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Old 06-12-2010, 03:13 AM
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Default A source of error in the MFT

I made a large MFT type cutting table using a spare set of MFT guide rail brackets.

I began calibrating it to cut at 90 degrees using the very handy calculator in Rick's Kapex manual and used shim stock to make corrections. I was shooting for an error of less than 0.004 @ 2'. (The great thing about the calculator is it tells you exactly how much shim is needed and which way you need to move the kerf.) Sometimes the corrections helped and sometimes they didn't. After some time I realized that the results were so erratic that I was overlooking something.

Finally discovered that the height adjuster doesn't reliably clamp the steel bracket perpendicular to the plastic block. If the bracket leans a little one way or the other it will supersede fine adjustments.

So, after adjusting the height but before fully tightening the clamp I put a machinist's level on the top of the steel bracket and adjusted it to match the pitch of the table. Even after clamping all the way it was still possible to fine tune the pitch of the steel bracket. From then on my shim adjustments produced predictable results repeatably and the error was corrected to zero.

The moral of the story is even if you have the fence/guide rail set perfectly square to the fence/guide rail, if you make an adjustment to the height, or even unclamp and retighten the guide rail mounting bracket you can end up loosing that perfect 90 degrees. Depending on your definition of perfect.
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Old 06-12-2010, 04:04 AM
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Default Re: A source of error in the MFT

I'd love to see some pictures illustrating your point, Michael. I've just finished a bottle of sake with Sharon, celebrating her birthday, and I can't quite figure out where your error is arising from. And it sounds like something I should know about...
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Old 06-12-2010, 04:29 AM
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Default Re: A source of error in the MFT

The problem arises from cheap rivets, lousy plastic and loose T slots....

Oh, and happy B Day to your wife.

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Old 06-12-2010, 04:36 AM
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Default Re: A source of error in the MFT

...forgot to mention the sagging mdf and the wobbly legs...


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Old 06-12-2010, 04:50 AM
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Default Re: A source of error in the MFT

I think .004 over 2 feet is pushing the limits of the saw, rails, clamps, and table. I'm not going to say it's impossible but you're going to find all kinds of little things...

I haven't used my rail clamps in months and I happen to have them sitting beside my computer (I've been thinking of selling them on eBay). I can't duplicate the play in the clamp itself. I was thinking maybe the bolts on the clamp were loose but if that is true then the clamp won't lock at the proper height - it just slides down. However, if you remove the clamp from the table you will notice 2 set screws to adjust the black plastic block that rides in the profile. Maybe that is where you are getting some movement? It might be worth trying to adjust them and see what happens.
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Old 06-12-2010, 04:52 AM
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Default Re: A source of error in the MFT

Michael, didn't you post about this potential for error when adjusting the height of the brackets some time ago? In any event, the edges of the black plastic base that the steel piece slides on and tightens to are slightly tapered. The bend in the mating steel piece is tapered too -- when viewed from the top, the outer edges flare outward a little. As you tighten the brackets these tapers are supposed to keep the two parts in alignment -- but, as you have noted, there is a little slop and the brackets may not tighten down at the same alignment whenever you adjust them.

The bulk of this slop -- not to infer that there is a huge amount -- can be compensated for by simply holding the steel piece snug to one side of the bracket while tightening the handle. I've been in the habit of pressing the side towards the back of the rail inward and towards the rail's splinter guard on either bracket as I push down on the handle. This does seem to remove some of the potential of error -- maybe not as good as your machinist's level method, but still helpful.

On Edit: Man, am I slow... I went to reply after Peter's post and found three post between after I posted.

Last edited by Corwin; 06-12-2010 at 04:55 AM.
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Old 06-12-2010, 05:06 AM
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Default Re: A source of error in the MFT

The tables T-slots really do not have a part in this, or those sagging or wobbly items either. And the set screws Steve mentions are to remove play between the 'stationary' portion of the bracket and the T-slot in the table. So, no, when you have the brackets tightened in place on the table, these issues are not the problem that Michael is speaking of.

The issue here is simply between the 'stationary' portion -- and by this I am referring to the portion that is tightened to the T-slots with the thumb screws, not the D-handle -- and the steel piece that you raise and lower as needed.

Last edited by Corwin; 06-12-2010 at 05:09 AM.
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Old 06-12-2010, 05:53 AM
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Default Re: A source of error in the MFT

Corwin totally gets it, not that it's anything complex.

I might have mentioned this as a potential limitation at some point but I never tried to get beyond "pretty good" so I haven't made an issue of it before.

I do think the wedge idea is a good one but the black plastic part is short and at least in this bracket the sides aren't completely straight and parallel so it relies on clamping pressure to keep the steel plate stationary. I might put some nylon or brass set screws into the plastic to get the contact points farther apart.


Originally I was going to make a really simple Steve Jones type cutting table which just has wooden stop blocks for the guide rail to butt against. Then I remembered there was a set of guide rail clamps sitting around so I made some mounting brackets for them and then got side tracked by the calibration problem. Should'a made the simple table....


Steve, you're right, when going for precision there are a bunch of things that have to be dealt with. I thought I had them all under control until the calibration process became wacky. After adopting the bracket leveling method the calibration went well and it's extremely accurate now. So much so that I have to do the full 4 cut miter calibration process to find out if it's changed because my squares aren't long enough to test visually.
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Old 06-12-2010, 06:40 AM
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Default Re: A source of error in the MFT

Quote:
Originally Posted by MichaelKellough View Post
Corwin totally gets it, not that it's anything complex.
...
Uhm, thanks Michael -- I think?
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Old 06-12-2010, 02:57 PM
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Default Re: A source of error in the MFT

Corwin, I meant that it's something that is overlooked because we assume it's just fine since Festool made it. As Mirko pointed out, there is a bunch of stuff made by Festool that could be improved. Particularly, plastic parts.

It's the plastic part of the guide rail mount that is allowing the steel bracket to move laterally, despite the wedge shape and clamping pressure, effectively changing the miter angle slightly. It would help a lot to make that plastic part taller, and the sides straighter.
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