Removing material to lower weight

[angelfj1]

Hello all. I will soon have a Grizzly G0704 mill. The plan is to eventually convert to CNC, but that wont be for some time.
I also have an Enco 9x24 Lathe.

I hope to make my own astronomy accessories. It is important to have strong, rigid and light weight parts. I am looking for any standard guidelines and/or recommendations for removing material from a part , such as an aluminum plate, without sacrificing strength or rigidity.

Below I have shown a commercial mounting plate. You can see that considerable material has been removed. Also notice the dovetail. This plate fits into a 'receiver' piece with a complimentary dovetail recess. This allows easy adjustments and locking once the scope is in the correct position.

These are very expensive and I am hoping to mill my own. I guess they are produced on a CNC mill - seems like a natural application for CNC. But could one be produced on a manual mill?



Any suggestions would be appreciated.

Regards, Frank )

 

[Tom Griffin]

I used to build a lot of equipment out of machined plates that flew, so weight was always an issue. Generally the best way to lighten a plate without sacrificing strength is to divide it up into sections as square as possible with diagonal ribs. The pics you show leave a bit to be desired, the sections are rectangular instead of square and the ribs should align in the center instead of being offset. Three square pattens with thinner ribs that align in the center would have been stronger than the two patterns they used with misaligned ribs. If you keep all of your patterns square, so the angle of the ribs is the same, parts like that are easy to machine in a manual mill. Just layout the pockets and mill to the lines.

Tom

 

[Bill C.]

Hello all. I will soon have a Grizzly G0704 mill. The plan is to eventually convert to CNC, but that wont be for some time.
I also have an Enco 9x24 Lathe.

I hope to make my own astronomy accessories. It is important to have strong, rigid and light weight parts. I am looking for any standard guidelines and/or recommendations for removing material from a part , such as an aluminum plate, without sacrificing strength or rigidity.

Below I have shown a commercial mounting plate. You can see that considerable material has been removed. Also notice the dovetail. This plate fits into a 'receiver' piece with a complimentary dovetail recess. This allows easy adjustments and locking once the scope is in the correct position.

These are very expensive and I am hoping to mill my own. I guess they are produced on a CNC mill - seems like a natural application for CNC. But could one be produced on a manual mill?

View attachment 62132View attachment 62133


Any suggestions would be appreciated.

Regards, Frank )

Yes, it could done. Cut the dove tails to slip fit the receiver then do the mounting holes. If you want to lighten the part after that then cut slots in between the mounting holes leaving enough material to support the holes. That is how I would do it. Good luck

 

[angelfj1]

Yes, it could done. Cut the dove tails to slip fit the receiver then do the mounting holes. If you want to lighten the part after that then cut slots in between the mounting holes leaving enough material to support the holes. That is how I would do it. Good luck

Thank you Tom and Bill.

There have been a number of posts describing the problems with milling dovetails. Am I kidding myself or are dovetails beyond the ability of a novice with a hobby level mill?

 

[f350ca]

I'd do the lightening first. When you start removing large amounts of material you relieve internal stress from the rolling or extrusion processes, this will cause the material to warp. The lightening will probably relieve the most stress but aren't critical dimensions, the dovetails are the most critical so should be cut last, unless you rough cut them then set up at the last to do a finished pass.

Greg

 

[angelfj1]

I'd do the lightening first. When you start removing large amounts of material you relieve internal stress from the rolling or extrusion processes, this will cause the material to warp. The lightening will probably relieve the most stress but aren't critical dimensions, the dovetails are the most critical so should be cut last, unless you rough cut them then set up at the last to do a finished pass.

Greg

Thank you Greg.

Gee, that's very interesting. I never would have thought about relieving internal stresses. These plates will be aluminum but not sure of the alloy. Any suggestions?

There have been a number of posts describing the problems with milling dovetails. Am I kidding myself or are dovetails beyond the ability of a novice with a hobby level mill?

Regards,

Frank

 

[f350ca]

Im by no means a professional, but have had no problems cutting dovetails, especially in aluminum. In steel or cast iron I'll often use the shaper, simply due to cutter cost.
Rough out what you can with a straight end mill then cut the profile with a milling cutter.
Measure across two dowel pins nested into the dovetails to determine your width.

Greg

 

[twstoerzinger]

For aluminum alloys:
6061 is widely available and tends to be low cost. It was originally developed in the 1930's for the aircraft industry, but it is widely used today for many applications. It is usually mill heat treated to improve strength and is commonly available in two heat treatments: T6 and T651. In addition to the internal stresses put into the metal by "working" during rolling, the heat treatment introduces some more internal stresses.
Others below have already recommended removing the "lightening" portions first (when most of the distortion occurs) and doing the precision cuts last.
There are also several "stress relieving" treatments for 6061 alloys using heat, cryogenic, or stretching - and combinations thereof. Some stress relief treatments are as simple as baking the metal in your wife's oven at 250 F or so for 4-6 hours before rough machining and again before final machining. (Be sure to remove all traces of oil and cutting fluid before you put it in the oven or you will be very unpopular). Do a search for "stress relieving 6061 aluminum" for a lot of reading.
MIC 6 is a cast aluminum plate developed by Alcoa. It is continuously cast and stress relived when produced. It is intended for use in precision applications where high strength and corrosion resistance are not the primary requirements (and welding is not required). It is designed for stability during machining. The casting process takes great pains to eliminate voids and impurities. MIC 6 is fairly easy to get (McMaster Carr and other on-line sources carry it). It is about 50% more pricey than 6061 (a 12 x 12 x 1 slab is about $100), but might be worth considering for a precision instrument part.

Terry S.

 

[lrsglory]

Just drilling lightening holes in a predetermined manner will greatly reduce weight. Rough the piece out. layout the mounting holes and dovetail etc. and then drill holes. Kudos to stress relieving it in mama oven prior to finishing it. A century ago I made a matched set of v blocks out of a piece of RDS. After I rough milled them and had heat treated they spent a week in mama oven. 2-3 hours at 350 then into the freezer. Next day from the freezer to oven . Repeated for 7-8 days. After I ground them in they just didn't move.

 

[Tom Griffin]

Thank you Tom and Bill.

There have been a number of posts describing the problems with milling dovetails. Am I kidding myself or are dovetails beyond the ability of a novice with a hobby level mill?

Dovetails are very much within reach of the novice, the biggest hurdle to overcome is the cost of the cutter. The comment about doing the lightening first is a good suggestion, but if you skin both sides first, warping will be kept to a minimum. Also, type 200 tooling plate (stress relieved 6061) and cast plate have less built up stresses. One thing you don't want to do when lightening a plate is to pocket all the way through. The skin on the opposite side of the pockets is where most of the strength comes form.

I am planning on doing a video on cutting and measuring dovetails. Maybe I'll have to move it up in the queue a bit.

Tom