Plunging with an end mill?

[imagineer]

I’m in the beginning stages of a project where I will be machining two large chain sprockets (picture a 40-tooth bicycle chainring, but at 20” diameter). For the curious, I am building 2 automatic, solar powered compost drum rotators.

The material will be 14ga steel (or stainless). To save time and make sure the sprockets end up the same, I plan to bolt the two blanks together and mill them at the same time. The blanks will be circles with the outside diameter being the top of each tooth.

Once set up, the plan is to plunge through with an endmill, then rotate the rotisserie table 6 degrees (milling), withdraw the end mill, rotate another 3 degrees, and plunge through again. I’ll repeat this process 40 times to create the sprockets. The 10” rotary table I have has a good perimeter degree gauge so I’m confident I can hit the angles accurately enough. I also plan on having a support block clamped to the mill table to brace the blanks while plunging.

My question, is there a type of endmill that is better for plunging through without walking or wobbling? The end mill I’ll need will be something like 23/64” or 9mm diameter.

 

[markba633csi]

I think you would find it easier to use the side of the endmill instead of plunging- move the blanks horizontally to cut each tooth
The fixture would need to hold the blanks solidly in position during the cut
A four-flute endmill would give smoother cutting with less vibration
Goes without saying you need plenty of cutting fluid
I would try one blank at a time so you don't ruin them both- in fact, it would be a good idea to do some test cuts first on a scrap piece
-Mark
Even with a center cutting endmill plunging into steel like a drill will be very challenging and will break tools unless the setup is incredibly rigid

 

[Winegrower]

There are two types of end mills, center cutting and non-center cutting. Make sure you have center cutting. If you don't, drilling a hole first is necessary, and is a good idea in any case.

 

[imagineer]

Steel is going to be slow going- can you not drill holes first to remove most of the material, then switch to an endmill?
-Mark

I can, but don't want to. The Z-axis travel and clearance needed to hold a drill chuck, versus using an end mill in a collet might warrant lowering the head on the mill. Being its on a round column mill (I know, I know...shut up), I'd like to avoid having to change the height mid-project.

I suppose I can look for the shortest possible drill bit, or cut one down such that I can switch from the drill chuck to a collet and still use the same head height.

Also, I don't mind it being a slow process. I've got plenty of time.

 

[markba633csi]

Edit: changed my reply

 

[Provincial]

All end mills wobble when plunge cutting. A stiff machine with tight spindle bearings will cut at least .005 oversize, and a mill drill will probably cut twice that much oversize.

One way to do it would be to experiment on a piece of scrap. Plunge with an end mill (2-flute, center cutting), a 3/8" if you have one. Measure the hole diameter and see how much oversize it cuts. Call that X. Decide the exact hole size you want in your sprocket and subtract X from that number. Is there an end mill available in that size? If not, but one is available that is about .010 undersize, consider using it, then reaming the holes to size afterwards.

How are you shaping the teeth of the sprocket other than the gullet? For your application, you might be able to freehand them with a grinder. Make a template that locates off the gullets and scribe lines to grind to.

 

[TorontoBuilder]

I’m in the beginning stages of a project where I will be machining two large chain sprockets (picture a 40-tooth bicycle chainring, but at 20” diameter). For the curious, I am building 2 automatic, solar powered compost drum rotators.

The material will be 14ga steel (or stainless). To save time and make sure the sprockets end up the same, I plan to bolt the two blanks together and mill them at the same time. The blanks will be circles with the outside diameter being the top of each tooth.

Once set up, the plan is to plunge through with an endmill, then rotate the rotisserie table 6 degrees (milling), withdraw the end mill, rotate another 3 degrees, and plunge through again. I’ll repeat this process 40 times to create the sprockets. The 10” rotary table I have has a good perimeter degree gauge so I’m confident I can hit the angles accurately enough. I also plan on having a support block clamped to the mill table to brace the blanks while plunging.

My question, is there a type of endmill that is better for plunging through without walking or wobbling? The end mill I’ll need will be something like 23/64” or 9mm diameter.

Sounds like a job for a plasma cutter to me.

But if end milling it is I'd side cut as mark suggested

 

[sdelivery]

I frequently plunge cut with an end mill, it is a great way to make a pocket or other deep recess. Be sure to use a center cutting end mill.

 

[imagineer]

This is what the final parts will look like. The blanks will start off as 19.23" round discs with a 1/4" hole at the center. There will be 4 other holes located near the outer edges of the wedge shaped openings that will be used for clamping to the t-slots in the rotary table. The wedge shapes are not integral to the design and will be cut after the gear profile is done. The end mill will be creating the bottom of the gear profile.

I'm also going to using aluminum instead of steel or stainless (seeing as all the chainrings on my road bikes have been aluminum, and were subjected to much more abuse than this contraption will impart).



I was hoping to place this on one of the waterjets here at work, but they're booked 24/7 for the next 6 months.

 

[Lo-Fi]

Are you starting with blanks that are at or near the finished outside diameter of the sprockets? If so, you won't be plunging the full diameter of the end mill. In fact, it's not bad practice to "nibble" rather than full depth side mill, leaving a small cut to side mill for a nice finish, particularly on not so rigid setups. It's often quicker too.