One of the single biggest cost drivers for machined parts is
the length of time it takes to machine it. The rigidity and
strength of the actual cutting tools often determines how
much time it takes. Very simply, the shorter a tool is, the
faster it can feed, and the less the part will cost to make. The
selection of these cutting tools is determined by the design of
the part and a few simple rules can really help reduce
machining time.
When designing parts that have pockets, or other features
with vertical inside corners, you will need to leave a radius as
the machining process uses rotating tools. Use the largestradii you can get away with. The tool that is used to machine a particular feature will obviously have a diameter of 2x
the radius that you put in your model. If you design a part with a 1/8” radius, it will require a minimum of a 1/4” tool to
cut that feature.
The larger a tool that can be used in that corner, the faster it
can feed through the material. As the length of that corner
increases, the length of the tool must increase as well and
that tool must be fed much more slowly to avoid deflection
and breakage. The relationship is worse than linear. For
every doubling in length, the feedrate is more than cut in
half. When figuring costs, assume that a double of the ratio
equates to a double of the cost of that feature. A good ratio
is less than 3:1. Once you get up to 4, 5, or 6 to one, the
feedrates are much slower. See figures 1 and 2. Under
normal circumstances, 8:1 is the upper limit and is very slow
and expensive to cut.
By using these simple guidelines, significant savings can be
achieved in the cost of your machined parts.Sometimes you just need to have a long small radius
because of assembly issues. There are still options to
reduce the cost of features like this. Figure 3 shows how
you can make a virtually square corner with very little
intrusion into the surrounding walls. This is a great
technique if for weight or assembly reasons you can't
tolerate a larger radius.
The key to this feature is to not put the center of the radius
on the intersection of the inside edges. Put the center point
inboard and then you can adjust it to fit your application.
Use the biggest radius that fits the application as well.
It isn't uncommon for engineers to put a radius both on the
floor and wall intersection as well as the vertical walls (see
fig 4). With the "apply round" or fillet feature on most 3D CAD systems, the easiest thing to do is to select both that
floor intersection and the wall intersections and just apply the same size radius to all those. But in fact, what saves
you a few seconds work to have just one feature, can cause enormous headaches for the machine shop and cost you
a lot of money in the long run.
It isn't obvious what it takes to machine the area in the corner. It is much more complicated if the floor radius is
smaller than the wall radius. Because of the equal wall and floor
radii, two tools must be used to clean up this area completely. The
wall needs to be cut with a ball end mill (an end mill with a full radius
on the tip). The floor of the part needs to be cut with a flat end mill,
but this will leave a triangular shaped section in the corner that
neither tool can
reach. (See fig 5).
This condition can
be avoided by
modeling the floor
radii smaller than
the wall radii (see
Fig. 6). This enables
the shop to machine
this entire area with
one tool that has a
flat bottom but also
has radii on its tips. In the last issue of Pro Tips we identified that the
larger the vertical corner radii can be, the faster the tool can travel
and the cheaper the part will be. Generally speaking the smaller thefloor radii can be, the better, with a 0 radius being the easiest of all.
In the US, tools are readily available with tip radii in .01"
increments up to .125". And when indicating a tolerance of this
floor radius on your drawing, make it as generous as possible to
allow the shop greater flexibility in choosing tools.
To put it into perspective, the equal corner radii detail will easily
cost 10x what the unequal corner radii detail costs. That should
offer enough incentive to spend a couple extra minutes modeling
the optimum radii on your part, and reap the benefits for the life of
the part.
One of the single biggest cost drivers for machined parts isthe length of time it takes to machine it. The rigidity andstrength of the actual cutting tools often determines howmuch time it takes. Very simply, the shorter a tool is, thefaster it can feed, and the less the part will cost to make. Theselection of these cutting tools is determined by the design ofthe part and a few simple rules can really help reducemachining time.When designing parts that have pockets, or other featureswith vertical inside corners, you will need to leave a radius asthe machining process uses rotating tools. Use the largestradii you can get away with. The tool that is used to machine a particular feature will obviously have a diameter of 2xthe radius that you put in your model. If you design a part with a 1/8” radius, it will require a minimum of a 1/4” tool tocut that feature.The larger a tool that can be used in that corner, the faster itcan feed through the material. As the length of that cornerincreases, the length of the tool must increase as well andthat tool must be fed much more slowly to avoid deflectionand breakage. The relationship is worse than linear. Forevery doubling in length, the feedrate is more than cut inhalf. When figuring costs, assume that a double of the ratioequates to a double of the cost of that feature. A good ratiois less than 3:1. Once you get up to 4, 5, or 6 to one, thefeedrates are much slower. See figures 1 and 2. Undernormal circumstances, 8:1 is the upper limit and is very slowand expensive to cut.By using these simple guidelines, significant savings can beachieved in the cost of your machined parts.Sometimes you just need to have a long small radiusbecause of assembly issues. There are still options toreduce the cost of features like this. Figure 3 shows howyou can make a virtually square corner with very littleintrusion into the surrounding walls. This is a greattechnique if for weight or assembly reasons you can'ttolerate a larger radius.The key to this feature is to not put the center of the radiuson the intersection of the inside edges. Put the center pointinboard and then you can adjust it to fit your application.Use the biggest radius that fits the application as well.It isn't uncommon for engineers to put a radius both on thefloor and wall intersection as well as the vertical walls (seefig 4). With the "apply round" or fillet feature on most 3D CAD systems, the easiest thing to do is to select both thatfloor intersection and the wall intersections and just apply the same size radius to all those. But in fact, what savesyou a few seconds work to have just one feature, can cause enormous headaches for the machine shop and cost youa lot of money in the long run.It isn't obvious what it takes to machine the area in the corner. It is much more complicated if the floor radius issmaller than the wall radius. Because of the equal wall and floor
radii, two tools must be used to clean up this area completely. The
wall needs to be cut with a ball end mill (an end mill with a full radius
on the tip). The floor of the part needs to be cut with a flat end mill,
but this will leave a triangular shaped section in the corner that
neither tool can
reach. (See fig 5).
This condition can
be avoided by
modeling the floor
radii smaller than
the wall radii (see
Fig. 6). This enables
the shop to machine
this entire area with
one tool that has a
flat bottom but also
has radii on its tips. In the last issue of Pro Tips we identified that the
larger the vertical corner radii can be, the faster the tool can travel
and the cheaper the part will be. Generally speaking the smaller thefloor radii can be, the better, with a 0 radius being the easiest of all.
In the US, tools are readily available with tip radii in .01"
increments up to .125". And when indicating a tolerance of this
floor radius on your drawing, make it as generous as possible to
allow the shop greater flexibility in choosing tools.
To put it into perspective, the equal corner radii detail will easily
cost 10x what the unequal corner radii detail costs. That should
offer enough incentive to spend a couple extra minutes modeling
the optimum radii on your part, and reap the benefits for the life of
the part.
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