Work Holding Ideas

JimDawson

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After machining a part, I realized that it would not work as designed. I found I needed to take 0.250 off of the height, and I had already machined off the holding tab that I used to hold it do the original machining. So I needed a quick and dirty method to hold the part while making modifications. Here is what I came up with.

First I bolted a piece of MDF to the table, then machined a pocket to fit the base of the part. MDF is a great base for many machining operations. There is a 0.093 flange at the base so I machined the pocket in the MDF 0.080 deep so the flange would sit a bit proud. Then I screwed a scrap piece of aluminum down, and machined a hole to fit the body of the part. The pocket in the MDF insures that when the clamp plate is removed that the part remains on center. That way I only have to set the 0 one time. The 0 is set on the center of the T-slot so the deck screws could penetrate the entire thickness of the MDF, and not hit the table.

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taycat

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we have local place that makes furniture where you can buy big sacks of offcuts cheap as fire wood.
might be worth looking if you have similar near you.
last bag i got had some nice rectangular hardwood blocks i kept for use putting sets of taps etc into.
loads of odd shaped bits that have been used to hold stuff on drill press or make jigs from.
 

gr8legs

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Jim, forgive me if I'm a little dim on this one - definitely a creative solution to a work-holding problem, but:

If this part was something that you milled on your CNC, why not just clamp down a new blank, reset the final Z-Axis cut parameter in the programming to what you need and mill a new one?

That would seem to me to be faster than building an MDF fixture, milling a pocket into the MDF, overlaying a bit of aluminum and milling a hole into it to hold down the part, then milling off the 0.250 excess height - and then cleaning / deburring / etc. the part.

Or - since the center hole looks circular, why not chuck it into your lathe 3-jaw chuck with reversed jaws gripping the I.D. then part off the 0.250 excess and clean/ deburr / etc. from there...

Maybe this was the last piece of stock you had on hand and absolutely had to re-work the part, or maybe I'm missing something else - Just my $.02

Stu
 

JimDawson

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Here is another work holding solution using MDF. It's a whole bunch cheaper than aluminum or steel for a backup when profiling parts.

The larger cap screws are bolted through the MDF to the T-nuts, the smaller screws are threaded into the drilled and tapped holes in the MDF just to keep the parts from twisting under the machining pressures. This started out as a solid plate the size of the MDF.

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JimDawson

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Jim, forgive me if I'm a little dim on this one - definitely a creative solution to a work-holding problem, but:

If this part was something that you milled on your CNC, why not just clamp down a new blank, reset the final Z-Axis cut parameter in the programming to what you need and mill a new one?

That would seem to me to be faster than building an MDF fixture, milling a pocket into the MDF, overlaying a bit of aluminum and milling a hole into it to hold down the part, then milling off the 0.250 excess height - and then cleaning / deburring / etc. the part.

Or - since the center hole looks circular, why not chuck it into your lathe 3-jaw chuck with reversed jaws gripping the I.D. then part off the 0.250 excess and clean/ deburr / etc. from there...

Maybe this was the last piece of stock you had on hand and absolutely had to re-work the part, or maybe I'm missing something else - Just my $.02

Stu
If this was a production part I would have done just what you suggest. Most of the work I do is one-off and prototyping so many times there is rework and design changes. Temp workholding fixtures is the norm in my shop.

Removing the 0.250 necessitated machining the internal profile again, it's only 0.112 deep so by the time I removed the top 0.250, the internal profile was gone so it had to be fixtured to do that. There are also 6 tool changes to do the whole part, so it takes a while to run, and then there is some lathe work also. I think it took about 15 minutes to build that temp fixture, and about 10 minutes to rework the part.

I'm building another one today with an octagon shape, I'll post pictures of the process in the Project of the Day forum when I get it done.
.
.
 

randyc

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ray

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I have never had good luck with screwing anything into MDF. With some vibration and little force it would loosen some when trying to secure metal to it.
 

JimDawson

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I have never had good luck with screwing anything into MDF. With some vibration and little force it would loosen some when trying to secure metal to it.
I've not experienced that, and so far I haven't had anything come loose. I also normally cut down the feed rate a bit if I don't have the most secure setup. I normally use 3/4 inch MDF and deck or drywall screws. Sometime I drill and tap the MDF and use a cap screw, and if I need extra holding power I will double up on the MDF and use longer screws. Where possible I will through-bolt to the T-nuts.
 

Joe Harlan

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I've not experienced that, and so far I haven't had anything come loose. I also normally cut down the feed rate a bit if I don't have the most secure setup. I normally use 3/4 inch MDF and deck or drywall screws. Sometime I drill and tap the MDF and use a cap screw, and if I need extra holding power I will double up on the MDF and use longer screws. Where possible I will through-bolt to the T-nuts.
I will pickup some MDF to try this in the future. I made this fixture for drilling and milling 2 holes in these caliper spacers I had plasma cut. Your idea may have bee cheaper and easier in the end. spacer.JPG
 

JimDawson

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Here is another example of work holding and order of operation in the lathe. I am building a new bearing retainer for the spindle drive system in my mill. http://www.hobby-machinist.com/threads/mill-spindle-direct-drive-conversion.49130/

This started out life as a 4x4x1 chunk of 6061 T6.

I didn't get any pictures of the work in the 4-jaw, but I center punched a divot in the center of the piece, and then just aligned that to the tailstock center. All of the internal work and face was done in one setup so the bores and face would be concentric and parallel.

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Then I grabbed a chunk of 2 inch shafting, and made a stub arbor to mount the part on to turn the OD. The snout is a press fit into the mating bore in the part. This insures that the OD is concentric to the ID. In this case, this sets the alignment of the bearing in the machine. Note the relief just larger than the locating diameter. I wanted the part to grab on the outer 1/4 inch or so of the stub.

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And the part mounted up
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I didn't want the part to touch the jaws, so I left about a 0.030 gap when bolted up tight.
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This should dispel some of the myth of interrupted cuts and carbide. I'm taking about 0.125 off of the diameter per pass at 460 RPM
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Once it's on size, then reverse the jaws and face off the excess material on the other face.
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And there it is in place. Still need to drill & tap the bolt holes.
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