Tram does play a role, but it depends on the situation too. If your task is for only doing a single pass profile cut for things that things you might hang on the wall, tram is less important vs you do thicker material with multiple passes where you’ll see the tram marks on the side. Or maybe doing only 3d tool paths, tram is less important. I hope that made sense…

I would say tram is about as important as proper motor calibration but both of their importance depends on your necessity of accuracy and clean cuts.

I await your findings

8

Here’s what I have learned over the course of experimentation today.

TL;DR - if you are using double stick tape, go slow, be careful. Remove less material at once.

Tool paths

My tool paths are too aggressive for my work holding technique (double-stick tape and hope).

What I did

I reduced depth of cut and feed rate and stopped having the machine yeet the work piece into next year.

  • Feed rate was reduced to 120 inches / minute.
  • RPM reduced to 12,000
  • Depth of cut reduced to 1/4"
  • Step over remained the same.

End result

  • Better cut quality.
    • Minimal chatter.
    • Minimal swirly marks that will need sanded off.
  • Work pieces stayed on the spoil board.

Tramming

I thought that the machine was in tram, but I decided to check after observing tooling marks that should have been removed. Tram was mildly out according to both of my tramming tools.

However, both tools have about half the stick out of the end mill in question. By virtue of accidentally remembering the Pythagorean theorem…

I had a suspicion that the tram was JUST enough off that the combination of an aggressive tool path, an aggressive up cut end mill, AND slightly poor tram meant I was going to have trouble keeping the work piece in place.

What I did

  • Trammed the machine.
  • Verified tram using a second tramming tool (this is not normal behavior) and a machinist’s square.

End result

  • I am no longer seeing tool marks where I did not expect them.
  • The work piece has yet to visit Jupiter to get more stupider.

Order of Operations

As we all know, order of operations is important. After the previous two fixes, I revisited the order of operations in CAM. After all: keeping a solid piece of material engaged with the work holding for as long as possible provides better work holding.

What I did

  • I updated all tool paths to leave 2mm of stock between the cutter and the bed until the last possible moment.
  • Additionally, when the tool path was removing a lot of material or was particularly deep, I added significant stock to leave on tool paths involving aggressive stock removal. This has the side effect of helping if the machine is out of tram.

What I saw

Would you believe it, the same thing as the other steps:

  • Fewer work pieces in orbit
  • Fewer tool marks on work pieces.

What’s Next

As much as y’all love clamps, I’m not working with square parts and I am loathe to leave a lot of material around that isn’t useful to me. People who are working with rectangular stock should absolutely clamp everything they can.

For now, I’ll be setting up different tool path profiles in Fusion and changing them based on the material and, potentially, based on stock dimensions.

Longer term I will be investigating vacuum workholding as a way of adding additional holding force. I’ll also be investigating other hold down mechanisms (clamps, t-tracks, hiring people with VERY fast hands).

Disclaimer

Yes, this is copied and pasted on at least one other site.
Yes, I work in metric. All SAE values are approximate. No imperial values have been used since nobody in the US believes in a 20oz pint of beer.

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9

I don’t always use square parts either.
Clamps are at disadvantage because they need to have something to hang onto… so cutting a shape out poses challenges. a disadvantage of vaccuum or double sided tape do not share. Vacuum holding is both costly and loud (according to Daniel and his experience)

I can make do with about 1" of excess material on each side for clamps, or (i haven’t used it yet) vectric has some sort of clamp avoidance thing now.

Thank you for your thoroughness in going thru the Problem solving process

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