How to fixture a part?? ...see post #10 for print

[Matt McColley]

Could someone please tell me what is the standard method for securing piece of plate from which you want to mill an outside perimeter profile on a milling machine?

Is clamping down to a sacrificial fixture plate best?

Or....

should I bolt a smaller size block to the bottom of the plate (with the bolt holes aligned to features that will later be milled out of the interior section) and then secure the block into a mill vise?

Or....

something else?

Thanks in advance,

Matt

[Geof]

Originally Posted by Matt McColley Is clamping down to a sacrificial fixture plate best? Or.... should I bolt a smaller size block to the bottom of the plate (with the bolt holes aligned to features that will later be milled out of the interior section) and then secure the block into a mill vise? Or.... something else? Thanks in advance, Matt

Yes.

Yes.

Yes.



You do that which is most convenient and quickest.

Your middle suggestion of bolting to a block which is held in the vise can sometimes be the best. The reason for this is that this block, or the holes it was attached to, give you a reference point when you flip the part for subsequent operations.

It is necessary to spend the time drilling the holes and making the block but you have to compare this with the time taken to move clamps if you have it clamped to a sacrificial plate.

[Matt McColley]

Most of my CNC machining experience is at work using gantry style table routers to mill wood and poly sheet goods.

With the higher grade wood panels, we are always concerned with the finish of the cut on the reverse side and having 100% contact with a waste board can help greatly with tear out.

Does any of this apply with the machining of metals?

[tobyaxis]

Originally Posted by Matt McColley Most of my CNC machining experience is at work using gantry style table routers to mill wood and poly sheet goods. With the higher grade wood panels, we are always concerned with the finish of the cut on the reverse side and having 100% contact with a waste board can help greatly with tear out. Does any of this apply with the machining of metals?

Yes.

Sometimes double sided tape, vacuum plates, fixture plates with clamps, vises etc. Cutting forces and operations dictate how well you have to hold a part.

Many of the same principals apply.

[Matt McColley]

I'd hate to machine a nice fixture plate and then have it hacked up....

is it SOP to clamp a thin plate between workpiece and fixture plate?

if so, what's a suitable (and inexpensive) material to use?

[Geof]

You don't have to worry about tear out. The only time it might be necessary to fully support metal for milling, and this probably means front and back, is if you are milling the perimeter of something thin and want to go very fast. High helix cuttesrs used on aluminum tend to lift the material strongly, but this is not a common situation.

[tobyaxis]

Originally Posted by Matt McColley I'd hate to machine a nice fixture plate and then have it hacked up.... is it SOP to clamp a thin plate between workpiece and fixture plate? if so, what's a suitable (and inexpensive) material to use?

In the case of light alloy machining say aluminum you can use an aluminum plate for the fixture.

It is always practical to use an inexpensive material for fixtures but keep in mind that it has to be strong enough for your operation.

[Matt McColley]

we've used some kind of resin based, uber hard material for spacer blocks as part of modular punch press tooling. That stuff would be great to use, but when I looked into using it for a work bench surface, it turns out that it's not at all cheap.

I wonder if Masonite that was smooth on both sides would be hard enough to use as a waste layer for machining Aluminum clamped down on a fixture plate.

I can get 1/8" Masonite dirt cheap.

Hmmm.....

Another thought I had was to just machine it in the vise with "tabs" cut in to the last 50 thou. or so and then manually trim the tabs to release the part.

Good idea? Bad idea?

I've obviously got a lot to learn

[Mcgyver]

start by telling us what you're machining. actually, start by using a subject heading that isn't' completely meaningless and gives a hint of what you're after, my pet peeve

tell us what it is that you're machining and what equipment you have as technique will be determined by this....and.... your learning will be better when you understand in the context of the job at hand why you're getting the advise you're getting.

there is a big difference in say milling the perimeter of a 4x 6 rectangle of 3/4" thick steel vs milling tons of slots and holes all over a 12x12" piece of 1/8" AL

The general principals are 1) you want sufficient clamping so things don't move (this determined by things like surface area and force and coefficient of friction, but its rarely calculated, you generally go for as much as you can get, being practical and back off the cutting force when a set up is more on the precarious or light side) and 2) you want support as close where the cutting force is applied (tool) as you can. beyond that, the work piece dimensions, machine, available tooling and your ingenuity determine the set up.

[Matt McColley]

the question was intended to be generic....

but the piece at hand is ....

1/4" 6061 AL plate....

fabricating small motor mount plates, as per this print.....

I can blunt the outside corners with a die grinder, but want a decent finish on exterior edges.

I'd prefer to do the job at home on my new X3 size benchtop mill and use a boring bar for the large hole.

But I can also do it at work in my off time on at knee mill & lathe.

[Mcgyver]

the question was intended to be generic....

fair enough, then its the 1 -2 combo of enough clamping force and support near the cutting action...beyond thats its inginuity and whats availalble

but the piece at hand is .... 1/4" 6061 AL plate.... fabricating small motor mount plates, as per this print.....

1/4 is beefy enough and the overall part is small enough that there many ways to do it. clamp it to the table on parallels, cut two sides and then move the clamps (one at a time) to cut the other two would be one. For speed of set up though , I'd probably C clamp to an angle plate and hold the angle plate in the vise (or better still on the table). Use a square and surface plate (or mill table) to get the plate square on the angle plate. simple and sturdy.

avoid fibre material. it'll be a mess with the collant and is not dimensionally accurate or stable (when accuracy matters). further, its flexiblity is not what we want, we want rigidity. of course there a exceptions, like how to hold something big a thin; mdf and double sided tape can work there, but in my experience that is an uncommon type of job.

also, don't forget the humble file, it would be a quick way to clean and square up the edges on non critical dimension...surprisingly accurate work can be done with a bit of care

bore the hole with the work held on some thin parallels in the vise or wider ones if directly on the table.