Alignment issue resurfaces

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TReischl
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Re: Alignment issue resurfaces

Post by TReischl »

If'n it were me. . . .

I would jettison those screws in favor of some brass pins. Actually, that is what I do on my machine. I had some 3/8 brass rod collecting dust so I made a set of pins. That would be the first thing I would do if it were me. Then, if the problem persisted. . . .

I would get very busy checking the squareness of the machine. I looked up your machine. Apparently it has a stepper motor on each side of the gantry. That is a good thing. The way I check my machine for squareness is to machine an edge along the base axis. The base axis is the axis that does not move, the one connected to the legs. On my machine that is Y axis. Could be X depending on how the machine is configured. Anyway. Then I take a piece of 1/4 MDF and mill two slots as far apart as I can get them perpendicular to the machined edge I created above. Then flip the piece over and butt the same edge against the machined reference and cut two more slots. Then measure them to see if they are equal. If not, the machine is out of square. This is a rif on the old draftsman trick of checking a square by flipping it over.

On my machine I only use one microswitch for zeroing the Y axis (it has two motors like yours). So yup, the axis can get out of square. But yup, having two switches can also guarantee the machine zeroes up out of square. So I have two fixed reference blocks that I can butt the gantry up against with power off to get back to square if need be. Then I can power up, and zero the axis as normal.

Hope this helps!

Oh, one other thing, go into Motor Tuning in Mach and make darn sure the settings for both of those gantry motors are exactly the same. They probably are, but it is an easy thing to check.
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Stickman
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Re: Alignment issue resurfaces

Post by Stickman »

My machine calibration is ok. That's not been a problem, but I rechecked it anyway. I also checked the spindle for squareness to the table before resurfacing it yesterday.

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Re: Alignment issue resurfaces

Post by Stickman »

TReischl - Thanks for that advice. When you flip that MDF, are you flipping it parallel to your fixed axis or perpendicular? My machine's fixed axis is x, so would it be a horizontal flip (how I do my projects) or vertical?

I'm not sure I get the pins vs. screws. Many of my pieces are fairly small, so there's not a lot of room for mounting. I already have to throw away a lot of material without making the blanks bigger. I've used screws both to align the piece on the table as well as to hold it down. I don't see how pins would align it any more accurately. I use #10 brass flathead wood screws, fat enough to get a good grip in the material, and those have the taper on the underside of the head which helps center the screw firmly down into the .25" hole in the material block. I turn the material over horizontally and check for any play before tightening them down. What seems odd is the misalignment isn't just straight x or y, but seems a slight rotation, so squareness of the machine would seem a good thing to check.

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TReischl
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Re: Alignment issue resurfaces

Post by TReischl »

Flipping it parallel to the fixed axis.

You may not "see" how pins would more accurately align something vs screws. Just something I know from starting my career in a machine shop: nothing is ever aligned with screws, ever. Dowel pins are used. Think of it this way, with a pin you have one full cylinder making contact with another full cylinder. Screws are not self centering either, nor are they manufactured to close tolerances. Driving screws in and out, in and out eventually results in slop. We are talking MDF here.

Trying a couple of pins should not wreck your life, so give it a try and see if it solves the problem! Sure is better than wondering what is wrong.

Curious, how are you checking the squareness of your X vs Y axis now?
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Stickman
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Re: Alignment issue resurfaces

Post by Stickman »

It might not wreck my life, but it will make things more complicated. I'll have to redo about 60 designs, enlarging the blanks (increasing wasted material and cost) to fit pins in symmetrical positions, plus the mounting screws. Using pins, though ... won't the softness of MDF still be an issue? If it doesn't work for screws, how many times can pin holes be reused before accuracy is gone? I've used pins (generally steel) to set up certain alignments, and found those bent a number of times. I know how much softer is brass rod, as I use that for attaching handles.

It does seem a bit odd, though, that just using screws would skew things the same way. I'll start by figuring out if the machine is properly squared.

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Re: Alignment issue resurfaces

Post by LittleGreyMan »

As I already wrote: use dowels, not screws to align your job. Thanks Ted for explaining it much more clearly than I'd be able in English.

You can even use wood dowel pins, they'll give better results than screws.

You can also use threaded inserts in you MDF board and use shoulder bolts. This kind oh bolts will both fix and align your material. But you'll have to find bolts with a shoulder size (diameter and length) that matches your design.

As Ted wrote, just give a try to a dowel alignment, with a cheap material, and see what happens.
Best regards

Didier

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TReischl
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Re: Alignment issue resurfaces

Post by TReischl »

Mr Stickman. Been giving your situation more thought. About screws, what they do for a living is cut material. Dowels do not cut material.

Another thought for you. Back when I was making things for sale that were double sided I had the same issue as you as far as wasted material. Seemed impossible to solve. Then it dawned on me, cut the blanks to the minimum size required but create gages so those cuts can easily be repeated. On my table saw I installed a wixey digital fence. I can definitely repeat a cut perfectly with no problems. I cut to length on a radial arm saw. I created a gage for that so I can repeat the stop block setup every time. It works! Once I did that, I just built a few vises with end stops and life was good. Still is. I always use the back jaw on the vises and a flip down stop on the right side. These can easily be located to set zero. Since I am controlling length, width and thickness I can flip parts over without using dowels and have holes/features line up properly. Something to think about. You can bet that in modern machine shops they do not build fixtures with pins in them to do multi sided work. BTW, you only have to control either the length or the width to be able to flip the parts.
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Re: Alignment issue resurfaces

Post by martin54 »

If I am doing a one off double sided project then I use wooden dowels for alignment most of the time. If I have projects that I know I will be machining a few times then I will generally make a jig up both for accurate alignment & for work holding. If this is repeat work that you are doing then using a jig will mean that you don't have to increase the size of your blanks, in fact you may even be able to reduce them in size :lol: :lol: :lol:

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Re: Alignment issue resurfaces

Post by garylmast »

If I have projects that I know I will be machining a few times then I will generally make a jig up both for accurate alignment & for work holding. If this is repeat work that you are doing then using a jig will mean that you don't have to increase the size of your blanks, in fact you may even be able to reduce them in size :lol: :lol: :lol:
I've done quite a few double-sided projects over the last several months, and what Martin has suggested is what I do on every project. I will setup a jig, and if for whatever reason my material is not exactly the same size as the job setup, I zero the X,Y datum in the center of the material. Dealing will dowels seems to be a lot of extra work, where a jig is a lot simpler.

Gary

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dealguy11
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Re: Alignment issue resurfaces

Post by dealguy11 »

When I had a Legacy machine (different model) the y axis was not initially square to the x. Had to mechanically realign them. Could be the case here.
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Re: Alignment issue resurfaces

Post by Stickman »

Well, this is a smack-myself-in-the-head moment. Looking at all the variables checked .... but not the table itself. It's removable, and I thought the position consistent when remounting, but over time things get a bit loose ... so there's my skew. I put on a longer piece, 24", and measured from mounting hole center to mounting hole center end-to-end to see the offset. After adjusting for that, I ran the program for that piece and it came out perfect ... or at least correctly. Now I recognize I'll have to check this calibration for each of the nearly 70 preset positions I have when I run those pieces, because there's no way to know if the table was skewed when those were mounted up. Still, that's easy enough to do, and kind of nice to find a low-level resolution to the problem. Thanks to all who gave me feedback here, because that really helped focus my attention.

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TReischl
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Re: Alignment issue resurfaces

Post by TReischl »

Happy to hear you worked through the issue.

One thing I learned way back, and I mean WAY, WAY back, was that problems that I encountered were usually of my own making. I have worked with machines before they were CNC, just plain old punch tape NC's. In all those years (since 1974) I have never encountered a software problem that was not obvious long before the program reached the machine. Especially motion problems. The math is just too simple to cause nuanced errors. Later in my career I wrote CAD/CAM software for a living. When I goofed up it was always spectacular, not some little subtle error. Multiply something by a cosine when you should have used a sine and WOW!

It is also rare to see "control" errors, it seems they either work or they don't.

Based on that experience, any time I have a problem I start with me. And yup, I have made some real beauties lemme tell ya!
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Re: Alignment issue resurfaces

Post by LittleGreyMan »

TReischl wrote:Happy to hear you worked through the issue... Based on that experience, any time I have a problem I start with me.
+1 :!:
Best regards

Didier

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Re: Alignment issue resurfaces

Post by Stickman »

The table alignment was 99% of the issue. However, there is still an issue that this didn't resolve, and I wonder if it has to do with backlash or such due to the bit always turning in one direction it travels in opposite directions on the X axis from side to side. I've just run three of the long pieces, and while the profile is now aligned, the bevels cuts with the fluting pathway are not cutting symmetrically to the same depth initially where the bit enters the material. If I look at the profile edge-wise, I can see a discrepancy that looks a bit like this:
Uneven edge from 2-sided fluting pathway
Uneven edge from 2-sided fluting pathway
I've double and triple checked this across numerous designs and this isssue seems consistent. I'm using a 1.125" chamfer bit to do the cutting. I can't see any differences in the program settings, so this has to be a mechanical issue. I'm wondering if I have to adjust for that spin in my calculations, which seems an awkward variable.

As for the use of dowels or pins vs. screws, I'm having a hard time figuring out how the material makes a difference, except with dowels I would have to then come up with a different solution for attaching the material solidly, and since I'm cutting my blanks as close as possible to the size needed for my projects, I'd have to enlarge my blanks, as I clearly need downward pressure for the hold, not just lateral, which will flex the material when I do the final profile cut-out. The holes in my material and mounting surface are exactly aligned. The wood screws I use are chamfered underneath, basically countersinking down into the hole on the material, so when I tighten them down, I can torque them (going around, like tightening lug nuts on a wheel or bolts on an engine head for even pressure) to see that they are centering in the hole and the material has no play before final tightening. Frankly, the amount of variation in the tolerances from one screw to the next is infinitesimal compared to the kind of variations I've had in my cuts.
The profile of my "centering screws".
The profile of my "centering screws".

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Re: Alignment issue resurfaces

Post by dealguy11 »

If your profile look asymmetric, then it suggests to me that the bit is not 90 degrees to the table. It may be that the table is not level (in which case you would see the cut getting shallower from one side to the other) or that the spindle is out of tram. If a bit set to the table height stays on the table at all points then it's a tram issue. If not, then you may need to shim the rails that your table are attached to.
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