A Measurement Question

[Anorak Bob]

Rather than bury this at the end of my previous post, I will ask here. Hopefully someone can help.

I need to cut a piece of cast iron as per the section below. The cutting is easy.
Can anyone suggest a way of accurately measuring the 70mm without removing the work from the mill?

I can blue and scribe the lines on a surface plate and observe the progress of the cutter with a 10X jeweller's loupe. Is there a better way?

BT

[Steamwhisperer]

Hi BT
It looks like the top 'T' slot is relative to the side ones. Could you machine the top 'T' slot then use that as the datum to measure from the bed surface down to the base of the top 'T' slot.

Phil

[Stustoys]

Hi BT,
I thought I'd come up with a clever idea, but now I'm not so sure as it assumes a perfectly square corner on the cutter.

Assuming the angle is 30 degrees and my maths is correct. If you machine the block as per drawing. Then machine the 30 degrees until the corner is gone. Its makes the 71.16 easy to measure but then I'm not sure you are better off.

Will you be using vertical or horizontal?

Stuart

[Anorak Bob]

Hi BT
It looks like the top 'T' slot is relative to the side ones. Could you machine the top 'T' slot then use that as the datum to measure from the bed surface down to the base of the top 'T' slot.

Phil

Hello Phil,

For what I am trying to achieve I only require the central tee slot. It is the 70mm horizontal dimension that is critical. Thinking about it some more, all I need to do is invert the swivel saddle that I have on a surface plate then using a pair of same sized rods, mike the dimension as you would a dovetail. Utilizing the same technique on the workpiece will be tricky because it will be upside down. I could use Plasticine ( if it's still around ) to hold the rods in place.

Bob

[Bryan]

If it were me I would probably make something square I could sit on top to line the edges up to. Which I guess is kind of like Stuart's idea but cruder. If it really has to be right, you can only get it so close by machining, then scrape it in.

Edit: Good idea with the pins Bob.

[Anorak Bob]

Hi BT,
I thought I'd come up with a clever idea, but now I'm not so sure as it assumes a perfectly square corner on the cutter.

Assuming the angle is 30 degrees and my maths is correct. If you machine the block as per drawing. Then machine the 30 degrees until the corner is gone. Its makes the 71.16 easy to measure but then I'm not sure you are better off.

Will you be using vertical or horizontal?

Stuart

Stu,

I have a 60 degree cutter that I would use on the saw arbor in the vertical head. Up one side and down the other assuring parallelism. So the idea goes.

Bob.

[RayG]

Hi BT,

I can think of a couple of ways to do it, but they all involve a DRO.. :)

Yes, I know that's not helping.. How accurate is your x axis handwheel? I'm assuming you will be taking the cut by advancing the X.

Regards
Ray

[Anorak Bob]

If it were me I would probably make something square I could sit on top to line the edges up to. Which I guess is kind of like Stuart's idea but cruder. If it really has to be right, you can only get it so close by machining, then scrape it in.

Edit: Good idea with the pins Bob.

And I'm talking to three blokes who are all scraping exponents!

As far as the being right goes Bryan, here's some background.

When I first saddled up the dividing head on the mill I thought I'd check the head's spindle runout. I set up my 0.0001" DTI in the bore and rotated the spindle. No needle movement so I thought the stylus wasn't making contact. It was. I can't get my 1um Mahr indicator inside the bore but I'm not bothered. The accuracy dissipates with the 3 jaw Burnerd chuck fitted but I do have a limited selection of Schaublin W20 collets for times when accuracy is required.

It would be a shame to squander the precision as a result of shabby workmanship.
The base I envision would be small. If scraping was required, it would be on a scale that wouldn't be too daunting. And I know where to seek advice.

BT

[Anorak Bob]

Hi BT,

I can think of a couple of ways to do it, but they all involve a DRO.. :)

Yes, I know that's not helping.. How accurate is your x axis handwheel? I'm assuming you will be taking the cut by advancing the X.

Regards
Ray

I was thinking power feed on the X. Y's by hand anyway and that's what I was thinking I'd use to increase the depth of cut. I can fit my 1um indicator in my "not a DRO" for a bit of critical cutter advancement. An indicator for the Z is another project.

As far as accuracy goes, the machine is a well maintained 49 year old veteran from the Weapons Research Establishment's Aeronautical Research Laboratory in Salisbury. I'd say pretty accurate.

BT

[RayG]

Hi BT,

Ok, if you set the Z axis and lock it. Advance the Y axis until it is just touching, then advance until you've cut exactly 15mm ( or maybe stop a little shy, so you can take a finishing cut for the 70mm dimension. Repeat on the other side.

If the cutter isn't exactly 60 degrees, the top 70mm dimension will still be correct.

I get the feeling I'm missing something? But what?

Regards
Ray

[Dave J]

Could you make the piece higher than needed? Then cut the angles so the 70mm measurement was slightly than needed.
You could then use the face mill to skim the top until it fits nicely. At least this way you would be able to test fit it with the other piece after each cut, and then calculate how much more needs to come off.

Dave