Fixture for two sided machining -- is this the right approach?

I have a Chinese 6040 router (with an Ethernet Smoothstepper and a G540) which I have been using successfully for plastic and Aluminium.

I am a complete newbie at this, although I'm very pleased with the results I am getting so far!

I want to do some two sided milling of 6mm aluminium, and I have some questions about making and using a suitable jig.

My plan is:

Make the fixture:

1. Use 6mm x 100mm x 100mm aluminium for the fixture plate -- this will be large enough to mount the stock I'm using.
2. Mount the fixture plate on my t-slot bed using the rather basic clamps the machine came with.
3. Machine two (or four?) sides of the fixture plate square with the axes of the CNC machine. (as a reference so that I can later remove and reattach the fixture)
4. Drill holes for dowels at appropriate places for the stock size I'm planning to use.
5. Insert dowel pins like these: Dowel Pins Hardened Ground 3mm X 10mm 10 PK Steel | eBay (steel into Aluminium -- will that be OK?)

Prepare the stock:
6. On another part of the bed drill the same hole pattern into the stock. (my parts are always cut from the center of the stock, so I don't need to worry about its edges or absolute alignment.
7. Slip the stock over the dowels -- hold down with clamps or double sided tape.

Machine the job:
8. Machine the first side
9. Flip the stock -- I'll ensure the centerline of the part is equidistant between the dowel pins.
10. Machine the second side

Remounting the fixture: (at some point I'll remove the fixture for some reason and need to reattach it)
Aligning:
11. Clamp the fixture as closely aligned as I can by eye/setsquare
12. Mount my dial gauge in my spindle chuck and sweep it along the machined edge, tapping the fixture until it is square with the motion of the axis. I only need to do that for one axis, as the other is perpendicular.
13. Find zero?


Questions:
1. Should I be using a mixture of dowels (for location) and threaded rod (to hold down the stock)?
2. Can I use a plastic spoil board between the fixture and the stock?
3. What's the best way to find a zero point in step 13 above?

Looking forward to comments, suggestions and corrections of terminology!

Thanks,
Tom

Tom

Drilled holes are not appropriate for dowel pins. You should drill and then ream.
If possible can you use a larger diameter dowel?
After pressing in the dowel pins you should re-cut the edge of the fixture to be parallel with the pins.
You can use a dial test indicator and sweep one of the dowel pins to use as your "zero.
Yes you can use a spoil board under your stock.

Gene

Hi Gene,

Thanks for the reply.

I'm not sure how to get a zero with a dial test indicator -- I understand how to sweep an edge to get it aligned with the axis of the router table, but how do I get an absolute zero?

I can certainly use larger diameter dowels -- I'll go for 5 or 6mm.

I had been thinking that would use four dowels, in the corners of the stock, but would just two -- one at each end of the centerline be ok/better?

I've been buying my end mills and drills on eBay (Kyocera Tycom mostly) -- any idea whether the cheap Chinese reamers any good? e.g. 6mm X 6mm X 92mm Straight Shank 6 Flutes HSS Machine Reamer Gray | eBay

What sort of reamer parameters do I need for Aluminium, i.e. straight or spiral flutes, HSS or carbide?

Thanks,
Tom

Tom

I'm getting the impression that I don't understand what you are trying to accomplish. Apparently you don't need any holes in your stock. I say that because you are willing to have 2 or 4 holes. It's not likely that you'll get 2 holes to fit over the dowel pins unless your center to center distances are within a few tenths in each part. Only by a generous hole clearance could this be overcome. Can you post a picture or a drawing?

Gene

Hi Gene,

Here's a picture of what I (think) I want to do -- showing just the stock/part, not the fixture.

https://plus.google.com/photos/11050...73029829155377

My part will be milled out of the middle of the stock, so the dowels will be positioned in holes in the 'waste' part of the stock, rather than desired holes in the part. There will be tabs connecting the part to the stock which I'll remove manually after milling.

I do have holes in the part, but they are only 2mm.

I hope that makes it clearer?

Tom

Tom

To allow for some deviation in the center to center holes in the stock make one of the dowels into a Diamond Point. Only the portion that protrudes into the stock should be the diamond shape. The portion that gets pressed into the fixture should be left round. Flip the stock as shown rather than in the other direction.

To indicate the round dowel to establish a reference point you'll need something like this. It doesn't have to be as fancy.

Here's an example

Gene

Thanks Gene -- more questions:

I understand the zeroing procedure now -- sweeping the indicator around the outside of the dowel by rotating the spindle (by hand!)

So the indicator needs a swivel mount to be able to be correctly positioned?

I'm finding it hard to google a suitable mount for my test indicator -- I have Mitutoyo with a 6mm (or is it 1/4 inch) mounting rod. Is there a magic search term?

Best wishes,
Tom

Quote:

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Originally Posted by tgdavies

.....I'm finding it hard to google a suitable mount for my test indicator -- I have Mitutoyo with a 6mm (or is it 1/4 inch) mounting rod. Is there a magic search term?

Best wishes,
Tom

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Model#?

Gene

Hi Gene,

It's a 513-401 -- it has a 6mm stem, which doesn't pivot. If it did I think I could just mount the indicator in my collet. (I could do that for finding the centre of a hole)

It also has dovetails, so I've ordered one of these: Dovetail Indicator Clamp Holder NEW | eBay -- I think I may have to cut it down as I'm short of Z height.

I'm certainly learning a lot -- I only just figured out that you can change the angle of the test indicator contact point!

Tom

Quote:

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Originally Posted by tgdavies

.....It also has dovetails, so I've ordered one of these: Dovetail Indicator Clamp Holder NEW | eBay -- I think I may have to cut it down as I'm short of Z height.
Tom

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Tom

That should do it. Remember to keep the axis of the indicator point as parallel as possible to the axis of the dowel. 15 degrees shouldn't be any problem. It really depends on the accuracy you need.

Gene

Tom

The dovetail indicator clamp that you ordered will do the job but the one shown here is much better. It has separate locking nuts, one for the swivel and one for the dovetail. It's much easier to use.

DIAL AND ELECTRONIC INDICATORS AND GAGES - STARRETT - Page n° 7 - PDF Catalogue | Technical Documentation | Brochure

Gene